Airbus control logic and alternate mode control
One problem that I see here with the Airbus approach to degraded mode operation is that the aircraft can be put into a flight mode that is not natural. In normal law the aircraft flies to a point in space where the pilot tells it to. In Direct Law the airplane is the same as a conventional aircraft, but in Alternate law the aircraft doesn’t have enough information to control the aircraft in a stable manner and it relies on the pilot for pitch stability. That is, when flying in normal mode the aircraft is controlled to point in the sky, and the autopilot system controls flight path by adjusting power and trim to match the control inputs of the pilot and the speed selected.
That’s all fine and good until something goes wrong with the inputs. When the control system goes into Alternate Law things get much more difficult. What I understand happens in that instance is that the pilot becomes the one who is controlling the aircraft in pitch and, the system is not stable in pitch. With a conventional trim system you set the trim angle and if you release stick pressure the aircraft will return to the speed required to match that pitch trim setting. This is the definition of a fundamentally speed stable system, remove the inputs and the aircraft goes back to a trimmed airspeed. With such a system, if the pilot made an incorrect input or responded to turbulence, all you have to do is relax stick force and the aircraft returns to a stable condition. With the Airbus the pilot must maintain pitch control with reference to the HSI and the trim will move around. Some of the Airbus pilots here say that isn’t excessively difficult, but it is surely a much higher workload to constantly keep the required pitch in the HSI while you are trying to fly through turbulence.
The Airbus system, in Alternate Law, will adjust the THS to “chase” the pilot inputs by changing the trim position. This is fundamentally unstable and consequently, unless you make absolutely no forward and aft inputs, the aircraft will climb or descend and the speed will increase and decrease. If the inputs are large enough and long enough, the trim will change and the aircraft will remain in a climbing or descending mode until the pilot applies input. It seems to me that Alternate Law without airspeed is really kind of a bastard system in that the control really doesn’t have enough information to fly the aircraft, so the pilot has to step in and provide stability.
In my opinion, the proper control system response to loss of airspeed information would be for the autotrim to be disabled at the same time as the autothrottle and set to the same condition as it was just before the autopilot dropped out. So I guess that in the case of loss of speed sensors I am thinking the system would be much better off in direct law.
Then the aircraft will be remain at a known pitch condition and therefore be speed stable. You don’t know what the speed is, but it was fine where it was and it isn’t going to change. The pilot then only has to control altitude with the throttle and keep the wings level and he is in a much safer place. If the pilot decides he wants more or less speed he can adjust the trim wheel to get it.
Somebody please correct me if I am wrong.