📖 The lesson
Feathering
When an engine fails on a multi-engine aircraft, a windmilling propeller creates large drag and asymmetric yaw, and may keep turning a damaged engine. Feathering turns the blades to about 90°, edge-on to the airflow, so the propeller stops and drag is minimised.
- Light twins (Hartzell/McCauley feathering types): governor oil pressure moves the piston to decrease pitch. Counterweights, a feathering spring and often a compressed air or nitrogen charge in the hub dome drive the blades to high pitch and feather.
- To feather, the pilot pulls the propeller lever fully aft through a detent to FEATHER. The governor dumps the oil from the hub to the engine, and the blades feather. Loss of oil pressure in flight also feathers these propellers.
- Feathering takes several seconds; the propeller must be turning at some speed for the oil to drain quickly — feather before the rpm falls too low.
- Start locks (latches or pins that work by centrifugal force) hold the blades at low pitch when the engine stops on the ground. Otherwise, the hub would feather as oil pressure decays, and starting against feathered blades would overload the engine. The pins latch below about 700–800 rpm and release by centrifugal force once the engine is running.
Unfeathering
- Light twins: an accumulator stores governor oil under pressure (with a nitrogen pre-charge). When the propeller lever is moved forward out of feather, the accumulator releases oil to the hub, moving the blades towards low pitch so the airflow windmills the propeller and the engine can restart.
- Without an accumulator, the engine starter is used to turn the engine until governor oil pressure moves the blades out of feather.
Hamilton Standard Hydromatic propellers
- A double-acting system for large piston and older turboprop aircraft. A piston in the dome moves fore and aft; a cam with rollers converts this into blade rotation through bevel gears on the blades.
- Governor oil acts on one side of the piston and engine oil on the other, so oil drives the blades in both directions. CTM assists in moving to low pitch.
- Feathering: the pilot presses the feathering button. A holding coil keeps it in, and an electric feathering pump supplies high-pressure oil (from a reserve in the oil tank) through the governor's distributor valve to the high-pitch side. When the blades reach the feather stop, the pressure rises and the pressure cut-out switch opens the circuit, releasing the button and stopping the pump.
- Unfeathering: the pilot holds the button in. The pump pressure rises higher still, shifting the distributor valve so oil is directed to the low-pitch side. The blades move out of feather and the propeller starts to windmill; the button is released at a set rpm.
Autofeather
- Used on many twin turboprops for take-off and landing. When armed (switch on, power levers above a set position), a loss of engine torque below a threshold for a set time automatically feathers that propeller.
- Once one propeller has autofeathered, the system on the other engine is disarmed, so a second engine cannot be feathered automatically. The system is tested on the ground before flight.
Reverse pitch and the turboprop beta range
- In flight (alpha range, from flight idle up to maximum power), the propeller governor holds the selected rpm and the power lever controls fuel flow/torque.
- On the ground (beta range, below flight idle), the power lever controls blade angle directly through a beta valve and mechanical feedback (beta rod, slip ring and carbon block on PT6-type installations). This gives fine control of thrust for taxi, down to reverse for braking on landing.
- PT6A / Hartzell example: a single-acting system. Governor oil pressure moves the piston to low pitch and reverse; counterweights and feathering springs move the blades to high pitch and feather. The primary governor (CSU) also contains the beta valve. The free-turbine engine allows the propeller to be feathered on shutdown.
- Reversing requires the propeller to pass through low blade angles, so it is locked out in flight by flight-idle stops, squat-switch solenoids (beta lockout) and gate detents on the power lever.
Electrical and electronic control
- Electrically controlled propellers (e.g. older Curtiss-Electric, some light aircraft) use an electric motor and reduction gearing in the hub to turn the blades, powered through slip rings and brushes. Rpm is controlled automatically by a governor or manually by a switch.
- Modern turboprops use an electronic propeller control — a propeller electronic control (PEC), engine electronic control (EEC) or FADEC. It senses propeller speed and blade angle and drives a pitch-control unit (PCU) with an electro-hydraulic servo valve. It provides constant speed, synchrophasing, beta/reverse scheduling, autofeather and overspeed protection, with a mechanical/hydraulic backup.
Overspeed protection
- Overspeed governor — a second, independent flyweight governor set a few percent above maximum rpm (≈ 106 %). If the primary governor fails, it releases oil from the hub to coarsen pitch. It is tested on the ground using a test solenoid that resets it to a lower rpm.
- Fuel topping governor (Np governor in the fuel control) — reduces fuel flow if propeller speed exceeds its limit, especially in beta and reverse, when the primary governor is not controlling.
- Pitch lock — a mechanical ratchet or lock in the hub that holds the blade angle if oil pressure is lost, preventing the blades from moving to fine pitch and overspeeding.
- Flight-idle (secondary low pitch) stop — prevents the blades going below flight fine pitch in flight, which could cause a severe overspeed and drag. A low pitch warning may alert the crew.
- Electronic controls add software speed limits and self-tests.