Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
NE-08-18
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 failure due to high vibratory
stress induced by worn or improper engine crankshaft dampers.
The FAA is concerned because excessive engine crankshaft wear, damage,
improper installation, or unapproved propeller/ engine installations
will result in a significant increase in propeller vibratory stress.
NOTE: Propeller vibratory stress is torsionally induced when the
engine cylinders fire. The vibratory stress is a high frequency
vibration typically unnoticed by the pilot. This differs from a
vibration such as a propeller out-of-balance condition or an airframe
induced vibration caused by blade passage as the propeller rotates.
Background
Poor maintenance of piston engines
Crankshaft dampers on piston engines have a significant influence on
propeller stresses. Except for small engines (less than 200
horsepower), most piston engines on aircraft use vibration dampers
attached to the crankshaft. The dampers are moveable weights attached
to the crankshaft by pins and bushings. The dampening effects are
designed to reduce peak vibratory stress occurring at specific
frequencies in certain parts of the engine or the propeller or both.
Propellers installed on dampened engines are vibrationally tested and
certificated based on the test results. The tests are performed on
engines with a specific damper configuration and the propeller approval
is granted for only those specific engine and propeller combinations.
Any variations in the crankshaft damper configuration can have
significant adverse effects on the propeller vibratory stress
characteristics. Such variations can occur from excessive wear or
damage in the pins and bushings of the crankshaft damper assemblies,
improperly installed crankshaft dampers, as well as situations where a
propeller was installed on an incompatible, or untested, engine/
propeller combination.
The following are possible sources of wear or damage to damper pins and bushings:
Prolonged operation without an oil change.
Excessive
time-between-overhaul (TBO). Operators have extended engine life by
doing “top overhauls” without splitting the engine case, which is
necessary for damper pin/bushing inspection or replacement.
Non-compliance with inspection or replacement requirements of damper pins and bushings during overhaul.
Ground strike or sudden stoppage of a rotating engine.
Improper (abrupt) handling of throttles during engine run-up to full static RPM or decrease from full to idle power.
The concern with regard to
crankshaft dampers is that there is no simple way to inspect for damper
problems. A defective damper can remain hidden because propeller
vibratory stresses are typically not evident to the pilot. A damper
problem might not be revealed until a propeller fails.
Recommendations
Piston engine maintenance
Perform proper and timely
maintenance of the aircraft, propeller, and engine to avoid a
significant increase in propeller vibratory stress.
Do not ignore vibration or changes in vibration patterns as this can be a sign of a more serious problem.
During engine overhaul,
follow manufacturer’s recommendations for replacement of crankshaft
damper pins and bushings. Use of new parts is important to ensure long
term, safe operation.
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.