More important than MCAS...
Thank you for starting a thread on this important topic.
Lost in the back and forth of the previous epic threads was the fact that at some point in (probably) all late-model 737s the ability of the aircrew to move the horizontal stabilizer using either manual (wheel) trim or manual (electronic) trim becomes impossible. Anecdotal contributions indicate that this design flaw traces its way all the way back to the 707 at least and may have been present in most or all 737s ever built.
The bottom line is this: The 737 series has been certified with a (possibly) unknown portion of the normal flight operations envelope within which you can inadvertently render your primary pitch control surface completely inoperable!! I have spent significant time reviewing the FAR’s as they relate to certification and I have yet to find the one that covers “Control surface stops functioning…” There are all kinds of tests for failure and overstress, but it never seems to have occurred to anyone that they would need to have a certification for “Control surface operates and is operable throughout its range in all quadrants of the performance envelope.
For you 737 drivers out there consider these thoughts:
1. What is the airspeed/altitude/pitch combination that you should not cross- after which you will not have control authority over the horizontal stabilizer??
2. If you TOGA on short-final and your PF holds in a bunch of nose-down trim as you accelerate out of 130Kts, at what point does the airplane become unrecoverable after you come over the top and start back down due to the severe out of trim situation?? It seems that 9 seconds of trim may well be enough to start this ball rolling downhill, so at what point does your airplane become unrecoverable due to a slow/non-moving horizontal stab trim due to aerodynamic overstresses??
I have never heard of an airplane being certified wherein a single control surface (the most important one really) can completely cease operation and lock-up within the normal flight envelope. And if such a situation was allowed to be certified you’d think there would be placards, warnings, airspeed bugs etc. to prevent one from unintentionally straying into uncharted territory.
It’s actually enough to make one look again at Rostov-on-Don and other incidents from the past, where airplanes went from happily flying to a nose-down dive and crash within 45 seconds or so after a TOGA. For some reason the 737 seems oddly prone to this type of unfortunate end.
MCAS is worth parsing to the nth degree of course, but this issue, the one that involves the total inoperability of the primary pitch control for the airplane, this is the one that has had me reading and researching everything I can find over the past month.
Regards,
dce