The Ethiopian Airlines pilots initially followed the advice to shut off the MCAS anti-stall system but later reversed the command counter to guidance at a time when they were travelling beyond maximum operating speeds, according to data contained in a preliminary report released on Thursday and experts on the jet. WHAT ARE THE PROPER PROCEDURES?
If MCAS misfires, forcing the nose down in a manoeuvre similar to a condition that pilots know as runaway trim, pilots are supposed to hit two cut-out switches at the plane's centre console to turn off power to the electric trim system. Under normal circumstances, trim is used to keep an aircraft flying level, but the MCAS makes automated nose-down movements.
Data from the Ethiopian Airlines flight indicates the aircraft was flying nose-heavy and not in a "neutral" attitude when pilots hit the stabilizer cutout switches to disable the MCAS system, the preliminary report showed. That would make the situation harder to manage, possibly accounting for their decision to turn the system back on.
Boeing's checklist for pilots tells them to "control airplane pitch attitude manually with control column and main electric trim as needed" before hitting cut-out switches and turning to a rarely used manual wheel to keep the plane's nose in the proper position. It does not describe a specific trim setting for the pilots to achieve. WOULD THE PROCEDURES WORK?
Experts are questioning whether the procedures outlined after the Lion Air crash were comprehensive enough to ensure pilots could recover from a real-life cockpit emergency with several distractions at a low altitude shortly after take-off rather than in a pre-planned simulator ride. A 737 MAX pilot said the resistance on the control yoke would be about four times normal and it could take a few dozen turns of the manual wheel to return the nose to the proper position, depending on the alignment when the switches were cut.
The preliminary report indicates the pilots tried to move their wheels together but were unable to raise the nose much at all by doing so. "It appears the flight crew reactivated electric trim," former Boeing engineer Peter Lemme said. "But they only made a very small nose up adjustment - I would have expected them to immediately and without stopping move the stabilizer back into trim. The last MCAS command comes 5 seconds after their last manual trim command."
WHY COULDN'T THEY RAISE THE NOSE MANUALLY? The proper response to MCAS emergencies, Leeham Co analyst Bjorn Fehrm said, is to correct the dangerous nose-down "trim" using electronic thumb switches, then turn off MCAS and trim manually with the wheel.
But if the aircraft is going too fast, those electronic switches may not be effective, European regulators said in a 2016 memo. And failing to fully fix the trim before MCAS is deactivated can make it physically impossible for pilots to control the plane, Fehrm said. Under normal circumstances, trim is used to keep an aircraft flying level.
At speeds up to 250 knots (288 mph) pilots can stabilise trim with the manual wheel. But when the speed rises towards 300 knots and higher, the wheel becomes impossible to turn as air rushing over control surfaces makes them harder to move, Fehrm said. At the time when both pilots were unable to move the wheel, they were travelling at over 340 knots, the maximum operating speed of the airplane and clacker alarms were sounding. By the end of the fatal flight they had reached 500 knots.
WHY WAS THE ETHIOPIAN JET GOING SO FAST? The plane's engines were at 94 percent thrust on take-off and remained there for the rest of the flight.
That is consistent with the pilots leaving the thrust setting in take-off mode throughout, aviation experts said. The 737's air data computer also uses angle-of-attack (AOA) information to adjust airspeed readings. If it mistakenly thinks the angle of attack is high, it can trigger pilot warnings that airspeed and altitude data are unreliable on one of the pilot's controls, according to former Boeing engineer Peter Lemme.
That leads to an unreliable airspeed checklist which involves turning off the autothrottle as well as setting engine thrust to 75 percent. The runaway stabilizer checklist to shut off MCAS also says to turn off autothrottle. However, according to the flight data recorder, the pilots never reduced the thrust from 94 percent.
"The report does not address information about unreliable airspeed procedures which should be considered because they had erratic airspeed," said Greg Feith, a former senior air safety investigator with the National Transportation Safety Board. (Reporting by Jamie Freed; additional reporting by Tim Hepher in Paris, Eric M. Johnson in Seattle and David Shepardson in Washington; Editing by Gerry Doyle and Alexandra Hudson)
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