Thursday, September 01, 2011

Propeller blade cracks

Some interesting stuff on inspecting propeller blades for cracks before flight. Australian Airworthiness Bulletin 61-008 "To provide guidance on propeller continuing airworthiness/maintenance practices"

A few quotes from the bulletin:

detection of the crack may only be possible from the rear of the blade

I always looked at the front - good advise - article has an good explanation as to why. Another quote:

Investigation has shown that cracks have propagated over a long  period, which in some cases exceeds thirty ground/air/ground cycles  i.e. thirty flights. There is no evidence to suggest that failures have  occurred where a crack may have propagated from initiation to final  failure in one ground/air/ground cycle i.e. one flight. Therefore  detection of the crack and prevention of failures of this nature should  be achievable.

A quick inspection before flight can prevent blade failures.

Some blade paint schemes are not conducive to easy inspection of 
the rear surface of the blade...

This is the big point! A thick durable layer of tough epoxy paint might hide the crack!

Piston engines and engine mounts were painted with a thin coat of brittle enamel paint. Through long experience we found that cracks would appear through the paint. The paint did not prevent inspection. Now the customer wants a thick powder-coat gloss finish. Cracks and corrosion are hidden; inspection is hindered or made impossible. Aerospace is more concerned with performance, endurance,  inspectability than cosmetics and bright colors under the hood.

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