Simulator tests conducted last year provide insight into flight scenarios that can leave Boeing 737 Max pilots struggling or unable to manually trim the aircraft back to level flight.
The tests, completed as part of the investigation into the March 2019 crash of Ethiopian Airlines flight 302, revealed that during certain “mis-trim” scenarios pilots might struggle to turn the manual trim wheel in a 737 Max’s cockpit at speeds of only 220kt.
Such difficulties occur when another pilot is simultaneously hauling back on the control column – a scenario apparently experienced by pilots of the crashed Ethiopian 737 Max 8, according to the Ethiopian Aircraft Accident Investigation Bureau’s interim report, released 9 March.
That report does not cite manual trim issues in its conclusions or recommendations, though the topic of manual trim difficulties has been discussed widely following the crash. Boeing still maintains it expects regulators will certificate the Max around mid-year.
Shortly after flight 302 took off, the Max’s Maneuvering Characteristics Augmentation System fired several times, trimming the jet’s horizontal stabiliser nose-down. To counter that nose-down trim, the pilots pulled back on the control column.
They also “cut out” the electric stabiliser trim system, a step recommended in a non-normal checklist. The cutout inhibits MCAS and electric trim switches in the cockpit from controlling the horizontal stabiliser, leaving the manual trim wheel in the cockpit as the remaining means to do so.
At the time, flight 302 was travelling at more than 340kt and the pilots were pulling back on the control column with an average of 43kg (94lb) of force, according to the Ethiopian report.
The pilots then discussed the trim system. The captain asked the first officer if he could “try it manually”, and the first officer replied, “It is not working”, says the report.
The pilots soon switched electric trim back on, and MCAS pitched the aircraft into a dive from which they did not recover, according to the report.
TO THE SIMULATOR
Investigators tested various out-of-trim scenarios in July 2019 using CAE-made 737 Max level D flight simulators in Addis Ababa.
They aimed to “assess the control column forces that were present and evaluate the manual trim wheel forces that were required to operate the manual trim wheel,” says the report.
First, the team reconstructed the circumstances of flight 302, setting the simulator’s stabiliser to the 2.3° setting – the same position at which flight 302’s stabiliser was set at the time the pilots struggled to turn the trim wheel.
The Max’s stabiliser swings 17.1°, with lower numbers representing more nose-down trim and higher numbers representing more nose-up trim, according to the Ethiopian and Lion Air accident reports.
Like flight 302’s pilots, the simulator pilots initially countered the 2.3° nose-down stabiliser trim by pulling back on the control column. Pulling together, the simulator pilots managed to climb, though “forces needed from both pilots to achieve this were considered significantly very high and unbearable for the duration held”, says the report.
Then, a simulator “pilot attempted to control the aircraft and return for landing with elevator authority only”, it adds, but states: “The attempt was unsuccessful.”
Investigators used the simulator to examine pilots’ ability to turn the trim wheel at varying nose-down stabiliser settings and differing speeds. Again, electric trim was cut off and one pilot was hauling back on the column.
At 220kt and the stabiliser set at 2.5° (slightly less nose-down trim than flight 302’s pilots faced) the simulator pilots found the trim wheel to be “barely moveable”, meaning they could not complete one turn. At 2.5° and 250kt airspeed, the wheel was “not movable”, says the report.
“For all speeds higher than 220kt and trim set at a value of 2.5 units, the difficulty level of turning the manual trim wheel was… trim wheel not movable,” it says.
Investigators say pilots must turn the trim wheel 15 times to adjust the stabiliser 1°, meaning the Ethiopian pilots would have needed to turn the wheel 40 times to return their Max’s stabiliser to a neutral position.
Peter Lemme, an aerospace consultant and former Boeing engineer, explains a scenario that can cause manual trimming difficulties.
In a blog post, he notes pilots are “likely” to oppose MCAS-induced nose-down trim by pulling back on the control column, as flight’s 302 pilots did.
But when the aircraft is trimmed nose-down, that back pressure on the column creates an out-of-trim condition and places greater force on the jackscrew, the mechanism that moves the leading edge of the stabiliser, says Lemme. That force can make manual nose-up trim ”very difficult”, he writes.
In such scenarios, pilots might resort to momentarily releasing back-pressure on the control column, which eases jackscrew loads and makes the trim wheel easier to turn, according to Lemme.
Although Ethiopia’s interim report highlights apparent manual trim difficulties, its conclusions and recommendations do not mention those difficulties.
Rather, it cites “inadequate” 737NG-to-737 Max differences training for pilots, shortcomings in Boeing’s MCAS “hazard assessment” and inoperability of the aircraft’s angle-of-attack disagree warning. It recommends that differences training include a review of MCAS and that MCAS should receive angle-of-attack data from the aircraft’s two angle-of-attack sensors, rather than a single one.
Ethiopia’s final accident report is yet to be published.
Indonesian investigators in October last year released their final analysis into the October 2018 crash of a Lion Air Max 8, which killed 189 people. That report outlined issues with Boeing, regulators, Lion Air and the aircraft’s pilots.