"The pilots on Ethiopian Airlines Flight 302 initially reacted to the emergency by shutting off power to electric motors driven by the automated system, these people said, but then appear to have re-engaged the system to cope with a persistent steep nose-down angle. It wasn’t immediately clear why the pilots turned the automated system back on instead of continuing to follow Boeing’s standard emergency checklist, but government and industry officials said the likely reason would have been because manual controls to raise the nose didn’t achieve the desired results. After first cranking a manual wheel in the cockpit that controls the same movable surfaces on the plane’s tail that MCAS had affected, the pilots turned electric power back on, one of these people said. They began to use electric switches to try to raise the plane’s nose, according to these people. But the electric power also reactivated MCAS, allowing it to continue its strong downward commands, the people said."
My guess it that they could not adjust the trim with the manual wheel due to stabilizer load. Satcom Guru just published a new blog article about this very topic
A mistrim situation can result if the stabilizer is trimmed nose down and the elevator is commanded nose up. The opposing lift components combine at the jackscrew to create a large upward force, making trimming stabilizer leading edge down difficult. While electric trim motor is designed to not stall out under mistrim conditions, the loads may make trimming by the manual trim wheel very difficult. Under these circumstances, it may be preferable to release the column and let the elevator return to the neutral position, lowering the jackscrew loads and making manual trim easier.
While it's easy to say this during analysis, it's not so easy to remember to do in an emergency situation. Especially since this was probably never covered in any training.
My guess is that since they were not able to trim with the manual wheel, then they re-enabled Electric Trim. Perhaps they got it somewhat leveled out, but now MCAS is kicking in. And if you're under high workload and panic, you could easily lose track of the stabilizer position for a few seconds (especially when they were turning back), and that's all it takes at their AGL to reach an unrecoverable dive.
Well (addressing in particular member dragon6172), so it looks like ET's flight crew did
read Boeing's bulletin, and did
try to apply the suggestion in the "Note" (counter excessive aerodynamic loads on the trim wheel using electric trim). They just didn't to it in the correct order: they should have first
brought the stabilizer back to near-neutral with electric trim
cutoff stab trim. Of course, that's easy to say behind a keyboard and with the benefit of hindsight, much less in an aircraft diving to the ground at 1200 ft AGL.
But guess what? Non-Normal Checklists are there precisely to guide pilots through the right sequence of steps (right actions in the right order
) to stabilize an abnormal situation. If the Runaway Trim NNC had actually been changed after Lion Air (introducing a mandatory step to use electric trim to neuter stab before
cutting off the actuator, and amending the misleading Condition: "Uncommanded stabilizer trim movement occurs continuously
", which makes applicability to MCAS runaway ambiguous) and
had MAX pilots undergone specific sim training on the new NNC, then MAYBE the ET's crew would have had a fighting chance (I'm not convinced at 1200 ft AGL, but maybe).
Of course, this would have done nothing to mitigate the unbelievable decision to certify and push into service an airliner vulnerable to catastrophic upsets on the failure of a single
sensor unless the crew reacts with the correct sequence of actions within seconds
. But at least it would not give the message that Boeing has been playing with the lives of people (and the business of clients) to cover-up the mistake of an over-ambitious sales manager promising to a client an impossible result.
Last edited by jollo
on Wed Apr 03, 2019 8:45 am, edited 3 times in total.