7 ms·

If you see two AoA sensors that aren't in agreement then you can safely assume either one sensor has failed or physics itself has.

phire 7y ago
The 737 has AoA sensors by default, the autopilot uses them and the flight computer corrects the flight speed based on the AoA and issues stalling warnings based on the values. But the display which displays raw AoA readings to the pilots is an optional extra, only 20% of 737 maxes ship with it enabled. Normally pilots (or at least commercial pilots) are trained to fly without referencing the AoA by staying well within the safety margins. So, if the AoA isn't shown anywhere in the default configuration, there is no way for the pilots to crosscheck them and see the fault. Which is why you need the AoA disagree warning message. Unfortunately, Boeing messed up and the AoA disagree warning was only enabled on the planes with the AoA display option enabled.
- acqq 7y ago
  > if the AoA isn't shown anywhere in the default configuration, there is no way for the pilots to crosscheck them and see the fault. Which is why you need the AoA disagree warning message. But the article buried the lede! There is much bigger problem reported now in this article: The warning wouldn't have helped even when flying with those companies that paid extra for it! Note: "“We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground and therefore not even take off,” said Dennis Tajer, a spokesman for the union representing American Airlines pilots. “That gave us additional confidence in continuing to fly that aircraft.” But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground." Note what happens then: at the moment you are above the ground, and the MCAS activates, we already know that if you "just follow the procedure" the forces against pilot corrections could be too big to do anything. That's what can be concluded from the analysis of the Ethiopian crash. That means the plane from the "20%" with "paid option" would still eventually crash as soon as AoA vanes fail, just there would be one more of many alerts in the cockpit for some short time before everybody dies. The sense of "security" Boeing gave the American pilots flying for the companies who paid for that additional feature was completely false. They would have been doomed too as soon as a single AoA vane that the MCAS depended on failed: The warning worked only at the moment it was completely useless. The U.S. MAX planes were grounded the last, yet they were in the same danger as the other planes.
  - dmix 7y ago
    That's certainly not how the information is presented. But if you read closely you can see how they set it up carefully in the first half. This is about adding to the narrative of Boeing's "general mismanagement" of the aircraft. Not about adding to the particular 737-800 crash investigation story (directly).
  - phire 7y ago
    On both the Lion Air and Ethiopian Air crashes, the AoA sensor had failed before takeoff. So the warning message could have potentially prevented both crashes and anything like it (assuming it activated during the takeoff roll, before V1 and the pilots were trained to abort take off when it happened) Though you are right, if the AoA sensor had failed halfway though a flight, they still would have had problems.
    - acqq 7y ago
      The new information in the article (buried in the last part of it!) is that apparently the warning message just didn't work on the ground at all, even with the already non-functioning sensors, and even if that option was paid for; read it again: "We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground" ... "But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground."
    - FabHK 7y ago
      > On both the Lion Air and Ethiopian Air crashes, the AoA sensor had failed before takeoff. Are you sure? The AvHerald narrative for the Ethiopian Airlines flight: "The takeoff run was entirely normal, both AoA sensors were in agreement. Shortly after becoming airborne the left AoA sensor however began to deviate and reached a position of about 85 degrees nose up, which obviously triggered the left hand stick shaker motor (and provides a very noisy and distracting cockpit environment), which in turn, different to the 737 NG behaviour (where the AoA does NOT correct the pitot data), also caused the IAS and ALT data to differ from the right hand system, in particular the left IAS became 12 knots higher than the right hand IAS (the preliminary report does not mention that the IAS DISAGREE warning activated however, which would invoke the unreliable airspeed procedures). The captain was pilot flying and focussed to keep the aircraft on track (via the flight director) due to terrain around while at the same time trying to understand why the stick shaker activated. Several required callouts by the first officer did not occur (e.g. speed, ...), as result the crew lost speed as well as thrust control completely out of sight." As I said somewhere below, I don't think the AoA disagree indication would have made much of a difference here. http://avherald.com/h?article=4c534c4a/0052&opt=0 http://avherald.com/h?article=4c534c4a/0052&opt=0
bitexploder 7y ago
Serious tangent, but the idea of physics failing amuses me. One night we had an Earthquake in an area that basically almost never has them. Gentle shaking. It woke me and my wife up. I remember looking out my window for some sign of nuclear weapons going off or something. Nothing. I blurted out “Earthquake” all in about 10 seconds of processing. Rolled over. Went back to sleep. My wife was anxious the rest of the night and could not sleep from “what if” scenarios. One of her concerns was “what if physics was broken”?!
- bobsil1 7y ago
  What if the moon’s too bright? https://en.wikipedia.org/wiki/Inconstant_Moon https://en.wikipedia.org/wiki/Inconstant_Moon
  - vidanay 7y ago
    Damn...I read that link as "Incontinent Moon"
  - selimthegrim 7y ago
    What if it’s only there when someone looks? https://www.quora.com/Is-the-moon-there-when-nobody-looks-If-Bells-inequality-has-been-experimentally-violated-by-subatomic-particles-like-photons-why-is-it-not-violated-by-objects-like-the-moon https://www.quora.com/Is-the-moon-there-when-nobody-looks-If...
- touisteur 7y ago
  That's kind of the plot of 'Ravages' (https://en.m.wikipedia.org/wiki/Ashes,_Ashes https://en.m.wikipedia.org/wiki/Ashes,_Ashes) a French SF novel by Barjavel. Electricity suddenly stops working.
  - Haga 7y ago
    So neurons stop working?
    - 7y ago
FabHK 7y ago
Sure. Problem is, if you can see the two disagreeing AoA indications, then you will get the "AoA disagreement" warning light/message, even though you don't need it. Conversely, if your airline has not ordered the optional AoA indicators, and you'd really want the "AoA disagreement" warning light/message, you don't get it.
sowbug 7y ago
If they're on either side of the fuselage and you're slipping, it seems like they'd give different readings. Pitots can give misleading readings if you slip away from the side they're on. I've never flown a plane with an angle of attack indicator.
- dreamcompiler 7y ago
  Most of the time when you're slipping close to the ground you're going to also have flaps deployed, in which case the MCAS is inoperative. Your AOA sensors will definitely disagree, but at least the MCAS won't act on them because it doesn't act when flaps are deployed. If you're slipping without flaps I guess it could still be a problem if the disagreement is outside the epsilon spec. I'm not sure if [normal] slip could ever cause that much disagreement.
- bigiain 7y ago
  I'm now imagining a 737MAX8 in a flat spin... (I've had the controls of a 172 and a Tiger Moth briefly, but have never _really_ flown a "real plane". I have flat spinned many many rc planes, mostly recovered fine, but also to their occasional doom...)
- inferiorhuman 7y ago
  If they're on either side of the fuselage and you're slipping, it seems like they'd give different readings. Which is why there's a threshold that much be crossed (IIRC ten degrees) before the disagree warning appears.
rosser 7y ago
While that's (partly) true, it's also pretty irrelevant to whether one can reasonably infer "AoA sensors agree" from the lack of an error notification indicating they don't, when the system that should be providing that notification is, in fact, disabled, because the airline one works for opted not to purchase some premium feature.
pdonis 7y ago
Or the sensors are on different sides of the plane and the plane is slipping or banking. Which happens routinely during normal flight.
SomeHacker44 7y ago
Looking at the mechanism of a vaned AoA sensor, I am guessing a fast enough roll would induce disagreement. Probably insufficient in degree to be material, though.

phire 7y ago

The 737 has AoA sensors by default, the autopilot uses them and the flight computer corrects the flight speed based on the AoA and issues stalling warnings based on the values. But the display which displays raw AoA readings to the pilots is an optional extra, only 20% of 737 maxes ship with it enabled. Normally pilots (or at least commercial pilots) are trained to fly without referencing the AoA by staying well within the safety margins. So, if the AoA isn't shown anywhere in the default configuration, there is no way for the pilots to crosscheck them and see the fault. Which is why you need the AoA disagree warning message. Unfortunately, Boeing messed up and the AoA disagree warning was only enabled on the planes with the AoA display option enabled.

acqq 7y ago
> if the AoA isn't shown anywhere in the default configuration, there is no way for the pilots to crosscheck them and see the fault. Which is why you need the AoA disagree warning message. But the article buried the lede! There is much bigger problem reported now in this article: The warning wouldn't have helped even when flying with those companies that paid extra for it! Note: "“We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground and therefore not even take off,” said Dennis Tajer, a spokesman for the union representing American Airlines pilots. “That gave us additional confidence in continuing to fly that aircraft.” But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground." Note what happens then: at the moment you are above the ground, and the MCAS activates, we already know that if you "just follow the procedure" the forces against pilot corrections could be too big to do anything. That's what can be concluded from the analysis of the Ethiopian crash. That means the plane from the "20%" with "paid option" would still eventually crash as soon as AoA vanes fail, just there would be one more of many alerts in the cockpit for some short time before everybody dies. The sense of "security" Boeing gave the American pilots flying for the companies who paid for that additional feature was completely false. They would have been doomed too as soon as a single AoA vane that the MCAS depended on failed: The warning worked only at the moment it was completely useless. The U.S. MAX planes were grounded the last, yet they were in the same danger as the other planes.
- dmix 7y ago
  That's certainly not how the information is presented. But if you read closely you can see how they set it up carefully in the first half. This is about adding to the narrative of Boeing's "general mismanagement" of the aircraft. Not about adding to the particular 737-800 crash investigation story (directly).
- phire 7y ago
  On both the Lion Air and Ethiopian Air crashes, the AoA sensor had failed before takeoff. So the warning message could have potentially prevented both crashes and anything like it (assuming it activated during the takeoff roll, before V1 and the pilots were trained to abort take off when it happened) Though you are right, if the AoA sensor had failed halfway though a flight, they still would have had problems.
  - acqq 7y ago
    The new information in the article (buried in the last part of it!) is that apparently the warning message just didn't work on the ground at all, even with the already non-functioning sensors, and even if that option was paid for; read it again: "We were told that if the A.O.A. vane, like on Lion Air, was in a massive difference, we would receive an alert on the ground" ... "But in the last several weeks, Boeing has been saying something different. Mr. Tajer said the company recently told American pilots that the system would not alert pilots about any sensor disagreement until the aircraft is 400 feet above the ground."
  - FabHK 7y ago
    > On both the Lion Air and Ethiopian Air crashes, the AoA sensor had failed before takeoff. Are you sure? The AvHerald narrative for the Ethiopian Airlines flight: "The takeoff run was entirely normal, both AoA sensors were in agreement. Shortly after becoming airborne the left AoA sensor however began to deviate and reached a position of about 85 degrees nose up, which obviously triggered the left hand stick shaker motor (and provides a very noisy and distracting cockpit environment), which in turn, different to the 737 NG behaviour (where the AoA does NOT correct the pitot data), also caused the IAS and ALT data to differ from the right hand system, in particular the left IAS became 12 knots higher than the right hand IAS (the preliminary report does not mention that the IAS DISAGREE warning activated however, which would invoke the unreliable airspeed procedures). The captain was pilot flying and focussed to keep the aircraft on track (via the flight director) due to terrain around while at the same time trying to understand why the stick shaker activated. Several required callouts by the first officer did not occur (e.g. speed, ...), as result the crew lost speed as well as thrust control completely out of sight." As I said somewhere below, I don't think the AoA disagree indication would have made much of a difference here. http://avherald.com/h?article=4c534c4a/0052&opt=0 http://avherald.com/h?article=4c534c4a/0052&opt=0

bitexploder 7y ago

Serious tangent, but the idea of physics failing amuses me. One night we had an Earthquake in an area that basically almost never has them. Gentle shaking. It woke me and my wife up. I remember looking out my window for some sign of nuclear weapons going off or something. Nothing. I blurted out “Earthquake” all in about 10 seconds of processing. Rolled over. Went back to sleep. My wife was anxious the rest of the night and could not sleep from “what if” scenarios. One of her concerns was “what if physics was broken”?!

bobsil1 7y ago
What if the moon’s too bright? https://en.wikipedia.org/wiki/Inconstant_Moon https://en.wikipedia.org/wiki/Inconstant_Moon
- vidanay 7y ago
  Damn...I read that link as "Incontinent Moon"
- selimthegrim 7y ago
  What if it’s only there when someone looks? https://www.quora.com/Is-the-moon-there-when-nobody-looks-If-Bells-inequality-has-been-experimentally-violated-by-subatomic-particles-like-photons-why-is-it-not-violated-by-objects-like-the-moon https://www.quora.com/Is-the-moon-there-when-nobody-looks-If...
touisteur 7y ago
That's kind of the plot of 'Ravages' (https://en.m.wikipedia.org/wiki/Ashes,_Ashes https://en.m.wikipedia.org/wiki/Ashes,_Ashes) a French SF novel by Barjavel. Electricity suddenly stops working.
- Haga 7y ago
  So neurons stop working?
  - 7y ago

FabHK 7y ago

Sure. Problem is, if you can see the two disagreeing AoA indications, then you will get the "AoA disagreement" warning light/message, even though you don't need it. Conversely, if your airline has not ordered the optional AoA indicators, and you'd really want the "AoA disagreement" warning light/message, you don't get it.

sowbug 7y ago

If they're on either side of the fuselage and you're slipping, it seems like they'd give different readings. Pitots can give misleading readings if you slip away from the side they're on. I've never flown a plane with an angle of attack indicator.

dreamcompiler 7y ago
Most of the time when you're slipping close to the ground you're going to also have flaps deployed, in which case the MCAS is inoperative. Your AOA sensors will definitely disagree, but at least the MCAS won't act on them because it doesn't act when flaps are deployed. If you're slipping without flaps I guess it could still be a problem if the disagreement is outside the epsilon spec. I'm not sure if [normal] slip could ever cause that much disagreement.
bigiain 7y ago
I'm now imagining a 737MAX8 in a flat spin... (I've had the controls of a 172 and a Tiger Moth briefly, but have never _really_ flown a "real plane". I have flat spinned many many rc planes, mostly recovered fine, but also to their occasional doom...)
inferiorhuman 7y ago
If they're on either side of the fuselage and you're slipping, it seems like they'd give different readings. Which is why there's a threshold that much be crossed (IIRC ten degrees) before the disagree warning appears.

rosser 7y ago

While that's (partly) true, it's also pretty irrelevant to whether one can reasonably infer "AoA sensors agree" from the lack of an error notification indicating they don't, when the system that should be providing that notification is, in fact, disabled, because the airline one works for opted not to purchase some premium feature.

pdonis 7y ago

Or the sensors are on different sides of the plane and the plane is slipping or banking. Which happens routinely during normal flight.

SomeHacker44 7y ago

Looking at the mechanism of a vaned AoA sensor, I am guessing a fast enough roll would induce disagreement. Probably insufficient in degree to be material, though.