Concerns Expressed Over 737 MAX Redundancy, Manual Trim

AOA sensors MAX 737
Credit: Boeing

WASHINGTON—Calls for an additional angle-of-attack indicator and concerns over the flight crew’s ability to manually trim the aircraft in an emergency are among the issues highlighted in the initial set of comments on the FAA’s proposed requirements to approve the Boeing 737 MAX’s service return. 

Boeing’s proposed fixes including software modifications that use data from both MAX angle-of-attack (AOA) vanes to activate the maneuvering characteristics augmentation system (MCAS) flight-control law implicated in two MAX fatal accidents that led to the grounding. If they disagree by more than a defined range, the MCAS, which provides nose-down horizontal stabilizer commands, will not activate.

“This is clearly an improvement on the original design” that used one sensor’s feed and meant MCAS could push the MAX’s nose down based on one feed of faulty AOA data, the British Airline Pilots’ Association (BALPA) wrote in comments to the FAA. “[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?” 

Guy Woolman, a former Southwest Airlines pilot with 12,000 hours of 737 experience but none in the MAX, seconded BALPA’s call. 

“There should be at least a third AOA and airspeed input,” he wrote in comments to the FAA. “I had several ‘IAS DISAGREE’ messages while flying [737 Next Generation variants]. There is certainly a lot of startle as the crew is forced to search for reliable airspeed ... Couple this with other seemingly dissimilar non-normal conditions and it is easy to see why things could get challenging in a hurry. Why not have a standby AOA gauge?” 

EASA is considering mandating introduction of a third AOA sensor, but it is not expected to be a prerequisite to allowing the MAX back into service. 

In both MAX accidents, Lion Air Flight 610 in October 2018 and Ethiopian Airlines Flight 302 (ET302) in March 2019, faulty AOA data triggered the MCAS, which commanded unneeded nose-down horizontal stabilizer movements. Neither crew reacted as Boeing thought pilots would, which included using cutout switches to disable the stabilizer motor and, if needed, manually turn a vertically-mounted trim wheel in the cockpit. Instead, they were confused by a cascading series of alerts and warnings, including a SPEED TRIM FAIL, and lost control of the airplane. The MCAS functionality was added to the 737’s speed trim system’s logic, and Boeing elected not to highlight it in pilot manuals or add a new alert light on the flight deck. 

“The caution and warning system in the 737 is as archaic as the airframe design. I flew jets made far earlier than the first 737 with a better system,” said Woolman, a former B-52 pilot and Rockwell B-1 bomber flight instructor. “Properly analyzing aircraft failures can be like hunting for Easter eggs, especially if struggling with basic aircraft control.” 

BALPA also raised concern about Boeing’s proposed manual-trim techniques. Investigators believe the ET302 pilots attempted to use manual trim, per Boeing’s instructions, to direct their 737-8’s nose up after the MCAS was disabled. But aerodynamic forces acting on the stabilizer made it too difficult to maneuver with the manually-linked wheel. 

In its updated MAX training, Boeing emphasizes that both pilots may have to crank the wheel, a spool-shaped device mounted with round sides vertical between them to generate enough force to move the stabilizer. A draft training aid distributed in June illustrates the concept, with each pilot using one hand to turn the wheel, and the other to fly the aircraft. 

“Requiring both crew members to turn the trim wheel simultaneously in a non-normal scenario is extremely undesirable and goes against all philosophies of having one pilot fly and one run the” quick reference handbook, or QRH, BALPA said. “No flight control system should require both pilots to operate it at any stage, let alone in an emergency.” 

BALPA added that the newer 737’s smaller trim wheel compared to the 737 Classics, which created room on the flight deck for new displays, renders the manual-trim scenario even more challenging. Moving a 737 Classic horizontal stabilizer one degree, or unit, required about 10 turns of the trim wheel. On newer 737s, including the MAX, moving the stabilizer the same amount requires about 15 turns of the smaller-diameter wheel. Full nose down equates to a stabilizer at 4.2 deg nose down, meaning pilots would need to rotate a MAX’s trim wheel about 60 times to bring the stabilizer to neutral.  

The public-comment period on the FAA’s proposals runs through Sept. 21. As of late in the afternoon on deadline day, more than 200 comments had been received, including many from members of the public that do not favor the MAX’s re-approval. 

Comments will be considered by the FAA and, if appropriate, incorporated into a final directive outlining what changes the MAXs must undergo before they can fly again, and what training pilots will receive. The FAA plans to mandate a flight control computer software upgrade, some writing modifications, and new training. Specifics of the training programs are being reviewed and finalized in a separate process. 

Sean Broderick

Senior Air Transport & Safety Editor Sean Broderick covers aviation safety, MRO, and the airline business from Aviation Week Network's Washington, D.C. office.


“Those who cannot remember the past are condemned to repeat it.”–George Santayana
These terrible accidents were a very expensive lessons, especially for the paying passengers, and Boeing. The lazy FAA Deep State Administrators should get off their fat asses and insist that ALL passenger carrying aircraft have redundancy in design, SO THAT NO SINGLE POINT FAILURE WILL RESULT IN A FATAL CRASH. ADDITIONALLY ALL PILOTS WILL (NOT SHOULD) BE TRAINED ON THE SYSTEM.

Any questions?

Thomas Owens
GV Series Pilot Examiner
FlightSafety Int'l (Ret)
You don't need a third AOA and airspeed input to identify "Erroneous Data". You overcome this by using dedicated memory that you burn "last known good data" into so you have Three (3) data sources, matching two of the three.

And if they allow using a single source airspeed instead of the same voting method as the AOA,, then you have the same anti-stall commands if you get a single sourced Airspeed that is Erroneously LOW.
The real problem is that MCAS kept doing it over, and over, and over, and over.....
There is a third Airspeed data, its restricted to the backup instruments (ISFD) but its in the middle, hard to see and over-crowded.

It also does not feed into the computer as a 2 out of 3 agree dispute resolution.

Not mentioned is the frozen motor possibility and the breaking out of the clutch on the Trim Unit via manual trim such that a pilot can overcome it. There are reports that two pilots cannot overcome it.

This is a separate issue from speed causing forces on the stab that means two pilots need to trim.

It also does not resolve can two slight pilots (be it male and or female in any combination) actually manage the manual trim for either mode.

The Classic 737 had two Stab Trim Motors. How those did or did not work together and why one was removed has not been addressed to the best of my knowledge.

So the 737NG and the MAX have a major change from the Classic and first 737 that should be part of the conversation as this is a Grandfather Cert but its changed so much from what it was that its a cert of a cert of a cert.

As noted you used to have a larger wheel AND two stab trim motors.

Grandfathering has gotten totally out of hand. The 787 is having more than enough problems and its not handi capped by that.
Separate, what does AOA do for an Airline Pilot?

It comes from the military world of fighters (where it is a tool) and the only reason it seems to be on commercial aircraft (larger ones) is that the stream of former fighter pilots wanted it.

Its not accurate, its not used for flying (speed is much closer for what you want or need) and you use the Attitude indicator set climb, level or decent via the bar and its degrees not AOA.