Stressed Out Propellers

faast

faastAre you operating your propeller under stress? Are you damaging your propeller? Will your propeller suddenly fail? You will likely respond with a confident – NO! But wait! How do you know for sure?

If you want to be sure, obtain and read FAA Special Airworthiness Information Bulletin SAIB NE-08-21, dated May 14, 2008. Yikes! 2008! Yes, it is dated, but the information could save your life; because what you don’t know can hurt you! Consider the following.

Does the piston engine aircraft you work on or operate have a range of restricted propeller RPMs? Does it have a maximum propeller RPM indicated on the tachometer? Many aircraft models do. Some aircraft may require a placard or marking that states, for example, “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. Operating in these ranges can result in some very significant vibration. Prolonged violation of such restrictions could result in structural damage to a propeller, leading to propeller failure or internal engine component failure, such as the crankshaft.

Ask yourself the following:
When was the last time you had your tachometer accuracy checked? It 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 without your knowledge.
Is it possible the restriction placards in the aircraft are no longer correct? If so, there is an increased risk of exposing the propeller to damaging vibratory stresses.

If a tachometer was replaced or modified, does it have the proper markings, such as redlines, yellow arcs, red arcs, green arcs, or other noted limitations?

Are instrument panel placards for RPM restrictions incorrect, illegible, or missing?

If a propeller and/or an engine was replaced or modified, are the propeller RPM restrictions or placards still providing correct information?
Ponder this: On June 18, 2012, a Piper PA-28-200, lost about 6″ of the tip of one propeller blade while still in flight! It can happen – and does happen! (BTW, although the pilot described “a violent vibration,” he made a successful precautionary landing at a nearby airport.)

If you keep the stress off the propeller, you will remove stress from yourself.

 

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