By Dominic Gates - Seattle Times Aerospace Reporter
The pilots of the Ethiopian Airlines 737 MAX that crashed last month appear to have followed the emergency procedure laid out by both Boeing and the Federal Aviation Administration — cutting off the suspect flight-control system — but could not regain control and avert the plunge that killed all 157 on board.
Press reports citing people briefed on the crash investigation’s preliminary findings said the pilots hit the system-cutoff switches as Boeing had instructed after October’s Lion Air MAX crash, but couldn’t get the plane’s nose back up. They then turned the system back on before the plane nose-dived into the ground.
While the new software fix Boeing has proposed will likely prevent this situation recurring, if the preliminary investigation confirms that the Ethiopian pilots did cut off the automatic flight-control system, this is still a nightmarish outcome for Boeing and the FAA.
It would suggest the emergency procedure laid out by Boeing and passed along by the FAA after the Lion Air crash is wholly inadequate and failed the Ethiopian flight crew.
A local expert, former Boeing flight-control engineer Peter Lemme, recently explained how the emergency procedure could fail disastrously. His scenario is backed up by extracts from a 1982 Boeing 737-200 Pilot Training Manual posted to an online pilot forum a month ago by an Australian pilot.
That old 737 pilot manual lays out a scenario where a much more elaborate pilot response is required than the one that Boeing outlined in November and has reiterated ever since. The explanation in that manual from nearly 40 years ago is no longer detailed in the current flight manual.
Just a week after the Oct. 29 Lion Air crash, Boeing sent out an urgent bulletin to all 737 MAX operators across the world, cautioning them that a sensor failure could cause a new MAX flight-control system to automatically swivel upward the horizontal tail — also called the stabilizer — and push the jet’s nose down.
Boeing’s bulletin laid out a seemingly simple response: Hit a pair of cutoff switches to turn off the electrical motor that moves the stabilizer, disabling the automatic system — known as the Maneuvering Characteristics Augmentation System, or MCAS. Then swivel the tail down manually by turning a large stabilizer trim wheel, next to the pilot’s seat, that connects mechanically to the tail via cables.
Boeing has publicly contended for five months that this simple procedure was all that was needed to save the airplane if MCAS was inadvertently activated.
In a November television interview on the Fox Business Network, Boeing Chief Executive Dennis Muilenburg, when asked if information had been withheld from pilots, cited this procedure as “part of the training manual” and said Boeing’s bulletin to airlines “pointed to that existing flight procedure.”
Vice president Mike Sinnett repeatedly described the procedure as a “memory item,” meaning a routine that pilots may need to do quickly without consulting a manual and so must commit to memory.
But Lemme said the Ethiopian pilots most likely were unable to carry out that last instruction in the Boeing emergency procedure — because they simply couldn’t physically move that wheel against the heavy forces acting on the tail.
“The forces on the tail could have been too great,” Lemme said. “They couldn’t turn the manual trim wheel.”
The stabilizer in the Ethiopian jet could have been in an extreme position with two separate forces acting on it:
MCAS had swiveled the stabilizer upward by turning a large mechanical screw inside the tail called the jackscrew. This is pushing the jet’s nose down.
But the pilot had pulled his control column far back in an attempt to counter, which would flip up a separate movable surface called the elevator on the trailing edge of the tail.
The elevator and stabilizer normally work together to minimize the loads on the jackscrew. But in certain conditions, the elevator and stabilizer loads combine to present high forces on the jackscrew and make it very difficult to turn manually.
As the jet’s airspeed increases — and with nose down it will accelerate — these forces grow even stronger.
In this scenario, the air flow pushing downward against the elevator would have created an equal and opposite load on the jackscrew, a force tending to hold the stabilizer in its upward displacement. This heavy force would resist the pilot’s manual effort to swivel the stabilizer back down.
This analysis suggests the stabilizer trim wheel at the Ethiopian captain’s right hand could have been difficult to budge. As a result, the pilots would have struggled to get the nose up and the plane to climb.
If after much physical exertion failed, the pilots gave up their manual strategy and switched the electric trim system back on — as indicated in the preliminary reports on the Ethiopian flight — MCAS would have begun pushing the nose down again.
Boeing on Wednesday issued a statement following the first account, published Tuesday night by The Wall Street Journal, that the Ethiopian pilots had followed the recommended procedures.
“We urge caution against speculating and drawing conclusions on the findings prior to the release of the flight data and the preliminary report,” Boeing said.