Hacker News new | past | comments | ask | show | jobs | submit login

There's a difference there though. That's Fly-By-Wire in two different contexts.

One way to do it is indirect linkage to control surfaces, and using the computer to generate the appropriate signaling to actuate the control surfaces in a more or less direct mapping of pilot intent to control surface output. Depending on design philosophy, more in depth envelope protection may be implemented, (see Airbus) but there's no given that says Fly-By-Wire must be able to countermand and pilot commands.

The other way Fly-by-Wire is used (and to me personally is an anti-pattern) is as a pilot/plane babysitter like MCAS. The plane should be free of bad aerodynamic behaviors so that even with total electrical system casualty, a passive glide and controlled landing is possible. Once MCAS is taken out of the picture, the MAX doesn't have that. It does not meat the full set of airworthyness regulations for Civil Transport Aircraft as laid out in FAR 25.173.

I understand some in the industry may scoff at having to comply with the directive as written, but it's now been clearly demonstrated what happens when you don't.




IMO MCAS (the version properly respecified and reimplemented, of course) really fits in the "envelop protection" category (even if slightly pre-emptive), why do you classify it otherwise?


I don't qualify it as envelope protection, because it doesn't actually prevent you from doing something daft. This is a characteristic that seems to be common in every system I've heard of described as an envelope protection system. Though in a sense one could consider it one I suppose in the absence of a malfunctioning AoA sensor.

If it were envelope protection. It would serve the purpose of preventing you from getting into an untenable situation, regardless of your control inputs. MCAS doesn't. You can go all the way to stall with it, and it just keeps cranking down the trim to keep the control stick force curve from inverting.

It creates the piloting experience (look and feel) of 737 piloting, but introduces new and lethal failure States due to the lack of error checking, and graceful degradation of functionality.

There's other issues, but that's what sticks out to me.

To be clear, my categories of the different applications of FBW are built by the distinction between what it is trying to do. Take the F-16, or F-117 as an example of Fly-By-Wire making an unstable aircraft controllable versus Airbus control law system, which basically takes a stable airframe, and exposes a common user interface so similar control schemes can be used for physically different airframes.

The first variety has no place in Civil Transport. The second absolutely does.


FAR 25.173 says nothing about electrical failure.

Indeed at the sizes of aircraft we’re taking about the pilot is just not going to be strong enough without hydraulic or electric assistance.


The regulation doesn't, no, the technical details of MCAS as a piece of software necessary to maintain airworthyness however, demand the flight control computer be able at all times to work as designed to maintain the regulated condition of airworthyness. Namely, no control stick force inversions on approach to stall.

It's kind of hard to do that with a computer if you've lost your electrics. Therefore either the system needs to be bulletproof, or you should have designed out the need for the crutch in the first place. Not gone and made a Frankenstein's monster of a plane.


leaving out the discussion of how much force is required and how strong a pilot is, the hand crank for the stab trim has mechanical leverage. the pilot turns a crank that is geared to move a cable. the amount of leverage is determined by mechanical design. it could be geared to move a stabilizer on a plane N times as large...more turns though.


It still runs into problems where in general, the electronic manipulation is what pilots muscle memory develops around, and the more mechanical advantage is utilized, the faster the wheel turns in the cockpit during normal operation.

Those wheels are already notorious for the punishment they can subject an errant kneecap to.




Applications are open for YC Winter 2020

Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact

Search: