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Boeing 787 Suffers Rare Dual Engine Failure on Landing (thepointsguy.com)
406 points by georgecmu 48 days ago | hide | past | web | favorite | 350 comments



I think the auto shutdown of engines in flight, in case it senses reverse thrust, is a great design.

The 787 is a replacement for my 767 (currently a pilot on the aircraft) and in 1991 a Lauda Air 767 suffered a failure in one of its thrust reverser and it became activated during cruise. The aircraft was destroyed in flight.

The thrust reversers should not activate during flight (a air/ground sensors prevents that) but as it is a catastrophic failure, having the engine shut down automatically seems like a great fail safe device.

In this case it seems that the 787 engines were reversed before the air/ground sensor had sensed ground and it is a potential problem.


As long at it is well documented and part of the training, and also ideally the system should indicate to the pilot why it is shutting down the engine(s), to avoid something like this:

https://en.wikipedia.org/wiki/TAM_Transportes_A%C3%A9reos_Re...

"The safety system automatically cut the power to the engine suffering the thrust reverser malfunction. There was no alarm in the cockpit to indicate that a thrust reverser had been accidentally deployed. The crew had no way of knowing what the true problem was. The copilot, seeing the right engine power lever automatically move to the closed position, thought that the lever had slipped back and pushed it back to the full power position alongside the left engine throttle lever. Once again, the automatic safety system closed the right engine throttle and the captain, who was conducting the takeoff, called for the autothrottle system to be switched off. After switching off the system, the copilot again pushed the right engine power lever fully forward and forcefully held it there."


there's a really interesting story around Lauda Air flight 004 where Boeing attempted to write it off as pilot error but Niki Lauda basically threatened to go fly one himself and recreate the conditions as proof it was not. Eventually Boeing conceded and he didn't have to actually risk himself or another of his planes, but still an interesting anecdote.


That's just the kind of person Lauda is.

He was severely burned in a Formula 1 race at the Nurburgring, and despite still suffering badly scarred lungs and weeping wounds he was back in the seat before the end of the season and only lost the championship that year because he refused to go out in the pouring rain to race the Japanese Grand Prix as he decided it would have been foolhardy.


He lost the championship by one point. The movie Rush on this story is also amazing.


Indeed - you can tell Ron Howard is a genuine fan of the sport - he's often at races.



TCMA, the system the article refers to, doesn't actually shut down the engines in flight. In fact, one of the explicit conditions for its activation is the aircraft being on the ground:

> The EEC commands shutdown of the affected engine when the: > • airplane is on the ground, and > • thrust lever is at idle, and > • engine is above idle speed and not decelerating normally > The EICAS caution message ENG FAIL (L or R) is displayed with an aural beeper once the engine falls below idle speed.

https://quizlet.com/27148021/engines-apu-787-ge-engine-flash...


Fyi, military pilot here, there are a handful of mil aircraft that can reverse thrust in flight. A great too for dropping down fast to avoid low/slow flight over hostile areas. There might be a relationship between that ability and this problem.


Also, as I recall, Concorde could do this too...


Lucky bastards, get all the fun features...


It can be crazy dangerous. Once a big plane is dropping like a rock, getting back into a climb is very difficult.


> In this case it seems that the 787 engines were reversed before the air/ground sensor had sensed ground and it is a potential problem.

What I don't understand is why does the reverse thrust input from the cockpit even influence the engine control when the air/ground sensor doesn't sense ground? It seems like an obvious lockout mechanism to have in place, if the result is shutting down the engines when the combination occurs.

Then just have a manual air/ground sensor override should that thing fail and you know you're on the ground and require reverse thrust.


I am very scared of flying (although I still do it, although it's not fun). Any advice/rationalism you can convey? I am a person that became sweaty and anxious reading your post.


My advice is to look at the statistics. Commercial flying is much, much safer than any other mode of transport.

Do you get scared when you drive to work? Or take a train? Both are more risky. (Driving significantly so.)

More specifically... The pilots have extensive training, modern airliners have fail safes everywhere, and the amount of research and engineering spent on aviation safety is mind boggling.

See: http://airlines.org/dataset/safety-record-of-u-s-air-carrier...


An accident during flight is pretty much a binary event when it comes down to whether you live or not, but with cars there is a whole slew of different types of accidents and vast majority of them don't cause instant death preceded by 45 seconds of absolute terror. Before you downvote what I said in no way discounts the statistics. It is much safer to fly of course than to drive, but when we think of accidents which is an inevitable fact of any mode of travel one mode is binary you live or die and another not. To our mammalian brains the statistics is a very hard sell.


Aircraft accidents aren't binary at all. Many crashes have few or some casualties, e.g. landing gear failure, running off the runway or hitting another aircraft on the ground. Ditto with in-flight failures that don't impact the flight (there's only one major thing that fails, picked up by the redundancies), engine failures that lead to emergency landings, extreme unexpected turbulence or ditchings etc. that might cause injuries and in some cases deaths. It's only the worst crashes (termed "air disasters" or similar) that lead to everybody on board dying. Of course, these get the most attention. I blame the news.


How can you argue against what I said, it takes profound willingness to ignore precise words to accomplish that. I did not say all accidents I said majority are. If you consider a mosquito hitting the windshield of an airplane when it's parked on the ground and no passenger in it an accident that's your problem. Of course I meant an accident that happens during flight as that's the vast majority of the time that people are in an airplane and when they're worrying about dying.


I don't think that's a super fair assessment of my comment. If we're just disagreeing on the definition of an in-flight accident, or what percentage between 30% and 90% of accidents constitute a vast majority, we should probably leave it at that, because that's not really disagreement :)

My point is, there's plenty of accidents caused by things that happened while the plane was in the air, that has many survivors or just some injuries. The news article at [0] has some examples, and it's possible to search for more. Granted, if you exclude the cases where the injuries or casualties were caused during an attempted landing or a forced landing due to malfunction, the picture would probably look much more bleak, but then you would also have ruled out a large portion of the accidents that happened.

It's thankfully _extremely_ rare that it's been impossible to make an honest attempt at a controlled landing or ditching during an accident, and in a large part of the cases where this was possible, things turned out okay for most passengers.

[0]: https://www.nbcnews.com/news/us-news/after-air-accidents-sur...


I agree with what you say, but I think it has more to do with (perceptions of) control than the failure modes.

When you’re in an automobile, you could choose to drive faster or slower, safer or more aggressively. You can tell yourself that you are more alert or a better driver to the people who die in fatal car accidents. You can tell yourself that you have a superior ability to avoid that drunk driver who might suddenly veer into your lane from oncoming traffic. And once your car comes to a stop, if you’re not seriously injured, you can just get up and walk away.

In an airplane there’s none of that. You can make vanishingly few choices to affect the safety of your flight. And you’re stuck in that tin can until it’s on the ground and you’re either alive or dead.


I was very scared of flying for a long time. I fixed it by booking a flight a week for 12 weeks, starting the first week heavily medicated, and following the advice of a book on agoraphobia (Freedom from Anxious Energy). Then cut down the drugs progressively each week until it was none. Worked a charm


Congrats on it working for you!


Airliners are incredibly durable machines. This article about about how much stress the wings can take is what I think about anytime I see the wings bouncing in turbulence.

https://www.travelandleisure.com/airlines-airports/airplane-...

Then there is this image:

https://www.wired.com/images_blogs/autopia/2010/03/index1.jp...


It is perfectly rational to be scared of flying. Hell, you're sitting in a pressurized cylinder at 30'000 ft flying at 700 mph. But you have to realize that the crew would not do this job if they weren't sure to come home to their families and dogs every night. Most pilots I know aren't thrill seekers but their love of flying are more a drive due to interest in travel and technology.


I advise you to forget statistics. They are very hard to relate to (we're not good visualizing large numbers anyway). Instead, think of the pilots who know these things inside and out and still choose to happily board each and every day. They raise families and eventually retire uneventfully. If they know the risks and still choose to go to work, surely you can board a flight.


That's a bad argument, every dangerous profession also has lots of experienced people that still choose to work there.


I feel for you. I was the same way. I just internalized the stats and remind myself of them every time I fly. As a numbers guy, I simply cannot justify my abject fear compared to being a pedestrian or auto driver.

Another thing I do is to remember the physics when I'm on a bumpy. Thanks to momentum, planes don't just fall out of the sky, and even bad turbulence is not likely to result in deaths.


A bit of an extreme solution, but I hear this workshop run by Air France is good. https://www.airfrance.fr/FR/en/common/guidevoyageur/pratique...


I have been there. If you think this anxiety has been on the rise (think carefully about if some time ago you had such strong emotions reading about accidents, etc), consider going to a doctor and treat your anxiety. Medication solves it, I can assure you. Rationalizing does not go very far.


Tons of good advice on this previous post: https://news.ycombinator.com/item?id=17654487


I was not scared of flying until one day while taking off from O’Hare lightning stroke to the tip of the left wing of my plane. I was sitting just on that row aisle side though. For a split second it felt like we stoped mid air. Kind of like when you drive over a bump with your car. After that for quite a while I was scared shitless of flying. But that didnt stop me I probably flew a hundred times since then and now when I do I dont even think about whether I am on a plane or train ( well my be for a bit). Airplanes are incredibly robust machines so my advice is just stop thinking about it and keep flying.


Summarized, I understand that you mean that a malfunction in the "reverse-thrust-detector" (which switches off automatically the involved engine(s) when the reverse-thrust is not operating within its envelope => referenced as "feature" below) is more desirable than not to have it?

(because if, when having a mulfunction of the detector's "feature" in mid-air the engines switch off you still have some time to fix/override it, but on the other hand when not having it if your reverse-thrust engages you're screwed immediately - and even if the feature fails nearby the ground you're still be slow & low enough for at least some passengers to have some chance to survive)

Correct?


Yes, no engines is bad but thrust reverser in flight is a instant killer.


Apparently it's bad if done at cruise (due to the small margins in thin air) but if done in mid-air it's controllable.

See this episode of Mentour Pilot:

https://youtu.be/OguKthkuaT4


it's a great fail safe but not if you lose your engines in flight and can't relight them like in the article.


> it's a great fail safe but not if you lose your engines in flight and can't relight them like in the article.

It is still better to lose all engines than to have thruster reversers deploy in unsafe situations. You may stall without an engine at low altitude in a landing configuration, but you should still be able to glide for some time. Deploy reversers (or even a single reverser) and you fall like a brick very quickly.

https://aviation-safety.net/database/record.php?id=19961031-...


This existing system wouldn't likely cause loss of all engines unless the PIC commanded all engines TR in-flight. Also, it doesn't make sense that such a system would go out untested because such a system should try command engines to idle before cutting fuel to what it thinks are runaway engines. Finally, it's still unknown if it's a Boeing software issue or a RR T1K issue... the OP article is purely speculative and guesswork "news" is not how aircraft safety is handled.


A single engine on reverse is enough to mess up the the flight violently enough that it'll break apart before hitting the ground (see the Lauda Air case).


I think there are a few options for a failsafe here.

none where it would deploy the reverse thrusters and probably crash regardless of other inputs to the system(could be one or more of many inputs wheel speed, slats deployed, airspeed, elevation, throttle position, etc.)

turning off the engine (presumably you want this because you are on the ground but your ground sensor is failing so you want to cut engine and apply brakes which is less preferable than the reverse thrusters but manageable normally.)

ignoring the input altogether.

not a failsafe at all and an unexpected failure mode of the system (I think this is probably the case since they couldn't relight the engine on the ground)

the point i was making is that if you have the failsafe turn off the engines under normal operating procedures it should be able to relight when in flight and it's not good if a software glitch turns your heavy into a glider without possibility to relight. (i am not a pilot but it's my understanding that you still have the turbines spinning and all you need to do is give it some fuel and fire up both ignition plugs. might need to use the compressor to spin them up to full speed but i doubt it.)


Have a mechanical latches on reverse cowls that will be operated separately of the code that deploys reverse. Landing without reverse is better than falling like a brick.


Having altitude and being able to glide in is a hell of a lot better than one of your engines reversing and throwing your plane into an irrecoverable spin.


I agree and that's not what I meant I replied in another comment.


Cutting power to all engines in response to an event that is most likely to happen while the aircraft is operating close to, but not actually on, the ground is not failing safe.


I am not sure there is a perfect solution here. you have competing issues that you need to resolve.

If it gets pulled in normal flight you would lose both engines but that's fine because you can glide and restart the engines. if the altitude sensor/ground sensor was broken and you want to override the reverse thrusters you have a conflicting goal there.

If you accidentally pull it during the approach you lose your engines and probably don't have time to relight before landing. in that case you might be able to still land by backup systems(FAU or just the ancillary tail turbine?) and glide?

I guess it boils down to how much trust you place in the pilots to not do the wrong thing or how much trust you place in the machine to not do the wrong thing/malfunction. It's a difficult question without a perfect solution imo.


Well, I thought that the design was that they would not engage if it gets pulled in normal flight.

If it's on the approach, I'd be worried. I imagine that that, at an airport like ORD, that might result in more risk than anyone really wants of the plane unexpectedly touching down on an interstate highway or something like that.

If it's like what Boeing described in the bulletin, where the engines only get shut down after the wheels are on the ground, that's maybe not super worrisome, but I can still see some room for concern. My reconsidered but still totally uninformed reading on the situation is that it takes some time for all the aircraft's systems to get the memo on whether the plane is on the ground, and that what's really going on here is that the order in which they get the memo isn't quite right. So the question of whether or not to allow a Lauda Air type situation isn't really at play here, but perhaps a related glitch to the one that caused this event could interfere with something like aborting a landing at the last minute.


With most types loss of engines on approach is FAR less dangerous than engaging your reversers in flight.


If the thrust reversers are deployed in flight, you're going to crash, even if near the ground. The airplane would be uncontrollable.

If the engines quit, you still have control.


Interestingly, some airplane types allow the use of thrust-reversal in flight, notably some fighter jets. It's apparently used when you do need to sink quickly..


Hmm. There might need for a sooner transition point for that particular inhibition. Perhaps somewhere between ground effect (well below minimums) and back trucks are turning.


There is Airplane pilot on HN? I am surprised, did you make a career switch from IT?


"Hacker: A person who delights in having an intimate understanding of the internal workings of a system [...]"


I'm always amazed how diverse the people on HN are. Not everyone is an IT professional but we share a passion for technology.


'Passionate' is not the adjective I would use to describe my feelings towards technology.


Maybe more like passion in the "crime of..." sense?


Modern planes are flying server farms. How is this surprising?

I've worked with several pilots in my career. Unless you have a ton of experience, IT pays better than flying.


Airplane pilot checking in. There are multiple. :)


Yes, some of my software engineer colleagues are pilots as well. You never know who you are talking to, especially now-days.


This is really strange. Not only did they have a dual engine failure, they couldn't restart while stuck on the runway. After 40 minutes, the aircraft had to be towed in. Then maintenance couldn't find a fault.

This is an ANA aircraft, which means it's probably maintained well and flown by competent pilots. Looking forward to a good analysis on this.

Most large aircraft have a "weight on wheels" switch, activated when the aircraft is on the ground. There might be some bad interaction between the "weight on wheels" sensing and the thrust reverser protection. Pure speculation: initial touchdown, pilots deploy reversers, small bounce into the air, weight on wheels switches open, engine controller detects flight condition with thrust reversers deployed and shuts down engines.

The other side of this kind of failure is dealing with unintended thrust reverser deployment in flight. Here's an overview.[1] Historically, it wasn't a big deal at altitude, but on some newer aircraft, it is, and so stronger steps have been taken to avoid it.

[1] https://lessonslearned.faa.gov/ll_main.cfm?TabID=3&LLID=34&L...


Your speculative scenario has actually happened, but on a significantly less computerized Boeing 737, in 1978.[1]

In that case, the aircraft touched down, reversers were deployed, a runway obstacle (a snowplow) appeared, and the pilots attempted to go around. They got airborne and cleared the obstacle, but as they lifted off, the "squat switches" on the main gear opened. One of the reversers was still deployed, and with the squat switches open, was no longer powered, and could not retract. The aircraft was uncontrollable, and crashed.

In that case, shutting down the reversed engine would likely have been preferable, although if we're doing counter-factuals, telling the snowplow driver about the airplane's revised arrival time is your go-to move, I think.

[1] https://en.wikipedia.org/wiki/Pacific_Western_Airlines_Fligh...


It might be less of a small bounce than the nose wheel not being on the ground and/or not having enough weight on it. I know Airbus and Boeing handle this differently, but I don't remember the details (I want to say Airbus allows reverse thrust once the mains are firmly on the ground but the nose isn't, but I need to check my manuals).

I really do find the inability to restart extremely interesting though. I'd bet it's a bug somewhere, but it would be alarming if Boeing programmed in a lock-out on the failed engine(s) (I've heard it talked about as a solution to the sudden deployment of reverse thrust in flight) .


The Boeing 787 is now about nine years old. I wonder how much of this is operator proficiency triggering latent bugs (aka, "pilots got too good at pushing buttons quickly").

The bugs in the Therac 25 were only discovered after the machine had been in production for a while (you had to be typing "too quickly" in order to get the software to trigger the bug with the safety interlock).


It's not very typical for pilots to push buttons quickly. Especially not the thrustreversers and engine and flight controls.

But it is entirely possible that nobody pulled the reversers while still airborne in 9 years, because it's a very strange thing to do. A bit like opening your car door while still driving 60 mph... Nobody does that in normal operation.


A bit like opening your car door while still driving 60 mph... Nobody does that in normal operation.

You don't open and shut a door when you notice it didn't fully close because you can hear the wind?


In airplanes, a lot of procedures and diagnostics involve not re-trying a failed action, not touching anything before reading a check list, nor flipping a switch twice if it didn't work the first time.


A better comparison might be putting the car in reverse at 60 MPH.


You never missed a gear, thinking you were driving a 6 speed rather than a 5 speed?


The sleeve of my coat caught on the gear shift while making a left turn and threw it in reverse while on the freeway.

It made a grinding noise and I yanked it back into drive.

Other than the noise nothing bad happened.


No, but for as long as I've been driving manual cars they've had gates to prevent that very action. (1995)


Not mine (Honda Jazz). But you physically can't do it, as the reverse gear has no synchro and won't mesh unless stationary.


It can be done, but it's neither easy nor a good idea: https://youtube.com/watch?v=ARJENV0qGdY

You'd certainly struggle to do it by accident.


it should make a loud grinding noise though,


At 60 mph? No. I listen to the wind until I get a safe opportunity to properly close it.

(I might try to pull it more shut while moving. But not open it.)


Good luck even opening it far enough if you're going at 60 ...


Non-suicide doors[1] in cars are very difficult to open at 60 mph. Between air pressure, and poor leverage, you are going to have a hell of a time doing it.

[1] https://en.wikipedia.org/wiki/Suicide_door


Just roll your window down first!


I've seen a number of videos on youtube of reversers been opened, if not fully spooled up, off the ground. It's not common but it does happen, especially in high cross-wind or otherwise iffy landings.

Here's the first video I found but there are certainly others https://www.youtube.com/watch?v=-RO66a_nvus


> But it is entirely possible that nobody pulled the reversers while still airborne in 9 years, because it's a very strange thing to do.

That's overstating the case a bit. I'm 100% a layman here but I'd agree that the behavior seems like some sort of bug. But…… some planes (typically not commercial, although the Concord is one notable exception) call for reversers in flight under certain circumstances. Additionally there are definitely circumstances where a pilot might deliberately think reversers before touchdown are a good idea.


Make me think about could we use a robot to fuzz physical inputs.


Probably not cost effective as opposed to either making the software more resilient or just letting some monkeys/interns loose in the cockpit and pressing everything (With some focused searches obviously)


I don't know, I suspect software resiliency is one of those "gets exponentially more expensive as you approach perfect" situations and a robot could bash inputs at speeds a human couldn't in thousands of orders, it would certainly let you test those weird code paths that blow up when they are never exercised until they are.


It does get exponentially more expensive, which is why you have to pick a certain level of ~perfection~ and then stop.

Also, the robots would have to bash at the speed of a human: The point is to mimick human inputs not necessarily test every code path (Which you could do in software)


When testing resiliency software testing isn't the whole of the thing.

I spent a lot of time at new job (well 18mths now) on making the software side more resilient, we still had a site wide outage of internet access and comms and internal systems because one of the women in the sales office unplugged a socket in the ingress comms cabinet to plug her hair straighteners in.

Coincidentally my request to have physical access control on all the comms cabinets shot right up the list.

The boundary layer between software and hardware itself has to be tested, the real world is a messy place :).


I wonder if selling tickets to watch would offset that somewhat?

I'd pay quite a lot of money to camp out in the desert for a week watching fuzzing robots try to crash airliners...


..or not requiring physical hardware to test?


How? Planes won't not have cockpits for decades surely.


The software has some interface to the physical controls.


Considering the level of fly-by-wire that goes into airframes nowadays, I could certainly see such features added to older airframes through flight-software updates after they are already in service. I have no idea if that's how it's done in the field, though.


Software updates are done. However the manufacture has to recertify the airplane with the FAA first. This is expensive, requiring months of testing and a lot of paperwork. Each plane model getting the update needs to be re-certified, it isn't good enough to say that it works on this other model even if the planes are identical for purposes of that software.


The Ryanair booking website has several bugs exactly like what you describe. If you use it too quickly it won't sell you plane tickets. If you use it more slowly it works fine.


Probably to try and prevent bots


Boeing’s answer seems to say if you accidentally pull the thrust reverser lever before the aircraft is on the ground it just force shuts down the engines.

That sounds like an odd safety override. Surely a better solution would be to just not activate reverse thrust. Unlike car engines a jet engine can’t just be quickly restarted if it accidentally shuts down. It typically takes 30-60 seconds to get going.

This and earlier incidents are highlighting the dangers automation can add to mission critical systems. No pilot wants to hear notices about “hey, so in case you didn’t know our programmers added some code that does this strange uncommanded thing when you push buttons a certain way”


Do you have experience with aircraft engine design, or landing protocols, or insight in to the meetings when the engines were designed?

If so, I'll take back my words but considering the manufacturer doesn't know exactly why this happened, and the engineers on the ground said it seems like a "software bug", it seems a bit presumptuous and frankly a bit comical to start saying things like:

"Surely a better solution would be to just"...

Ah well, glad a 5 sentence comment on the internet can resolve an issue in a hardware, software, and social engineering challenge decades in the making.


Reminds me of the quote,

"For every complex problem there is an answer that is clear, simple, and wrong." -- H.L. Mencken

One of the things I have experienced in my career is the mind boggling complexity of the software in systems that are capable of killing people. The more people it can kill the more complex the software.

As a result when I read articles like the one posted I find it tantalizing to speculate about the requirements behind the story. Taking the story at face value that Boeing has thought a lot about it and that is how the software has to be, what prevents the other solution of disabling the reverser? My guess is that you want the reverser to work in the event of software failure so it has to always work, but if the pilots pull it while you are flying that would probably rip the engines off the plane. Perhaps the compromise is to take the reverser off during flight and relight the engines using aerial pressure to spin up the engines. (which has me wondering if airlines still have a turbine they can drop down to start an APU which can then be used to start the engines). Which leads me to wondering if the pilots turn off the reverser right away do they have enough residual engine power to relight? Versus when they got to the end of the runway and had nothing? Clearly the 777 doesn't have an APU running when it lands since they would have used that to do an engine start at the end of the runway or if it does, it isn't running.

It is one of those things that my like minded systems friends could sit around lunch and make a good hour and a half discussion out of.


Relevant article: "Reality has a surprising amount of detail"

http://johnsalvatier.org/blog/2017/reality-has-a-surprising-...


This is very good.


If the reverser could rip the engine off the wing, it wouldn't be safe to engage on the ground. Just thought that was worth pointing out.

A reverser only redirects thrust, and only generates about 60% of max thrust pushing backwards.

It's only on for a limited time on the ground because under a certain speed, you run the risk of blowing FOD into the intake path, putting the engine at risk.

I'm not aware of any reason to outright lockout a reverser in flight other than to hedge your bets against a very, very poor set of configuration choices by the pilot (reversing near stall speed too low to recover). I could foresee scenarios where being able to use reversers in the air could save the aircraft with the right combination of subsystem casualties.

Not a pilot or engineer, but absolutely LOVE aviation.


The cruising airspeed of an airplane is faster than its landing speed. I don't know if that alone is enough to rip the engine off the wing, but it's something that it sounds like you did not consider in making your point.

[EDIT remove "roughly speaking an order of magnitude"]


>The cruising airspeed of an airplane is roughly speaking an order of magnitude faster than its landing speed.

No. No it is not. 787 approach speed is 145-153 knots. It does not cruise at 1400+ knots.


Thanks for bringing more precision to the conversation (really).

What would you like to call the factor? 3? Call me a weasel, but I was hoping "roughly speaking an order of magnitude" would go down to 3. I suspected it was comfortably above 2. Am I right about that?


> I was hoping "roughly speaking an order of magnitude" would go down to 3.

I'd call that half a order of magnitude, considering log₁₀(3) is around 0.477. So you're off by more than a factor of two.


You know, I considered saying that the "roughly" buys me a factor of two in log space. I didn't want to be that person, but I'm glad you were ;).


It's surprisingly non-trivial to pin down exactly what the 'correct' scale[0] for a given measurement is. I do agree about "roughly" generally being up to a factor of two in the appropriate scale[1], although I'm the sort who thinks a 19% increase (or 16% decrease) should be called a quarter of a factor of two.

0: Uniform, linear and logarithmic are obvious candidates, but depending on the domain you can end up with some really wierd scales (eg floating-point ULPs, which can look logarithmic or linear, but aren't either).

1: hence > So you're off by more than a factor of two.


3 is half an order of magnitude (because it is a good approximation to 10^0.5)


re go down to 3: Use different words and see if you think it was reasonable

roughly 10x going down to 3x would also mean it would go up to 17x. That's a pretty wide range, so I don't think that order of magnitude is going to ever really be similar to 3x of something on the basis of what it means.


but what about a rough order of magnitude? There is definitely some qualifiers you can put on "order of magnitude" to make it include 3.


"within an order of magnitude"


It seems unlikely that a jet engine could rip itself off the wing in any scenario based on thrust alone. The forces involved are not that large.


Your wording makes it where you aren’t factually wrong, but a plane was in fact wripped apart due to the thrust reverser engaging on one engine of a 767:

https://en.m.wikipedia.org/wiki/Lauda_Air_Flight_004


The difference isn't that large in the grand scheme of things. The engine mount would have to be able to fail in it's operational envelope in order for the reverser kicking in to be realistic.

In fact, the reversed kicking in would decrease the loading on the aircraft by decreasing it's airspeed.

This isn't a case of acceleration being able to break the mount from the frame. If it ever could, one wouldn't want it on the plane in the first place.

Doesn't mean you couldn't ruin your day with it, but it isn't an instant catastrophic failure either.


> but it isn't an instant catastrophic failure either.

Read the accident report and follow up on Lauda Air Flight 004. Boeing specifically was forced to issue a statement that it was virtually impossible to overcome a catastrophic failure outcome from a thrust reverser deploying at cruising speed.


I stand corrected. Way to go Boeing. Seems like that should have come up in a design meeting at some point.


Apropos, the Wikipedia article (https://en.wikipedia.org/wiki/Thrust_reversal) mentions number of events where an engine reverser was deployed in flight, one involving a 767 which resulted in the loss of the aircraft.

That counts as a pretty serious thing to avoid :-).


Yes, Boeing still puts the RAT - ram-air turbine - electrical backup system on their planes.


Why "still"? Even if the chances of a failure of both engines is almost zero, it has happened that there are losses of fuel. The ram-air turbine allows the pilots to have some instrumentation. That's super useful, for example in this accident: https://en.wikipedia.org/wiki/Air_Transat_Flight_236


> The ram-air turbine allows the pilots to have some instrumentation.

Ideally, yes. I submitted a recent incident where the RAT deployed on a 777 because it's so unusual (and was also not due to lack of fuel):

https://news.ycombinator.com/item?id=19039978


“Still” because if you read the parent to my reply he uses that term in his query.


I always love it when you have worked on something for months and someone new comes by and says "Why don't you just X?".


It's worse when they are right though. I wish I could say that had not happened to me.


Is that from other experts with a very close understanding of the specifics, or random people online?

What I find is you start with several viable approaches, pick one and go down the path enough to figure out the downsides. At which point you need to decide to backtrack or keep going. That’s the hard part not simply coming up with a seemingly simple solution.


Not "random people online", but I've got a few friends I think of as "terrifyingly smart", and one of their common characteristics is how they've all, in areas of expertise I've been investigating/researching/working in for weeks or months and which they have barely a passing interest in, quite obviously thought about a problem I'm describing and thought through a bunch of the obvious options, categorised them, and made conclusions about which avenues are workable and which should be discarded, and come up with either a workable solution or options I'd not even considered yet - all just in the course of a conversation over coffee...

(Somewhat frighteningly, two of those people are doing that at Facebook right now...)


> Is that from other experts with a very close understanding of the specifics, or random people online?

These groups are not mutually exclusive, membership can only be assigned by a domain expert and is finally irrelevant if the other is right.


Constraints define good decisions. A new team member making a viable suggestion is very different from a random person tossing out a wild ass guess that happens to be right. The difference is the random commentator has no real way to judge how viable something is, and thus is simply tossing out ideas.


This can be right when things change over time and people still operate under their initial assumptions. Times change constrains change but on some teams assumptions and choices are not revisited.


I am fine with that. In most cases they aren't though. Or they don't understand the whole environment.


My experience is about 50/50, both with my own mistakes and with others.

Generally when one is very close to the problem, one sees the environment as immutable. Because, well you spent very many hours building that environment, for very good reasons. And your complex solution "has to" work within the constraints of that complex environment.

Whereas, the "mind of a child" that doesn't grok the environment, also doesn't have a fixed notion of it. This is anecdotal of course, but maybe half the time what I see happen is that it's easier/better to change the environment and this can only be seen with fresh eyes.

It's not the spoon that bends, and it's not you that bends around the spoon. There is no spoon.


Had someone do that this morning. They ran across a problem that has been the focus of a multi-year effort involving dozens of people. Fixing the problem is a big part of the literal #1 priority for the company.

Anyway, he hit an example of the issue, and wanted to just put in a bug ticket to one team, and didn't understand why that was not useful or necessary.

My analogy is that it's like showing up at NASA in 1965, and wanting to submit a ticket that says "Your rockets can't actually go to the moon. Fix rocket so that it can go to the moon."


I found that I now distrust a lot of articles as people tend to get the things in my field blatantly wrong all the time.

Things are complicated and rarely as simple as they seem.

On the other hand, I've worked on things for hours before for someone to glance at the problem and solve it. Always a humbling experience.



I never knew there was a name for this, but I feel like everyone has come across this feeling at some point in their life.


The best part is when your project is running late or having issues, and the non-technical manager plans a meeting with all the other dead-weight people. "Explain the problem to us, and since we know absolutely nothing about it, our stupid questions might give you new insights". Facepalm.


Exactly. "Let's all take a step back" or "I'd like to challenge that". Makes you sound insightful while knowing nothing.


I for one love this type of question, and was the one asking it as I started out. 10-20% of the time, the asker has a valid suggestion. The rest of the time, the explanation as for why it will not work or is not a good idea fosters a better understanding for the asker, and reinforcement brush up for the answerer.


I always appreciate those comments. Sometimes they include something obvious I never thought of.


Yeah, it shows someone is trying to understand the problem. I mean, sure, most often the response is, "because that would require data that does not yet exist when the decision must be made," or, "there are legitimate circumstances where that would be the exact wrong response," but it does get you thinking.

It is less appreciated when people phrase things in a condescending manner, though.


They are right more often than you’d expect. Irrational escalation, loss aversion, confirmation bias, etc play their part everywhere causing senseless projects to be pushed to completion.


> They are right more often than you’d expect.

When the "they" is at least another expert in the same field and not a total stranger completely ignorant of pretty much every relevant detail, and the "you'd expect" part starts at a baseline of nearly 0% for people walking in out of the street unfamiliar with the specific problem at hand, then yes, "they" are right more often than I'd "expect."


In this case, possibly years


HN is the worst for it. Especially when it comes to tech stacks.

If you use anything other than non-SPA, Go, PostgreSQL then it's over engineered and doomed to fail.


Tech is the worst. Fetishizing disruption leads to neophytes thinking they're breaking new ground when they're just re-discovering long dismissed ideas.


My answer to the question is no, but I am a pilot, and my expectation is an invalid input is recorded but is a no op. It is fly by wire so why not just no op the request? The idea such input suggest sabotage, is more paranoid than cautious. And shutting down the engines is hardly fail safe. It's just less fail danger than engaging the thrust reversers.

Quite a few turboprops have beta range that is supported for use in-flight. The purpose is to increase the rate of decent, similar to slipping the plane. So it's not always an invalid input; it's make/model/phase of flight specific.


Welcome to the internet, where everyone's an expert, the problems are real, and the points don't matter.


So the bulletin says that activating the thrust reversers too soon can cause un-commanded high trust. Activating it too soon by itself does not cause a shutdown. But one monitor that could stop the engine would be the overspeed monitor. The un-commanded high thrust might have tripped the overspeed monitor on the turbines and shut both of them down. There are numerous other monitors on the engines that might also play a role. So determining root cause might still take a while still.


This is an issue carmakers have had to deal with, too, since the invention of the automatic transmission: what if you put it in reverse while driving on the highway?

There are a number of YouTube videos with people who have tried it. The answer, in a modern car: pretty much nothing. It just stays in Drive. On one car, it turned on the backup camera display!


Standard transmissions nowadays often include mechanical features as well that make it very difficult / impossible to engage reverse (and first gear) while going forwards at more than a few km/h.


Even a 1950s era transmission will still be very hard to force into a gear if the speed difference is too great. Likewise you'll have a very difficult time shifting into reverse or 1st at speed. That's just how syncros work. You might be able to overcome it by double clutching but it would require conscious effort. Even just shifting to reverse while rolling is difficult.


Still possible though. Years ago a friend and I pulled a manual transmission in a junk yard. He got it home, installed it, and then discovered reverse was completely stripped.


A stripped reverse gear would be indicative of failure to put it in reverse.

If the connecting rods had fled the engine that would indicate that someone succeeded in getting it into a gear it didn't want to be in.


I thought that most modern aircrafts were not allowing the reverse to be deployed if the wheels didn't touch the runway.

Talking about an odd safety override. It's kind of a self-destruct button.


It's not an odd safety override. An overthrust situation is always dangerous for various reasons. One of the more spectacular is the possibility of liberating airfoils from the engine and shooting them into the cabin.

It's not a self-destruct button, it's anti-destruct limit.

As for the thrust reverser, there are many integrated systems on the aircraft. It's possible that the cockpit detected weight-on-wheels, but the flight mode hadn't yet transitioned for the engine controls.


By self destruct, I meant switching off the engines in flight seems to me to be a terribly dangerous behavior.


Switching off is much safer than a turbine overspeed past certain limits. Because an engine failure (one engine) is a situation all pilots are trained for and it has an almost certain safe outcome. Airplanes fly and land just fine on a single engine. We all train for engine failures all the time.

On the other hand, a runaway engine fire or uncontained turbine failure is much much more likely to cause a crash.

So almost all jet engines are designed to have a shutdown (sometimes helped by the built-in fire extinguishers) as a worst case outcome. The quick response drill for an engine overspeed or temperature past certain limits is to shut it down immediately and pull the fire extinguisher handle.


But why not throttling instead of switching off then?


Good question, you could theoretically throttle a runaway engine by reducing fuel flow (unlike a piston engine where you can limit air intake). But in 9 out of 10 cases you've already done that by pulling back the power levers. The next step is to cut off the fuel, because the throttles didn't get it under control.


Overspeed protection takes place once everything else has failed. The engine controls have already attempted to throttle down and such. If that doesn't work then overspeed kicks in.


Adding on to this... things inside an engine happen FAST. Too much energy can spin up the turbines to the point of failure in milliseconds.


Yep, as anyone who has ruined a fan by shooting a canned air duster at it can attest. :(


I think almost every plane can glide safely for a lot longer than it takes to restart the engines. Could be dangerous at low altitudes still.


That and restarting an engine in-flight is faster than starting it on the ground because it's already spinning (by virtue of sailing through the air, like a windmill). This is, aptly, called windmill starting.


You would be surprised at just how safe and 'normal' shutting off the engines on a plane is.

Related, British Airways Flight 268, a B-747, when taking off from LAX had a problem with one of it's engines so they shut it off, and continued flying all the way to London, albeit to Manchester instead of Heathrow, with one less engine. https://en.wikipedia.org/wiki/British_Airways_Flight_268


Shutting down one engine is one thing. Shutting down all engines is what surprised me.


that's why this is news. it's very rare.


I was watching Air Crash Investigation yesterday, where a reverser deployed right after take off and caused the plane to crash. They said now reversers can't be activated unless all wheels are on the ground, as you've said.


Lauda Air flight 004 crashed because of that.


I'm willing to bet that the plane does prevent the pilot from activating reverse thrust when not on the ground.

But what if a thrust reverser self-activated, without being commanded? The previous safety mechanism wouldn't help because it's being bypassed.

So there is a secondary safety system that detects such situations. Something along the lines of "I think the trust reversers shouldn't be activated right now, yet they appear to be activated. The engine is clearly malfunctioning, lets shut it down"

It appears that this secondary safety system has been activated. There was probably a bug, or a sensor malfunction that triggered it.


Thrust reversers can never be deployed while airborne. This engine shutdown system is only enable on the ground. It's designed to shutdown an engine if the engine thrust doesn't match the commanded thrust, so an engine malfunction on the ground cannot cause the plane to taxi out of control.

But it is still possible for the pilots to deploy reverse thrust too soon, after touchdown but before there there is enough weight on the wheels to provide sufficient steering. I'm guessing the pilots deployed too soon, and discovered a new corner case.


Definitely you can use thrust reversers before touchdown at least on B737. I wonder how big is a difference between software on B737 and B787. Aviation industry doesn't have tendency to build software from scratch. Here is example video: https://youtu.be/-RO66a_nvus


Hey, thanks for this video. I was on a flight that did that some years ago when I was doing a lot of traveling. I don't remember anymore where exactly in Europe it was, but no one I later talked to believed me that the pilot would engage thrust reverser before touchdown.


Maybe the 787 won't allow airborne use, but other aircraft allow it. It's useful for steep descent. If the feature were more common, obstructions near airports (like mountains) would be less of an issue. We could build new runways at some airports.

https://en.wikipedia.org/wiki/Thrust_reversal#In-flight_oper...


Is that true even if the turbines are still spinning very fast?


A thrust reverser isn't like the reverse gear on your car.

It's a vent that opens to blow exhaust gases to the front instead of behind.


I think you've misunderstood. The system you're describing kicks in when (a) the aircraft is on the ground, (b) commanded thrust is idle and (c) the engines continue to produce thrust above idle (when there is an overthrust condition). Boeing does not cut power to the engines to prevent reverse thrust application in air - that would be absurd.

On this particular occasion the system is thought to have malfunctioned in some way (e.g. it might have not registered the application of reverse thrust) triggering a shutdown.


I thought that they did as a mitigation for uncommanded thruster reverser application.


A tough call. Your solution would disappoint a pilot who actually wanted to slow down ASAP more than Boeing's solution, and might contribute to a runway overrun.


No, it would not. If the engine is shut down moments before touching down it would mean that there would be no reverse thrust after touching down since it takes a considerable amount of time to re-start a turbo-fan (not 60 seconds as OP wrote as the engine is still spooling but still a significant amount of time and I don't think that any pilot would go through the workload and checklists of re-starting an engine during the landing roll anyway). This leads to a longer braking distance.

The sane choice would be to not engage reverse thrust at all (until the pilot has reset the reverse thrust throttle) or to only engage reverse thrust once the landing gear has weight on it.


> or to only engage reverse thrust once the landing gear has weight on it.

Having reverse thrust depend on a sensor that could fail seems like a poor choice. What if the landing gear don't drop? Is there a situation where you would still want reverse thrust without landing gear?


Sometimes you'd like to have reverse thrust in-flight if you want to descend really, really quickly. Airliners don't have that feature anymore, but the C-17 and C-5 do.

Actually, maybe some ex-Soviet airliners can still do it. DC-8s being used by cargo airlines could possibly still technically do it, but don't use the ability.


Specifically, it wasn't a design goal of reverse thrust, and it's operation mid-flight was directly responsible for over 200 innocent lives lost[0]. Thrust reversers are very much a "nice to have" feature on turbofan aircraft, with every one qualified to both land and initiate a rejected takeoff at maximum weight without using them, though not without some maintenance after.

[0]: https://en.wikipedia.org/wiki/Lauda_Air_Flight_004


Most things in aircraft automation systems depend on sensors that could fail (those sensors could be redundant etc. but the same can be done with the sensors on landing gears). It is sane design.

> What if the landing gear don't drop?

I don't think that there is need for reverse thrust in such a scenario.

> Is there a situation where you would still want reverse thrust without landing gear?

Even if there is, there could be a manual override.


> I don't think that there is need for reverse thrust in such a scenario.

How do we slow down planes that still need to come down despite a stuck landing gear, then?

Indeed, a manual override must exist.


How would that work exactly? Surely the engines will be among the first things to be destroyed on 'landing' if the gear aren't down.


It depends on where the engines are mounted. There was a Tu-154 that landed gear-up in Greece, then took off again, dropped the landing gear, and landed normally.

https://en.wikipedia.org/wiki/Mal%C3%A9v_Flight_262


There are plenty of examples of non-catastrophic belly landings. I'm no pilot or aircraft engineer, so I don't know for sure if you'd want reverse thrust in such a situation. https://en.wikipedia.org/wiki/Belly_landing


Just have multiple sensors like they do already.

It's also a sensor that you can simply put on each wheel to determine if there is weight applied.


> or to only engage reverse thrust once the landing gear has weight on it.

Or if the manual override is turned on.


hundreds of people are going to die.


Only if the people in this thread are designing aircraft. Presumably the actual aircraft designers have more experience reasoning through these kinds of problems.


It's weird to call it a "danger" when the automation is making a suboptimal response to override a human error that could crash the plane.


The Professional Pilots Rumor Network (PPRUNE) is a pretty awesome forum full of aviation professionals discussing aviation at a high level, I always go there for aviation news commentary. The thread on this isn't very extensive yet, but has some interesting commentary https://www.pprune.org/rumours-news/617426-ana-787-engines-s...


"Boeing said that selecting full reverse too quickly upon landing before the aircraft has fully transitioned to ground mode could cause the system to activate."

That sounds like apple's "you are holding it wrong" when "antenagate" happened. Not something I want to hear from an aircraft company where people's lives depend on it working.


Reading between the lines, and having read other comments here, I can interpret Boeing’s remark as “if the pilots or the on-board software try to reverse engines when it still is really, really dangerous, the Thrust Control Malfunction Accommodation system (TCMA) will try to make matters less bad, in some cases by shutting down the engines instead”.

They may have to fine-tune that system, but from a safety perspective, this event had a good ending, and making its decision system more complex also carries risks, so, maybe, nothing has to change.


> when it still is really, really dangerous

More apt would be "when the computer is not sure that isn't really, really dangerous".


It doesn't at all. It's an explanation why it might happen.They never said that this behavior won't be fixed or changed. It's not blaming the pilots.


There is a notion of blame wrapped up with "too quickly".


I'm having a hard time thinking up an alternative choice of words that is similarly clear and concise.

I also think that the phrase "too X" is polysemous. Depending on how it's used, it may imply a notion of blame. But it can also just be a way of describing an incompatibility. "This clearance is too low for that truck" and "This truck is too tall for that clearance" are entirely equivalent statements, IMO. Neither implies that the truck or the bridge is wrong, just that the driver would be wrong to try and drive under it.

Even further out there, when describing a timing-based bug that isn't known to be 100% deterministic as, "If X is happens too quickly after Y, Z might happen" seems to me like it's just a much more straightforward way of saying, "If X happens within some unspecified interval after Y, then Z might happen." Nine syllables shorter, same meaning.


>I'm having a hard time thinking up an alternative choice of words that is similarly clear and concise.

There's "beyond a certain speed", but still something of a mouthful.


There's a further implication that an instruction, but not a failsafe, exists to prevent the given condition. E.g. "do not reverse thrust until ground mode has fully activated." but no check to actually prevent the crew from doing so.

I'm not a lawyer; couldn't tell you where the fault would split in that case, but if my hunch about the lack of a failsafe for a given instruction is correct... it's still a surprise to me. I'd expect existing avionics production procedures to catch this sort of thing.


>> I'd expect existing avionics production procedures to catch this sort of thing.

The older I get, the more I believe your expectation is wrong. Lessons learned are rarely transferred to new people who were not present when the lesson was initially learned.

I've even worked at companies that try to compile a database of "lessons learned", but they never instruct anyone to read through the whole thing. Even if they did, when confronted with a large amount of material how much of it actually sticks?

The we move on to more procedural methods like fault-tree analysis, FMEA, etc... That's great and can help a lot, but it's still a GIGO process and new people need to learn how to do it well. There are always new people learning new things.


In software, we usually encode lessons learned as tests and static analysis. There is a reasonable level of success on that.

Aviation usually encode them on checklists. They have a much higher degree of success (probably because of culture, not medium), but failures happen some times too.


"Too quickly" appears to be the article's wording, not Boeing's, although I couldn't find a copy of the actual bulletin to confirm that.


Aircraft are very different from cell phones. There are a number of things you can do as a pilot that will cause them to crash or stall especially if something unusual like a sensor or engine failure occurs. Pilots spend a lot of time in simulators practicing how to fly the plane in a variety of such situations as well as under normal operating conditions.

Or as my neighbor says: "I've dealt with hundreds of engine fires!" (He flies A320s.)


Going off your quote it just looks like they're describing the symptom?


It’s a completely neutral statement, and air travel has a strong culture of not accepting “just don’t screw up” as a solution.


NTSB is probably the best example of a safety investigation agency. Air travel in the US is extremely safe for many good reasons.


Airbus has a limitation on one of their airplanes for Rudder Range - https://www.usatoday.com/story/travel/flights/2012/11/19/air...


Next from Boeing: a car where, if you unlatch your seatbelt before the engine stops, the airbags deploy.


Sort of off topic, but reverse thrust sort of boggles my mind. If you're sucking air in from the front of the aircraft and ejecting it out the front, don't those forces counterbalance each other?


To elaborate on seiferteric: If the engine were just an unducted fan, the input and output volumes and velocities would be the same, and if they're pointed in opposite directions, there would be zero net force.

In a jet engine though, you have a nozzle on the back, so the input area is greater than the output area. Heating the air also causes it to take up much more volume. These put together, mean that the output velocity is much greater than the input velocity, so there is a momentum transfer from the atmosphere to the plane.


"Sucking" doesn't actually generate any thrust - In highschool, my physics teacher did a demo with a T-shaped pipe with a fan in the middle, suspended from some string.

Without the T, the pipe moved as you'd expect. With the T directing the airflow in equal proportions perpendicular to the axis of the pipe, the pipe stayed still as if the fan wasn't on at all.


physics says no. There was something wrong with that demo if this was the conclusion.


Feynman says yes, except at very low speeds. https://en.wikipedia.org/wiki/Feynman_sprinkler


that's very interesting! but it's about angular momentum and I'm not confident that I can take the lesson over to linear momentum.

Here's my caveman f=ma thought experiment:

1. make it 2-d.

2. replace the fan with a person sitting on a chair on a frictionless surface.

3. instead of air it's an endless field cinder blocks ahead of him.

the person reaches out, and pulls in a cinder block. f=ma says they each move toward the other while the center of mass of the combination of them does not move.

Now, if he throws the cinderblock behind him, he moves further forward - this would be analogous to an airplane propeller. or a fan in an open pipe.

If he, um, splits the cinderblock in two and places each half directly off to his sides there is no net force exerted on him by this. This is the fan in a T-shaped pipe.

the fan+pipe grabs air from ahead, moves this mass backward and then sets it aside. it's not a jet-engine, but it is moving the air mass toward itself and must be moved equally and oppositely.

I don't think it's essential to worry about how the air/blocks rearrange themselves after this - but if the blocks surround and jostle, that's just another effect layered in super-position over this one, and if we don't agree so far then it will only make things more confusing


You have to look for the equal and opposite reaction. A normal jet creates a stream of fast air behind it. There's no such thing for a reverse jet that sucks air in.

With your concrete block example, as I start pulling the block towards me I experience an impulse forward. But when it approaches my body I slow it down to zero speed, creating an opposite impulse. So although I might have moved forward a few inches during the motion, my momentum is zero at the end. When you scale this up to large numbers of air molecules, the result is the same.


thanks for following up. I think this is one of those conversations that works better in front of a white board - it's surprisingly tricky to express in words.

if the demo was mainly to show that it's the jet of air expelled out the back that's providing thrust, then, fine it does that and its a valuable lesson. I guess I'm hung up on the technicality that there is actually a real movement of air mass even without that rearward jet and that has to be felt by the apparatus - I guess it's just unnoticeably small in the real world demo.

anyhow, my confidence in physics intuition has been shaken. thanks bunches.


Nah the demo was right. The impulse from the sucking (ie air molecules bouncing off the inside of the fan blade) is countered by the impulse from the air molecules bouncing off the inside off the back T-pipe. Different at a low Reynolds number with a reversible flow.


Reynolds number, compressibility, reversibility don't really matter for this - the principle under discussion is a simple momentum-balance. Find the force on the pipe by analyzing what is happening at the boundaries, you can ignore what happens inside. (in engineering this is called control-volume analysis).

The exiting flows out the sides neatly cancel. So what's happening from front-to-back? There is air flowing in at some velocity x cross-sectional area x air density. this momentum has to be balanced completely for the pipe to stay still - but there is no source of momentum in the other direction so the pipe will feel this force and move.

if the pipe was open at the back, there would be momentum exiting the pipe balancing the incoming momentum - or even over-balancing (as in a jet engine)

here's a video going through the math on a similar problem which is a little more complex in detail, but the same in principle: https://www.youtube.com/watch?v=hXApWf1r0Eo


Reverse thrust doesn't come directly out the front of the engine - it's vented out the side (or, in very old engines, redirected from the back exhaust):

https://upload.wikimedia.org/wikipedia/commons/d/df/F-GTAR_A...


That makes it even more surprisingly counterintuitive.

I think the important thing to consider is the relative velocities of the intake and exhaust air (the latter being much larger).


that is a misleading way to put it - the reverse-thrust air does indeed move toward the front of the plane. it also moves outward, and is often taken from the sides of the engine, but a component of the (reversed) thrust is definitely going opposite the usual direction.

here is a thermal image of a modern jet with thrust reverses on : http://www.rusaviainsider.com/russias-mc-21-tested-for-rever...


Your forgetting that the cool dense intake air is expanded (greatly) in the engine via combustion with fuel, so the volume of air being exhausted is much greater than went in and at much higher pressure, this is where the thrust comes from in the first place. It's not just a big fan :)


Modern turbofans are basically ducted fans spun by a smaller turbojet engine, so they are basically huge fans!


But is it the hot air out of the engine being reversed or the air from the big fan? I thought it was the latter.


It's mostly bypass air.


Think of the net momentum balance. Some mass of air is entering the intake at low speed, the engine does work on the air, then the air exits the front at very high speed. The intake speed is approximately the airplane's speed (150 mph), while the exhaust speed is probably 500 mph.


If the suck force and the blow force were equal, you wouldn't need to burn fuel inside the engine during normal operation. Combustion produces a lot higher volume of gasses at higher energy than the air that goes in the front.


I initially thought this as well, but it turns out that the reverse thrust is actually made of bypass air in most engines


No - because it's going faster when it exits the engine than when it enters.


The net effect is moving a mass of air one way, and generating an opposite force on the aircraft.


Just draw arrows for how the air is moving the whole time.

The force on the plane is in the opposite direction.


Oh Dreamliner, still[0] making me look bad 12 years later.

[0] https://news.ycombinator.com/item?id=9477941


I have never ever thought avionics software development would be in such a condition that you described in the link.

I am surprised to say the least.

I have always imagined avionics developers in a different way. Like having a thick rulebook to go through to add a new line to codebase etc.


Keep in mind my experience was almost exclusively with communications systems, which are important but nowhere near as vital as things like autopilot. Things could very likely be better there, or at a different company. And even many of the things I worked on were like you describe. There is a lot of red tape and testing to make certain changes, and the industry documents are often strict. But it wasn't always clear how much of that actually made things better/safer and how much was just fluff.


How many easter eggs will emerge from Boeing. In last 2 months, we had similar incidents from Boeing where an undocumented software safety kicked in and causing trouble for everyone


It's not just Boeing unfortunately. Every software driven aircraft seems to suffer from these bugs. The F35 lost radio communication after crossing the international date line the wrong direction IIRC, and the pilots had to use hand signals to land.


> The F35 lost radio communication after crossing the international date line the wrong direction IIRC, and the pilots had to use hand signals to land.

From what I remember, the first time 4 USAF F22 were flying across international date line to deploy to Hawaii(?), they had software related issues. As they flew across the international date line (first time in a real flight), all computers on the jets crashed, including navigation/etc.

Luckily they were flying in close formation with a tanker, signaled with flashlights, and were able to follow the tanker back to another base safely.

I believe USAF requires a multi jet (usually tanker) to accompany small jets when they are flying across the ocean.


Deploying to Guam. Hawaii is on the same side of the dateline as the continental United States. Also, as I recall, they simply rebooted the computers.


You're correct, it was the F22, not the F35.


and causing trouble for everyone

You mean killed 189 people, right? https://en.wikipedia.org/wiki/Lion_Air_Flight_610


That aircraft was being knowingly flown with [edit: a broken sensor]. It was unsafe on takeoff.

The final problem that brought the aircraft down - the trim run-away - happens all the time in airliners you've flown in. When the run-away starts, the pilot is supposed to flip two switches to disable the trim system motors. In the Lion Air crash, it is indeed believed that a software bug from the misbehaving sensors started the run-away, but again, handling trim run-aways are something that pilots are supposed to train for and deal with.

737's are even safer here than many airliners, because after you disable the electric trim motors, you have manual wheels in the cockpit that you can rotate to set the trim back to what it should be. Some other airliners in common use don't have these manual trim controls - you have to disable a trim run-away in time, or it's game over.


> the trim run-away - happens all the time in airliners you've flown in.

It doesn't. A trim run-away is a very very serious incident for any pilot.

There are procedures to deal with it, and we check trim override in pre-flight checks, but it's absolutely not an everyday thing.

> 737's are even safer here than many airliners, because after you disable the electric trim motors, you have manual wheels in the cockpit that you can rotate to set the trim back to what it should be. Some other airliners in common use don't have these manual trim controls - you have to disable a trim run-away in time, or it's game over.

This is not true at all either. There are no planes with a single electric trim that you cannot override. The FAA and EASA would refuse to certify those.


Embraer 190? There are no non-electrical trim controls in the cockpit. http://www.smartcockpit.com/docs/Embraer_190-Flight_Controls...

Whereas a 737 has physical cables connected from wheels in the cockpit to the stabilizer jackscrew.


I've never flown an E190, but if you Google the basic systems description you'll find that it has two sets of trim controls and on top of that separate override controls.

Both circuits are electric, but they are separate systems on separate power busses to ensure you always have control.


Thanks!


> That aircraft was knowingly flown with multiple broken sensors. It was unsafe on takeoff.

You'll need to cite that, since it contrary to any information I can find on the Lion Air accident e.g.:

> The chief executive officer of Lion Air, Edward Sirait, said the aircraft had a "technical issue" on Sunday night, but this had been addressed in accordance with maintenance manuals issued by the manufacturer. Engineers had declared that the aircraft was ready for takeoff on the morning of the accident

Your claim that it was "unsafe to takeoff" and had "multiple broken sensors" is pretty remarkable. So I'll definitely need to see a source backing up such remarkable claims.


From the same Wikipedia article that was just linked:

> "The aircraft suffered an airspeed indicator problem for its last four flights, including the flight to Denpasar. Thinking that it would fix the problem, the engineers in Bali then replaced one of the aircraft's AoA sensors, but the problem persisted on the penultimate flight [...] [the crew] recorded a twenty-degree difference between the readings of the left AoA sensor and the right sensor."

> "On 28 November, Indonesia investigators said the Lion Air jet was not airworthy on flight before crash."


Right, there were repeated problems with the angle of attack sensor. If I had to guess the sensor itself is and was just fine and the problem lay elsewhere in the pipeline.

More important though is that the 737 MAX differs wildly from earlier 737s in how much it relies on the AoA data. Mechanics and pilots not experienced with the MAX were probably operating under the (false) assumption that a bad alpha vane wouldn't be the end of the world. In fact displays indicating the angle of attack and warnings about disparity between the alpha vanes is an optional feature on the 737. It's considered that unimportant.

The key differences from earlier 737s are that the MAX uses the AoA data to calculate airspeed and that the MAX may use a single AoA input to try to kill you. I believe the former was disclosed, but considered how short the differences training is may have been easily overlooked. The latter, of course, was not disclosed until the crash.


Those are prior flights. The aircraft was fully repaired:

> The chief executive officer of Lion Air, Edward Sirait, said the aircraft had a "technical issue" on Sunday night, but this had been addressed in accordance with maintenance manuals issued by the manufacturer. Engineers had declared that the aircraft was ready for takeoff on the morning of the accident

So you still haven't supported your extraordinary claim that the aircraft was:

> unsafe on takeoff.

In fact we know it was safe on takeoff.


> we know it was safe on takeoff

Or rather, as airworthy as any other MAX flying around.

Not sure why you're being downvoted, as I haven't seen any evidence linked contradicting what you said.


Quoting from the same comment you just replied to:

> "On 28 November, Indonesia investigators said the Lion Air jet was not airworthy on flight before crash."

"not airworthy" means that it was unsafe.


on flight before the crash. After which it was repaired, and sent on the fatal flight.


It was also repaired before the previous flight.

It goes like this:

several flights report problem

maintenance "fixes" the problem

next flight reports that another, worse problem has appeared - bad enough that regulators have now said that the plane was "not airworthy" during this flight

maintenance "fixes" the problem

final flight impacts ocean at high speed

Your assertion is that since maintenance cleared the plane after the second fix, the plane must have been fine. To that, I point to the previous time maintenance cleared the plane, when it was demonstrably not fine.


No, my claim merely was that the quote does not make the claim about the specific flight (although I didn't word it quite clearly). The findings about bad maintenance culture certainly suggest that the plane wasn't fixed properly.


Indeed, in a completely different flight than the one we're discussing.


https://www.google.com/amp/s/amp.theguardian.com/world/2018/...

“Indonesian investigators have said the Lion Air Boeing 737 jet that plunged into the sea, killing 189 people in October, was not airworthy on a flight the day before it crashed.

They further found that Lion Air must improve its safety culture and better document repair work on its planes.

The flight from Bali to Jakarta on 28 October had experienced similar technical issues to the doomed flight the next day from Jakarta to Pangkal Pinang, said Nurcahyo Utomo, head of Indonesia’s national transport safety committee (KNKT).

The pilot of the 28 October flight chose to press on to Jakarta after shutting down the plane’s anti-stall system, Utomo said.

“This is the basis of our recommendation to Lion Air. In our view, the plane was not airworthy,” he told a news conference in Jakarta.

...

But its investigators said that Lion Air kept putting the plane back into service despite repeatedly failing to fix a problem with the airspeed indicator in the days leading up to the fatal flight.”

-

So the aircraft spent several days flying with a broken sensor, sometimes without even an attempt at repair.

I've removed the multiple sensor part.


> “Indonesian investigators have said the Lion Air Boeing 737 jet that plunged into the sea, killing 189 people in October, was not airworthy on a flight the day before it crashed.

And was repaired between the two flights as I quoted above. Are you selectively ignoring the information in the very post you replied to?

Here's that information again to refresh your memory:

> The chief executive officer of Lion Air, Edward Sirait, said the aircraft had a "technical issue" on Sunday night, but this had been addressed in accordance with maintenance manuals issued by the manufacturer. Engineers had declared that the aircraft was ready for takeoff on the morning of the accident

You and the other poster seem to be basing your whole position on time not moving forward in a linear fashion: Failure, repair, flight. In that order.

Pointing out the previous day over and over while ignoring what occurred in the interim isn't a real argument.


Let's say you hear a knocking sound from your car engine. You take it to a mechanic for repairs, and the mechanic says he fixed it. But you still hear the knocking sound. If you repeat this cycle for a few times, when the mechanic says for the Nth time that he fixed the problem, is it actually fixed?


To save people the reading, the safety feature triggered was an automatic dive maneuver initiated by the flight computer as a result of an incorrect angle-of-attack sensor reading causing the plane to believe it was in a stall, when it actually wasn't

I didn't look into it any further, but based on that alone I would agree that this seems like a shitty feature, perhaps a loud alarm or other warning would be more appropriate than putting the plane in a dive without human intervention


The 737 is a short and stubby airliner with its center of thrust far below the rest of the aircraft. The 737MAX version added upgraded, powerful engines. It is theoretically possible for the engines to flip the aircraft up and out of control if power is applied too quickly in certain fight situations.


A big issue with that flash was that there was a way to disable the feature, but Boeing didn't tell anyone about it and it wasn't documented anywhere.


You can still disable the stabilizer trim from switches in the cockpit, though. This was in the checklists that the pilots had, and done by other pilots in that aircraft in the days before the accident.


Yeah, it's hard to make the argument that a recovery procedure is so incredibly difficult and obscure when several previous flights of that plane experienced the same issue and successfully followed that procedure... And apparently without considering it extremely outside of the norm.


Probably more to come for the foreseeable future:

https://www.theguardian.com/technology/2012/may/29/cyber-att...


brutal comment.


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