5 ms·
Layman question here: Are there real scenarios where you want to have both control surfaces opposing each other so heavily? Are there real cases for having the
by fgonzag 8y ago
Layman question here:
Are there real scenarios where you want to have both control surfaces opposing each other so heavily? Are there real cases for having the trim trying to pitch the plane down while the pilot is trying to pitch up with the stabilizer?
Or should we limit the trim so it never actually opposes the pilots intentions? (Maybe set it to a neutral position and disengage auto function if this condition is detected?)
- msbarnett 8y agoI'm not really an expert but my understanding is that the stabilizer will always out-muscle the elevator due to the physics of their respective positions on the aircraft. In yet another change in the MAX series, there used to be what was called an aft column cutout switch which did more or less what it sounds like: engage when the stick was pulled back, cutting out autopilot commands to the stabilizer, so that it never attempted to outmuscle the pilot's input commands to the elevator. This was removed in the MAX. Possibly this was to meet certification requirements: if the MCAS was needed to push down on the nose to meet the requirement that "During the approach to the stall, the longitudinal control pull force should increase continuously as speed is reduced from the trimmed speed to the onset of stall warning." If the MAX tends to pitch nose up at a certain point, the MCAS is intended to create a counter force against the pilots commands as it approaches that, rather than have the pilot pulling back at some point suddenly buck the nose upwards, then the MCAS needs to be active when the pilot is pulling back on the stick. But it does argue that a LOT more thought needed to go into MCAS failure modes if it was necessary for it not to be cut out when the pilot is operating elevators.
- Gibbon1 8y agoI think the issue is the difference between the force needed to trim the aircraft and the acceptable control gain between the yoke and the elevator. I think with light aircraft trim is what you think of it. Slightly centers the yoke so the aircraft will fly level without control input. I think what's going on with the 737-MAX is the needed trim is way beyond what's needed to fly the plane. I'm now wondering is the MACS is needed because the tendency to pitch up also exceeds the pilots control inputs which is why it had to be on the elevator and have that much authority. Meaning once the aircraft pitches up the pilot doesn't have enough authority to push the nose back down.
- linuxftw 8y ago> Meaning once the aircraft pitches up the pilot doesn't have enough authority to push the nose back down. Or, alternatively, the pitch up happens so quickly and severely the risk of stall is too great above a certain AoA and the pilot won't have adequate time to respond. If either is the case, it implies the aircraft is not safe to fly without MCAS operational, and possibly not at all.
- rootusrootus 8y agoMCAS is not an emergency system, it is a trim control system designed to make the plane behave predictably in an otherwise unstable flight regime. If you wanted it to move fast, you would use a stick pusher; moving the trim takes many seconds.