Sunday, April 7, 2019

What happened on ET302?

The preliminary report for ET302 reveals many details, but not enough to know exactly every and all aspects. The following is a look at what can be learned.

Tuesday, April 2, 2019

Trim Cutout with Severe Out-of-Trim Stabilizer can be difficult to recover

The Wall Street Journal reports that the crew on ET302 used the stabilizer trim cutout switches in response to MCAS commands, but appeared to be unable to raise the nose. They then released the cutout switches to use electric trim, but encountered MCAS commands again without recovering.

One possible explanation is that the loads on the jackscrew due to the severe stabilizer nose down out-of-trim situation were too great for the pilot to overcome using the trim wheel with folding handle. The pilots restored electric trim as a means to trim.

Boeing published a technique in the past that discussed this issue and the need to release the column briefly in a series of "roller coaster" or "yo yo" maneuvers, by cranking in stabilizer trim alternatively with large column commands.

This is in a 737NG Flight Crew Ops Manual

This is in the 737NG training manual:
Manual Stabilizer TrimIf manual stabilizer trim is necessary, ensure both stabilizer trim cutout switches are in CUTOUT prior to extending the manual trim wheel handles.Excessive airloads on the stabilizer may require effort by both pilots to correct the mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually.Anticipate the trim changes required for the approach. Configure the airplane early in the approach. When reaching the landing configuration, maintain as constant a trim setting as possible. If a go-around is required, anticipate the trim changes as airspeed increases.
This is in a 737 QRH (Ref 9.15)
This is from the Airworthiness Directive. It seems quite pertinent to emphasize using the electric trim to get the stabilizer near to an in-trim condition before hitting the cutout switch.
REVISED 11:40pm 3 Apr - Added discussion at end regarding cutout switch change
REVISED 7:43pm 4 April - Adjusted the Stab Nose Down trim representation to be zero lift
REVISED 9:20am 5 Apr - Added Stab Nose Down trim at Autopilot travel limit