Re:The Case Against Boeing
by pdonis on Saturday November 16, 2019 @06:38PM
(#21555638)
> Boeing has a point about MCAS issues manifesting exactly like another well-known event in other models and having the exact same solution.
No, they don't, because (a) the MCAS issue does not manifest exactly the same as another well-known event (runaway trim), and (b) the solution to the MCAS issue is not the same as the solution to runaway trim.
Re (a), normal runaway trim on a 737 manifests as a continuous automatic adjustment of trim. The trim starts moving and doesn't stop until the pilot disables the automatic trim system entirely.
The MCAS issue, however, manifests as an intermittent automatic adjustment of trim, at times that seem completely random to the pilot. It is not at all the same as normal runaway trim.
Re (b), the procedure for normal runaway trim on previous 737s is to shut off the automatic trim system. You can then use the manual electric trim system to put the trim back where it belongs. The critical fact here is that shutting off the automatic trim system does not shut off the manual electric trim system.
But on the 737 MAX, shutting off the automatic trim system, which is what you need to do to disable MCAS, also shuts off the manual electric trim system. So now the only way to get the trim back where it is supposed to be is to use the mechanical trim wheel; and MCAS has enough control authority to put the trim in a place where it is impossible for the pilot to exert enough mechanical force on the trim wheel to move the trim back to where it belongs. So the normal runaway trim response procedure does not work for an MCAS failure.
There is a way to recover from an MCAS failure, which is to wait until you are in between intermittent MCAS adjustments of the trim, use the manual electric trim system to put the trim where it is supposed to be, and then shut off the automatic trim system (which, as noted, also disables the manual electric trim system in the 737 MAX). Then you have MCAS disabled and you have the trim in a place where you can use the mechanical trim wheel if needed. But that procedure is not the standard procedure that 737 pilots are trained to do.
No, they don't, because (a) the MCAS issue does not manifest exactly the same as another well-known event (runaway trim), and (b) the solution to the MCAS issue is not the same as the solution to runaway trim.
Re (a), normal runaway trim on a 737 manifests as a continuous automatic adjustment of trim. The trim starts moving and doesn't stop until the pilot disables the automatic trim system entirely.
The MCAS issue, however, manifests as an intermittent automatic adjustment of trim, at times that seem completely random to the pilot. It is not at all the same as normal runaway trim.
Re (b), the procedure for normal runaway trim on previous 737s is to shut off the automatic trim system. You can then use the manual electric trim system to put the trim back where it belongs. The critical fact here is that shutting off the automatic trim system does not shut off the manual electric trim system.
But on the 737 MAX, shutting off the automatic trim system, which is what you need to do to disable MCAS, also shuts off the manual electric trim system. So now the only way to get the trim back where it is supposed to be is to use the mechanical trim wheel; and MCAS has enough control authority to put the trim in a place where it is impossible for the pilot to exert enough mechanical force on the trim wheel to move the trim back to where it belongs. So the normal runaway trim response procedure does not work for an MCAS failure.
There is a way to recover from an MCAS failure, which is to wait until you are in between intermittent MCAS adjustments of the trim, use the manual electric trim system to put the trim where it is supposed to be, and then shut off the automatic trim system (which, as noted, also disables the manual electric trim system in the 737 MAX). Then you have MCAS disabled and you have the trim in a place where you can use the mechanical trim wheel if needed. But that procedure is not the standard procedure that 737 pilots are trained to do.