WITH THE 737 MAX RE-CLEARED FOR TAKE OFF GROUNDED AUTHORS
ASK 8 CRUCIAL QUESTIONS

By Captain Shem Malmquist and Roger Rapoport

EIGHT UNANSWERED QUESTIONS ON THE
UNGROUNDED BOEING 737 MAX

While some airlines begin the process of camouflaging the identity of their Boeing 737 MAX jets, passengers want to know if they can board this ungrounded plane with confidence. Our answer after writing a book on this subject and the future of aviation safety is we don’t know.

The reason is that we have not been able to receive answers from government authorities to key questions, just six weeks before American Airlines is scheduled to begin flying the MAX. They will be followed in the spring by Southwest.

Here’s some helpful background on the accidents that led to the fleet grounding in March 2019 and our eight unanswered questions.

While the MAX resembled the 14 earlier versions of the 737, test pilots reported to the FAA that it handled differently. This was certainly no surprise to customers who could see that much larger and heavier engines had been pushed forward on the 737 airframe. To solve this problem Boeing added additional software to the flight control computers called Maneuvering Characteristics Augmentation System (MCAS).

What MCAS did was just look at a certain condition, flaps up and high angle of attack. If these parameters were met it would automatically adjust the horizontal stabilizer (the large horizontal sur- face at the tail) to make the aircraft want to pitch downwards, reducing the plane’s angle of attack. The idea was to make it respond more like older 737 models. This also translates into lower training costs for pilots who didn’t know that MCAS existed when they began flying the new plane with zero simulator experience.

Tragically this safety system designed to “protect” the aircraft unexpectedly led to the October 2018 and March 2019 crashes of Lion Air and Ethiopian Airlines MAX aircraft taking the lives of 346 people. The FAA determined that due to “a continuous erroneous high Angle of Attack sensor value …multiple MCAS commands resulted in a significant horizontal stabilizer mistrim condition which the flight crew could not counter using only elevator control.”

According to the FAA, Boeing has now changed flight control laws to ensure that if the MCAS system is activated “it will generate a single nose-down command” and not repeat the command in a manner that could potentially recreate the scenario leading to an aerodynamic stall and both crashes.

The FAA’s proposed corrective action stipulated that, “MCAS will not command repeated movements of the horizontal stabilizer. The revised flight control laws permit only one activation of MCAS per sensed high Angle of Attack event. Any subsequent activation of MCAS would only be possible after the airplane returns to a low Angle of Attack state.”

To date no one working at Boeing or the FAA on the upgraded MAX and its recertification has been able to answer our key questions fundamental to flight safety.

1. 1. Have you addressed common cause anomalies that could lead to an indication of a low-Angle of Attack? If so, how?

2. 2. Has there been an assessment of Radio Frequency interference associated with other crashes. What about radom damage? If so, how have they been addressed?

3. 3. Were probability estimates used? If so, estimating probability on unknown combinations of events is what got us into this situation in the first place. This key error was a central factor in both MAX crashes as well as an earlier automation failure that led directly to the 2009 loss of Air France 447, an Airbus 330 with 228 people on board.

   The FAA also mandated “that in the event that MCAS is activated, the MCAS system preserves the flight crew’s ability to control the airplane after the activation.”

   Here the proposed corrective action was a change in “flight control laws to include a limit for MCAS commands. The MCAS will stop commanding stabilizer movement at a point that preserves enough elevator movement for sufficient pilot control of aircraft pitch attitude for current operating conditions.”

   Here’s what we want to know:

4. What is the definition of "preserves enough elevator movement" and under what conditions? Was a lookup table used to determine the amount of stabilizer movement based on Angle of Attack and mach number?

5. Can it obtain the wrong value due to, for example, reading a mach number lower than what the aircraft is actually flying?

   We also have several other related questions critical to the MAX and MCAS, a system not present on any other Boeing aircraft.

6. We understand that testing with the stabilizer inoperative and stuck at a very nose-down position has been accomplished. What were the parameters of the testing and what were the results?

7. Are there any unexpected corner cases where the stabilizer could be stuck down forcing the pilots to utilize what is popularly called the "roller coaster" where back pressure on the control column is sufficiently relaxed to allow for many trim movements?

8. If a pilot does not execute the expected responses correctly how much allowance has been made in the design of the systems and procedures to ensure that this does not lead to an accident?

Until these questions are answered it is not possible to rule out new problems on the upgraded MAX. We look forward to answers on all these questions and will share them as soon as they arrive from the FAA or Boeing.

Shem Malmquist, visiting professor at Florida Institute of Technology is a veteran aviation accident investigator. He and FSI senior editor Roger Rapoport, are authors of the new book GROUNDED; How to Solve The Aviation Crisis (Curt Lewis Aviation Books/Lexographic Press) and Angle of Attack: Air France 447 and the Future of Aviation Safety, He is also the producer of the award winning feature film Pilot Error.