4 ms·
> Eventually, the trim wheel made an abrupt twitch forward at the high rate, but only for a fraction of a turn. This, our instructor told us, was the MCAS syste
by gregmac 6y ago
> Eventually, the trim wheel made an abrupt twitch forward at the high rate, but only for a fraction of a turn. This, our instructor told us, was the MCAS system becoming active or “waking up”. What was happening behind the scene was the MCAS logic took a “snapshot” of the existing trim position when its threshold AOA was reached. It then calculated a maximum amount of trim that could be added. Should the trim ever meet this computed limit, the speed trim and MCAS system becomes inhibited for the remainder of the flight.
This seems like one of those overly complex behaviors that's indistinguishable from "randomly decides to just stop working" when someone doesn't fully understand (or remember) all the details. Is this common in aircraft, especially for critical flight systems?
Does a normal fly-by-wire system do things like this?
- rhodozelia 6y agothe key is that it has to hit the limit for the maximum amount of trim that should ever reasonably need to be added before it is disabled for the remainder of the flight. If it hit the limit it is an indication to the control system that something isn't operating correctly and it should stop automatically operating the outputs (trim) based on the inputs (AOA) as it isn't having the anticipated effect and it should punt the problem to the human pilots. Presumably there is some annunciation for both the function activating and also being disabled.
- Toutouxc 6y agoHey, can I ask you to reword the quotation? English isn't my first language and I have trouble understanding it the way it's written. (I know all the aviation lingo and stuff, it just doesn't make sense.)