Originally Posted by
GordonR_Cape
- MCAS was designed to activate at a much lower AOA, independent of the stick shaker.
- MCAS was designed to activate under many areas of the flight envelope, independent of the stick forces and airspeed.
2 things -
1) Do you have a reference for what AOA MCAS activates at when compared to the stickshaker AOA, preferably from a primary source, i.e. the manufacturer, a regulatory body, or similar? I've looked for that but have been unable to find it... it's getting difficult to find original data among all the poorly written media interpretations.
2) I do not believe that is true. Airspeed is used both as in input to determine the MCAS activation AOA, and it is used to determine the magnitude of the stab trim input. The first time that we learned anything about MCAS was in the November 8/9/10 timeframe in a letter from Mike Michaelis, APA safety committee chairman. It was an unpublished letter to APA members but was extensively reported in the media, and claimed to include a description of MCAS that was provided by Boeing. Here's a contemporary PPRUNE link -
Indonesian aircraft missing off Jakarta
The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.