Originally Posted by
737 Driver
I got a brief on the new software yesterday. First, I would point out that while a third AOA would be nice, it is not necessary. The 737 has a number of double redundant systems. If there is a disagreement, we get a warning and we refer to a procedure to determine the bad system. We don't need a third AOA to figure out the bad one because we can cross check the airspeeds, mach numbers, altitudes, and power settings for reasonableness. The pilots are, in effect, the "third" system that sorts the good from the bad.
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.
This is news and this is very important especially the STS MCAS relation thing to be heard by a Boeing representative. Maybe not so much for pilots but for safety engineers and probably lawyers. It has a lot of implications on what has (not) been done and should have been done in the first place and what was state of the art and good practice within Boeing.
A feast.