Originally Posted by
phil gollin
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Going back a bit ...... the problem SEEMS to have arisen due to a single (?) AOA sensor failure. This then SEEMS to have activated the stick shaker on the side of the aircraft associated with that failed sensor , which happened to be the PF's side (correct ?) In addition this single failure then gave rise to the MCAS starting the POSSIBLE disastrous changes in trim.
IF that is basically correct.
1: It has been stated on here that there were two AOA sensors on the plane, so how did the FCS decide which one was at fault ? IF they had had, say, three AOA sensors, then at least the electronics could have "voted" two-to-one which were correct and which "failed".
2: Whilst the stick-shaker seems to have been a warning there has been no mention (I may have missed it) of any particular flashing button, or message on a glass screen coming up to note that a particular AOA sensor had failed. I note that there will always be a problem with overloading pilots with information when there is an emergency but asking overloaded pilots to remember the possibility of a failed AOA sensor causing one-sided stick-shaker operation seems optimistic.
I am an avionics maintenance engineer. I have not worked on any model of the 737, but am very familiar with AOA systems on a variety of other transport category aircraft.
If the probe dropped offline completely - stopped sending any position data to the aircraft systems - that would usually trigger some kind of CAS message to indicate a failure of the probe has occurred. On all aircraft I am familiar with, this would also trigger a warning that the stall warning and protection system is degraded. A single AOA probe can trigger the stick shaker if it senses high alpha, but normally both probes have to be in agreement as to the high alpha to trigger the stick pusher. With a failed probe, you would not have the pusher function available.
The MCAS system is a new concept to me. I would think that in a well-designed system, you would want to have agreement as to current AOA from BOTH probes before automatically applying nose down trim, just as with the voting required to activate the pusher on every aircraft model I have worked on.
In this incident, it appears that the left probe was still sending data, thus no “AOA fail” message - but the AOA data from the left probe appears to have been massively different than that from the right side.
I do not have access to a 737 Max AMM, so do not know the post-installation functional checks required after replacing a faulty probe. Most probes contain a guide pin which inserts into a matching hole in the fuselage to insure that the probe body is physically oriented correctly in relation to the airframe, but on all aircraft models I work on, installing a new probe also requires a post-install rigging check to insure that the output position data is correct. Typically this is done with a rigging adapter that attaches to the probe and fuselage, containing a calibrated angle scale and pointer, and a mount to attach a calibrated digital protractor. The check usually involves rotating the probe to specific angles, while checking the generated data to insure it is accurate.
IF the 737 Max requires a rigging check when an AOA probe is replaced, the question is: did the Lion Air engineers perform one, or did they just replace the probe and sign it off? A rigging check would have revealed any inaccuracy in either the new or old probes. If no rigging check is required in the AMM procedure for replacing an AOA probe, then that is on Boeing, not Lion Air.
An AOA rigging adapter is a precision piece of test equipment that has to be calibrated and certified. A large airline would likely have one on hand, a smaller airline not so likely.