MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

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Originally Posted by Water pilot

Ah yes, thank you. I see that you are right, media reported that indicators had been repaired but I think it was the airspeed indicators that were replaced. But I still don't see why reporting the stick shaker is the key missing part here, since my understanding is that the stick shaker activates based on airspeed and not AOA. The captain did report that the airspeed indicators were wrong. A technician might have concluded from this information that the recently replaced AOA indicator was the root cause, but for whatever reason he didn't. Hindsight is 20/20.

From the report:


The point that I am trying to convey is that the problem was not one of carelessness or stupidity -- the pilots had a mental model of what caused the failure. The Airspeed indicator was wrong which caused the STS system to run in reverse. This is what they reported to the techs, and this is what the techs tried to solve. The fact that the AOA sensor failure was critical was in nobody's mental model because Boeing had not told anybody about MCAS. In the electronic log, the pilot reported a critical failure that caused him to hand off control to the FO, so I don't really see any dereliction of duty here. According to his mental model of the problem, he reported everything that was wrong with the plane. It is lawyerly to go through and look for some omission that perhaps should have, could have, led to the key insight into the MCAS problem -- and it ignores the fact that had Boeing documented MCAS and the known vulnerability that it has to a duff AOA (not speed) sensor, everybody would have been "in the loop" and the accident would probably not have happened.

Quote: "since my understanding is that the stick shaker activates based on airspeed and not AOA".Since the MAX type is based of the NG 737, here is the information from the OM for the 737NG with regard to "stick shaker":Two independent, identical stall management yaw damper (SMYD) computers determine when stall warning is required based upon:
• alpha vane angle of attack outputs
• ADIRU outputs
• anti–ice controls
• wing configurations
• air/ground sensing
• thrust
• FMC outputs.
The SMYD computers provide outputs for all stall warning to include stick shaker and signals to the pitch limit indicator and airspeed displays and the GPWS windshear detection and alert.