Quote:
Originally Posted by p911dad
"The following is an example of just the type of scenario that you must avoid. Again, the message here is to always disengage first!
The autopilot suddenly goes into a nose down situation. The autopilot and autotrim are engaged and the pilot reacts by applying backpressure. Unless the pilot immediately disconnects the autopilot, autotrim does its job by further applying down trim. As you can imagine, things can get ugly fast, especially at low altitude."
This comes out of an old article talking about Century autopilots and autopilots in general. I have no jet or turbine time in my logbook, this is from a Piper Owners site, and relates to the Aztec, which I am familiar with. I realize that modern aircraft are way beyond anything I ever flew, but it seems the advice above should be well known to any pilot flying 737's. We had a simple trim/autopilot disconnect button on the yoke, push the button and the autopilot disconnects instantly. Don't Boeing aircraft have such a device? Or are the computers so powerful the pilot can't disengage the trim if the computer says no?
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In this case, it would appear to be just the opposite (and thus an even more unintuitive system, IMO). The MCAS system (AOA trim nanny) activates when the AP is disengaged. They were already flying manually, and it
may have helped their situation if they had actually re-engaged the AP. That said, it
should have been easily override-able with manual trim inputs. They could have flipped the stab-trim switches down (cut-out), grabbed hold of the console trim wheel, or even temporary relief from the electric trim switches on the yoke.
But as Jeff and Matt mentioned above, negligence WRT to maintenance was certainly a contributing factor. Seems like a text-book example of the "error chain theory" at work here.
And from what Jeff describes, I am surprised it hadn't bit them sooner.