First Look at JT610 Flight Data

Partial release of flight data from JT610 (h/t to Don Thompson). In short summary, the left Angle of Attack (AoA) vane was substantially offset from the right vane from taxi out. The left stick shaker activated on liftoff and stayed on for most of the accident flight. MCAS trimmed nose down when flaps were up. In a deadly game of tag, the Pilot stopped MCAS by manual trim nose up, but MCAS would start again five seconds later.  In each excursion, the stabilizer was held roughly in trim. There are 26 occurrences of MCAS trim down, pilot trim up.

It may have been the point when the captain transferred control to the first officer that MCAS was successful in two successive cycles, trimming the stabilizer nose down over four units without opposition.

The captain seems to have taken control back in the end, and pulls the column hard aft. MCAS is able to trim yet another one unit nose down before the captain stops it, but there is no subsequent  significant command to trim stabilizer nose up. The airplane dive exceeded 450 knots by the point of impact.

The prior flight showed similar behavior. But on that flight, the flight crew appear to have quickly cutout the electric trim altogether, and completed the flight trimming the stabilizer with the wheel.

Search for the CVR continues.

737 FCC Pitch Axis Augmentation - Command Integrity Mandate for Dual Channel, Fail-Safe

A line is drawn between primary flight controls that the pilot handles directly through control column, rudder pedal, and stabilizer trim; and automatic control, such as the autopilot, which manages the same surfaces through computer command. 

The Yaw Damper is an example of an automatic flight control system added to the airplane to augment flight characteristics. The Yaw Damper operates regardless whether pilot manual flight control or automatic flight control.

FCC Pitch Axis augmentation (Mach Trim, Speed Trim, and MCAS) commands may be based on a single sensor input. These commands should be checked against a calculation based on a second sensor set before becoming valid. A software update to the FCC may provide support for a dual channel mandate.

737 MCAS - Failure is an Option

The 737MAX introduced a new feature, Maneuvering Characteristics Augmentation System (MCAS). MCAS commands the stabilizer to trim down (only while flaps are up) in steps.  From reports, the aft column cutout switch is disabled while MCAS is active.  MCAS commands nose down trim at 0.27 deg/sec for about 10 seconds at a time. It then pauses briefly (duration not known, but presumed five seconds).  The time history of a fabricated scenario, starting at a stabilizer position of 8 units, shows it would take about 55 seconds to reach the nose-down limit. Starting at a more nose down position takes less time, e.g from 4 degrees about 35 seconds).

MCAS Commanded Stabilizer Trim Without Stopping

Stabilizer Trim

LionAir JT610 plunged into the sea from what may have been the result of runaway stabilizer trim.  The details are not released, but the FAA has issued an Airworthiness Directive drawing attention to the recovery from such an event.  What is stabilizer trim and why would it runaway?

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgad.nsf/0/83ec7f95f3e5bfbd8625833e0070a070/$FILE/2018-23-51_Emergency.pdf