|Quoting XaraB (Thread starter):|
Is the aircondition fan speed somehow directly connected to the engine revolution speed? Especially during approach, when thrust levels vary gratly, aircondition fan speed seems to follow these variations closely rather than being at a constant level. What is the rationale for this system design anyway?
Like most jets, the Fokker 70/100 uses bleed air from the engines to power the air cycle machine. As the engine changes speed, the pressure at the bleed air tap also changes. In most designs, a regulator prevents changes in pressure at the bleed tap from going into the ECS system. However, when you're close to idle as you are in descent, even the highest bleed pressure may not be at the regulator set point so the regulator is wide open. When that happens, changes in engine speed translate directly to changes in bleed pressure and air cycle machine speed.
What you're hearing probably isn't actually a fan, it's changes in the airflow and air cycle machine speed. It's also possible that the recirculation fans are tied into the pack flow to hold a constant re-circulation percentage, in which case the re-circulation fan speed would also modulate with engine bleed pressure when the bleed pressure is low.