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posted on November 10, 2018 09:22
737 MAX8 Emergency Airworthiness Directive
The recently released
Emergency Airworthiness Directive directs pilots how to deal with a known issue, but it does nothing to address the systems issues with the AOA system, which may be the causal system in the Lion Air accident. This is significant. The positive takeaway is that we are advised, as pilots, that once we recognize the issue, we can
stop the negative impacts by taking the trim system out of the loop.
At the heart of this investigation is the MCAS system (description from Boeing):
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.
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.
This is the first description you, as 737 pilots, have seen. It is not in the AA 737 Flight Manual Part 2, nor is there a description in the Boeing FCOM. It will be soon.
APA Safety recommends that you familiarize yourselves thoroughly with the information provided by CA XXXXXXX, 737 Fleet Captain, and the AA 737 fleet team. We have been working closely with CA XXXXXXX to get you accurate information as quickly as it becomes available. The AA 737 fleet team has placed this information in CCIs to 737 pilots, in bulletins, and in changes to flight documents.
At the present time, we have found no instances of AOA anomalies with our 737 MAX8 aircraft. That is positive news, but it is no assurance that the system will not fail. It is mechanical and software-driven. That is why pilots are at the controls.
Awareness is the key with all safety issues. You are aware this anomaly may occur and there is a mitigation procedure. No different than should you experience an engine failure.
As we continue to receive details, we will provide them in emails only to the 737 group. We chose to send this initial email to all pilots because it is a subject that is generating a great deal of interest.
Should you have questions, please do not hesitate to email or call us here at APA Safety: XXX-XXX-XXXX.
Captain XXXXXXX
DFW 737I
APA Safety Committee Chairman