A report that pilots of the crashed Ethiopian Airlines Boeing 737 Max followed procedures that Boeing and the Federal Aviation Administration said would address a software issue has raised new questions about circumstances that preceded the deadly crash.
The 3 April report in the Wall Street Journal, which cites unnamed sources, has led to questions about other factors that might have played a role.
It has also highlighted to at least one pilot the remarkable speed at which the 737 Max's maneuvering characteristics augmentation system (MCAS) can apparently put a 737 Max into a dive.
"It's like a Tasmanian devil in there," says Dennis Tajer, a 737 pilot and communications chair for Allied Pilots Association, which represents American Airlines' pilots.
"Every time you make a gain, it makes a double gain back on you," Tajer says of a scenario in which MCAS repeatedly trims the aircraft's nose down due to erroneous flight data.
Asked to comment about the new reports, Boeing warns against drawing conclusions before investigators release more details.
"We urge caution against speculating and drawing conclusions on the findings prior to the release of the flight data and the preliminary report," says the company, which has steadfastly insisted its prime focus is ensuring the 737 Max is safe. Boeing has also developed a software fix it will deliver to the FAA for review in the coming weeks, it says.
The Journal report says the pilots of the Ethiopian 737 Max 8 responded to an MCAS activation shortly after takeoff by switching electric trim off. Then, however, still struggling for control, they turned the system back on. The aircraft crashed, killing all aboard.
If true, the pilots initially followed steps Boeing has long said enable crew to address unwanted MCAS activation. The steps, which Boeing highlighted following last year's crash of a Lion Air 737 Max 8, include electrically adjusting the stabiliser using trim switches on the control column, shutting off electric trim altogether and reverting to manual trim using the trim wheel in the cockpit.
The FAA also highlighted those procedures in an airworthiness directive last year.
Several pilots interviewed by FlightGlobal say the Journal's report bears more investigation, especially considering its reliance on unnamed sources.
Still, the article has left pilots wondering if the Ethiopian pilots faced circumstances that led them to conclude their best chance for a safe landing rested in reactivating the control system that caused the problem in the first place.
If so, what were those circumstances?
Industry observers on 3 April also mulled reports that manuals for Boeing's first-generation 737 mentioned that, under certain conditions, pilots might struggle to adjust the stabiliser using the manual trim wheel.
Specifically, that condition might occur when the stabiliser is in a nose-down position while a pilot is pulling back on the column, placing the elevator in a nose-up position and resulting in countering forces.
Pilots interviewed by FlightGlobal said their 737NG and 737 Max flight manuals include no such warnings and that they were unaware such conditions could exist on 737 Classics.
But they wondered whether such a scenario could exist on newer 737 variants, particularly at high speeds, when forces on control surfaces are greater.
More details will be available when investigators release more information.
APA's Tajer says his understanding of MCAS has led him to view the system as a "monster" that, when activated, can seemingly quickly overwhelm efforts by the pilots to counter it.
MCAS adjusts the stabiliser at a rate faster than can be countered by pilots using electric trim switches on the control column, he says.
The system can activate only when the flaps are up and the autopilot is off. When activated, it drops the aircraft's nose by moving the stabiliser for about 10s at a rate of 0.27 "units" per second, for a total stabiliser movement of about 2.5 units. Tajer is unsure if units translate directly to degrees.
After those 10s, MCAS deactivates, but can reactivate again after 5s. As a result, MCAS can quickly put the stabiliser into maximum nose-down position, he says.
At that point, pulling back on the control column will not arrest the dive because the elevator lacks sufficient opposing force, he notes.
Pilots can counter MCAS stabiliser movement using the electric trim switches.
But when the flaps are up, the electric trim – when activated by the switches – moves the stabiliser at only 0.18 units per second.
If MCAS keeps tripping, and if pilots do not shut off electric trim entirely, the result is what Tajer describes as a two-steps-back, one-step-forward scenario, with MCAS maintaining an edge.
"The MCAS knows but one speed, which is 0.27, which is the most-aggressive speed," Tajer says. "If you look at the balance sheet on it, MCAS is winning, and you are losing."
Source: Cirium Dashboard