23. MODULE 17A (B1) — PROPELLER

Magnetic particle inspection (MPI) is the standard method for detecting surface and near-surface cracks in ferromagnetic (steel) components such as propeller hubs.

Large aircraft de-icing systems may use either AC or DC power depending on the aircraft design; both supply types are encountered in service.

Centrifugal latches are spring-loaded and engage when the propeller is stationary; they lock the blades to prevent inadvertent pitch change during ground handling and maintenance.

These propellers use governor oil pressure directed into the hub cylinder to push the piston and rotate the blades to a higher (coarser) pitch angle.

CTM arises from the centrifugal force acting on blade mass elements that are offset from the pitch-change axis, causing a twist about that axis toward the flat (fine) pitch position.

Any multi-engine propeller aircraft can benefit from synchrophasing to reduce the beat-frequency cabin noise created by slight RPM differences between propellers.

By sensing engine torque, the system can detect engine failure (near-zero torque) and automatically adjust pitch to feather or reduce drag, minimising yaw forces after failure.

Blades are placed at full feather for off-wing storage to allow even access to internal pitch change components and to ensure oil drains appropriately.

An ammeter (or current indicator) in the flight deck shows the current draw of each de-icing heater circuit, confirming that elements are energised and functioning correctly.

If the engine had truly lost torque, auto-feathering or RPM changes would be apparent; the engine continuing to run and perform normally indicates only the gauge has failed.

The PCU control lever varies the governor set point for the free turbine shaft, allowing propeller RPM to be changed independently of gas-generator speed.

The foam core fills the internal volume, damping vibrational energy within the blade structure and preventing resonant oscillations that could cause fatigue damage.

A compound twin-spool engine is a turboprop that features a two-spool compressor arrangement, allowing each compressor stage to operate at its optimum speed.

A single-acting governor uses oil pressure to fine the blades against a return spring; if the pump fails and oil pressure drops, the spring returns the blades to coarse pitch.

If the propeller retaining nut is insufficiently torqued, the cones can micro-move under load, causing fretting and galling damage to the seating surfaces.

Higher airspeed increases the apparent pitch of each blade, tending to load the engine and reduce RPM; the CSU responds by increasing blade angle (coarsening) to maintain the selected RPM.

Engine torque reaction to a clockwise-turning propeller creates an equal and opposite anticlockwise (left) rolling moment on the airframe.

The ammeter (current gauge) for the de-icing circuit shows current draw; a steady reading confirms the system is operating in continuous mode with elements energised.

The propeller brake interlock is wired to the HP (high pressure) fuel cock to ensure the fuel supply is cut before the brake can be applied, preventing engine damage.

Auto-feathering is disarmed in the cruise because the risk of spurious feathering outweighs the benefit; it is typically armed only for take-off and initial climb.

The hydraulic safety stop blocks fine pitch movement; only movement toward coarse pitch remains possible, preventing the propeller from going to an uncontrolled fine pitch.

Synchrophasing allows the pilot to adjust the angular phase relationship between the rotating propellers so that blade pressure pulses interact in the most acoustically favourable way.

Propeller synchronisation systems typically operate within a ±50 RPM band (100 RPM total range) of the master engine speed.

An epicyclic (planetary) reduction gearbox is used in turboprop engines because it provides a high reduction ratio in a compact, lightweight package.

A wooden propeller must be wrapped loosely to allow the wood to breathe and equalise with ambient humidity, preventing moisture trapping, warping, or splitting.

In the beta range, the power lever directly controls blade angle; advancing the lever (increasing fuel) simultaneously commands a higher (coarser) blade angle to absorb the additional power.

Coarser pitch increases blade load, slowing the propeller (RPM decreases); the engine now has to work harder against the increased pitch, raising manifold pressure.

Auto-feathering is armed only for critical phases such as take-off; it is intentionally disarmed in cruise to prevent inadvertent feathering from transient torque fluctuations.

The dressed area must be polished with very fine sandpaper to restore a smooth aerodynamic surface and remove any stress-raising scratches left by the repair.

The blade root gear segments engage with the rotating (moving) cam; as the cam rotates, it drives the blade segments to change pitch angle.

A more highly cambered blade section generates greater lift for the same pitch, increasing the aerodynamic load on the engine and therefore absorbing more power.

Each indentation is first assessed individually against the single-defect limit, then all indentations are collectively assessed to ensure the combined damage area is also within limits.