By Tim Hepher, Eric M. Johnson and Jamie Freed
PARIS/SEATTLE/SINGAPORE (Reuters) - Minutes after take-off, the pilots of an Ethiopian Airlines 737 MAX were caught in a bad situation.
A key sensor had been wrecked, possibly by a bird strike. As soon as they retracted the landing gear, flaps and slats, it began to feed faulty data into the Maneuvering Characteristics Augmentation System (MCAS), designed to prevent stalls.
Flying faster than recommended, the crew struggled with MCAS. But the high speed made it nearly impossible to use the controls to pull the nose up.
Moments later, the Boeing Co jet hit the ground, killing all 157 people onboard after six minutes of flight.
Ethiopian authorities said on Thursday that the pilots followed all the correct procedures in trying to keep MCAS from sending the plane into a fatal dive.
But 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 an impressive 8,100 hours flying time, 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.
By the end, the aircraft was traveling at 500 knots (575 mph, 926 kph), far beyond its design limits.
That and some 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 being left in take-off power while leveling off at that speed is not a normal procedure," said one U.S. pilot, who declined to be named because he was not authorized 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, have left the world's largest planemaker 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 its chief executive, Dennis Muilenburg, said would prevent similar events from happening again.
Sources who reviewed the crash data said the problems started barely 12 seconds after take-off.
A sudden data spike suggests a bird hit the plane 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 forcefully with the aircraft's "trim" system, normally used to maintain level flight.
The first time the MCAS software kicked in, the Ethiopian Airlines pilots quickly countered the movement by flicking switches under their thumbs - they had recognized the movements as the same type all flight crews had been warned about after the Lion Air flight.
But data suggest they did not hold the buttons down long enough to fully counteract the computer's movements. At that point, they were a mere 3,000 feet above the airport, so low that a new warning - a computerized 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 the maximum level, while the control yoke noisily vibrated with another stall warning called a "stick shaker."
This time, the pilots countered MCAS more effectively. But when they turned off the system - as they were instructed to do by Boeing and the U.S. Federal Aviation Administration (FAA) in the wake of the Lion Air disaster - the nose was still pointed downward, leaving the jetliner vulnerable.
The combination of excess speed and cutting off the system while the plane was still leaning downwards meant up to 50 pounds of force would be needed to move the control column, and moving the manual trim wheels was impossible.
'PULL UP, PULL UP'
The captain called out "pull up" three times. The co-pilot reported problems to air traffic control.
In the meantime, the aircraft's speed remained abnormally high.
The bird strike and loss of airflow data would have affected airspeed information too. In such cases, pilots know to turn off automatic engine settlings and control thrust manually.
But the report says "the throttles did not move," without elaborating. Data confirms the engines stayed at nearly full power. Other 737 pilots say that made the crew's job tougher by making the controls much harder to move.
Some experienced pilots said there were an array of stressful factors sapping the pilots' attention, which Muilenburg addressed on Thursday.
"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 the distractions was a "clacker" warning telling the pilots their aircraft was going too fast.
As the nose gradually fell, the pilots turned to a last-resort device to adjust the plane's trim.
The captain asked the young co-pilot to try to trim the plane manually using a wheel in the center console to lift the nose and make it easier to recover from the dive.
But it was too hard to move the wheel. Both men then tried to pitch the nose up together. The captain, according to the report, said it was not enough.
In a possible last-ditch attempt to level the plane, data suggests the pilots turned MCAS-related systems back on. That would also reactivate the electric trim system, and perhaps make it easier for the pilots to force the reluctant nose higher.
Reactivating MCAS is contrary to advice issued by Boeing and the U.S. Federal Aviation Administration after Lion Air. The report did not address that.
The pilots managed to lift the nose slightly using the electric thumb switches on their control yokes. But data suggest they may have flicked the switches too gingerly.
With its power restored, a final MCAS nose-down command kicked in, eventually pushing the nose down to a 40 degree angle at an airspeed of up to 500 knots, far beyond the plane's operating limits.
As the 737 MAX plunged, G-forces turned negative, pulling occupants out of their seats and possibly inducing a feeling of weightlessness as the plane hurtled toward the ground.
Just six minutes after take off, the plane crashed into a field.
(Additional reporting by Jason Neely in Addis Ababa, Tracy Rucinski in Chicago, David Shepardson in Washington, Allison Lampert in Montreal; Editing by Gerry Doyle)