Last week FAA spokesman Greg Martin said
“the angle of attack values used by several systems, including the air data, the fight controls, the stall warning, etcetera, the safety analysis for each of these systems are currently being reviewed.”
On the face of it, a fairly boilerplate statement under the circumstances. However, I've been wondering if the MCAS would have been the only stall protection triggered by erroneous AoA data. The -800 FCOM states:
Stall Identification
Stall identification and control is enhanced by the yaw damper, the Elevator Feel Shift (EFS) module and the speed trim system. These three systems work together to help the pilot identify and prevent further movement into a stall condition.
During high AOA operations, the Stall Management/Yaw Damper (SMYD) reduces yaw damper commanded rudder movement.
The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure. The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are: on the ground, radio altitude less than 100 feet and autopilot engaged. However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.
As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables speed trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane. With the column aft, the amount of column force increase with the onset of EFS module is more pronounced.
Can anyone familiar with the systems comment on whether the EFS and/or speed trim protections would likely have been active? Looking at the Runaway Stabilizer checklist, I'm struck by the fact that in the circumstances either or both the EFS and/or speed trim protections may have posed a serious impediment to recovering from an MCAS induced stab trim pitch down.
Control airplane pitch attitude manually with control column ...
may be somewhat problematic if the EFS is increasing forward control column force to approximately four times normal feel pressure, and
... and main electric trim as needed.
If the trim system senses that the AoA is already at or aboveabove stickshaker AoA will it recognise nose up trim commands using the main electric trim? (
ie will it allow the aircraft to be trimmed further into the 'perceived' stall?)