Originally Posted by
SPUDO
It is this governing action that is at the heart of my question. According to the Flightsafety Pilot Training Manual, the power lever angle (PLA) determines the N2 speed (consistent with every other twin spool engine I have experience with).
So, without touching the power levers, switching on the engine anti-ice should result in the N2 remaining exactly the same, should it not? The PLA does not change, so the N2 should not change.
But it does - and the N1 stays the same. How on earth?
N2 governing is quite unusual, IMHO. N1 is much more typical. That's because in moderate and high bypass ratio engines (the PW530 is ~3.7 I believe), the majority (~75%) of the thrust comes from the fan. Hence, keeping N1 constant keeps thrust constant much better than keeping N2 constant. The behaviour you observed is certianly entirely consistent with keeping N1 constant, not N2. Hence, I question if the Manual may have been misinterpreted, though it should be clear?
The use of a hydro-mechanical fuel control system vs a FADEC should not be an issue because either can be used to control the engine in any chosen manner, a FADEC can just implement more advanced schedules easier, such as for acceleration, so that performance can be maximised without unnecessary conservatism to avoid engine stall, as an example.
Is there anything in the public domain that describes the control system? I can't find anything. I checked the EASA type certificate, but it just stated it "The PW530A and PW535A models are controlled by a hydromechanical system the PW535B, PW535E are controlled by a dual channel FADEC".
Cheers...
PS. How do you know the engine was not operating temperature limited? Was the EGT observed to be below the limit? If the enigne was on the limiter, the limit schedule would override the RPM governing (which ever spool it is).