3 ms·

> Just to clarify, with two AoA sensors, you can know that there is a problem (if they disagree), but you don't know which one is erroneous. The AoA disagree a

> Just to clarify, with two AoA sensors, you can know that there is a problem (if they disagree), but you don't know which one is erroneous. The AoA disagree alert is an optional feature on the 737[1]. My understanding is that the AoA display is optional as well[2] but does not break down the info per vane. I don't know if the gauge and alert are bundled together or available separately. So maybe you can know, maybe not. 1: https://ad.easa.europa.eu/blob/2018-23-51_Emergency.pdf/EAD_US-2018-23-51_1 https://ad.easa.europa.eu/blob/2018-23-51_Emergency.pdf/EAD_... 2: https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/432x481/e7b83569_c06b_4d91_a0b7_2d397e0f608f_c460dc9136ee5730ca34bfc573d8b923e98b11bd.jpeg https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/432x481/e7...

FabHK 8y ago
Interesting. What I meant to say is that even if the pilots had no way of knowing, the computer should notice and drop into a failure mode (that does not involve trimming down again and again, until the pilot sticks an umbrella in the trim wheel).
VBprogrammer 8y ago
I'd be surprised if this was the case. Typically redundancy like this is handled by having A and B systems on commercial aircraft. In the case of flight instruments this is usually divided by pilot and co-pilot systems. They have a separate AHRS (Attitude and Heading system) and their flight instruments show data from each system independently. If you watch a cockpit video of an airliner taking off you will usually hear the co-pilot announce "80 knots" and the pilot reply "cross-checked". What they are doing is checking that their air-sensor data agrees (within a reasonable margin) for the most critical information at that stage of flight (since takeoff speed is very important with the modern wing shape on an airliner). Similarly they have A and B autopilot systems which are driven independently by two AHRS units (except in special cases like during auto-land where both systems are operational). Which is all to say that I think they likely have two separate AoA sensors. Although, perhaps being an optional element the failure of one doesn't automatically trigger a AHRS disagree message.
- inferiorhuman 8y ago
  > Which is all to say that I think they likely have two separate AoA sensors. Correct, the 737 NG has two separate alpha vanes[1] and I believe the MAX does as well. However the "alpha vanes disagree" alert is a paid option per the emergency AD. Likewise the AoA indicator is a paid option. There is redundancy, but the plane may be configured such that the pilot cannot determine if there is a failure. Failure of one or both alpha vanes on an NG isn't a good thing, but failure of an alpha vane on a MAX could cause MCAS to essentially try to kill you and without that AoA disagree alert you may not know why because you've never been informed about this system, and at low altitude you likely wouldn't have time to figure out what's going on. Edit: if that all sounds fucking insane, it is. That's why American and Southwest pilot unions are livid[2]. 1: https://www.faa.gov/air_traffic/separation_standards/ase/2016_Event/documents/2.5_B737_Design_Differences.pdf https://www.faa.gov/air_traffic/separation_standards/ase/201... 2: https://www.seattletimes.com/business/boeing-aerospace/u-s-pilots-flying-737-max-werent-told-about-new-automatic-systems-change-linked-to-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/u-s-p...
  - ahartmetz 8y ago
    Okay, so Boeing added an electronic safety feature (with a deadly failure mode) necessitated by physical changes and regulation. They didn't mention it for marketing reasons. They made an indicator of the deadly failure mode of the feature a paid option. Got it.
    - VBprogrammer 8y ago
      Reading a little between the lines this is probably related to the pitch - power coupling present on all modern airliners. This is due to the thrust line being below the centre of lift meaning that increasing power causes a pitch up moment. In the 737 Max this probably got exacerbated to the point that it was possible to fly the plane into a stall by sharply increasing power in a high AoA situation (typically in a go-around). This was probably different enough to the 737NG that they felt it necessary to add the MCAS system to prevent having to do, what they considered, excessive differences training in that phase of flight.
village-idiot 8y ago
That a feature that notifies you that two flight critical sensors are disagreeing is optional is mind blowing to me. It’s like discovering that brakes are optional on the new Mercedes C Class.

FabHK 8y ago

Interesting. What I meant to say is that even if the pilots had no way of knowing, the computer should notice and drop into a failure mode (that does not involve trimming down again and again, until the pilot sticks an umbrella in the trim wheel).

VBprogrammer 8y ago

I'd be surprised if this was the case. Typically redundancy like this is handled by having A and B systems on commercial aircraft. In the case of flight instruments this is usually divided by pilot and co-pilot systems. They have a separate AHRS (Attitude and Heading system) and their flight instruments show data from each system independently. If you watch a cockpit video of an airliner taking off you will usually hear the co-pilot announce "80 knots" and the pilot reply "cross-checked". What they are doing is checking that their air-sensor data agrees (within a reasonable margin) for the most critical information at that stage of flight (since takeoff speed is very important with the modern wing shape on an airliner). Similarly they have A and B autopilot systems which are driven independently by two AHRS units (except in special cases like during auto-land where both systems are operational). Which is all to say that I think they likely have two separate AoA sensors. Although, perhaps being an optional element the failure of one doesn't automatically trigger a AHRS disagree message.

inferiorhuman 8y ago
> Which is all to say that I think they likely have two separate AoA sensors. Correct, the 737 NG has two separate alpha vanes[1] and I believe the MAX does as well. However the "alpha vanes disagree" alert is a paid option per the emergency AD. Likewise the AoA indicator is a paid option. There is redundancy, but the plane may be configured such that the pilot cannot determine if there is a failure. Failure of one or both alpha vanes on an NG isn't a good thing, but failure of an alpha vane on a MAX could cause MCAS to essentially try to kill you and without that AoA disagree alert you may not know why because you've never been informed about this system, and at low altitude you likely wouldn't have time to figure out what's going on. Edit: if that all sounds fucking insane, it is. That's why American and Southwest pilot unions are livid[2]. 1: https://www.faa.gov/air_traffic/separation_standards/ase/2016_Event/documents/2.5_B737_Design_Differences.pdf https://www.faa.gov/air_traffic/separation_standards/ase/201... 2: https://www.seattletimes.com/business/boeing-aerospace/u-s-pilots-flying-737-max-werent-told-about-new-automatic-systems-change-linked-to-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/u-s-p...
- ahartmetz 8y ago
  Okay, so Boeing added an electronic safety feature (with a deadly failure mode) necessitated by physical changes and regulation. They didn't mention it for marketing reasons. They made an indicator of the deadly failure mode of the feature a paid option. Got it.
  - VBprogrammer 8y ago
    Reading a little between the lines this is probably related to the pitch - power coupling present on all modern airliners. This is due to the thrust line being below the centre of lift meaning that increasing power causes a pitch up moment. In the 737 Max this probably got exacerbated to the point that it was possible to fly the plane into a stall by sharply increasing power in a high AoA situation (typically in a go-around). This was probably different enough to the 737NG that they felt it necessary to add the MCAS system to prevent having to do, what they considered, excessive differences training in that phase of flight.

village-idiot 8y ago

That a feature that notifies you that two flight critical sensors are disagreeing is optional is mind blowing to me. It’s like discovering that brakes are optional on the new Mercedes C Class.