Originally Posted by
MurphyWasRight
I do disagree though that had step6 (re-enable manual electric trim only) been available it would not have helped ET:
Once they realized that manual mechanical trim was not available, due lack of training on use of flip out handles and/or aero loads alone, step 6 would have allowed them to re-trim the aircraft, even though they had not followed the memory items perfectly.
Possibly, but let me throw out one more thought that relates to both why the procedure was changed and why the crew might not have made the decision you think they would.
The Boeing trim system found on the 707, 727, and 737 has evolved over time. Initially, there was a fast motor for the pilots use, and a slower motor for the autopilot (back when the autopilot was the only other source of electric stab trim). That logic continued even as systems were added and more things could move the stabilizer (Mach Trim, Speed Trim). The thumb switch moved the trim quickly, the automatics moved it slowly. I don't know about the 737 classics, but on the 737NG the logic also included flap position. Flaps down trim was always faster than flaps up trim, and pilot trim was always faster than automatic trim. However, this logic now created four distinct trim speeds (0.4/0.2/0.27/0.09 deg/sec)
Part of the old procedure in isolating the malfunctioning trim system was a subjective evaluation of whether it was moving fast or slow. If if was moving fast, then the Main Electric Trim was suspect. If it was moving slow, then one of the automatic systems. However, with the existence of four distinct speeds and the fact that the "fast" automatic was now faster than the "slow" Main Electric, there was some concern that pilots might misidentify the malfunctioning system, use the wrong cutout, and aggravate the problem. Keep in mind that a runaway stab is not anything you want to dally with - prompt and correct action are critical. This was one of the drivers for simplifying the procedure in the first place.
One of the issues that has been highlighted with the original MCAS design is that it operated at a higher speed than the Main Electric Trim in the flaps up configuration. Thus, it is entirely possible that this faster movement could have been interpreted as a problem with the Main Electric Trim, and not one of the automatic sources. Logically, then the crew would attempt to restore the automatic trim thus 1) reintroducing the runaway while 2) taking away the most effective tool to stop it - namely the Main Electric Trim.
In fact, this is effectively what happened with ET302. They did not properly run the Runaway Stab procedure, they cutout the trim in a significant out-of-trim conditions, and in an act of desperation they restored the malfunctioning system which promptly drove the stab to the stop. It should be noted this crew also had the opportunity to reactivate the cutout switch, but did not, probably because the MCAS movement at the excessive speed they were flying created such a strong negative g force that they were startled or thrown off balance.