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The "warning light" is an "AOA DISAGREE" text that appears on the main flight display: http://www.boeing.com/resources/boeingdotcom/commercial/737max/updates/A
by AnssiH 7y ago
The "warning light" is an "AOA DISAGREE" text that appears on the main flight display:
http://www.boeing.com/resources/boeingdotcom/commercial/737max/updates/AOA_VaneIndicator_AoA_Disagree_lg.jpg http://www.boeing.com/resources/boeingdotcom/commercial/737m...
So there was no non-functional physical light on the cockpit.
- lqet 7y agoAh, I was not aware of that. But surely the pilots were under the impression that such a warning message existed?
- nraynaud 7y agoSo 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.
- yaur 7y agoIn “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.
- cjbprime 7y agoCivilian 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.
- nraynaud 7y agoit'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.
- nraynaud 7y agoI 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.
- lqet 7y agoBut 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?
- nraynaud 7y agoThe 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.
- salawat 7y agoThat's the rub. 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.
- inferiorhuman 7y agoMilitary 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.
- nraynaud 7y agoI am trying to explain Boeing’s point of view before the accident, you’re busting an open door.
- inflatableDodo 7y agoI 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. https://apstraining.com/the-angle-of-attack-debate/ https://apstraining.com/the-angle-of-attack-debate/ >"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."
- wnkrshm 7y agoAir 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.
- danaliv 7y agoCritically 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.
- inflatableDodo 7y agoFucking hell. That is a fucker of a dark UX pattern.
- danaliv 7y agoIndeed. And it goes to show that rejecting “impossible” values is a dangerous strategy.
- 7y ago
- phire 7y agoYes. An in the aftermath of the Lion Air crash, Boeing were telling pilots that this warning message would alert them to that kind of problem before takeoff.
- inferiorhuman 7y agoBoeing were telling pilots that this warning message would alert them to that kind of problem before takeoff. I doubt that, the alpha vanes aren't particularly useful until the plane is actually moving at a fairly decent clip.
- AnssiH 7y agoYes, it does not work. According to the OP article, Boeing said to American Airlines pilots in November that the warning would work on the ground, but later said it in fact does not.
- phire 7y agoBy before takeoff, I assume that means "during the takeoff roll" And I hope that also means "before V1". I know the DC 8 couldn't do it's elevator check until it was moving quite fast, as there was no direct connection between the pilots controls and the elevator and it couldn't respond without airflow.
- danaliv 7y agoWhat speed is that? I don’t know about 737s in particular but some AoA sensors work at speeds as low as 20 knots—far less than V1 on any runway a 73 is going to use.
- Maxious 7y agoSo if you were to get a value that doesn't make sense on the ground when you aren't moving, that would be a assuring sign that you have correctly detected a fault.
- sokoloff 7y agoThe angle of attack of a wing that has zero airspeed is undefined. You could take two traditional weathervanes and put them inside your house and they wouldn't tell you the direction of the wind outside.
- briandear 7y agoIf that annunciator isn’t in the checklist, you wouldn’t think about it being there or not. It wouldn’t be part of your scan.
- kirykl 7y agoSo is that text warning always available and just software enabled if the packaged is purchased, or is there extra hardware involved?
- ineedasername 7y agoThat's kind of a distinction without a difference. A standard function was non-functional.
- Chris_Chambers 7y agoWhy are there only two sensors? Are these large and ridiculously expensive devices? Why aren’t there a hundred sensors?
- jrochkind1 7y agoI think that makes it worse on Boeing's part, that it was a software-only feature that was disabled from those who didn't pay. They didn't even have to "install" any hardware. It sounds like the software accidentally coupled functionality of this light to another feature. It is in fact a software bug. To fix the bug would have required a software redeploy -- which is definitely non-trivial thing in safety-critical software, and may have required some kind of recertification, or at any rate significant expense. Instead, they could have done what they in fact _have done_ only _after_ people died: > In the months after the Lion Air crash, Boeing quietly worked to appease some customers, according to a person briefed on the matter. In several instances, it activated the angle of attack indicator for free, which then turned on the disagree alert. Well, quietly, for some customers, who know to insist on it and have the purchasing power to get what they insist on. All Boeing had to do was flip a switch to turn the software feature on, without charging for it.