Rollercoaster Technique Could Have Helped 737 MAX Crews

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Image: Boeing

An old-school technique tested by a U.S. flight crew in a 737 simulator might have helped the Lion Air and Ethiopian Airline crews had they known about it. Colloquially referred to as the “roller coaster,” the procedure requires the aircraft experiencing an out-of-trim condition to descend with reduced elevator input so that the horizontal stabilizer (used as pitch trim in the 737) could be “unloaded” enough to be manually adjusted. Then elevator inputs are resumed to arrest or slow the descent, and the procedure repeated until the aircraft is back in trim.

As reported by Aviation Week, the simulator crews set up the accident scenario from the Ethiopian Airlines flight and were able to demonstrate that despite following procedures in place after the Lion Air crash, they were unable to add enough nose-up trim manually without this special procedure. “Keeping the aircraft level required significant aft-column pressure by the captain, and aerodynamic forces prevented the first officer from moving the trim wheel a full turn,” said the report.

After the Lion Air crash indicated that the MCAS was erroneously driving nose-down trim, Boeing recommended using the trim cutout switches to disable electric trim as part of the recovery procedure. Removing power from the electric trim also deactivated MCAS. Boeing did not, however, indicate that the flight crew might not be able to manually move the trim wheel. Based on these simulator runs, this appears to be a possible scenario: that the MAX was simply too far out of trim and going too fast for the crew to successfully re-trim with the manual wheel alone.

We do know that the Ethiopian crew used the electric trim to offset the initial MCAS inputs, but they apparently moved on to other troubleshooting avenues before getting the 737 MAX completely in trim. They described the trim system as “not working,” which is widely understood to mean the manual system. This supposition is backed up by the U.S. crew’s recent simulator experience.

According to the Aviation Week report, “Boeing’s assumption was that erroneous stabilizer nose-down inputs by MCAS, such as those experienced by both the [Lion Air] and ET302 [Ethiopian] crews, would be diagnosed as runaway stabilizer. The checklist to counter runaway stabilizer includes using the cutout switches to de-power the stabilizer trim motor. The ET302 crew followed this, but not until the aircraft was severely out of trim … Unable to move the stabilizer manually, the ET302 crew moved the cutout switches to power the stabilizer trim motors.” This step is contra-indicated by the checklist, in part because it would put MCAS back online.

As we’ve reported, changes expected with the MAX’s software are expected to eliminate the chance that MCAS will continue to offer corrections. The issue of revised simulator training to precede the MAX’s return to service is still being discussed.

In other MAX news, Ethiopian Airlines CEO Tewolde Gebremariam told NBC News on Monday that he’s unsure if his airline will fly the MAX again. “At this stage I cannot, I cannot fully say that the airplane will fly back on Ethiopian Airlines. It may, if we are fully convinced and if we are able to convince our pilots, if we are ever to convince our traveling public. We have not got a time to discuss on the return to service and we have made it very clear on several occasions we would not be the first one to return their airplane back to air.”

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