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I think you’re slightly mischaracterizing the cause in the AF447 flight in that there was a lot more going on than just the airspeed indicator: AF447 crashed b

by js2 8y ago

I think you’re slightly mischaracterizing the cause in the AF447 flight in that there was a lot more going on than just the airspeed indicator: AF447 crashed because of a cyclical series of erroneous inputs based on unreliable prompts from aircraft systems; a cycle that fed on itself to the extent that control was never regained. The BEA’s final report attributes multiple factors to the AF447 mishap: temporary and repeated inconsistencies and loss of airspeed indication; inappropriate control inputs; failure to identify unreliable indicated speed; the approach to stall; the full stall state; and the inability to apply an appropriate response in these flight regimes. https://sma.nasa.gov/docs/default-source/safety-messages/safetymessage-2012-07-31-lossairfranceflight447.pdf?sfvrsn=e3ae1ef8_4 https://sma.nasa.gov/docs/default-source/safety-messages/saf... Also, in the AF447 case the computer had to defer to the human because it didn’t know what the air speed was. In contrast, one thing that’s surprising to me in Lion case is that the MCAS seemed to be willing to trust a faulty AoA sensor: The DFDR recorded a difference between left and right Angle of Attack (AoA) of about 20° and continued until the end of recording. http://avherald.com/h?article=4bf90724/0009&opt=0 http://avherald.com/h?article=4bf90724/0009&opt=0 So it’s got a pair of AoA sensors that aren’t in agreement (split brain, as it were) and it’s going to command nose down anyway? (I’m sure there’s a reason there aren’t triple redundant sensors where two have to agree or some such. I think the flight computer does require the various different kinds of sensors to agree, but I don’t think there’s triple redundancy of each sensor.) (Edit: sorry folks, I edited this a bunch of times so the replies below may not make perfect sense. I should really start typing my replies offline.)

cperciva 8y ago
Air France 447 was an Airbus 330. I think 333 is a typo. It was a typo, but for "332" (which is the jargon for "330-200").
- raverbashing 8y ago
  Yes, it's the ICAO aircraft type: A332 https://en.wikipedia.org/wiki/List_of_ICAO_aircraft_type_designators https://en.wikipedia.org/wiki/List_of_ICAO_aircraft_type_des...
carlivar 8y ago
> the pilot’s action of pulling up the nose was an irrational thing to do Actually the inexperienced COPILOT did this. The pilot realized that was a problem eventually, but too late. And unfortunately the Airbus takes the average of both pilot inputs.
- learc83 8y ago
  >And unfortunately the Airbus takes the average of both pilot inputs Really? That's the best way to handle that?
  - carlivar 8y ago
    Since I read that Popular Mechanics article, I think the exact same thing every time I board an Airbus plane.
    - richardhod 8y ago
      Airbus has since updated its flight control feedback systems
      - foldr 8y ago
        Source for this?
        richardhod 8y ago
        I may be mistaken; I had thought I had read that Airbus had acted, but on checking up, I think not in ways which would make my brief implication useful.
  - nolok 8y ago
    From what I read, it's a case of "can't do it the right way originally because of patents/legal/whatever; can't change it to the right way ever since because then every airbus pilot would need to requalify, every book rewritten, etc ...". Sounds like a terrible case of rotten legacy support.
bitwalker 8y ago
They can't agree though - angle of attack and airspeed are two different measurements. Together they can be used to detect a stall condition, but independently are insufficient. Furthermore, if either are incorrect or inoperable, then a stall may be misreported or not reported when it should be. AoA is really the weak point in my opinion, the probe can get stuck or iced up; they always seemed extremely fragile to me for something so critical. Not to mention, redundancy is limited by the number of probes, of which there are only two on all the aircraft I've worked on, so if they ice up, that's it. IIRC that's what started the chain of events which brought down the Air France fight.
- js2 8y ago
  I edited my comment and removed the airspeed reference but what I meant it that MCAS is only supposed to activate at airspeeds approaching stall and high AoA (and some other things). In the Lion case the plane had a pair of AoA sensors in disagreement and the airspeed shouldn’t be near stall in level flight, should it? Yet the MCAS commands nose down anyway.
  - baybal2 8y ago
    AoE sensors may well be in disagreement even without malfunction: local turbulence, flying in somebody's wake, etc
    - neuronic 8y ago
      AoA sensors might disagree to a point, but 20 degrees difference is ... unusual.
  - chipsa 8y ago
    Define "airspeeds approaching stall". You can stall a wing at any airspeed, if the angle of attack is great enough. And being able to tell if your angle of attack is great enough is the point of the AoA sensors.
- burfog 8y ago
  Given the expense and size of airliners, it might be worth determining AoA via tracking minute dust particles with LIDAR. That should be highly reliable, with very direct speed measurement in airflow that isn't in any way disturbed by the aircraft. You also get airspeed. Maybe there is an alternative to AoA. The whole point is to determine how air will be affecting surfaces such as the wing, so one could instead just put pressure transducers all over the aircraft. From that you can determine how close you are to stalling. You can even tell where on the aircraft a stall is imminent.
zkms 8y ago
A specific aspect of the AF447 incident that has not been mentioned here is the fact that the incident aircraft's computers declared data from the Angle of Attack (AOA) sensors INVALID if the measured airspeed was LESS than 60 knots. By itself, this is a sensible thing to do, since the AOA sensors are composed of a little vane that is free to rotate and align itself with the direction of airflow, connected to an angle resolver that measures the angle of the vane. If the airflow is not fast enough, there will not be enough force exerted to reliably and continually align the vane, thus the instantaneous angle of the vane may not conform to the angle of the airflow. To quote the BEA report: > If the CAS [calibrated air speed] measurements for the three ADR [air data reference] are lower than 60 kt, the angle of attack values of the three ADR are invalid and the stall warning is then inoperative. This results from a logic stating that the airflow must be sufficient to ensure a valid measurement by the angle of attack sensors, especially to prevent spurious warnings. However, the pilot flying had gotten the airspeed under that 60kt threshold, which inhibited the stall warning. When he reacted by pushing the control stick forwards (the correct reaction to a stall), the airspeed increased (a good thing, you need airspeed for lift) and the AOA decreased (also a good thing). However, the airspeed quickly increased over the 60kt threshold -- well before the AOA had a chance to decrease past its threshold. Therefore, the stall warning sounded again. This does not fit into any mental model of how aircraft operate. A pilot knows, from their training and experience, that when you pull back on the stick, you end up stalling your plane (which leads the stall warning to sound); and to recover from a stall, you must push forwards on the stick, which unstalls the plane. After you're out of the stall, after you've recovered by pushing the stick, you expect the stall warning to be silent. The crew were likely not aware that the stall warning cannot sound under 60kt airspeed, and could not, were not given the opportunity or information to realize that "stall warning NOT sounding" can signify that the plane is in fact, very seriously stalled. The crew reacted with control inputs to shut that stall warning up, to avoid a stall, because "silent stall warning" is supposed to mean "not stalled". Acting on that stall warning, unaware of its treacherous reversed semantics -- sounding with the stick pushed forwards, and silenced when pulling back -- sent that crew to their doom: > Until the end of the flight, the angle of attack values changed successively from valid to invalid. Each time that at least one value became valid again, the stall warning re-triggered and each time the angle of attack values were invalid, the warning stopped. Several nose-down inputs caused a drop in the pitch attitude and the angle of attack, whose values then became valid, such that a clear nose-down input resulted in the triggering of the stall warning. It appears that the PF reacted, on at least two occasions, with a nose-up input, whose consequences were an increase in angle of attack, a drop in measured speed and consequently stopping the stall warning. Until the end of the flight, no valid angle of attack value was less than 35°. There was no AOA instrument on the flight deck either! The computers saw the AOA increase when the pilots pulled the stick back and saw the AOA decrease when the pushed forwards, but the pilots themselves never saw that, because they were not shown a single AOA number/readout. The information they were given that was derived from AOA (the stall warning's status) was completely and fatally misleading; to the point that the pilots never diagnosed a stall: > The crew never formally identified the stall situation. Information on angle of attack is not directly accessible to pilots. The angle of attack in cruise is close to the stall warning trigger angle of attack in a law other thann normal law. Under these conditions, manual handling can bring the aeroplane to high angles of attack such as those encountered during the event. It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the aeroplane and take the actions that may be required. > Consequently, the BEA recommends that EASA and the FAA evaluate the relevance of requiring the presence of an angle of attack indicator directly accessible to pilots on board aeroplanes.
deleted 8y ago
[deleted]

by js2 8y ago

cperciva 8y ago

Air France 447 was an Airbus 330. I think 333 is a typo. It was a typo, but for "332" (which is the jargon for "330-200").

raverbashing 8y ago
Yes, it's the ICAO aircraft type: A332 https://en.wikipedia.org/wiki/List_of_ICAO_aircraft_type_designators https://en.wikipedia.org/wiki/List_of_ICAO_aircraft_type_des...

carlivar 8y ago

> the pilot’s action of pulling up the nose was an irrational thing to do Actually the inexperienced COPILOT did this. The pilot realized that was a problem eventually, but too late. And unfortunately the Airbus takes the average of both pilot inputs.

learc83 8y ago
>And unfortunately the Airbus takes the average of both pilot inputs Really? That's the best way to handle that?
- carlivar 8y ago
  Since I read that Popular Mechanics article, I think the exact same thing every time I board an Airbus plane.
  - richardhod 8y ago
    Airbus has since updated its flight control feedback systems
    - foldr 8y ago
      Source for this?
      - richardhod 8y ago
        I may be mistaken; I had thought I had read that Airbus had acted, but on checking up, I think not in ways which would make my brief implication useful.
- nolok 8y ago
  From what I read, it's a case of "can't do it the right way originally because of patents/legal/whatever; can't change it to the right way ever since because then every airbus pilot would need to requalify, every book rewritten, etc ...". Sounds like a terrible case of rotten legacy support.

bitwalker 8y ago

They can't agree though - angle of attack and airspeed are two different measurements. Together they can be used to detect a stall condition, but independently are insufficient. Furthermore, if either are incorrect or inoperable, then a stall may be misreported or not reported when it should be. AoA is really the weak point in my opinion, the probe can get stuck or iced up; they always seemed extremely fragile to me for something so critical. Not to mention, redundancy is limited by the number of probes, of which there are only two on all the aircraft I've worked on, so if they ice up, that's it. IIRC that's what started the chain of events which brought down the Air France fight.

js2 8y ago
I edited my comment and removed the airspeed reference but what I meant it that MCAS is only supposed to activate at airspeeds approaching stall and high AoA (and some other things). In the Lion case the plane had a pair of AoA sensors in disagreement and the airspeed shouldn’t be near stall in level flight, should it? Yet the MCAS commands nose down anyway.
- baybal2 8y ago
  AoE sensors may well be in disagreement even without malfunction: local turbulence, flying in somebody's wake, etc
  - neuronic 8y ago
    AoA sensors might disagree to a point, but 20 degrees difference is ... unusual.
- chipsa 8y ago
  Define "airspeeds approaching stall". You can stall a wing at any airspeed, if the angle of attack is great enough. And being able to tell if your angle of attack is great enough is the point of the AoA sensors.
burfog 8y ago
Given the expense and size of airliners, it might be worth determining AoA via tracking minute dust particles with LIDAR. That should be highly reliable, with very direct speed measurement in airflow that isn't in any way disturbed by the aircraft. You also get airspeed. Maybe there is an alternative to AoA. The whole point is to determine how air will be affecting surfaces such as the wing, so one could instead just put pressure transducers all over the aircraft. From that you can determine how close you are to stalling. You can even tell where on the aircraft a stall is imminent.

zkms 8y ago

A specific aspect of the AF447 incident that has not been mentioned here is the fact that the incident aircraft's computers declared data from the Angle of Attack (AOA) sensors INVALID if the measured airspeed was LESS than 60 knots. By itself, this is a sensible thing to do, since the AOA sensors are composed of a little vane that is free to rotate and align itself with the direction of airflow, connected to an angle resolver that measures the angle of the vane. If the airflow is not fast enough, there will not be enough force exerted to reliably and continually align the vane, thus the instantaneous angle of the vane may not conform to the angle of the airflow. To quote the BEA report: > If the CAS [calibrated air speed] measurements for the three ADR [air data reference] are lower than 60 kt, the angle of attack values of the three ADR are invalid and the stall warning is then inoperative. This results from a logic stating that the airflow must be sufficient to ensure a valid measurement by the angle of attack sensors, especially to prevent spurious warnings. However, the pilot flying had gotten the airspeed under that 60kt threshold, which inhibited the stall warning. When he reacted by pushing the control stick forwards (the correct reaction to a stall), the airspeed increased (a good thing, you need airspeed for lift) and the AOA decreased (also a good thing). However, the airspeed quickly increased over the 60kt threshold -- well before the AOA had a chance to decrease past its threshold. Therefore, the stall warning sounded again. This does not fit into any mental model of how aircraft operate. A pilot knows, from their training and experience, that when you pull back on the stick, you end up stalling your plane (which leads the stall warning to sound); and to recover from a stall, you must push forwards on the stick, which unstalls the plane. After you're out of the stall, after you've recovered by pushing the stick, you expect the stall warning to be silent. The crew were likely not aware that the stall warning cannot sound under 60kt airspeed, and could not, were not given the opportunity or information to realize that "stall warning NOT sounding" can signify that the plane is in fact, very seriously stalled. The crew reacted with control inputs to shut that stall warning up, to avoid a stall, because "silent stall warning" is supposed to mean "not stalled". Acting on that stall warning, unaware of its treacherous reversed semantics -- sounding with the stick pushed forwards, and silenced when pulling back -- sent that crew to their doom: > Until the end of the flight, the angle of attack values changed successively from valid to invalid. Each time that at least one value became valid again, the stall warning re-triggered and each time the angle of attack values were invalid, the warning stopped. Several nose-down inputs caused a drop in the pitch attitude and the angle of attack, whose values then became valid, such that a clear nose-down input resulted in the triggering of the stall warning. It appears that the PF reacted, on at least two occasions, with a nose-up input, whose consequences were an increase in angle of attack, a drop in measured speed and consequently stopping the stall warning. Until the end of the flight, no valid angle of attack value was less than 35°. There was no AOA instrument on the flight deck either! The computers saw the AOA increase when the pilots pulled the stick back and saw the AOA decrease when the pushed forwards, but the pilots themselves never saw that, because they were not shown a single AOA number/readout. The information they were given that was derived from AOA (the stall warning's status) was completely and fatally misleading; to the point that the pilots never diagnosed a stall: > The crew never formally identified the stall situation. Information on angle of attack is not directly accessible to pilots. The angle of attack in cruise is close to the stall warning trigger angle of attack in a law other thann normal law. Under these conditions, manual handling can bring the aeroplane to high angles of attack such as those encountered during the event. It is essential in order to ensure flight safety to reduce the angle of attack when a stall is imminent. Only a direct readout of the angle of attack could enable crews to rapidly identify the aerodynamic situation of the aeroplane and take the actions that may be required. > Consequently, the BEA recommends that EASA and the FAA evaluate the relevance of requiring the presence of an angle of attack indicator directly accessible to pilots on board aeroplanes.

deleted 8y ago

[deleted]