Originally Posted by Capt Cashman
If we get a wake-up or a mode transition of the autothrottle for a low-speed condition, say, we had to transition the pilots authorizing that command mode change. They would have had two systems controlling speed. We had to rectify that situation. In that case, we would end up with two mode changes and a thrust increase, and if this low-speed condition came about because of an engine problem, we might also be pushing thrust up into the asymmetric thrust conditions. We would have potentially two mode changes uncommitted by the pilot, a thrust increase, possibly asymmetric thrust increase, and we would be violating our philosophy of the pilot having authority. We thought the less confusing of the situations was the design we chose.
Hang on. For the speed to decrease,
whatever is controlling the speed (with Pitch) has stuffed it up (in this case, pilots pulling the nose up). So a min speed protection feature would work to stop the speed slowing further. If the nose goes higher, the thrust comes up further to prevent a further slowdown. If the nose gets lowered, the ATS reduces thrust to maintain Vmin. While technically it is a case of "two systems controlling speed", practically it's not, certainly after the "WTF is it doing?!" screech by the pilots and they either hit TOGA or re-engage normal (VS, FPA) modes. As for the thrust being put up automatically up during assy, that would be just as good an idea as if both donks were running and the speed got below Vmin, I would have thought...