The Boeing 737 MAX 8 cockpit

For 2 1/2 minutes as Ethiopian Airlines Flight 302 roared above Addis Ababa, the jet's survival depended on the pilots turning a pair of wheels in the cockpit.

Black with white stripes, each disk was the size of a dinner plate and manually adjusted the nose of the Boeing Co. 737 Max up or down. The motor that normally adjusted this angle was shut off because a safety system had gone haywire and it was trying to drive down the jet's nose.

Cranking the wheels, mounted near the pilots' thighs, was the key to averting disaster.

"It is not working," copilot Ahmed Nur Mohammod Nur said, according to cockpit recordings paraphrased in a preliminary report on the March 10 crash.

Whether they failed or Nur and Captain Yared Getachew simply couldn't manage to turn them as they faced a chaotic onslaught of alarms and failures isn't clear. It appears that in the panic, one or both of them managed to turn the wheels backward.

Analyzing whether a range of pilots are capable of turning the wheels in emergencies has been part of safety reviews being conducted before the plane can be returned to service, although the U.S. Federal Aviation Administration disputed a report that those evaluations are delaying the certification process.

The conclusions will help determine how the FAA revamps pilot training requirements and procedures in the wake of the crashes -- not only for the Max but for thousands of earlier 737 models that share the same system design.

Boeing updated the software for its 737 simulators twice in recent weeks because they didn't accurately replicate the forces on the wheels.

On Flight 302, the jet hit the ground at 575 mph after a dive so severe that mobile phones and other loose items in the plane would have slammed against the ceiling with twice the force of gravity.

The inability of the Ethiopian Airlines pilots to use the wheels -- safety experts argue over whether it was more the result of Boeing's design or pilot actions -- is a critical part of what led to the crash that killed 157 people and the subsequent grounding of Boeing's best-selling model. It's an issue that the crash's investigators will almost certainly want to explore in depth, safety advocates say.

Unable to restore control with the wheel, the pilots made the fatal decision to abandon the Boeing procedure designed to address their emergency. The plane then pitched nose-down violently and the pilots couldn't respond.

Ethiopian government officials and the airline have steadfastly defended the pilots. Transport Minister Dagmawit Moges in April said they followed "all the procedures repeatedly." Airline Chief Executive Officer Tewolde GebreMariam has bristled in interviews over implications that the crew didn't perform properly.

Boeing and a spokesman for the airline didn't respond to requests for comment.

This description of the battle between pilots and the machine on Flight 302 is based on the Ethiopian Ministry of Transport preliminary report on April 4 and interviews with safety consultants, former investigators and several people familiar with the probe who asked not to be identified.

The flight took off in light winds with a few scattered clouds at 8:38:39 a.m. The onslaught of alarms began seconds later.

Something -- possibly a bird -- slammed into a sensor on the left side of the plane, prompting it to erroneously conclude its nose was pointed 75 degrees above the onrushing air. That triggered a loud stall warning on the captain's side of the cockpit and also threw out of whack speed and altitude readings on his instruments.

The startled pilots left the plane's engines racing at nearly full power, an error that would make it harder to recover. Jetliners are designed to use full power for only short periods during takeoff and shortly afterward. Leaving the throttles so high is never done on a normal flight and their jet rapidly accelerated to dangerous levels that made it harder to control.

At 8:40:00 a.m. a flight-control feature known as Maneuvering Characteristics Augmentation System or MCAS began driving down the nose. Boeing said it added the now-infamous feature to the Max in an effort to duplicate the feel of earlier 737 models.

MCAS has since become the focus of investigations over whether Boeing and regulators erred by not telling pilots about it. The fact that it can be triggered by the failure of only one of the two sensors that measure the plane's pitch relative to oncoming air has also drawn criticism. MCAS is even the subject of a criminal probe.

When MCAS activated on the Ethiopian flight, the pilots stopped using thumb switches on their control yokes to adjust -- or "trim" -- the up-and-down angle of the plane's nose. The switches rotate the wing-like horizontal stabilizer on the tail.

Boeing updated the software for its 737 simulators twice in recent weeks because they didn't accurately replicate the forces on the wheels

Trim is a control function at the heart of the two 737 Max accidents, though it's not well understood outside of the aviation world. It is a skill taught to all pilots from the time they learn to fly, said Roger Cox, a former accident investigator with the U.S. National Transportation Safety Board who also flew 737s and other planes during a career as an airline pilot.

Flicking those switches, which either pilot can do, relieves pressure on the control yoke so they can hold a climb or stay level with no effort. Without trimming, it may still be possible to fly, but it requires extra force on the control column that can feel like lifting a large barbell. The forces grow as speed increases and as the plane is trimmed more nose-down.

Pilots typically make repeated trim adjustments in the minutes after takeoff before switching on the autopilot to keep those forces in check. Indeed, on Flight 302, Getachew, who was flying the plane, adjusted the trim seven times during the first 45 seconds of the flight, even as he and Nur tried to diagnose their emergency.

Once MCAS began firing, however, something changed. A Boeing bulletin issued last November after the first 737 Max accident twice mentions the need for hitting the trim switches in response to an MCAS failure. The emergency procedure for handling a malfunctioning trim system says the same thing.

Yet the Ethiopian pilots didn't successfully retrim the plane, according to the investigation's preliminary report.

Over a 28-second span, MCAS twice moved the stabilizer and each time pilots responded. But they didn't move the horizontal stabilizer back to its original position. It was moved a total of 4.6 units by MCAS and only 2.2 units in the opposite direction by pilots.

It was at this point, at 8:40:35 a.m., or about 1 minute, 56 seconds into the flight, that Nur called out "stab trim cut-out" twice. Getachew agreed and one of them flicked two switches that cut power to the trim motor, the key step in the Boeing procedure to address the emergency. With the power cut, MCAS was no longer a threat.

But with the plane badly out of trim -- its stabilizer set to dive -- they were left with new hazards.

It was taking more and more force on the control column to keep the plane climbing. "Pull up," Getachew called out three times at 8:40:44 a.m. as Nur acknowledged. The captain, seated in the left seat, was flying, but Nur helped him pull back on the control column several times, according to sensors recorded in the plane's flight-data recorder.

The only way left to relieve the pressure on the control column was to crank the trim wheels in the nose-up direction. In normal flight, they're easy to rotate and pilots can simply run their hands back and forth on the top of each wheel to fine tune the trim. Each wheel contains a crank arm that folds out and can be used to apply more leverage.

Pilots on a Lion Air 737 Max flight encountered an identical malfunction on Oct. 28, the day before the same plane crashed after takeoff from Jakarta. They were able to crank their trim wheel repeatedly during a 90-minute flight and the aircraft landed safely, according to Indonesian investigators.

But on the Ethiopian jet's horizontal stabilizer, there were now massive forces as it was set to lower the nose while Getachew was pulling the elevator panels in the opposite direction and the speed increased. Known as a "jackknifed" stabilizer, the forces buffeting the small wing at the tail made it hard to turn the manual trim wheels.

The issue was well known since the introduction of the 737 in the 1960s. Early models included instructions in pilot manuals for making repeated climbs and dives to relieve the stress on the stabilizer to make it easier to crank the wheels in an emergency. Current models, including the Max, caution pilots that "it may be necessary to aerodynamically relieve the airloads to allow manual trimming."

The scant data released on the accident so far by investigators suggests it was at least possible to turn the wheels in one direction.

During the 2 1/2 minutes the trim motor was shut off, the stabilizer moved 0.2 units in the nose-down direction, according to the report. The only way for that to have happened was if the wheels were rotated by hand, according to two people familiar with the probe.

That wasn't a trivial movement of the wheels. It would take three full revolutions to tilt the stabilizer that far, according to one of the persons.

What that means is a puzzle. Was it a mistake by Nur, who probably would have been cranking on the wheel while Getachew tried to fly? Was he unable to turn it to bring up the nose, so tried the opposite direction out of desperation? These are questions investigators will try to answer by teasing out sounds from the cockpit voice recorder, studying flight data and conducting tests.

At 8:43:04 a.m., after more than two minutes of struggling to keep the plane in control and vague discussions about the trim not working, Getachew asked Nur to "pitch up together" and said "pitch is not enough."

Seconds later, even though the Boeing procedure said pilots shouldn't switch power back on to the trim motor, the desperate crew did so. One of the pilots made two small electric trim movements that rotated the stabilizer by only 0.2 units in the nose-up direction.

The power had also reactivated MCAS and it then fired for five seconds, cranking the stabilizer more than six times as far, from 2.3 units to 1.0 units. That sent the jet into its final dive.

The Wall Street Journal earlier reported that Boeing and FAA are studying the trim-wheel forces. The agency denied that the work was slowing Boeing's submission of its proposed fix for the 737 Max.

John Cox, a former 737 pilot and union safety representative, said he tried replicating the Ethiopian flight in an airline simulator for the 737 Max. Once speeds reached about 370 miles per hour -- still slower than the Ethiopian plane was flying -- the trim wheels became difficult to turn, even with the heft of both pilots.

Cox acknowledged that the simulators airlines use don't perfectly replicate the actual forces the Ethiopian pilots would have faced. Boeing announced in May it was adjusting its simulator modeling for trim wheel to make the devices more accurate. Nevertheless, the simulator provides a window into how confusing it might have been for Getachew and Nur to find the trim wheel so difficult to turn, Cox said.

The entire flight was a cacophony of alarms and warnings, along with a malfunctioning MCAS system trying to dive, he said. The combination made the pilots' response understandable, in his view. "This is a breeding ground for confusion and task saturation," he said.

Chesley Sullenberger, a retired pilot who gained fame by safely ditching a disabled jetliner on the Hudson River off Manhattan, also experienced a recreation of the disaster in a simulator.

"Even knowing what was going to happen," Sullenberger told lawmakers at a congressional hearing Wednesday, "I could see how crews could have run out of time and altitude before they could have solved the problems."

Roger Cox, also a former pilot who investigated dozens of accidents at the NTSB, sees it differently.

Keeping a plane in trim and within a normal speed range are so basic that they're assumed and don't need to be explicitly restated throughout pilot manuals, he said. Pilots who fail to do so are making basic errors with severe consequences.

"You can get an airplane so badly out of trim that it's hard to recover," Roger Cox said.