5 ms·

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 instrum

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.
    - FabHK 8y ago
      Could well be; I read somewhere that it was related to the ever bigger engines (for more efficient (higher) bypass ratio), which presumably have a lower centre of thrust.
      - VBprogrammer 8y ago
        They lengthened the nose gear 20cm to fit the new engines with the same ground clearance. Must have dropped the thrust centre line nearly half of that.
        inferiorhuman 8y ago
        Well the Seattle Times quoted an ex-Boeing employee thusly: > A former Boeing executive, speaking on condition of anonymity because discussion of accident investigations is supposed to be closely held, said that Boeing engineers didn’t introduce the change to the flight-control system arbitrarily. > He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.

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.
  - FabHK 8y ago
    Could well be; I read somewhere that it was related to the ever bigger engines (for more efficient (higher) bypass ratio), which presumably have a lower centre of thrust.
    - VBprogrammer 8y ago
      They lengthened the nose gear 20cm to fit the new engines with the same ground clearance. Must have dropped the thrust centre line nearly half of that.
      - inferiorhuman 8y ago
        Well the Seattle Times quoted an ex-Boeing employee thusly: > A former Boeing executive, speaking on condition of anonymity because discussion of accident investigations is supposed to be closely held, said that Boeing engineers didn’t introduce the change to the flight-control system arbitrarily. > He said it was done primarily because the much bigger engines on the MAX changed the aerodynamics of the jet and shifted the conditions under which a stall could happen. That required further stall protection be implemented to certify the jet as safe.