Propellers on most multiengine airplanes, and even some singles, have an unique capability to feather, to be brought to a stop in the event of an engine failure. This dramatically reduces drag, as the stopped blades twist to nearly align with the slipstream and no longer present a disc to the relative wind. The result is substantially improved glide performance for those few feather-capable single engine airplanes, and the difference between a slight climb capability and a steep descent in most piston twins. But there’s a trap that may befall the pilot of a feather-capable airplane if an in-flight engine restart isn’t successful. How can we avoid the propeller unfeathering trap?
Controllable Props 101
Controllable-pitch propellers come in several forms, but the vast majority share a common design. Prop blade angle is controlled by the motion of a piston inside the propeller dome. This piston, in turn, is moved by oil pressure on one side, and a spring (sometimes augmented by a charge of compressed air) on the other. As oil pressure changes the piston moves and the blade angle changes through gearing between the piston and the blades themselves.
In single-engine airplanes the gearing is designed so that if oil pressure drops below a minimum value the blades twist into the low pitch/high propeller RPM position. The logic is that an oil leak or engine failure will spring-load the prop to the high-speed position for as long as the engine is putting out any power-not a bad idea when flying behind a single powerplant.
In multiengine airplanes, however, there’s one or more other engine(s) that may be able to keep the airplane aloft so long as drag is reduced on the ‘dead’ engine side. It twins, then, the prop logic works the other way-if oil pressure drops below a set minimum the propeller blades drive to the HIGH PITCH/LOW RPM position. In most twins the propeller goes to so high a pitch they flatten out (relative to their direction of rotation) and drag increases to the point the propeller stops completely. The prop ‘feathers,’ twisted to the lowest-forward-drag position to permit maximum flight performance on the remaining, ‘live’ engine(s).
You can feather a propeller manually as well, done as part of the engine failure procedure. After detecting and confirming a failed engine, and exhausting all restart attempts (assuming you have altitude and time to try a restart), pull the propeller control handle through a detent to the FEATHER position. This opens a valve that dumps all oil from the prop dome and drives the blades into feather.
Unfeathering, and Accumulators
In almost all cases a pilot who feathers a propeller should land at the earliest opportunity on remaining power, leaving the ‘dead engine’ propeller in this lowest-drag position. Training for the multiengine rating, however, requires at least one actual engine shutdown and prop feathering in flight, and so also calls for an in-flight restart and unfeathering to resume training and avoid the heightened risk of a real-world single-engine landing.
Unfeathering the propeller involves making sure fuel, ignition and air are available to the engine, then moving the prop control out of the feather position. The procedure should not be rushed; use the appropriate checklist to get it right. Oil again flows to the prop dome and the blades twist out of feather. Once they’re in a low rpm position slipstream air may cause the propeller to slowly spin up, which also spins the engine’s gear-driven fuel pump and magnetos and restarts the engine. Sometimes air force isn’t enough to get the propeller spinning again, and the pilot must ‘bump’ it around a few times with the starter before it’ll unstuck from feather.
This is where unfeathering accumulators come in. An unfeathering accumulator is simply an oil sphere or cylinder, usually mounted in the engine nacelle behind the firewall. A dedicated pump in the engine’s oil system crams oil into the accumulator under high pressure. Inside the accumulator this oil pushes against one side of a diaphragm and is opposed by a charge of pressurized air on the other side.
When a propeller is feathered and oil dumped from its prop dome, an accumulator valve is also closed, trapping the accumulator oil. When the prop control moves forward out of feather, the accumulator valve opens and this high-pressure oil, boosted by air pressure on the other side of the accumulator, rushes back into the propeller dome to rapidly twist the prop blades to a high-rpm position where they spin more freely in the slipstream…making the restart much easier.
INSIDER’S TIP: Although unfeathering accumulators are often marketed as a ‘plus’ for all multiengine airplanes, their true value (offsetting the added weight and complexity of a pair of accumulator systems on a twin-engine airplane) is seen best in airplanes used for multiengine training, where in-flight shutdowns and air restarts are an everyday necessity.
Anti-feathering Lock Pins
If you’ve ever feathered a propeller in flight you know it does three things in addition to reducing drag. First, it looks weird-there’s something unnerving about looking out there and seeing the blades stopped while thousands of feet above the ground. Second, it causes vibration-the engine shakes and rattles in its mounts as the prop comes to a halt against the prop dome’s oil-free stops. Third, it makes it difficult to get a restart without accumulators, as the prop-dome piston and gears move without benefit of internal lubrication.
Hence, it is undesirable to feather unless dictated by an emergency or a specific training objective.
To keep some oil in the prop dome and avoid all this friction and vibration every time you shut down the engines, each prop dome contains devices to keep the blades out of feather on the ground. These devices are called the propeller anti-feathering lock pins. Held out of contact by flyweights when the engine is running, the lock pins engage when propeller speed drops to between 600 and 800 rpm. In an in-flight emergency or training scenario with air load driving the propeller blades, moving the prop control to feather causes the prop blades to twist to feather pitch before the lock pins engage-and the propeller feathers. During a normal, on-ground shutdown, however, the air load is absent and when the engine stops the prop reduces speed slowly enough that the anti-lock pins drop into place as the rpm drops through the 600 – 800 rpm range. The prop blades will twist no further, so they don’t go into feather. No vibration, no friction on oil-starved prop dome gears, and no cranking against dry metal on the next start-up.
The Propeller Unfeathering Trap
These anti-feather lock pins, vital to long-term health of the propeller mechanism, present a potential trap for the unwary pilot. Let’s say you’ve shut down an engine and feathered its propeller, whether for training or in a real-world emergency. Now you’ve handled the exercise or resolved the malfunction and are going to attempt an in-flight engine restart.
WARNING: Never attempt a restart following an unexpected engine failure when the cause of the failure is unknown or may cause further damage or a fire.
You process the appropriate checklist and move the propeller control forward out of feather…and the propeller begins slowly ticking around. Something’s wrong with fuel flow, induction air or ignition, however, and the engine won’t roar back to life. If the propeller isn’t spinning above 600 to 800 rpm, the anti-feather lock pins will drop into place and you cannot re-feather the prop. Where you were airborne under control in a low-drag configuration before the attempted restart, now you’re aloft with a high-drag, windmilling propeller, with far less aircraft capability. Your training exercise has become a real-world emergency, or the successful outcome of your shutdown-driving emergency, at first safely handled, is now definitely in doubt.
You can’t get 800 rpm out of a propeller with the starter. If you are very lucky you might be able to spin the prop to that rate with air load in a high-speed dive (assuming you’ve got the altitude), but that’s not certain or safe either. Your best bet is, in an actual in-flight emergency, to dismiss thoughts of an engine restart unless you’re certain your earlier failure came from an in-flight repairable situation (example: running an auxiliary fuel tank dry when you have ample fuel remaining in a main tank). Even then, don’t assume the relight will come off as planned. Wait until you’re over a runway and in a position to land in case your restart leaves you with a dead, windmilling propeller.
Same goes for shutdowns during training or checkrides. Be sure you’re at a safe altitude and near an adequate runway before shutting the engine down, and stay there, in a position to land, in case your practice air restart doesn’t work.
BOTTOM LINE: If you feather a propeller ‘for real,’ do not attempt a restart unless you’re absolutely certain the engine will restart. If you get a propeller out of feather and the engine won’t restart, you may be in a far worse situation than if you were in before.