Propellers, Feathering, and Systems

A controllable-pitch propeller whose pitch is automatically varied in flight by a governor, maintaining constant rpm despite varying air loads (PHAK ch. 7).

• Two controls: the throttle controls power output; the propeller control regulates engine rpm
• Once an rpm is selected, the governor adjusts blade angle to maintain it — an increase in airspeed or decrease in propeller load increases blade angle; a reduction in airspeed or increase in load decreases it
• The constant-speed range is defined by the high and low pitch stops; as long as the blades are within that range and not against a stop, constant rpm is maintained (PHAK ch. 7)

The direction the oil pressure works is reversed.

Most single-engine airplanes: non-feathering, oil-pressure-to-increase-pitch — increased governor oil pressure drives the blade toward high pitch, low rpm.

Most multiengine airplanes: full feathering, counterweighted, oil-pressure-to-decrease-pitch — increased governor oil pressure drives the blade toward low pitch, high rpm, away from feather.

In effect, the only thing keeping these propellers from feathering is a constant supply of high-pressure engine oil. That is deliberate: it enables feathering in the event of a loss of oil pressure or a governor failure (AFH ch. 13).

• Aerodynamic forces on a windmilling propeller tend to drive the blades to low pitch, high rpm
• Counterweights on the blade shanks tend to force the blades to high pitch, low rpm. Inertia acting through the counterweights is generally slightly greater than the aerodynamic forces
• High-pressure governor oil pushes the blades toward low pitch, high rpm

So a reduction in oil pressure allows the counterweights to drive the blades to higher pitch and decreases rpm (AFH ch. 13).

The propeller control is brought fully aft. All oil pressure is dumped from the governor and the counterweights drive the blades toward feather.

As centrifugal force on the counterweights decays with falling rpm, additional force is needed to complete the travel — that comes from either a spring or high-pressure air stored in the propeller dome. The entire process may take up to 10 seconds (AFH ch. 13).

Because of anti-feathering lock pins. Below approximately 800 rpm, a reduction in centrifugal force allows small pins in the pitch-changing mechanism of the hub to move into place and block feathering.

Therefore, if a propeller is to be feathered, it needs to be done before engine rpm decays below approximately 800 (AFH ch. 13). One popular turboprop engine does feather with each shutdown — it has no such centrifugally-operated pins, due to a unique engine design.

Feathering only alters blade angle and stops engine rotation — it stops rotation with the blade streamlined to the relative wind, minimizing propeller drag.

To completely secure the engine you additionally turn off the fuel (mixture, electric boost pump, and fuel selector), the ignition, the alternator/generator, and close the cowl flaps. On a pressurized airplane there may also be an air bleed to close for the failed engine. Some airplanes have firewall shutoff valves that secure several of these with a single switch (AFH ch. 13).

No — completely securing may not be necessary or even desirable, depending on failure mode, altitude, and time available.

The position of the fuel controls, ignition, and alternator/generator switches of the failed engine has no effect on aircraft performance, and the pilot might manipulate the incorrect switch under conditions of haste or pressure (AFH ch. 13). The one exception on that list: closing the cowl flap of the failed engine does affect climb performance.

The engine must be rotated so oil pressure can be generated to move the blades out of feather (AFH ch. 13):

1. Ignition on prior to engine rotation, throttle at low idle, mixture rich
2. Propeller control to a high rpm position
3. Engage the starter — the engine windmills, starts, and runs as oil pressure moves the blades out of feather
4. As the engine starts, immediately reduce propeller rpm until it has had several minutes to warm up; monitor cylinder head and oil temperatures

Always follow the AFM/POH for the exact procedure.

A device that permits starting a feathered engine in flight without the use of the electric starter. It stores a small reserve of engine oil under pressure from compressed air or nitrogen (AFH ch. 13).

Moving the propeller control out of the feather position releases the accumulator pressure; oil flows to the propeller hub and drives the blades toward high rpm, low pitch, whereupon the propeller usually begins to windmill. With fuel and ignition present, the engine starts and runs. High oil pressure from the governor recharges the accumulator just moments after rotation begins, making it available for another cycle. If the accumulator fails to bring the propeller out of feather, the electric starter may be engaged.

A prop sync eliminates the "drumming" or "beat" of propellers whose rpm are close but not precisely the same. The pilot coarsely matches rpm and engages the system, which adjusts the slave engine to precisely match the master engine (AFH ch. 13).

Prop sync should always be off for takeoff, landing, and single-engine operation. Disengage it when selecting a new rpm and re-engage after the new rpm is set.

A synchrophaser goes further — it also compares and adjusts the positions of the individual blades in their arcs, reducing noise and vibration. It is commonly called prop sync, though technically that's imprecise.

Crossfeed lets an engine draw fuel from a tank in the opposite wing. On most multiengine airplanes, crossfeed is an emergency procedure used to extend range and endurance in OEI flight. A few models permit it as a normal fuel-balancing technique, but these are not common (AFH ch. 13).

• Never use crossfeed during takeoff, or for normal landing operations with both engines operating
• Crossfeed is ordinarily not used for completing a flight with one engine inoperative when an alternate airport is nearby
• Prior to landing, terminate crossfeed and return the operating engine to its main tank
• A landing on one engine using crossfeed may be necessary if setting normal fuel flow would cause the operative engine to fail

A quick repositioning of the fuel selectors on the ground does nothing more than ensure freedom of motion of the handle (AFH ch. 13).

A real functional check: during run-up, operate each engine individually from its crossfeed position at moderate power (1,500 rpm minimum) for at least 1 minute to confirm fuel flow can be established from the crossfeed source. Then run each engine at least 1 minute at moderate power from the main (takeoff) tanks to reconfirm fuel flow before takeoff.

This check is not required before every flight, but crossfeed lines are ideal places for water and debris to accumulate unless used from time to time and drained via their external drains during preflight.

Each engine has an alternator or generator. Paralleling circuitry matches the output of each so the electrical load is shared equally.

If one fails, the inoperative unit can be isolated and the entire electrical system powered from the remaining one. Depending on the capacity of the remaining unit, the pilot may need to reduce electrical load — load shedding (AFH ch. 13). The AFM/POH has the system description and limitations.

A small furnace that burns gasoline to produce heated air for cabin comfort and windshield defogging. Most are thermostatically operated with a separate hour meter for maintenance tracking (AFH ch. 13).

Over-temperature protection is a thermal switch that cannot be accessed in flight — resetting it requires a visual inspection of the unit for heat damage. Manufacturers usually specify a cool-down: outside air circulating through the unit for at least 15 seconds in flight, or the ventilation fan for at least 2 minutes on the ground. Failure to cool down usually trips the thermal switch and renders the heater inoperative until reset.

Because it spans the entire range from negligible to airframe-sized (AFH ch. 13).

Feathered: parasite drag from the propeller is at a minimum; in a typical multiengine airplane, a single feathered propeller is a small part of the airplane's total drag.

Near flat pitch, windmilling at high rpm: parasite drag can be as great as the parasite drag of the entire airframe, and enough to make the airplane difficult or impossible to control.

That range is why feathering, done in a timely manner, often permits continued flight to a suitable airport following an engine failure.

Because a windmilling propeller looks like a working one. The AFH names this directly: a windmilling propeller has in many cases given the improperly trained multiengine pilot the mistaken perception that the engine is still developing useful thrust, producing a psychological reluctance to feather — since feathering stops the propeller from turning (AFH ch. 13).

A competent instructor teaches the critical importance of feathering in a timely manner and demonstrates the performance difference between a zero-thrust (simulated feathered) propeller and a windmilling one.

Not necessarily. Not all engine failures result in complete power loss. If there is a performance loss when the throttle of the affected engine is retarded, some power is still available — and the pilot may consider letting it run until reaching a safe altitude and airspeed for single-engine flight (AFH ch. 13).

While shutdown may prevent additional engine damage in some circumstances, shutting down an engine that can still produce partial power may increase risk for an accident. In cruise, leave the engine running if there is any doubt as to needing it for further safe flight.

The exception: catastrophic failure with heavy vibration, smoke, blistering paint, or large trails of oil indicates a critical situation — feather, complete the securing checklist, divert to the nearest suitable airport, and declare an emergency (AFH ch. 13).

No. Both feathering and starting a feathered reciprocating engine on the ground are strongly discouraged by manufacturers due to the excessive stress and vibrations generated (AFH ch. 13).

Similarly, repeated feathering and unfeathering is hard on the engine and airframe, and in training is done only as necessary to ensure adequate training. Plan any unfeathering and restart in training to be complete no lower than 3,000 ft AGL — at some elevations, in many popular trainers, that may be above the single-engine service ceiling, so level flight will not be possible (AFH ch. 13).

Depending on system design, you may need to select between main and auxiliary tanks, or actively transfer fuel from one tank to another. Complex fuel systems often carry limitations restricting some tanks to level flight only, or requiring a reserve in the main tanks for descent and landing (AFH ch. 13).

Electric fuel pump operation varies widely among models, particularly during tank switching or transfer — some pumps are to be on for takeoff and landing, others off. There is no substitute for thorough systems and AFM/POH knowledge.

Sharp turns onto the runway combined with a rolling takeoff are not good practice and may be prohibited by the AFM/POH due to the possibility of "unporting" a fuel tank pickup. The takeoff itself may be prohibited under any circumstances below certain fuel levels (AFH ch. 13).

The same physics limits slips: some multiengine airplanes have AFM/POH limitations against slips in excess of a certain time period — 30 seconds, for example — to prevent power loss from fuel starvation as fuel in the lowered wing's tank flows toward the wingtip, away from the pickup (AFH ch. 13).

By a loss of manifold pressure — not a loss of rpm, which is the fixed-pitch indication (AFH ch. 13).

Select carburetor heat (carbureted) or alternate air (fuel-injected). On some fuel-injected engines the alternate air source is automatically activated when the normal source is blocked.

An increase in manifold pressure to a value corresponding to the ambient air pressure at the altitude where the failure occurred (PHAK ch. 7).

For reference: with the engine not running the gauge indicates ambient pressure (29.92 "Hg at sea level standard); once started, the indication drops below ambient (roughly 12 "Hg at idle). The gauge face carries a green arc for the normal range and a red radial line for the manifold pressure upper limit.

It proves nothing about approval. The presence of anti-ice and deice equipment, however elaborate, does not necessarily mean the airplane is approved for flight in icing conditions — consult the AFM/POH, placards, and even the manufacturer (AFH ch. 13).

Anti-icing prevents ice on protected surfaces: heated pitot tubes, heated or non-icing static ports and fuel vents, propeller electrothermal boots or alcohol slingers, alcohol-spray or electrically heated windshields, defoggers, heated stall warning lift detectors, heated turboprop intake lips.

Deicing removes ice already formed — generally pneumatic boots on wing and tail leading edges, inflated from pneumatic pumps and deflated with vacuum assistance.

Also required for icing flight: alternate induction air, alternate static source, and ice-tolerant antennas. An ice light on the left engine nacelle lets you monitor wing ice at night.

Because continuous use of the autopilot masks trim and handling changes that occur with ice accumulation. Without that control feedback the pilot may not realize ice is building to hazardous levels — and the autopilot suddenly disconnects when it reaches design limits, handing back an airplane with unsatisfactory handling characteristics (AFH ch. 13).

A servo that moves the rudder in response to inputs from a gyroscope or accelerometer detecting yaw rate or lateral G, reducing motion about the vertical axis caused by turbulence (AFH ch. 13).

The yaw damper should be off for takeoff and landing, and there may be additional restrictions against its use with one engine inoperative. Most yaw dampers can be engaged independently of the autopilot.

Latch and lock security is a vital preflight item. When improperly secured, the door may open and the contents may be drawn out — usually into the propeller arc, and usually just after takeoff. Even when the compartment is empty, airplanes have been lost when the pilot became distracted by the open door (AFH ch. 13).

Most airplanes continue to fly fine with a nose baggage door open — some buffeting, more noise. Never become so preoccupied with an open door of any kind that you fail to fly the airplane. Also inspect the interior: tow bars, inlet covers, sun screens, oil containers, chocks, and hand tools accumulate there and must be secured against shifting.