Aircraft Systems

For any given rpm, there is a manifold pressure that should Operating Handbook (POH), but was unable to reach the not be exceeded. If manifold pressure is excessive for a given airport and was forced to conduct an off-field landing. rpm, the pressure within the cylinders could be exceeded, placing undue stress on the cylinders. If repeated too It was further explained that a determination was made that the frequently, this stress can weaken the cylinder components propeller experienced a failure causing the blade pitch change and eventually cause engine failure. mechanism to move to the low pitch stop position. This caused the propeller to operate as a fixed-pitch propeller such that it A pilot can avoid conditions that overstress the cylinders changes rpm with changes in power and airspeed. The low by being constantly aware of the rpm, especially when pitch setting allows for maximum power during takeoff but increasing the manifold pressure. Consult the manufacturer’s can result in a propeller overspeed at a higher airspeed. recommendations for power settings of a particular engine to maintain the proper relationship between manifold pressure A performance evaluation of the flight condition was and rpm. performed for the particular aircraft model involved in this incident. This evaluation indicated that an airspeed lower When both manifold pressure and rpm need to be changed, than the best glide speed would have resulted in increased avoid engine overstress by making power adjustments in thrust enabling the pilot to maintain level flight. There are the proper order: numerous variables in aircraft, engines, and propellers that affect aircraft performance. For some aircraft models, • When power settings are being decreased, reduce the published best glide speed may not be low enough to manifold pressure before reducing rpm. If rpm is generate adequate thrust for a given propeller installation in reduced before manifold pressure, manifold pressure this situation (propeller blades at low pitch stop position). automatically increases, possibly exceeding the manufacturer’s tolerances. The operators of aircraft with variable pitch propellers should • When power settings are being increased, reverse the be aware that in certain instances of propeller overspeed, the order—increase rpm first, then manifold pressure. airspeed necessary to maintain level flight may be different • To prevent damage to radial engines, minimize than the speed associated with engine-out best glide speed. operating time at maximum rpm and manifold The appropriate emergency procedures should be followed to pressure, and avoid operation at maximum rpm and mitigate the emergency situation in the event of a propeller low manifold pressure. overspeed; however, pilots should be aware that some reduction in airspeed may result in the ability for continued The engine and/or airframe manufacturer’s recommendations safe flight and landing. The determination of an airspeed that should be followed to prevent severe wear, fatigue, and is more suitable than engine-out best glide speed should only damage to high-performance reciprocating engines. be conducted at a safe altitude when the pilot has time to determine an alternative course of action other than landing Propeller Overspeed in Piston Engine Aircraft immediately. On March 17, 2010, the Federal Aviation Administration (FAA) issued Special Airworthiness Information Bulletin Induction Systems (SAIB) CE-10-21. The subject was Propellers/Propulsers; The induction system brings in air from the outside, mixes Propeller Overspeed in Piston Engine Aircraft to alert it with fuel, and delivers the fuel-air mixture to the cylinder operators, pilots, and aircraft manufacturers of concerns where combustion occurs. Outside air enters the induction for an optimum response to a propeller overspeed in piston system through an intake port on the front of the engine engine aircraft with variable pitch propellers. Although a cowling. This port normally contains an air filter that inhibits SAIB is not regulatory in nature, the FAA recommends that the entry of dust and other foreign objects. Since the filter the information be read and taken into consideration for the may occasionally become clogged, an alternate source of safety of flight. air must be available. Usually, the alternate air comes from inside the engine cowling, where it bypasses a clogged air The document explains that a single-engine aircraft filter. Some alternate air sources function automatically, experienced a propeller overspeed during cruise flight at while others operate manually. 7,000 feet. The pilot reported that the application of throttle resulted in a propeller overspeed with no appreciable thrust. The pilot attempted to glide to a nearby airport and established the “best glide” speed of 110 knots, as published in the Pilot’s 7-7