Originally Posted by
Smythe
The 'fix' includes using 2 AoA sensors. That is not a fix. The ac needs 3, so Boeing is minimizing the cost and impact of the fix.
They almost always disagree, so a 3rd is needed in the decision process. What happens if, like in the crash, one is damaged on DEP?
If MCAS is inop can the ac be dispatched? How does a pilot know MCAS INOP?
If the system is not active until 400 AGL, how does a pilot know the system is inop?
Since the first Lion Air incident was on final, where are the parameters for that?
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.