Originally Posted by
Uplinker
OK, thanks skkm. So like the Airbus, the Boeing FMA speed box will always display the Autothrust status, and only if it is inactive and unarmed, will the speed box ever be blank?
You will always have a mode displayed if the A/T is active.
If the A/T is disconnected, the FMA will always be blank. However, the A/T can (and usually is) still be armed in this case (by having the arm switches on the MCP on), and as such is able to perform its stall protection "wake up" function depending on vertical mode. There is no indication other than the position of the arm switches of this status.
The only time in normal operations that you'd see the left FMA blank is a manual thrust landing.
I realised that, my point was that in the example the speed box was blank while the speed
was being controlled, (by the elevators), leading to a potential negative as PEI3721 picked up.
I think this is a difference in design philosophy, then? The left FMA box is not a
speed mode indication; it's an
autothrottle mode indication. As such, a blank left FMA ought be a warning sign that while the speed may be being managed by a vertical mode, there's no support from the A/T. As I mentioned above, it is not normal or expected to see a blank left FMA except in specific circumstances, so the inferred expectation bias shouldn't exist.
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There is the opposite issue, though, which is the crux of the Asiana accident: while you have an indication in the left box (HOLD) whilst in a speed-on-elevator vertical FD mode, the autothrottle
will not wake up to provide speed protection should you get slow in manual flight.