Originally Posted by
tdracer
The PW2000 and F117 engines are controlled to EPR at power, not N1 (assuming normal operation - there are failure modes where control reverts to N1).
N2 speed control is used at/near idle, as is minimum burner pressure (PS3) (i.e. at idle, the control will control to N2 or PS3, doing a 'select high').
Turning on bleed doesn't change the control loop response, but if the N2 rotor is flirting with separation or stall (not an uncommon problem with this engine type, mainly the earlier builds), the rotor speed changes can simply be more rapid than the control can respond to - there are significant lags in the time between the control senses N2, the fuel flow changes, and the resultant torque change results in an N2 speed change. We're talking fractions of a second here, but that's all it takes to set up an oscillatory rotor speed response. When controlling to EPR, the time lags get worse, because of the inherent lags in sensing EPR compared to rotor speed, making an oscillatory response even more likely.
I was talking about what a controller might do at high power in response to AI being selected. No pilot action was involved - sorry if I gave that impression.
OK - so we are taking about an engine rated on EPR rather than N1, but the principle is similar. The OP mentions AI is claimed to be advantageous in mitigating overspeed. We are asked why. One way that this could be true is if AI selection dials in a rating debit (an EPR debit in this case) which would reduce rated power and hence increase speed margin (margin which may be partially consumed during the transient response onto the EPRrating from idle).
It was an idea - not necessarily the right answer .... Are rating debits applied for AI for this engine at takeoff?