In the end, pilots could have easily and quickly saved the flights by simply cutting off the power to the stabilizer trim system as soon as a problem was noticed. In the case of Lion Air, the crew of the flight prior to the accident flight did just that. The problem wasn’t resolved, however, because it’s hard to report an MCAS problem when you don’t even know the system exists.
The Max has larger engines installed farther forward than the engines on previous 737s, which destabilized the airplane’s flight characteristics at high angles of attack. This resulted in the installation of MCAS, which will roll in nose-down trim to aid in recovery and make the Max handle similarly to other 737 models. This also preserves a common type rating among all the models. In both accidents, a faulty (or missing or otherwise damaged) angle-of-attack sensor caused the airplane to think that it was in an excessively nose-high attitude. Dutifully, MCAS started adding nose-down trim to counter what it thought was a dangerously nose-up condition. Unlike light airplanes that use small trim tabs to trim the elevator, trim on jet airliners is achieved by moving the entire stabilizer. Moving that massive surface makes the trim extremely powerful—more powerful than two pilots can overcome. That’s what apparently doomed both the Lion Air and Ethiopian Airlines flights after a similar series of up/down gyrations.
Pilots of all airplanes with autopilots utilizing electric pitch trim should know how to disable the trim in case of a runaway.The baffling part, however, is why the pilots didn’t use the stabilizer trim cutout switches, which would disable the electric trim and allow manual trimming of the stabilizer. This is the crux of Boeing’s defense for not mentioning the existence of MCAS.
Whether trim is running away because of a malfunction or because of MCAS, the fix is the same: turn off the electric power to the stabilizer. The Ethiopian crew eventually got to those switches but, by the time they did, it’s speculated that the airplane was so far out of trim that aerodynamic forces on the tail made manual trimming impossible. While this investigation is ongoing, there are already important lessons learned from these accidents.
Pilots of all airplanes with autopilots utilizing electric pitch trim should know how to disable the trim in case of a runaway. Typically, holding the trim switch in the direction opposite to the runaway will stop it. Holding the red autopilot disconnect button should also do the trick. Or turn off the autopilot’s main power switch. It’s also a good idea to know where the pitch trim and autopilot circuit breakers are, if all else fails. Place red or yellow collars on them so they’re easy to find in the heat of battle. There are many ways to disable automation in aircraft, and pilots should be very familiar with all of them.
While the Lion Air and Ethiopian Airlines investigations are still ongoing, I have to question the background and flight time of the pilots involved. It was revealed that the first officer in the Ethiopian crash had only a few hundred hours total time. If true, it doesn’t take much imagination to realize that he was likely little to no help in attempting to figure out the problem. And the captain, now essentially flying solo, would have been wrestling the controls while attempting to sort out the problem, leading to task saturation.
In the end, it’s apparent that Boeing should have at least made pilots aware of MCAS’ existence and that the fix for a runaway MCAS is the same as for a runaway stabilizer trim. Boeing also is well on its way to recertifying MCAS to rely on two angle of attack sources with comparator logic and limiting MCAS’ authority on stabilizer movement.