On the 3rd of August 2020 almost 17 months after issuing an Emergency Order of Prohibition against the operation of the Boeing 737-8/-9, the Federal Aviation Administration (FAA) has issued a Preliminary Summary of the Review of the Boeing 737 MAX.
FAA Approval of the Boeing 737 MAX
The FAA preliminarily determined that Boeing’s proposed changes to the 737 MAX design, flight crew procedures and maintenance procedures effectively mitigate the airplane-related safety issues that contributed to the Flight 610 and Flight 302 accidents.
The FAA further determined that the proposed design change also addressed additional safety concerns beyond those identified during the accident investigations.
What safety issues have been raised and have addressed by Boeing?
1. Use of Single Angle of Attack (AOA) Sensor
The updated Flight Control Computer (FCC) software with revised flight control laws would use inputs from both AOA sensors to activate MCAS. A comparator logic has also been introduced to detect a failed AOA sensor. If the difference between the AOA sensor inputs are above a calculated threshold, the FCC would disable the Speed Trim System and MCAS function, for the remainder of that flight, and provide a corresponding indication of such deactivation on the flight deck.
2. MCAS Reset Generates Repetitive MCAS Commands
The flight control laws changes introduced ensure that MCAS will not command repeated movements of the horizontal stabilizer, now it would permit only one activation of MCAS per sensed high-AOA event. A subsequent activation of MCAS would only be possible after the airplane returns to a low-AOA state, below the threshold that would cause MCAS activation.
3. MCAS Trim Authority
New flight control laws limit the magnitude of any MCAS command to move the horizontal stabilizer, such that the final horizontal stabilizer position (after the MCAS command) would preserve the flight crew’s ability to control the airplane pitch by using only the control column.
4. Flight crew Recognition and Response
Boeing has revised eight non-normal flight crew procedures and proposed additional training that provides the pilot with additional information to recognize erroneous stabilizer movement and the effects of AOA sensor failures.
5. AOA Disagree Message
Boeing has also introduced a revised Max Display System (MDS) software enhancement that would alert the flight crew if there is a disagreement between the two AOA sensors, there is a sensor failure or a significant calibration issue by generating an AOA DISAGREE alert message.
6. Other Possible Stabilizer Runaway Failures
In the new design, Boeing has implemented FCC cross monitors to effectively detect and shut down erroneous stabilizer commands from the FCCs. This makes continued safe flight and landing for this type of failure not dependent on pilot reaction time.
7. Maintenance Procedures Related to MCAS
The Indonesian KNKT (Komite Nasional Keselamatan Transportasi) investigation and final report for flight JT610 accident shows several maintenance actions related to repair and installation of a replacement AOA sensor. The KNKT verified the existing Boeing Aircraft Maintenance Manual (AMM) procedures for AOA sensor installation will correctly identify a mis-calibrated AOA sensor.
The FAA expects that the 737 MAX design changes, once the FAA completes its compliance review, will address the unsafe condition noted in the accident investigations and also enhance the safety of the airplane’s flight control system architecture, operational procedures, and maintenance procedures.
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