Applies To Piston Engine Airplanes With Variable Pitch
Props
The FAA has issued a Special Airworthiness Information Bulletin
(SAIB) to alert operators, pilots, and aircraft manufacturers of
concerns for an optimum response to a propeller overspeed in piston
engine aircraft with variable pitch propellers. At this time, the
FAA has concluded that the airworthiness concern is not an unsafe
condition that would warrant airworthiness directive (AD)
action.
Recently, a single-engine aircraft experienced a propeller
overspeed during cruise flight at 7,000 ft. 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 Operating Handbook). The
pilot was unable to reach the airport and was forced to conduct an
off field landing.
It was determined that the propeller experienced a failure that
caused the blade pitch change mechanism to move to the low pitch
stop position. This caused the propeller to operate as a fixed
pitch propeller such that it changes RPM with changes in power and
airspeed. The low pitch setting allows for maximum power during
take off but can result in a propeller overspeed at a higher
airspeed. A performance evaluation of the flight condition was
performed for the particular aircraft model involved in this
incident. This evaluation indicated that an airspeed lower than the
best glide speed would have resulted in increased thrust and
enabled the pilot to maintain level flight.
There are numerous variables in aircraft, engines, and
propellers, which affect aircraft performance. For some aircraft
models, the published best glide speed may not be low enough to
generate adequate thrust for a given propeller installation in this
situation (propeller blades at low pitch stop position).
The graph below shows thrust versus airspeed for a typical
propeller model set at a 12-degree blade angle and maintaining
2,700 revolutions per minute (RPM). From the graph, note that at
over 118 knots this propeller is incapable of generating a positive
thrust and airspeed has to be lower than 95 knots to generate
enough thrust to overcome the aircraft drag, i.e. maintain level
flight. While different propeller models have different thrust
characteristics and different aircraft have different drag
characteristics, the concept remains the same – the lower the
airspeed the more thrust there is available at a given RPM.
A review of pilot’s operating handbooks from several
aircraft manufacturers showed they did not include emergency
procedures for a propeller overspeed. In cases where overspeed
procedures were published, the procedure said to simply reduce
throttle. More appropriately, the procedures should advise
the pilot to control a propeller overspeed by throttle reduction
and a reduction in airspeed as much as practical with an adequate
margin above stall speed such that level flight can be
maintained.
The FAA says that the operators of aircraft with variable pitch
propellers should be aware that in certain instances of propeller
overspeed, the airspeed necessary to maintain level flight may be
different than the speed associated with engine-out best glide
speed. The appropriate emergency procedures should be followed to
mitigate the emergency situation in the event of a propeller
overspeed; however, pilots should be aware that some reduction in
airspeed may result in the ability for continued safe flight and
landing. The determination of an airspeed that is more suitable
than engine-out best glide speed should only be conducted at a safe
altitude when the pilot has time to determine an alternative course
of action other than landing immediately.