17.3.3 Overspeed Protection Flashcards
What is the primary propeller governor also referred to as?
The primary propeller governor is also referred to as the CSU (Constant Speed Unit).
What does the primary propeller governor control?
It controls any propeller overspeed up to approximately 102 to 103% Np (prop speed).
What happens when propeller speed exceeds approximately 104 to 106% Np?
The propeller overspeed governor operates and returns oil from the propeller dome back into the reduction gearbox.
What does the oil overspeed governor house?
It houses a set of flyweights connected to a control valve driven by a bevelled gear mounted on the propeller shaft.
What do the flyweights in the governor act against?
The flyweights act against two springs: a speeder spring and a reset spring.
What occurs when the propeller speed reaches a specified limit?
The governor flyweights lift the control valve and bleed off propeller servo oil into the reduction gearbox sump, causing the blade angle to increase.
What effect does an increase in blade pitch have on the engine?
An increase in blade pitch puts more load on the engine and slows down the propeller.
How is the overspeed governor tested?
The speed reset solenoid is activated, and servo oil pressure pushes against the reset piston to cancel the effect of the reset spring.
What is the purpose of the second solenoid valve in twin installations?
It is used in conjunction with the aircraft auto feather system to dump propeller servo oil into the reduction gearbox sump in the event of an engine malfunction.
What happens when the auto feather system is armed?
In the event of an engine malfunction, it energises the solenoid valve to dump propeller servo oil into the reduction gearbox sump.
What is the additional overspeed protection device in some aircraft powered by a PT6A engine?
It is located inside the main propeller governor and operates at 106 to 107% Np and above.
What occurs when the flyweights inside the main propeller governor open a bleed air link?
Pressurised air, referred to as ‘Py’ pressure, leaks into ambient air, causing fuel flow to be reduced to the engine.
What is the role of the evacuated bellows inside the Fuel Control Unit (FCU)?
They act as the ‘brain’ of the FCU, balancing air pressure components Px and Py.
What happens if the air pressure balance is upset in the evacuated bellows?
The FCU reacts by reducing fuel flow to the engine, thus slowing down the gas generator/compressor and reducing propeller speed.
