degrees but are sufficiently large to trigger MCAS? Is this viewed as an extremely improbable event?” “What if there are two “valid erroneous” AoA values that do not differ by more than 5.5. There are other systems onboard the aircraft requiring AoA input, so how will they deal with two sensors that disagree?“īALPA also questions the effect of the modification to the flight control computer, which disables the speed trim system including MCAS if the two AoA-sensors disagree by more than a 5,5-degree angle of attack. The revised MAX will have a second AoA-sensor for redundancy, but even this is not sufficient, BALPA says: “This is clearly an improvement on the original design, but it would be preferable for the system to utilize three AoA sensors (as per the Airbus A320 family of aircraft) in which case ‘voting’ can be implemented to discard an erroneous AoA value. When this failed on both the Lion Air and Ethiopian flights, it caused the infamous Maneuvering Characteristics Augmentation System (MCAS) to take over the aircraft from the pilots in such an aggressive way that it caused both flights to crash. In its first version, the MAX has had only a single sensor.
A 45-day comment period closed on September 21.Ī third AoA-sensor has been suggested by the British Airline Pilots Association (BALPA). The suggestions have been made in 229 comments on the FAA’s Notice of Proposed Rule Making as it prepares for a new Airworthiness Directive for the MAX.
Among the recommendations is the installation of a third Angle of Attack (AoA) sensor to feed the MAX flight computers for extra redundancy as well as enhancing crew training. Pilots organizations, technical experts, and others have called on the FAA to take additional measures to make the Boeing MAX safer before it can return to service.
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