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cochise
11th Nov 2006, 14:02
Hey all,
I have always understood that operating at maneuvering speed will allow the aircraft to stall before the airframe is overstressed but is there another side to the coin?
I seem to be butting heads :ugh: with some of the other pilots on the line when it comes to the issue of maneuvering speed in the Dash-8. I am trying to find out what protection the specific maneuvering speed for the weight and configuration of the aircraft gives whilst in a turn with the autopilot engaged? And what are the problems associated with executing a turn below maneuvering speed with the autopilot engaged. Any factual information would be much appreciated.
Thanks

Genghis the Engineer
12th Nov 2006, 11:13
I don't know the Dash-8 at-all, but can do basic certification practice and theory.

Va is dependent upon weight, but bank angle is irrelevant. So, if the AFCS is banking you, Va remains the same.


Another and less well understood consideration is the load upon flying controls. The controls are stressed for the load cases of full deflection at Va and 1/3 deflection at Vd (which is somewhere around 11% above Vne). So, in theory any deflection more than 1/3rd aileron (which is the obvious player) above Va could constitute an overstress. However, one sincerely hopes that the AFCS was designed to prevent this.

However, there is a case that only came to light with the Airbus accident at Queens NY in 2001 which most AFCS are still not, so far as I'm aware, programmed for. This is that the "full deflection up to Va" requirement assumes steady state conditions. So, if taking the ailerons to (say) 70% one-way at just below Va, then once a maximum bank roll rate is achieved reversing them rapidly to 70% the other way - this would almost certainly be an overstress case.

In practice of-course, I don't think that any civil aircraft AFCS should use large control inputs or large rolls rates and bank angles, so it should be a non-issue - but that is the area where theoretically you have degraded protection.


As for problems with executing a turn below Va with AFCS engaged - there shouldn't be any. You've still got stall protection (in both senses of the phrase) and so long as nobody and nothing tries rapid pitch or yaw input reversals, you aren't going outside the standard load cases.


Just emphasising again of-course, this is all generic, and I know nothing whatsoever about Dash-8 AFCS modes.

G