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Old 18th Nov 2018, 05:50
  #1373 (permalink)  
Hi_Tech
 
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Originally Posted by QuagmireAirlines
True there. Odd & tragic & stupid how just ONE AOA (alpha) sensor can cause nose down pitch trim. One can argue about the Human Factors involved here concerning stress, panic, time to decide what to do, etc., but I tend to come down on the side of "don't over-stress tired human brains", from the Flight Controls Engineer viewpoint I have.

Assuming the crash was caused by the AOA alpha vane ... continuing on:

"Analytical Redundancy" would have prevented this problem, as it would add a third alpha to the dual vane measurement to break the tie between 2 vane sensors. ... Note that alpha = theta - gamma, received through air data & inertial sensors apart from the alpha vanes. That caclulated third alpha makes the system triple redundant, along with using median value selection.

You could even apply a (fourth redundancy!) reasonableness test on the alpha vane's sensor values by writing more software that applies plain old logical common sense: --> Can that high alpha value, seen all of a sudden, and when the airspeed is good, and when g's aren't being pulled, be a valid value of alpha? Remember we have pitch rate sensors, accelerometers, pitot-static tubes, etc. to help the alpha fault detection out. Remember alpha-dot = pitch rate - (accelerometer / speed); we can use all the kinematic laws to do this.

All the above is what a very fast, sharp, smart pilot or flight engineer would do in a millisecond if they saw an alpha vane sensor measurement hard-over (high) all of a sudden. It would be obvious to an observer actually seeing the sensor values along with the trimmed aircraft state to decide it's a bad sensor. That is if a human could see & monitor the raw sensor values in real-time to make that clear judgement. Computers can.

Algorithms such as the above, along with complementary or Kalman filtering to filter out spectral noise and emphasize the short term estimation of valid alpha would also help. I won't go into the signal processing or kinematic blending algorithms here, but I will say you wouldn't need much.

And, finally, where was the FTA & FMEA in all this? I guess they thought the nose-down trim would be easily countered by pilots & was not deemed catastrophic. Again, Human Factors, and I'm on the side of "don't put the plane in peril and expected tired or paniced pilots figure it out".
In answer to the question raised in post#1363 above and several other posts after this one, I had made a query to get an answer anyone trained on MAX and has access to MAX AMM (Maintenance Manual). The question of omission or rejection of wrong data is nicely dealt in B777 ADIRU, which also has only two AOA sensors (This acft is an early 1990 design). I am sure B737 could not be certified without similar protection. If they have not done so, it is a serious shortcomin in design. Sensors do fail or damaged by FOD. If in 1990 design this was foreseen, why overlook it in 2015 design?
B737 also has two ADIRUs like B777. So I wanted to find out from any one the correct config on B737 MAX. But no one has replied so far.

My Post #841
There has to be more protection in the system design for this not to happen. (Single source failure)
In the B777 which I am familiar with, each of the two ADIRUs (Air Data Inertial reference unit) receive both AOA inputs (There are two AOA sensors on most aircraft, same config on B737 also). This is compared with 'Calculated AOA' and a mid value is used. This is the redundancy built in the system on B777. Also each of the AOA sensor has two outputs, feed into two different computational channels. See the redundancy. There are actually 4 signals from two AOA sensors. Also there are two computed AoA values. So in total 6 signals.

The full text from the B777 AMM is as below.
AOA Redundancy Management
The AOA redundancy management logic uses a modified midvalue selection.
The modified mid-value selection chooses the mid-value of these three AOA values:
* Left corrected AOA
* Right corrected AOA
* Calculated AOA.
The AOA redundancy management logic receives inputs from the inertial and air data systems to calculate the calculated AOA.


Has any one in this forum have access to B737 MAX AMM (Pages from AMM Chap 34-20-00) and if you can post the same system info for B737 MAX redundancy management of AOA signals.
I am just curious, and hope it does not bore other users. END

However no one has replied. So we have wait for the interim report to come out to understand what happened to LION Air. For other concerned B737 MAX pilots, they have to follow the new procedure, and we have to keep speculating endlessly.
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