According to a preliminary accident report, the pilots of an Ethiopian Airlines Boeing 737 MAX that crashed on March 10 were unable to manually counter significant nose-down trim before losing control of the airplane in a dive that reached 500 knots. The report, released by the Ethiopian government Thursday, confirms that the MAX 8’s MCAS subsystem, a stall-protection add-on, rolled in nearly maximum nose-down trim in response to a faulty angle-of-attack sensor.
But the report, which draws no final conclusions, is silent on whether the crew simply didn’t know how to use mechanical manual trim or if trim input was inhibited because the airplane was flying at such high speed. The pilots retained takeoff/climb power throughout the accident sequence. Ethiopian Airlines said that following the crash of a Lion Air MAX 8 last October in Indonesia, the crew was briefed on Boeing-provided information on how to disable MCAS. And although some mainstream news organizations have reported that the Ethiopian pilots followed Boeing’s checklist, the report suggests they departed from it in one key detail: After initially disabling electric trim to isolate MCAS, they reengaged it later, allowing the malfunctioning system to trim the airplane nearly full nose-down.
MCAS—Maneuvering Characteristic Augmentation System—was added to the MAX series because the engines are heavier and mounted farther forward than on previous 737 models. As a result, in flight test, the airplane demonstrated a pitch-up tendency at high angles of attack and/or high load factors. To counter this, MCAS automatically adds nose-down trim at high angles of attack when the airplane is hand flown with the flaps up. Boeing has described it as a stall-protection system, but it also increases perceived pitch force before stall angle of attack is reached.
MCAS is fed by a single AoA sensor and in both crashes, the sensor furnished inaccurate information to the flight computer. The Ethiopian report said that the left-side AoA indicated 74.5 degrees less than a minute after takeoff, while the right side indicated 15.3 degrees. This activated the left-side stick shaker and MCAS eventually responded by rolling in nose-down trim. It also gave the crew airspeed and altitude disagree alerts between the left and right side displays.
Following Lion Air, Boeing’s guidance for this situation—published in the Ethiopian preliminary report--called for several steps that combine its existing standard runaway trim with the MCAS’s peculiarities. Specifically, the checklist calls for disengaging the autopilot and autothrottles and, if the runaway continues, setting electric trim to the cutout position, disabling electric trim. Boeing said it should remain off for the remainder of the flight. The checklist advises to trim manually with the mechanical wheel and to “anticipate trim requirements.” Following Lion Air, Boeing also said that a significant out-of-trim condition caused by a runaway could first be corrected with electric trim before the cutouts are used. Flight data appears to show that the Ethiopian crew didn’t do this.
At 5:38:58, the captain, who was flying, called for the first officer to engage the autopilot, opposite of Boeing’s guidance. At that point, the crew had a left side stick shaker, some of the fault warnings and a master caution light Boeing listed, including airspeed and altitude disagree. It’s unclear if this had any bearing on the accident scenario, since MCAS is disabled when the autopilot is engaged. (The autopilot disengaged 33 seconds later.)
Shortly after the autopilot disengaged, the FDR showed that automatic nose-down trim activated for nine seconds, confirming that MCAS was reacting to and trying to resolve the erroneous AoA indication. The captain countered this with electric trim with his yoke thumb switch and later asked the first officer to “trim up with him.” The aircraft began a series of pitch and altitude excursions, but the power was never reduced below 94% N1, an unusually high power setting.
After struggling against the automatic pitch trim and excursions for several seconds, at 5:40:35—about two and half minutes after takeoff—the first officer called “stab trim cutout” twice. The captain concurred and the report indicates the cutouts were used. The 737’s trim system is operated by an electric motor/jackscrew arrangement that trims by moving the entire horizontal stabilizer. In the event of a trim runaway, the cutout switches on the lower pedestal disable the electric motor. But the 737 still has manual mechanical trim wheels on either side of the pedestal that are accessible to both pilots.
Six seconds later, the FDR showed that more automatic nose-down trim was commanded, indicating that MCAS was still sensing high AoA. However, the data showed the stabilizer didn’t respond to this command, confirming that the cutout switches were engaged. MCAS can only move the stabilizer if electric trim is active.
The report indicates that shortly after the cutouts were used, the trim gradually moved nose-down from 2.3 to 2.1 units. It’s unclear why this happened, since electric trim was disabled. At this point, according to the report, both pilots were exerting pitch-up force on the control columns, after the captain asked the FO to assist him.
At 5:41:46—a little over four and a half minutes after takeoff—the captain asks the FO “if the trim is functional.” The FO replied that it wasn’t and asked the captain if he could trim manually. What’s unclear is if the FO meant trim manually with his yoke-mounted electric trim switch or the 737’s mechanical wheel. The training manual for the 737 MAX includes a warning that excessive airloads on the stabilizer could require the effort of both pilots to correct a mistrim condition. "In extreme cases, it may be necessary to aerodynamically relieve airloads to allow manual trimming. Accellerate or decelrate towards the in-trim speed while attempting to trim manually," the training guidance says.
The data indicates the stabilizer was never moved manually mechanically. However, 32 seconds before the crash, the FDR trace revealed two momentary manual electric trim inputs commanding nose-up from 2.1 to 2.3 units.
This indicates that counter to Boeing’s guidance, the crew reengaged electric trim, allowing MCAS to once again regain control of the stabilizer. In five seconds, it moved the trim nose down from 2.3 to 1.0 unit, a value that’s nearly maximum nose-down in the 737, according to sources AVweb contacted for this story. The aircraft reached 40 degrees pitch down before impacting at 500 knots, according to the FO’s data. According to the report, power was never reduced from the takeoff/climb value.
Ethiopian Airlines said that its pilots did follow the Boeing guidance and in a press statement, the airline said, ”We are very proud of our pilots' compliances to follow the emergency procedures and high level of professional performances in such extremely difficult situations."
For its part, Boeing continues work on a new software package for the 737 MAX, but it’s not known when it will be available. For the time being, more than 370 MAX series 737s remain grounded throughout the world.
In a press statement, Boeing said, “The preliminary report contains flight data recorder information indicating the airplane had an erroneous angle-of-attack sensor input that activated the Maneuvering Characteristics Augmentation System (MCAS) function during the flight, as it had during the Lion Air 610 flight.
"To ensure unintended MCAS activation will not occur again, Boeing has developed and is planning to release a software update to MCAS and an associated comprehensive pilot training and supplementary education program for the 737 MAX.
"As previously announced, the update adds additional layers of protection and will prevent erroneous data from causing MCAS activation. Flight crews will always have the ability to override MCAS and manually control the airplane."
