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 Authority — cutting off the suspect flight-control system — but could not regain control and avert the plunge that killed all 157 on board.
The Wall Street Journal, citing people briefed on the crash investigation’s preliminary findings, reported Tuesday night that 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.
If the preliminary investigation confirms that the Ethiopian pilots did cut off the automatic flight-control system, this is a nightmarish outcome for Boeing and the FAA.
It would suggest emergency procedures that Boeing laid out for pilots worldwide after the Lion Air crash, and that the FAA alerted airlines to the next day, is wholly inadequate, missing vital information that was in Boeing pilot manuals nearly 40 years ago but isn’t any longer.
A local expert, former Boeing flight-control engineer Peter Lemme, recently predicted how the emergency procedure could fail disastrously, and he 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 Boeing outlined in November and has reiterated ever since.
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 the horizontal tail upward — 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 automatic system — known as the Maneuvering Characteristics Augmentation System, or MCAS — and then swivel the tail down by manually 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 media interview on the Fox Business Network, Boeing chief executive Dennis Muilenburg, when asked if information had been withheld from pilots, cited the procedure as “part of the training manual” and said Boeing’s bulletin to airlines “pointed to that existing flight procedure.”
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.
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, MCAS would have begun pushing the nose down again.
More detailed instructions that conceivably could have saved the plane in this situation are provided in the 1982 pilot manual for the old 737. As described in the extract posted by the Australian pilot, they require the pilot to do something counterintuitive: to let go of the control column for a brief moment.
As Lemme explains, this “will make the nose drop a bit,” but it will relax the force on the elevator and on the jackscrew, allowing the pilot to crank the stabilizer trim wheel. The instructions in the old manual say that the pilot should repeatedly do this: Release the control column and crank the stabilizer wheel, release and crank, release and crank, until the stabilizer is swiveled back to where it should be.
The 1982 manual refers to this as “the ‘roller coaster’ technique” to trim the airplane, which means to get it back on the required flight path with no force pushing it away from that path.
“If nose-up trim is required, raise the nose well above the horizon with elevator control. Then slowly relax the control column pressure and manually trim nose-up. Allow the nose to drop below the horizon while trimming (manually). Repeat this sequence until the airplane is trim,” the manual states.
The Australian pilot also posted an extract from Boeing’s Airliner magazine published in May 1961, describing a similar technique as applied to Boeing’s first jet, the 707.
Clearly this unusual circumstance of having to move the stabilizer manually while maintaining a high stick force on the control column demands significant piloting skill.
“We learned all about these maneuvers in the 1950-60s,” the pilot wrote on the online forum. “Yet, for some inexplicable reason, Boeing manuals have since deleted what was then — and still is — vital handling information for flight crews.”
The preliminary investigation report into the Ethiopian crash is expected soon and will offer more details on what happened in the cockpit.