Pilots organizations, technical experts, and others have called on the FAA to take additional measures to make the safer before it can return to service. 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.

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. A 45-day comment period closed on September 21.

A third AoA-sensor has been suggested by the British Airline Pilots Association (BALPA). In its first version, the MAX has had only a single sensor. 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.

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. There are other systems onboard the aircraft requiring AoA input, so how will they deal with two sensors that disagree?“

BALPA 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. “What if there are two “valid erroneous” AoA values that do not differ by more than 5.5. degrees but are sufficiently large to trigger MCAS? Is this viewed as an extremely improbable event?”

The UK union also recommends making further changes to checklist procedures, specifically when the two pilots simultaneously would have to manually trim the horizontal stabilizer for pitch control. This would be “extremely undesirable and goes against all philosophies of having one pilot fly and one run the QRH. No flight control system should require both pilots to operate it at any stage, let alone in an emergency.” The UAE’s GCAA is also worried about manual trim: “The manual wheel trim forces were certified by analysis and not by flight testing (or tested on non B737MAX aircraft). Heaviness on the manual wheel trim following a failure, like runaway stabilizer must be fully understood and experienced by crew during training and test. (…) The
least FAA and Boeing can do is to assist the Authority and the operator by providing necessary data associated to this certification and manual trim techniques.”

In general, BALPA recommends that regulators take a different approach to future aircraft designs in that “fundamental aircraft flying or handling quality deficiencies are aerodynamically designed out and not masked by flight control system augmentation.” Also, substantial design changes should result in certification as a new type and provide the crew with a commensurate of training.

Integrity AoA-system insufficiently checked
Curtis Ewbank, who as a Boeing system engineer revealed various MAX design flaws, isn’t satisfied with the changes by the FAA either. According to him, the integrity of the AoA-system is insufficiently checked: “Other systems on the 737 MAX, including the Air Data/Inertial Reference Unit, Stall Management/Yaw Damper, and the MAX Display System, among others, also use AoA data. These systems will all respond to erroneous AoA data, as some of them did on the flights.”

Besides a third AoA sensor, Ewbank recommends the MAX will get the synthetic airspeed function from the 787, or the addition of an inertial AoA signal similar to the 787. He also criticizes the AD for the lack of tackling two other issues: the erroneous stick shaker that confused the crew on both fatal MAX crashes, as well as the overspeed warning.

At the same time, Ewbank says the FAA should require that air data alerts on the MAX alert pilots both visually and audibly: “The FAA’s proposed AD includes a revision to the software of the MAX Display System to ensure that the AOA DISAGREE alert is functional, as a software error caused the alert to be non-functional on the flights. This is a good step, but a holistic review of crew alerting shows that this step is insufficient to ensure the future safety of the 737 MAX.”

This point is also made by the National Safety Committee (NATCA), which consists of FAA safety engineers. They want alert systems to be designed in such a way that they minimize false and nuisance alerts

In general, the National Transportation Safety Board (NTSB) is satisfied by the way its seven safety recommendations made in September 2019 have been shared by the FAA. This included design enhancements, software updates, pilot training, and other changes to the aircraft flight manual. “The NTSB believes these changes are generally consistent with the intent of the second part of this recommendation as it relates to the MCAS.”

In its comments, Airlines for America highlighted the need for numerous corrections in FAA and Boeing service bulletins as well as conflicts between the proposal and policies regarding the minimum equipment list of the flight control system.

The regulatory agency of the United Arab of Emirates calls on the FAA to re-examine the impact of additional crew training on airlines with mixed fleets of both the MAX and 737NG, like for example flydubai. “It is imperative that the FAA and Boeing re-examine this aspect and provide the concerned National Authorities and operators the necessary documentation, requirements and guidance on the succeeding effect of the MCAS remedial actions if the exercise is limited to 737 MAX only.”

Families ET302 victims oppose proposals
Among the comments is one from the families of victims of Ethiopian ET302. They oppose the proposed rule and recertification of the MAX “because the airplane is aerodynamically unstable and does not comply with modern aircraft certification standards. The FAA has not revealed to the public the data supporting its determination that the changes mandated by the Airworthiness Directive would ensure that the 737 MAX is safe to fly, and the proposed AD leaves unanswered critical questions regarding the airplane’s safety.”

The families recommend the FAA conducts “a complete aerodynamic evaluation of the 737 MAX in to understand and mitigate the airplane’s dangerous pitch-up tendency” and “reject the proposed “MCAS fix” as insufficient because the software modification does not address the underlying aerodynamic problem, introduces greater complexity, and may create additional modes and other dangers.“ “If the airplane’s pitch-up tendency cannot be mitigated through an aerodynamic fix, require that Boeing integrate a state-of-the-art triple redundancy Angle of Attack (AOA) sensor system on the 737 MAX.”

The FAA will now review all comments and decide if amendments to its proposed AD are needed.

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