In order to prevent the feathering spring and counterweights  from feathering the propeller when the engine is shut down  and the engine stopped, automatically removable high-pitch  stops were incorporated in the design. These consist of spring- loaded latches fastened to the stationary hub that engage  high-pitch stop plates bolted to the movable blade clamps.  When the propeller is in rotation at speeds over 600–800 rpm,  centrifugal force acts to disengage the latches from the high- pitch stop plates so that the propeller pitch may be increased  to the feathering position. At lower rpm, or when the engine  is stopped, the latch springs engage the latches with the high- pitch stops, preventing the pitch from increasing further due to  the action of the feathering spring. As mentioned earlier, the  engine load would be excessive, especially on fixed-turbine  turboprop engines. One safety feature inherent in this method  of feathering is that the propeller feathers if the governor oil  pressure drops to zero for any reason. As the governor obtains  its supply of oil from the engine lubricating system, it follows  that if the engine runs out of oil or if oil pressure fails due  to breakage of a part of the engine, the propeller feathers  automatically. This action may save the engine from further  damage in case the pilot is not aware of trouble. Unfeathering can be accomplished by any of several methods,  as follows: Normal in-flight unfeathering is accomplished when the pilot  positions the propeller condition lever into the normal flight  (governing) range.  [Figure 7-26]  This causes the governor to  disconnect the propeller oil supply from drain and reconnects  it to the governed oil supply line from the governor. At that  point, there is no oil available from the engine oil pump to  the governor; therefore, no governed oil is available from the  governor for controlling the propeller blade angle and rpm.  As the engine is started, its speed increases, the governor  supplies oil to the propeller, and the blade angle decreases.  As soon as the engine is operating, the governor starts to  unfeather the blades. Soon, windmilling takes place, which  speeds up the process of unfeathering. In general, restarting and unfeathering of propellers can  be classified as reciprocating engine restart unfeathering,  turboprop engine restart unfeathering, and accumulator  unfeathering. When reciprocating unfeathering is used, the  engine takes a little longer to start turning enough to provide  oil pressure to the governor and then to the propeller. This  delay can cause vibration as the propeller is unfeathered.  Many aircraft can use an accumulator to provide stored  pressure to unfeather the propeller much quicker. Special unfeathering systems are available for certain aircraft  where restarting the engine is difficult or for training purposes.  The system consists of an oil accumulator connected to the  governor through a valve.  [Figure 7-26]  The air or nitrogen  pressure in one side of the accumulator pushes a piston to  force oil from the other side of the accumulator through  the governor to the propeller piston to move the propeller  blades from feather to a lower blade angle. The propeller  then begins to windmill and permits the engine to start. When  the unfeathering pump is used, it is an additional pump that,  once the propeller control is in the correct position, the (full  increase rpm) pump is actuated and the oil pressure from the  pump unfeathers the propeller. Ice formation on a propeller blade, in effect, produces a  distorted blade airfoil section that causes a loss in propeller  efficiency. Generally, ice collects asymmetrically on a  propeller blade and produces propeller unbalance and  destructive vibration and increases the weight of the blades. A typical fluid system includes a tank to hold a supply of  anti-icing fluid.  [Figure 7-27]  This fluid is forced to each  propeller by a pump. The control system permits variation  in the pumping rate so that the quantity of fluid delivered to  a propeller can be varied, depending on the severity of icing.  Fluid is transferred from a stationary nozzle on the engine  nose case into a circular U-shaped channel (slinger ring)  mounted on the rear of the propeller assembly. The fluid  under pressure of centrifugal force is transferred through  nozzles to each blade shank. Because airflow around a blade shank tends to disperse  anti-icing fluids to areas where ice does not collect in large  quantities, feed shoes, or boots, are installed on the blade 7-17 1.  Start the engine, so the governor can pump oil back  into the propeller to reduce pitch. In most light twins,  this procedure is considered adequate since the  feathering of the propeller would happen infrequently.  Vibration can occur when the engine starts and the  propeller starts to come out of feather. 2.  Provide an accumulator connected to the governor  with a valve to trap an air-oil charge when the propeller  is feathered but released to the propeller when the rpm  control is returned to normal position. This system is  used with training aircraft because it unfeathers the  propeller in a very short time and starts the engine  wind milling. 3.  Provide an unfeathering pump that provides pressure  to force the propeller back to low pitch quickly using  engine oil. Anti-Icing Systems Ice Control Systems  Propeller Auxiliary Systems Unfeathering