Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
NE-08-21
May 14, 2008
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin (SAIB) alerts you,
owners, operators, pilots, mechanics, and certificated repair
facilities of the potential for propeller failures on piston engine
aircraft due to prolonged operation in a restricted RPM (revolutions
per minute) range as a result of inaccurate tachometers, missing or
improper tachometer markings, or missing or
improper instrument panel placards.
Piston engine aircraft typically have a maximum propeller RPM indicated
on the tachometer. Many aircraft models also have a range of restricted
propeller RPMs. As an example, an aircraft may require a placard that
states to, “Avoid continuous operation between 2,000 and 2,250 RPM”.
Such limitations typically result from certification testing when
increased propeller stresses are observed during certain operating
conditions. Prolonged violation of such restrictions could result in
structural damage to a propeller leading to propeller failure.
Today, many general aviation aircraft are over 30 years old.
Replacement or modification of the tachometer, propeller, engine, and/
or instrument panels might have occurred over the years. Also during
this time, periodic tachometer calibration may also have been neglected.
The FAA is concerned that many tachometers and restriction placards in
older aircraft are no longer correct, thereby increasing the risk of
exposing the propeller to damaging vibratory stresses. The concerns are
that:
If a tachometer was replaced
or modified, the tachometer might not have the proper markings
(redlines, yellow arcs, red arcs, green arcs, or other noted
limitations).
Tachometers might be out of
calibration resulting in propellers being operated in a restricted RPM
operating range or causing propellers to exceed their maximum propeller
RPM.
Instrument panel placards for RPM restrictions might be incorrect or missing.
If a propeller and/ or
engine was replaced or modified, the propeller RPM restrictions or
placards might not be correctly updated.
Non-compliance with Airworthiness Directives that require changes to RPM restrictions are not reported.
Background
Propeller manufacturers determine a propeller’s operating vibratory
stresses during certification. RPM zones of localized high vibratory
stress are a relatively common finding during testing. If a zone of
localized high vibratory stress occurs within the propeller operating
range, then the propeller may be approved with the addition of an
operating restriction, placard, or life limit.
Operating restrictions for the propeller may be found in the Aircraft
Maintenance Manual, Pilots Operating Handbook or Approved Flight
Manual, Type Certificate Data Sheet (TCDS), or Supplemental Type
Certificate (STC) documentation. The documentation may mandate the
installation of a red or yellow arc on the aircraft tachometer and/ or
an instrument panel placard. A life limit, where applicable, is
included in the Airworthiness Limitations Section of the Aircraft or
Propeller Instructions for Continued Airworthiness. An operating
restriction or life limit may also be mandated by an Airworthiness
Directive.
Failure to comply with tachometer markings and instrument panel
placards could result in prolonged operation within a restricted RPM
range and increase the potential for a propeller failure.
Mechanical tachometers do not necessarily retain their accuracy for the
life of the aircraft. Reports of tachometer errors of 50 RPM are
common. Errors of 150 to 250 RPM have been reported. Using an
inaccurate tachometer could result in a restricted RPM range or maximum
RPM red line being unknowingly violated by the pilot, which could
result in repetitive or prolonged exposure to damaging propeller
stresses.
The most common failure associated with this condition is a blade tip
separation that results from a fatigue crack, but failure of the
propeller hub and/ or blade retention feature can also occur. Prolonged
operation within a restricted RPM range, or above the maximum RPM, will
be further aggravated by the presence of surface conditions such as
nicks or corrosion pits.
Recommendations
Check the aircraft records
for replacement or modification of the tachometer, and changes to the
propeller model, engine model, or installation changes.
Verify that the proper RPM restrictions are accurately marked on the tachometer and instrument panel placard.
Check the accuracy of the tachometer to ensure that the readings are accurate.
Check the accuracy of mechanical tachometers at intervals not to exceed 60 months.
Contact the propeller manufacturer for corrective action if the propeller was operated in a restricted range.
For Further Information Contact
Jay Turnberg, Propeller Specialist, Standards Staff, FAA, Engine
and Propeller Directorate; 12 New England Executive Park, Burlington,
MA 01803; e-mail: jay.turnberg@faa.gov; phone: (781) 238-7116; fax:
(781) 238-7199.
For Hamilton Sundstrand, Avia, Dowty, Hoffmann, or MT Propellers:
Terry Fahr, Aerospace Engineer, Boston Aircraft Certification Office,
FAA, 12 New England Executive Park, Burlington, MA 01803; e-mail:
terry.fahr@faa.gov; phone: (781) 238-7158; fax:
(781) 238-7170.
For Hartzell Propellers:
Tim Smyth, Aerospace Engineer, Chicago Aircraft Certification Office,
FAA, Propulsion Branch, 2300 East Devon Avenue, Des Plaines, IL 60018;
e-mail: timothy.smyth@faa.gov; phone: (847) 294-7132, fax: (847)
294-7834.
For Sensenich Propellers:
James Delisio, Aerospace Engineer, New York Aircraft Certification
Office, FAA, Propulsion Branch, 1600 Stewart Avenue, Suite 410,
Westbury, NY 11590; e-mail: james.delisio@faa.gov; phone: (516)
228-7321; fax: (516) 794-5531.