777 PIO?
Join Date: Feb 2023
Location: Aachen
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As an example of a typical control architecture (the feel block is misssing):
Source: Hydraulically operated flight control system
You are talking about this incident: https://assets.publishing.service.go...XWBC_09-22.pdf ?
Ok, then let's first define what I was talking about: I was talking about direct law => direct stick to surface relationship. Well it isn't really direct (proportional), most of the times a LUT is between the stick and the surface position demand, plus rate limiting.
When we're comparing fly by wire with conventional controls in terms of deflection speed and latency of control surfaces (as a result of stick input), frankly we can only compare direct laws/modes... (As conventional/ mechanical is direct, which seems also beeing true for the 777 "normal law" in roll)
When flight control loops are present in the path behavior of the surfaces can't be directly derived from stick input. You were talking about the surface action on the ground i.e. direct law...
Now let's have a look on the incident (the one you were most likely talking about):
In red I marked the zone where the aircraft was most likely in direct law (ground mode). It's a guess, nowhere is mentioned if, or when the aircraft was in ground mode. I added black markings as an aid to compare both graphs better with each other with respect to time.
I don't see any of the behavior you mentioned, it's perfectly direct. No surface running after the stick and trying to reach the position even though the stick is travelling in the opposite direction. Everything before and after the red rectangle is with flight control loops beeing engaged...
(It's also pretty likely that the aircraft is already fading into normal law at the end of the rectangle as RA height is increasing, it rotated)
Ok, then let's first define what I was talking about: I was talking about direct law => direct stick to surface relationship. Well it isn't really direct (proportional), most of the times a LUT is between the stick and the surface position demand, plus rate limiting.
When we're comparing fly by wire with conventional controls in terms of deflection speed and latency of control surfaces (as a result of stick input), frankly we can only compare direct laws/modes... (As conventional/ mechanical is direct, which seems also beeing true for the 777 "normal law" in roll)
When flight control loops are present in the path behavior of the surfaces can't be directly derived from stick input. You were talking about the surface action on the ground i.e. direct law...
Now let's have a look on the incident (the one you were most likely talking about):
In red I marked the zone where the aircraft was most likely in direct law (ground mode). It's a guess, nowhere is mentioned if, or when the aircraft was in ground mode. I added black markings as an aid to compare both graphs better with each other with respect to time.
I don't see any of the behavior you mentioned, it's perfectly direct. No surface running after the stick and trying to reach the position even though the stick is travelling in the opposite direction. Everything before and after the red rectangle is with flight control loops beeing engaged...
(It's also pretty likely that the aircraft is already fading into normal law at the end of the rectangle as RA height is increasing, it rotated)
Look more closely. I superimposed the curves on one another.
Initially, the curves are easy to match. But during the tailstrike, exactly the part you squared in red, there is a clear lag.
Keep in mind the timescale. One square is two seconds. You can see a lag between a half and two seconds.
The worst example is when the stick goes from full nose up to full nose down. It does so in around three seconds.
However, the elevators remain almost at full nose up for almost two seconds. Two seconds when your tail is about to hit the planet is a ****long time. There is no technical term to my knowledge to describe this kind of lag.
Then, you can see triangles on the curve. Triangles mean the elevators, being rate limited, are running after their target.
Okay, if after all these explanations you still don't see what I mean, just look at the first vertical red line that you drew. It's a perfect point. The two lines have a close position, a bit off, but both are vaguely nose up. Stick is around one third nose up, elevators are around two thirds nose up. The stick goes nose down. Are the elevators going nose down at the same time ?
Are the elevators going nose down at the same time ?
It is overcontrolling laterally. Common among lesser experienced pilots, no disrespect intended. Fair enough to come up with technical explanation but I have witnessed this frequently as a 737 instructor on the aircraft and simulator. Often I would tell the trainee to relax and reduce control input and lo and behold due inertia and good ol’ Newton the aircraft continues in a straight line. There is no need to fly the aircraft like Lady Penelope’s driver, Parker in Thunderbirds. You may need to be slightly, ahem, older to get that reference. I would humbly suggest more frequent manual flying when appropriate on the line rather than depending on automation too much.
