4 ms·

As far as i know you use three sensors, so you can automatically compensate for one failing/misreporting sensor without human intervention.

darkpuma 7y ago
You can't compensate because you don't know which one is wrong. However you can make MCAS inactive whenever the two sensors are disagreeing with each other.
- Macha 7y ago
  With the current two sensor setup, that is true. This is why the parent poster is arguing for 3 sensors (and the real reason for tripling). If not all sensors agree but 2/3 do, it is more probable that the two sensors are correct than the 1 sensor disagreeing.
  - darkpuma 7y ago
    Right, but the MCAS system as it was implemented during those two crashes did not disable itself when the two sensors disagreed. It could have, but didn't. Boeing is apparently changing that, and I believe that change to be sufficient. MCAS being automatically disabled when the sensors agree but leaving the pilots with electronic trim control seems like a perfectly adequate solution. The MCAS system was never even necessary for flight, it was only necessary for certification. In the situations where it's meant to be active, which are a limited subset of all high angle of attack scenarios, it's fine system to have if it's working correctly. But it should never be active outside of that limited set of scenarios. It should never be active when both angle of attack sensors aren't indicating a high angle of attack within a reasonable distance of each other. Frankly, if two sensors are indicating a high angle of attack and one is not, it's probably still sensible to disable MCAS. I don't have any of the real numbers, but the chance of a 737 being in a low speed high angle of attack scenario is low in the first place, possibly sufficiently low that "two sensors being wrong and the aircraft being in level flight" might be more likely than "aircraft is near stall and one sensor is wrong." To know for sure we'd need at least the hard data on what modes of failure these sensors have, how likely any of those modes is to occur and what the expected readouts from those failure modes are, and how likely a 737 is to encounter a low speed stall scenario. We, or at least I, don't have any of that. But my gut says that two sensors are sufficient iff the MCAS system is only active when they agree. Furthermore, the chance of two sensors being wrong actually goes up if you have three sensors, rather than two. Correct me if I'm wrong, stats was never my strong point, but it seems to me like the cumulative binomial distribution is relevant here: #lang racket (require math/number-theory) (define (general-binomial p k n) (* (binomial n k) (expt p k) (expt (- 1 p) (- n k)))) (define (cumulative-general-binomial p k1 k2 n) (apply + (map (λ (k) (general-binomial p k n)) (range k1 (add1 k2))))) Chance of a single sensor failing, if each has a 1% chance of failure (sanity check): > (cumulative-general-binomial .01 1 1 1) 0.01 Chance of two or three out of three sensors failing, if each has a 1% chance of failure: > (cumulative-general-binomial .01 2 3 3) 0.00029800000000000003 Chance of two out of two sensors failing, if each has a 15 chance of failure: > (cumulative-general-binomial .01 2 2 2) 0.0001 Two out of three sensors failing is three times more likely than two out of two sensors failing! (In other words, two broken sensors voting out a third working sensor is more likely than two out of two sensors being broken.)
    - jarym 7y ago
      I think your maths is sound but the question is what are the odds that multiple sensors fail on the same flight (and I don't know the answer)? Because as soon as one sensor fails that aircraft will go in for maintenance.
      - darkpuma 7y ago
        That was kind of my point.. MCAS should turn off as soon as a single sensor fails, and three sensors are therefore unnecessary. Two sensors are sufficient to detect when a single sensor fails. If you only have two sensors and two fail, then MCAS remains active and your plane crashes. But that's not likely to happen. However that's more likely to happen if you have three sensors and try to use two sensors to vote out a third, in order to keep MCAS active when a sensor fails. In that configuration, a double failure (causing a crash) is three times more likely. My conclusion is turn off MCAS as soon as even one sensor disagrees with the others. And if that's how MCAS is configured, then three sensors is unnecessary overkill. If you want overkill, you may as well double up the sensors on both sides and have four instead of three; all the better right?
- tropo 7y ago
  A good rule would be to keep MCAS active, choosing to use the sensor that produces the less-extreme adjustment.
  - darkpuma 7y ago
    Eh, that kind of makes a certain amount of sense, but I don't think that's quite right. Determining which of the two sensors is less-extreme supposes knowledge about the airframe's current state. If he aircraft is currently in a very high angle of attack low speed stall, then the sensor that says everything is A-Okay is the extreme sensor. I understand that's not what you mean, that the sensor indicating a situation closes to nominal flight should be chosen, but I think the sticky part is that MCAS isn't a system meant for normal flight conditions. MCAS is only supposed to be active when the aircraft is in an extreme scenario. So in fact if the sensor with "less-extreme adjustment" is preferred, in a way that actually means the system functions as I suggested: MCAS is disabled if the sensors disagree. But not quite. In your scheme if both the sensors are extreme but disagree, MCAS would be active to the lesser extent. But in my scheme, if both sensors are extreme but disagree, MCAS would be totally inactive (while leaving the pilot with electronic stabilizer trim control of course, allowing the pilot to manually do anything MCAS would be capable of doing.)

darkpuma 7y ago

You can't compensate because you don't know which one is wrong. However you can make MCAS inactive whenever the two sensors are disagreeing with each other.

Macha 7y ago
With the current two sensor setup, that is true. This is why the parent poster is arguing for 3 sensors (and the real reason for tripling). If not all sensors agree but 2/3 do, it is more probable that the two sensors are correct than the 1 sensor disagreeing.
- darkpuma 7y ago
  Right, but the MCAS system as it was implemented during those two crashes did not disable itself when the two sensors disagreed. It could have, but didn't. Boeing is apparently changing that, and I believe that change to be sufficient. MCAS being automatically disabled when the sensors agree but leaving the pilots with electronic trim control seems like a perfectly adequate solution. The MCAS system was never even necessary for flight, it was only necessary for certification. In the situations where it's meant to be active, which are a limited subset of all high angle of attack scenarios, it's fine system to have if it's working correctly. But it should never be active outside of that limited set of scenarios. It should never be active when both angle of attack sensors aren't indicating a high angle of attack within a reasonable distance of each other. Frankly, if two sensors are indicating a high angle of attack and one is not, it's probably still sensible to disable MCAS. I don't have any of the real numbers, but the chance of a 737 being in a low speed high angle of attack scenario is low in the first place, possibly sufficiently low that "two sensors being wrong and the aircraft being in level flight" might be more likely than "aircraft is near stall and one sensor is wrong." To know for sure we'd need at least the hard data on what modes of failure these sensors have, how likely any of those modes is to occur and what the expected readouts from those failure modes are, and how likely a 737 is to encounter a low speed stall scenario. We, or at least I, don't have any of that. But my gut says that two sensors are sufficient iff the MCAS system is only active when they agree. Furthermore, the chance of two sensors being wrong actually goes up if you have three sensors, rather than two. Correct me if I'm wrong, stats was never my strong point, but it seems to me like the cumulative binomial distribution is relevant here: #lang racket (require math/number-theory) (define (general-binomial p k n) (* (binomial n k) (expt p k) (expt (- 1 p) (- n k)))) (define (cumulative-general-binomial p k1 k2 n) (apply + (map (λ (k) (general-binomial p k n)) (range k1 (add1 k2))))) Chance of a single sensor failing, if each has a 1% chance of failure (sanity check): > (cumulative-general-binomial .01 1 1 1) 0.01 Chance of two or three out of three sensors failing, if each has a 1% chance of failure: > (cumulative-general-binomial .01 2 3 3) 0.00029800000000000003 Chance of two out of two sensors failing, if each has a 15 chance of failure: > (cumulative-general-binomial .01 2 2 2) 0.0001 Two out of three sensors failing is three times more likely than two out of two sensors failing! (In other words, two broken sensors voting out a third working sensor is more likely than two out of two sensors being broken.)
  - jarym 7y ago
    I think your maths is sound but the question is what are the odds that multiple sensors fail on the same flight (and I don't know the answer)? Because as soon as one sensor fails that aircraft will go in for maintenance.
    - darkpuma 7y ago
      That was kind of my point.. MCAS should turn off as soon as a single sensor fails, and three sensors are therefore unnecessary. Two sensors are sufficient to detect when a single sensor fails. If you only have two sensors and two fail, then MCAS remains active and your plane crashes. But that's not likely to happen. However that's more likely to happen if you have three sensors and try to use two sensors to vote out a third, in order to keep MCAS active when a sensor fails. In that configuration, a double failure (causing a crash) is three times more likely. My conclusion is turn off MCAS as soon as even one sensor disagrees with the others. And if that's how MCAS is configured, then three sensors is unnecessary overkill. If you want overkill, you may as well double up the sensors on both sides and have four instead of three; all the better right?
tropo 7y ago
A good rule would be to keep MCAS active, choosing to use the sensor that produces the less-extreme adjustment.
- darkpuma 7y ago
  Eh, that kind of makes a certain amount of sense, but I don't think that's quite right. Determining which of the two sensors is less-extreme supposes knowledge about the airframe's current state. If he aircraft is currently in a very high angle of attack low speed stall, then the sensor that says everything is A-Okay is the extreme sensor. I understand that's not what you mean, that the sensor indicating a situation closes to nominal flight should be chosen, but I think the sticky part is that MCAS isn't a system meant for normal flight conditions. MCAS is only supposed to be active when the aircraft is in an extreme scenario. So in fact if the sensor with "less-extreme adjustment" is preferred, in a way that actually means the system functions as I suggested: MCAS is disabled if the sensors disagree. But not quite. In your scheme if both the sensors are extreme but disagree, MCAS would be active to the lesser extent. But in my scheme, if both sensors are extreme but disagree, MCAS would be totally inactive (while leaving the pilot with electronic stabilizer trim control of course, allowing the pilot to manually do anything MCAS would be capable of doing.)