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Old 4th Apr 2019, 19:06
  #3140 (permalink)  
GarageYears
 
Join Date: Jun 2009
Location: VA, USA
Age: 58
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Originally Posted by Just This Once...
The autopilot behaviour differed considerably from the Boeing documents:

By 05:38:45 the left AoA sensor was reporting (but not displaying) over 74deg AoA, yet the LH AP was engaged successfully some 37 seconds later and remained engaged until time 05:39:55. By that time MCAS had completed 3 discrete pitch-down trim inputs before the AP actually disengaged.

George the autopilot wants to go up whilst HAL the MCAS wants to go down.

Eeek.
Really?

Aside from why the AP was successfully engaged, the three trim ND FCC commands (and one NU for good measure), DURING autopilot engagement are brief and likely STS driven - not MCAS. That comes after the AP is disconnected.

The first MCAS ND command occurs on deselection of the AP, and runs for 10 seconds as the system is expected to do. You can clearly see that in the FDR traces.

The crew then input a shorter NU pickle switch trim input (which did not equal the MCAS ND demaind).

MCAS reset and 5 seconds after the crew ceased their input, it runs again for less than 10 seconds this time, since the crew again provided NU pickle switch trim, which shut off MCAS for another 5 seconds, at which point the crew had selected the cutoff switches.

MCAS ran again (since it was reset) but had no effect, since it was disabled by the cutoffs.

At this point the aircraft was roughly pitch trim of 2.5 units (so ND). The crew then tried to use manual trim (which failed) and then hauled back on the column and tried to control the aircraft via the yoke.

Later two very short pickle switch inputs NU were input indicating the crew re-engaged the electric trim, but they were very short and had a very small effect on trim.

5 seconds later MCAS, having been reset by the crew electric trim input, and now being active again, ran for less than 10 seconds and seems to have been interrupted by the Left AOA value changing.

Unfortunately at this point the aircraft was very fast >375kts and the ND input was too much to overcome.

- GY


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