Two tragic flights, 12 problems

Before takeoff

Problem 1

Even before takeoff with 189 people on board, the data on Lion Air 610 showed evidence of a problem. As the plane taxied, the two angle-of-attack (AOA) sensors on the nose of the jet recorded starkly different values. The left sensor was clearly wrong — the plane was still on the ground — but the aircraft didn’t recognize the discrepancy.

The angle of attack is the angle between the wing and the airflow. The sensors include vanes that align themselves with oncoming airflow. The angle of the vane changes as the plane’s nose rises or falls.

The first minutes

Immediately after takeoff, the Lion Air plane began giving the pilots warnings as a result of the faulty sensor. The captain’s control stick began shaking, an indicator of a potential stall. The crew asked ground controllers to confirm their altitude and airspeed, due to disagreements on their instruments caused by the same faulty angle-of-attack sensor. They reported problems with the flight controls.

After climbing above 2,000 feet, the pilots retracted the flaps on the wings. The MCAS system, which the crew was unaware of, is designed to turn on once the flight crew retracts the flaps and disengages the autopilot. Seconds later, just a couple of minutes into the flight, the MCAS safety system engaged, pushing the nose down. The plane descended.

On Ethiopian Airlines Flight 302, with 157 on board, there was no sign of a problem before takeoff. Flight data shows the angle-of-attack sensors tracking each other identically. That stayed consistent while the plane was on the ground, but it was about to change, just not in the same way that it did on Lion Air 610.

Shortly after takeoff, Ethiopian 302's angle-of-attack sensors recorded a sudden disagreement. It’s unclear what caused the fault, though the suddenness and severity of the change suggests a bird strike may have cut off one of the AOA vanes, which protrude from the front of the plane.

Problem 3

The plane’s left sensor, the one that apparently controlled the MCAS system, recorded an angle of attack at 74.5 degrees — far higher than would be conceivable. A stall would typically be a risk before 20 degrees.

The sensor’s value remained stuck. But even though the sensor was no longer showing even slight fluctuations and was stuck at an inconceivable value, MCAS continued considering it valid data.

Problem 4

This set off a cascade of warning indicators in both cockpits — airspeed, altitude and stall — for the pilots to consider. The MAX was supposed to come with an indicator to tell the pilots that there was a disagreement between the two angle-of-attack sensors. However, due to a software glitch that Boeing discovered back in 2017 but hadn’t fixed, that light wasn’t actually functional for most customers, including both Lion Air and Ethiopian Airlines.

Problem 7

Five seconds after the Ethiopian pilots disengaged the autopilot, MCAS took control. The system moved the trim down 2.5 units, sending the plane into descent. As first reported in The Seattle Times, FAA officials believed, based on Boeing’s initial system safety assessment of the MAX aircraft, that MCAS was only supposed to be able to move the trim by 0.6 units, not 2.5.

Problem 8

In addition, because MCAS repeatedly engaged, it effectively acted with unlimited power. The pitch of the jet is marked on a scale that’s visible on the stabilizer trim wheel beside the throttles. On this scale, 0 is maximum nose-down pitch and 4.7 is level flight. The green band on the wheel shows the range of where the pitch should be in normal flight. At one point in the Ethiopian 302 flight, MCAS pushed the stabilizer to just 0.4 units, a severe nose-down pitch.

Then, just 35 seconds after the sudden takeover of MCAS, the pilots appeared to have relied on Boeing guidance on how to shut it off. The first officer called out “stab trim cutout” two times. The captain agreed, so the crew flipped the two “Stab Trim” cutout switches (seen at bottom right) to block electric movements of the horizontal tail.

Problem 9

Separate data released to the Indonesian parliament shows the pilots desperately pulling the control column back. On the previous 737 model and other Boeing planes, a pilot pulling back hard on the column would trigger a cutout switch that would halt automatic commands in the opposite direction. In order to make MCAS work, Boeing had to disable that feature when the new flight control system was active.

While the Ethiopian Airlines pilots had cut off the electric power that moved the horizontal tail, the tail was still in a position to pitch the nose down. The pilots pulled on the control column and continued ascending thanks to the elevator tabs on the trailing edge of the horizontal tail.

The raised elevator tabs at the back and the leading edge of the horizontal tail swiveled upward form a sort of V shape that creates opposing nose-up and nose-down forces.

Boeing’s previous 737 model had two separate cutout switches, one specifically for electric power to the automatic controls, the other for electrical power to the manual thumb switch on the control column. However, the option to operate these separately was never included in the manuals. And on the MAX, the two switches had identical functions. One primary and one backup, either one cut out all electric power, automatic and manual. Standard procedure was to flip both at once.

Problem 11

Boeing’s emergency directive, issued after the Lion Air crash, included a line near the bottom that mentions a way to avoid high forces making the manual trim wheel hard to move. It says that pilots “can” use the electric trim switches on the control column to neutralize those forces before hitting the cutout switches.

But that was not emphasized as a necessary step to avoid disaster. It appears that the Ethiopian pilots, eager to stop MCAS quickly and avoid the fate of the Lion Air flight, jumped immediately to the cutout switches without the vital step of first using the thumb switches to ease the forces on the horizontal tail. That’s why they couldn’t move it manually.

Problem 12

The pilot pulled on the control column with all possible force. But with the column only controlling the elevator tabs at the back of the horizontal tail, he was unable to fully counter the nose-down pitch from the MCAS swiveling of the larger portion of the horizontal tail.

The Ethiopian Airline pilots inexplicably hadn’t throttled back the engines, which were still at takeoff power. As a result, the speed of the plane rose well above 400 mph — higher than the maximum design speed of the airplane. That triggered overspeed warnings in the cockpit that include an alarming clicking sound called a “clacker.” At that speed, the forces on the tail would have made it impossible to move manually.

The pilots were likely using sizable physical effort to keep the plane level by pulling back on the control column. They then made a call: Their efforts to keep the airplane pitch were not enough. They decided to re-engage the electric controls. With the electric power to the tail restored, they used the thumb buttons to trim the nose up — but, for some reason, not substantially, just two short flicks. One expert theorized that the plane’s high speed may have caused an alarming shudder when they moved the trim.