Originally Posted by
737 Driver
I got a brief on the new software yesterday.
MCAS is part of the Speed Trim System (STS). There are two STS channels, and one is always required for dispatch. If one or both STS/MCAS systems became inop, we receive a warning annunciator on the flight deck. If both systems are inop, the annunciator comes on immediately - it doesn't wait until 400'. If both STS channels fail in flight, we have a non-normal procedure to address it - just like we have non-normals to address the failure of engines, hydraulics, electrics and all sorts of other things we would require for dispatch.
The new Flight Control Computer (FCC) software will inhibit any STS/MCAS input if the difference between AOA's is greater than about 5 degrees. This is more stringent that the current "AOA Disagree" trigger of greater than 10 degrees. The new software also has some other tests for reasonableness. and the total nose down input is adjusted for the conditions - it will input more stab trim at low airspeeds and less at high airspeeds.
Not sure what you are asking with your last question, but maybe it would be a helpful reminder that MCAS only activates with the flaps retracted. If the flight crew is operating the aircraft normally, the flaps will always be extended below 1000 feet.
There was the Boeing spokesman that said the AoA disagree only worked above 400ft after they found it actually was not active all all.
So if that is true, then how can the MCAS part of the STS be tested before flight? Lion Air seems faulty AoA fitted - would the annunciation illuminate in this case?
The current MCAS AoA had no limit of disagree, and there was no problem withe the STS operation previously - MCAS really looks more than just a sub-sytem of STS.
Now the that is worrying - variable amounts of input! So if you are going slow with a high angle of attack - how long do you watch that trim wheel spin for?
From the previous thread Mr Driver your instinct would be to trim have you then disabled MCAS and stopped it from its certification requirement?
Would your current practice still stand? - keep flap out, or Autopilot on to bypass MCAS from being able to activate?