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Old 30th Nov 2018, 12:07
  #72 (permalink)  
Join Date: Jan 2008
Location: uk
Posts: 780
Originally Posted by wiedehopf View Post
Some people said the AoA sensor needs to be tested with a special jig to orient it while someone in the cockpit is checking the output value.
Now either that test was not performed or they made a mistake here.
On the classic the last step of the install procedure 27-32-11/401 is to perform system test 27-32-00/501 which includes testing the stall warning system end to end by using the calibrator,
turning the vane and checking at what point stick shaker triggers.

I don't have the relevant bits of NG or MAX AMM to see if it is the same, but it seems likely it will be similar. We know from the log that they recorded doing an "installation test", but we don't know exactly what. However, if as with the classic, the test is specified as the last step of the install procedure, do you actually need to note the test number given that you have recorded completing the install procedure? Not sure.

There is further oddity in the traces too - the 20deg offset is actually not consistent. At the beginning of the previous flight, while on the ground, it looks to be about 10, falling and briefly going -ve, then rising to a constant 20 once the airspeed goes up. At the end of that flight, when airspeed drops, the offset appears to increase a little, and that higher offset is the same at start of next flight, until airspeed comes up and then it goes back to 20.

So we may be looking at a sensor fault that is dependent on airspeed and therefore wouldn't have been found on a ground test anyway. On the other hand the vanes are not alive without airspeed and may settle at different points anyway. But that drop at the start of the previous flight may be significant, probably at the end of taxi, the right AOA bounces a bit too, but the left much more. Something came loose? Image to show what I am rambling on about:

Note that if you are thinking sticky vane (I was), the test procedure on the classic contains the following:
slowly rotate sensor vane between stops using light finger pressure. Check that vane rotates without binding or variations in torque
infrequentflyer789 is offline