I A P 2 0 2 3 . 0 0 1 - A F M / I R I S T E X A N I I P A G E | 26
FOR SIMULATION USE ONLY – NOT A TRAINING AID
are aligned nearly straight into the wind. When in low pitch, the propeller blade
angle is approximately 15° from the reference plane.
The propeller blades will be at low pitch at low speeds and low throttle settings.
High pitch is variable between feather and low pitch to maintain NP at a constant
2000 RPM for the given condition.
Normally, propeller governing is automatically set by the PMU and PIU. The PMU
controls the propeller RPM by varying the propeller blade angle with oil pressure
through the PIU.
The PIU increases the pressure of the oil that is transferred from the PIU, through a
tube and stationary transfer sleeve into the hollow rotating propeller shaft.
Pressurized engine oil forces the piston forward to decrease pitch toward fine.
When oil pressure against the piston is reduced, the blades turn toward coarse
pitch. With the PMU functioning, the mechanical overspeed governor modulates oil
pressure to the propeller pitch change piston to limit NP below 106%, while the
electronic governor will maintain NP at 100%.
This arrangement keeps the mechanical governor in an underspeed condition with
the PMU functioning.
If PMU function is lost or deactivated, the mechanical flyweight overspeed governor
modulates oil pressure to the propeller pitch change piston to maintain NP at or
below 100±2%.
Manual governing is accomplished by centrifugal force moving the counterweights
outward causing oil pressure to dump. With the decreased oil pressure, the
feathering spring will drive the blade toward course pitch to keep NP within limits.
NP may peak above 100% during power changes and then return to the governed
range.
In the event of an engine failure with the power control lever (PCL) out of the cut-
off position, the propeller will slowly begin to move toward feather due to the loss
of oil pressure, but may not fully feather.
There are two methods to feather the propeller. If the engine is shut down with the
PCL and the PMU is in NORM, the PMU sends a signal to the prop servo valve to
drain propeller oil pressure.
Placing the PCL to cut-off also activates micro switches that power the feather
dump solenoid valve which also dumps oil pressure from the propeller. The feather
dump solenoid valve receives power through the PROP SYS circuit breaker located
on the battery bus circuit breaker panel in the front cockpit.
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