So there a weird thing about angle of attack: the autopilot uses it, but civilian pilots don’t use it (they use airspeed and descent angle). It’s used in the military, and airlines who recruit a lot of military pilots tend to buy the angle of attack option to get them comfortable. Ignoring the little MCAS issue, it was a fine reasoning, the autopilot can detect a disagree (I guess they just drop out, but I don’t know about cat 3 landings), ex-military pilots need a disagree because they read it, and other pilots don’t need to know.
I found it amazing that civilian pilots don't use angle of attack so went looking and found this article about the debate on installing angle of attack indicators on commercial aircraft, which cleared up for me why this would be the case. But not in the most reassuring manner.
>"From day one, pilots should be—and in most cases are—taught that stall occurs when the critical angle of attack is exceeded. They are then told to memorize the stall speeds in various configurations. By the time they have become proficient, they relate stall to a certain airspeed, and don’t really relate it to an AOA anymore."
Air France 447 immediately comes to mind, where the crew (or maybe specifically the copilot who kept pitching up for minutes) lost their airspeed indication and thought the stall warning was frivolous despite falling vertically at terminal velocity for minutes.
Critically in that case, the stall warning stopped when the nose was pulled up even further, because the computers rejected the indication as too large to be correct. So every time the nose was lowered, the stall warning activated.
AF447 is really a terrible accident, just hard to analyze. It's not even clear would breaking one link in the chain save the plane, since so many thing happened.
True, UX was inadequate. But there was one more detail, the indications were incorrect and mismatched for a short period, while the pilot inputs were continual. It's not hard to interpret that the pilot actually knows something you don't (speaking from the computer's POV) and disengage ("Wait, we're 40 degrees pitch-up, and the man keeps pulling up, alternate flight law engaged, sensors might be incorrect, listen to the man").
I seem to remember reading that the ability to cause a power-on stall is both part of standard pilot training and a surprise for most pilots when demonstrated.
Military or not you should know about the AoA disagree and the presence of MCAS as the combination can be quite deadly. Knowing the actual angle of attack could be useful, but knowing what sort of cascading failures to expect is critically important.
But how can you even diagnose a failing AoA sensor, if there is no warning message but you expect it to be indicated by one? Is there any testing done on ground by the engineers after each flight?
Let's plan a theoretical test case. At a minimum, you'd need two maintenance guys, maybe one and a pair of tools. One guy to tweak the sensor outside (as several hundred mile per hour airstreams aren't exactly in plentiful supply, or easy and trivial to reconfigure in a hangar near you), and a guy in the cockpit, reading off the measurement on the computer against some sort of calibration chart.
The "tweaking" part would probably be done with some sort of precision test mount (that I doubt they even build into the plane.)
Also, odds are, you'd only detect very specific types of disagrees on the ground. Bad springs should be trivially deducible from the forces required to create measurable deflections.
Bad potentiometer connections should be deduceable from checking output voltage against the component spec sheet while deflecting.
Unfortunately, this dive into Electrical Engineering won't stop until you've taken into account every length of wire and eventually, every circuit board and line of code between that sensor and the avionics suite. If some FOD for instance, was slowly wearing through the wiring and insulation leading from the sensor to the computer, or an inductive cross-talk event occurred, or something shorted. Etc, etc.
And all of this work and diagnostics would need to be done by someone whose documentation told them the AoA sensor was a non-critical component so the manufacturer could cut corners.
Oh yes.
<double facepalm>
Been here before.
Way.
Too.
Many.
Times.
Not in safety critical systems , mind, but oh the web's I've seen woven.
The more I learn about the story and the more I realize the bureaucracy that is airplane flying. The boeing line is that they had to use the runaway trim checklist and that’s it. They missed the auto throttle item on the list so it’s their fault.
I feel like it’s all about reading documents and never trying to use their brain.
A lot of my bubble about diagnosing things and connecting information together has been bursted.
In “the complete private pilot” they definitely talk about AoA in the context of stalls. I’m pretty surprised to hear that civilian pilots don’t use it.
it's a very important concept, but it's a far cry from having a (redundant) gage and introducing it into the scanning pattern. Air speed is generally a very good indicator of how far you're from stall, unless you're in the army doing crazy attitudes very close to the ground.
I'm pretty sure that small aerobatic planes don't have an angle of attack indicator, while the pilot would know everything there is to know about attitude control, having a number is not useful.
I would add an analogy: wing loading is an extremely important concept, and there is absolutely zero plane with a direct force reading on the wing spars.
Civilian planes (Cessnas etc) don't even have AoA indicators, so civilian pilots couldn't use them even if they wanted to. They have started to add them just in the last few years. But of course that means they're still not in the planes most private pilots are flying.
