Readings from the recorders recovered from last month’s crash of an Ethiopian Airlines Boeing 737 MAX jet reportedly suggest that the pilots tried using the recommended procedure for overriding a balky automated flight control system — but that the system was re-engaged and forced the plane into its fatal dive.
The reports by The Wall Street Journal and Reuters, based on interviews with unnamed sources who have been briefed on the post-crash investigation’s preliminary findings, raise deeper questions about the safety of the flight control system, known as the Maneuvering Characteristics Augmentation System, or MCAS.
Boeing added the MCAS system to the 737 MAX as a safeguard against stalling, but investigations into the Ethiopian crash on March 10 — and last October’s crash of a Lion Air 737 MAX in Indonesia — have focused on the possibility that spurious data from a single angle-of-attack sensor caused the system to force the planes into catastrophic nose dives.
The Indonesia crash killed all 189 people on board, and the Ethiopian crash killed 157 people. In the wake of the crash in Ethiopia, all 737 MAX planes have been grounded worldwide. Boeing is working on a software update that it says should resolve the MCAS issue, but that fix is still thought to be weeks away.
In the past, Boeing has stressed that pilots could remedy the scenario that led to the crashes by disconnecting the MCAS system and taking manual control of the jet’s stabilizer trim mechanism. But the latest reports quote sources as saying the Ethiopian Airlines pilots tried that procedure but didn’t fully execute it. Instead, the MCAS system was re-engaged, leading to the final, fatal plunge.
The Journal’s sources speculated that pilots re-engaged the automated system because they couldn’t raise the nose using manual controls, while Reuters’ sources held out the possibility that the MCAS system could have re-engaged itself.
The Seattle Times quoted a former Boeing flight control engineers, Peter Lemme, as saying that the pilots might have been stymied by excessive aerodynamic loads on the stabilizer trim control system.
I *assume* the mistrim situation created excessive load opposing the manual jackscrew authority from the trim wheel. From what is reported, they must have tried to restore electric trim to get the stab to come up, but then MCAS swept in again.#ET302https://t.co/1AdEyqlCro pic.twitter.com/TLpqplcN9j
— Peter Lemme (@Satcom_Guru) April 3, 2019
Leeham News and Analysis also laid out a scenario by which excessive loads could have foiled efforts to stabilize the jet.
The Times noted chatter on an online aviation forum about an alternate procedure, outlined in a 1982 pilot training manual, that might have averted the manual lockup by repeatedly letting go of the control column and turning the cockpit’s stabilizer trim wheel manually.
Boeing said it was premature to comment on the specifics of such reports. “We urge caution against speculating and drawing conclusions on the findings prior to the release of the flight data and the preliminary report,” the company said.
The 737 MAX crashes are the subject of investigations in Ethiopia and Indonesia, with participation by Boeing, the Federal Aviation Administration, the National Transportation Safety Board and other entities. The FAA’s inspector general is conducting its own investigation into the process by which the 737 MAX was certified for flight, and the Justice Department has reportedly launched a grand jury investigation with participation by the FBI.
Subpoenas have gone out to Lemme and other potential witnesses, The Seattle Times reported.