Not only a longer grounding, but some airlines are canceling orders and many passengers don't want to fly in them.
The 737 Max could become the Edsel of airplanes.
The 737 Max could become the Edsel of airplanes.
If they don't both give the same reading, MCAS turns off.
There’s no problem if one fails but what if both fail and both corroborate a bad reading?
Certainly the cost of 3 such sensors would not be as high as the cost of another crash.
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.
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)))))
> (cumulative-general-binomial .01 1 1 1)
0.01
> (cumulative-general-binomial .01 2 3 3)
0.00029800000000000003
> (cumulative-general-binomial .01 2 2 2)
0.0001
(In other words, two broken sensors voting out a third working sensor is more likely than two out of two sensors being broken.)
Because as soon as one sensor fails that aircraft will go in for maintenance.
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?
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.)
If I understood correctly, MCAS trimmed it so much that the pilots were unable to bring it back manually.
This sounds like Boeing engineers didn’t even put a limit on the maximum trimming that MCAS can do, not even to the maximum theoretical limit for the plane/engines.
Not only that, MCAS controlled an electric motor which turned the trim wheel and which could apply more torque than a human turning it manually. The only way to disable MCAS was to cut power to that motor. MCAS pointed the aircraft at the ground, and it was impossible to trim the aircraft to achieve level flight again.
Mind, MCAS was supposed to be a crutch to fly at the edges of the flight envelope, something that a regular pilot would never encounter because under normal conditions pilots wouldn't find themselves in a regime where MCAS would be activated. That was the idea, instead we had two hull losses within 6 months!
The issue from the Ethiopian crash was that, in the extreme trim angle, there was simply too much force on the screw to turn manually. This condition is trained for, and the solution is to let the plan angle down to relieve the aerodynamic pressure on the control surface. That gives you an opportunity to manually control the trim wheels.
The pilots did not have the benefit of altitude to perform this maneuver, so (likely) instead re-enabled the electric trim in a bid to use it to reduce the nose-down trim before the MCAS would re-engage. This proved fatal.
Scroll down slightly for the empennage diagram, right hand side, item 3.
https://www.seattletimes.com/business/boeing-aerospace/boein...
If Boeing really needed to cut these corners in order to remain competitive, then having to redo those corners might make them too expensive or inefficient for the market.
I also doubt customers will really vote with their feet given that the number one factor when booking a ticket is price. If some people choose not to fly on them it will create supply/demand imbalances in the market which would be interesting.
Agreed. As the article points out:
"Passengers eventually returned to flying the 787 and to other airplanes that were grounded: the McDonnell Douglas DC-10, the Douglas DC-6, the Lockheed Constellation and even the de Havilland Comet."