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Hartzell AD Followup



 
 
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Old April 21st 07, 04:23 AM posted to rec.aviation.owning
Doug Vetter
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Posts: 81
Default Hartzell AD Followup

This is a follow-up to a message I sent to the group recently about the
Hartzell propeller AD.

I flew out to the prop shop today and had them inspect my hub using the
eddy current method. Interesting technology and quite accurate for
finding the smallest of cracks on or very near the surface. The
inspection only called for an evaluation of the areas surrounding the
weight retaining holes but the tech surveyed the area around the blade
shanks as well simply because it was easy to do while he had the spinner
off.

The good news is that my hub was fine (as I expected it to be). The bad
news is that while that office hadn't failed any hubs, the Georgia
office had apparently failed six so far. That's not a huge number, but
it does prove it's not an isolated incident. There is now some "real
world" data to support the issuance of the AD.

I also took a look at the UK accident report the shop had handy and let
me tell you -- the people in that airplane (a 1987 Tobago, IIRC) were
EXTREMELY lucky. They applied maximum power on a touch and go and the
blade let loose just as they were rotating. If it had failed in flight
there is no way they would have survived. The engine was ripped off its
mounts and came to rest facing about 70 degrees to the left and pitched
upward about 30 degrees. The picture of the hub showed the blade took
off and split the hub in half.

When I asked the tech for his opinion of why some hubs fail and others
don't -- like the one that's been in service on my airplane for 30 years
-- he said it's probably a combination of things. There may (or may
not) be a design and/or manufacturing process defect. Hartzell will
never admit that, obviously. There may also be an installation-specific
issue at hand -- like the coupling of this prop with an engine that
produces high harmonic vibrations. The O-360 lacks a counterbalanced
crankshaft and that fact is responsible for the red arc in the
upper-middle portion of the RPM range. He seemed to think that the
biggest factor is the fact that most people assume that their props are
"smooth enough" and don't need dynamic balancing when in fact they could
benefit from it.

We looked back at the dynamic balancing history of my prop and found
that it came out of the last overhaul with 0.04 IPS with no weights
applied. I asked the tech "how common is that?" He said "very
rare...we usually can improve the vibration specs of a freshly
overhauled propeller".

So, it looks like we're good to go for another 100 hours. Hartzell is
offering new hubs at a 50% discount if ordered before September 2007, so
that will be the next decision for us. $3K in parts and labor buys a
lot of inspections and I still say there's something to be said for a
regular propeller inspection -- new hub or old. Guess we'll make the
call later this summer.

-Doug

--
--------------------
Doug Vetter, ATP/CFI

http://www.dvatp.com
--------------------
 




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