Originally Posted by
MurphyWasRight
It is not clear whether lack of manual trim after cutout was due to lack of familiarity with the flip out handles or aero loads or both.
Something had to be happening in the period while electric trim was disabled, we don't know what since the CVR transcript has not been released.
To this respect, the information transmitted by the interim report is :
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer confirmed stab trim cut-out
So, the cutout switches have been switched off between 05:40:35 and 05:40:41,
Then later:
At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try.
At 05:41:54, the First-Officer replied that it is not working.
This can only indicate that the crew was unsure if the cutout switches had disabled the electric trim switches or not.
The information about what was the exact function of these switches had not been transmitted to the pilots or assimilated by them.
So they WERE trying to trim up and they discovered that they had neither electric trim nor manual trim available.
There are 2 other points which I noticed in the report and found much informative.
The first point is about uncommanded stabiliser movements:
From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the control columns which remained aft of neutral position.
As this happens during the time the cutout switches were on the off position, I understand that this means that the stabiliser has moved under aerodynamic forces.
The second point is:
At 05:42:10, the Captain asked and the First-Officer requested radar control a vector to return and ATC approved.
At 05:42:30, ATC instructed ET-302 to turn right heading 260 degrees and the First-Officer acknowledged.
At 05:42:43, the selected heading was changed to 262 degrees.
This sequence starts shortly after the FO has reported that manual trim was not working.
The airplane had already started to bank right significantly (between 5° and 15°) at 05:41:30.
When ATC instructed heading 260°, the bank angle is increased gradually from about 10° to 20°.
It is between the 40 seconds that follow the ATC instruction that the crew decided to reset the cutout switches since, at 05:43:10, the DFDR records 2 short electric trim impulses that successfully (but very moderately) moved the stabiliser nose up.
At the time of the short pulses, the bank angle was between 20° and 25°.
When MCAS kicks in 5 seconds after the pulses at 05:43:20, the bank angle increases briefly to 30°.
I understand that the final loss of control happens at a point when the attention of the crew is divided between controlling pitch and another objective (here turning from heading 110° to heading 260°) and that the turn may have played a role in their decision to reset the cutout switches.
Keep in mind that, during all this sequence, the stick shaker is active and the EFS feedback force against aft command is multiplied by 4.
I am not disputing that this crew did not make the best possible decisions.
But apportioning blame is totally useless.
The important stuff for improving the system that led to this situation is understanding how and why these decisions were made.
Apologies for being lengthy.