Boeing's "Battle over blame": reduce power or not?

[TailwheelPilot]

I feel airspeed is a red herring. They should have reduced thrust to remain within limits because all it would have taken to get control was nose up manual (electric) trim and then flip the cut out switches.

Look at the FDR readout (AvHerald has one).

The manual (electric) trim worked fine, they even trimmed nose up before flipping the cut out switches. Unfortunately they flipped the cut out switches prior to trimming enough. I wonder if the PNF flipped the cut out switches while the PF was trimming nose up and the PF did not realize or if the PF was overloaded and thought the manual (electric) trim was no longer working.

They were not descending uncontrollably. They were generally climbing with two or three shallow descents until the final MCAS nose down command (after the cut out switches were switched back to normal, apparently).

Unless something broke due to a reasonably small overspeed, speed has minimal to no bearing on this crash. Aerodynamic forces may have prevented using the cable trim, which may be an issue, but the manual (electric) trim was working properly and kept them in the air and climbing as long as they did.

Basic airmanship should tell us to trim out excessive control forces and to continue trimming since everything was responding as expected - command nose up trim; pitch trim was moving nose up; yoke position (and presumably force required) decreased. No checklist required.

[Eric Janson]

Unless something broke due to a reasonably small overspeed, speed has minimal to no bearing on this crash. Aerodynamic forces may have prevented using the cable trim, which may be an issue, but the manual (electric) trim was working properly and kept them in the air and climbing as long as they did.

I'm not sure that's correct.

The high airspeed resulted in a high load on the stabiliser so that the trim wheel was hard/impossible to move manually. If the airspeed had been lower this wouldn't have been the case.

The fact that they made the trim wheel smaller on this model of 737 only made things worse. I would imagine this is being looked at as part of the design/certification review.

I've had the electric trim fail on a previous model of 737 and moving the trim manually was very easy and we landed using manual trim.

[TailwheelPilot]

Just to be clear, I was referencing the annotated FDR readout from avherald. The manual cable trim is not recorded. There is automatic pitch trim that is commanded by software (including MCAS) using the electric trim, manual (electric) pitch trim that is the pilot commanded trim changes using the electric trim, and pitch trim that is the stabilizer position.

The plane did not get significantly above Vmo until going into a dive at the end. All I am saying is the electric trim appears to have been working until it was cut out. Additionally there was minimal descent towards ground until the very end of the flight. They were climbing through the majority of it.

Trim wheel size sounds like it was a factor, manual cable trim could have helped save the plane. As could have Boeing training pilots about the system, potential failure modes, and how to minimize MCAS problems or the pilots trimming the plane before using the cut out switches.

[photofly]

I may have misunderstood something, but if the aircraft hadn't entered an uncontrolled descent, it wouldn't have hit the ground. To the extent that there's some controversy about whether the pilots should have reduced power I think that controversy relates to the final portion of the flight, when it was in an uncorrected descent.

[digits_]

If we can assume the plane started the uncorrected/uncontrolled descent because the pilots were unable to raise the nose while they were still under Vne, it makes me wonder if airplanes can be certified if there are situations in which the control forces on the yoke with full opposite trim exceed a certain force? Are there any requirements that way? Obviously it will be very uncomfortable to fly with a plane fully trimmed in the opposite direction, but does the regulator require it to be possible?

[photofly]

If we can assume the plane started the uncorrected/uncontrolled descent because the pilots were unable to raise the nose while they were still under Vne, it makes me wonder if airplanes can be certified if there are situations in which the control forces on the yoke with full opposite trim exceed a certain force? Are there any requirements that way? Obviously it will be very uncomfortable to fly with a plane fully trimmed in the opposite direction, but does the regulator require it to be possible?

From the way I read FAR25.255, it's only a requirement to be able to generate 1.5G, with 125lbs of pull, given as much incorrect trim as can be generated in three seconds of trim operation. Not full trim. Obviously 1.5G will enable you to pull out of the dive if you happen to be in one:

25.255(f)In the out-of-trim condition specified in paragraph (a) [3 seconds of trim etc.] of this section, it must be possible from an overspeed condition at VDF/MDF to produce at least 1.5 g for recovery by applying not more than 125 pounds of longitudinal control force using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system.

That section goes on to say that if you can't recover with the elevator alone, and you need to trim, the trim has to be moveable etc. etc.

Obviously a cursory reading of one regulation doesn't substitute for a full analysis of what the certfication requirements are, but I don't see anything that says a manual recovery from full nose-down trim has to be achievable.

[Ki-ll]

If we can assume the plane started the uncorrected/uncontrolled descent because the pilots were unable to raise the nose while they were still under Vne, it makes me wonder if airplanes can be certified if there are situations in which the control forces on the yoke with full opposite trim exceed a certain force? Are there any requirements that way? Obviously it will be very uncomfortable to fly with a plane fully trimmed in the opposite direction, but does the regulator require it to be possible?

Do you have a copy of “Handling the Big Jets”?
Similar case is described there. From what I remember it was a case of a stalled stabilizer drive due to aerodynamic forces and the certification outcome was reduction of nose down travel limit.
P.S. Attaching the reference.

[photofly]

Good call... I just finished scanning the pages but you beat me to it!