Minutes after take-off, the pilots of an Ethiopian Airlines 737 MAX were in a bad situation.
A key sensor was wrecked, possibly by a bird strike. As they retracted the landing gear, flaps and slats, it fed faulty data into the Manoeuvring Characteristics Augmentation System (MCAS), designed to prevent stalls.
Flying faster than recommended, the crew struggled with MCAS. High speed made it almost impossible to pull the nose up.
Moments later, the Boeing jet hit the ground, killing all 157 people aboard after six minutes in the air.
Ethiopian authorities said the pilots followed correct procedures trying to keep MCAS sending the aircraft into a fatal dive.
The full picture of what happened in the cockpit of Flight 302 on March 10 is emerging from a preliminary report and a newly released data plot showing how crew and technology interacted.
The airline’s youngest-ever captain, a 29-year-old with 8,100 hours in his logbook, and his rookie 25-year-old co-pilot may have made a crucial mistake by leaving the engines at full take-off power, according to data and other pilots.
Atthe end the aircraft was travelling at 500 knots well beyond design limits.
That and other potential missteps may have left them unable to fight flawed Boeing software that eventually sent the jet into an uncontrollable dive, experts said after studying the data.
“Power left in take-off power while levelling off at that speed is not normal procedure,” said a US pilot, who declined to be named because he was not authorised to speak to the media. “I can’t imagine a scenario where you’d need to do that.”
The Ethiopian Airlines crash, and another in Indonesia five months earlier, left the world’s largest aircraft manufacturer in crisis as its top-selling jetliner is grounded worldwide, and Ethiopia scrambling to protect one of Africa’s most successful companies.
Boeing is working on a software fix for MCAS and extra pilot training, which chief executive, Dennis Muilenburg, maintains will prevent similar events happening.
Sources who reviewed the crash data said the problems started 12 seconds after take-off.
A sudden data spike suggests a bird hit the jet as it was taking off and sheared away a vital airflow sensor.
As with the Lion Air crash in Indonesia, the damaged ‘angle of attack’ sensor, which tells pilots what angle the aircraft has relative to its forward movement, may have set off a volatile chain of events.
In both cases, the faulty sensor tricked the plane’s computer into thinking the nose was too high and the aircraft was about to stall, or lose lift. The anti-stall MCAS software then pushed the nose down with the aircraft’s trim system, normally used to maintain level flight.
The first time the MCAS software kicked in, the Ethiopian Airlines pilots countered the movement by flicking switches under their thumbs – they recognised the movements as the same type all flight crews were warned about after the Lion Air crash.
Data suggests they did not hold the switches down long enough to fully counteract the computer’s movements. At that point, they were 3,000 feet above the airport, so low a new warning – a computerised voice saying “don’t sink” – sounded in the cabin.
When MCAS triggered again, the jetliner’s trim was set to push the nose down at almost maximum level, while the control yoke vibrated with another stall warning called a “stick shaker.”
This time, the pilots countered MCAS more effectively. When they turned off the system – as instructed by Boeing and the US Federal Aviation Administration (FAA) in the wake of the Lion Air disaster – the nose was still pointed down, leaving the jetliner vulnerable.
The combination of excess speed and cutting off the system while the aircraft was leaning down meant up to 50 pounds of force would be needed to move the control column and moving manual trim wheels was impossible.
‘PULL UP, PULL UP’
The captain called “pull up” three times. The co-pilot reported problems to air traffic control.
Meantime aircraft speed remained abnormally high.
The bird strike and loss of airflow data would affect airspeed information as well. In such cases, pilots turn off automatic engine settlings and control thrust manually.
The report states “the throttles did not move,” without elaborating. Data confirms engines stayed at nearly full power. Other 737 pilots say that made the crew’s job tougher by making controls harder to operate.
Some experienced pilots said there an array of stressful factors sapped pilots’ attention, which Muilenburg addressed.
“As pilots have told us, erroneous activation of the MCAS function can add to what is already a high-workload environment,” Muilenburg said. “It’s our responsibility to eliminate this risk. We own it and we know how to do it.”
Among distractions was a “clacker” warning telling the pilots the aircraft was going too fast.
As the nose gradually fell, pilots turned to a last-resort device to adjust aircraft trim.
The captain asked the co-pilot to trim the plane manually using a wheel in the centre console and recover from the dive.
It was too difficult to move the wheel. Together both tried to pitch the nose up. The captain, according to the report, said it was not enough.
In a possible last-ditch attempt to level the aircraft, data suggests the pilots turned MCAS-related systems back on. That would reactivate the electric trim system and perhaps make it easier to force the nose higher.
Reactivating MCAS is contrary to advice issued by Boeing and the FAA after Lion Air. The report did not address that.
The pilots managed to lift the nose slightly using electric thumb switches on the control yokes. Data suggest they may have flicked the switches too gingerly.
With power restored, a final MCAS nose-down command kicked in, eventually pushing the nose down to a 40 degree angle at airspeed of up to 500 knots, well beyond operating limits.
As the 737 MAX plunged, G-forces turned negative, pulling occupants out of their seats and possibly inducing weightlessness as the jet hurtled toward the ground.
Six minutes after take-off, the 737 MAX crashed into a field.