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
--------------------

On Apr 20, 8:23 pm, Doug Vetter wrote:
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

--------------------

1) Hartzell has admited that the current hub has a design flaw and is
offering a beefed up hub to avoid the inspection.
2) The new hub deal requires you have servicable blades. They will not
be able to put 30 year old blades on a new hub. Figure $6K to replace
blades and hub.

-Robert

Robert M. Gary wrote:
1) Hartzell has admited that the current hub has a design flaw and is
offering a beefed up hub to avoid the inspection.

Where did they admit this? Improving a design is not the same as
admitting that there is a design flaw in the previous version. I'm not
defending them...just pointing that out.

2) The new hub deal requires you have servicable blades. They will not
be able to put 30 year old blades on a new hub. Figure $6K to replace
blades and hub.

Since when does age have anything to do with it? If the blades meet
serviceable limits (various physical criteria), they'll be fine. I've
already been told that.

In any case, the higher the cost the more I can justify routine
inspections. We won't have the airplane forever.

-Doug

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

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

That's a prop shop policy, not regulation. If the blades are servicable, you
can find a shop that will put them on with a new hub.

--
-------------------------------
Travis
Lake N3094P
PWK
"Robert M. Gary" wrote in message
ps.com...
On Apr 20, 8:23 pm, Doug Vetter wrote:
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

--------------------

1) Hartzell has admited that the current hub has a design flaw and is
offering a beefed up hub to avoid the inspection.
2) The new hub deal requires you have servicable blades. They will not
be able to put 30 year old blades on a new hub. Figure $6K to replace
blades and hub.

-Robert

On Apr 21, 5:19 am, Doug Vetter wrote:
Robert M. Gary wrote:
1) Hartzell has admited that the current hub has a design flaw and is
offering a beefed up hub to avoid the inspection.

Where did they admit this? Improving a design is not the same as
admitting that there is a design flaw in the previous version. I'm not
defending them...just pointing that out.

The letter I received said that the current design is subject to blade
separation and that the new design addresses this.

2) The new hub deal requires you have servicable blades. They will not
be able to put 30 year old blades on a new hub. Figure $6K to replace
blades and hub.

Since when does age have anything to do with it? If the blades meet
serviceable limits (various physical criteria), they'll be fine. I've
already been told that.

If the prop does not have enough metal left, it is illegal to
reinstall it. I'm assuming if your prop is 30 years old enough metal
has been removed during facing and O/H's that there will not be
enough. If not, good for you.

-Robert

On Apr 21, 9:10 pm, "Robert M. Gary" wrote:
On Apr 21, 5:19 am, Doug Vetter wrote:

Robert M. Gary wrote:
1) Hartzell has admited that the current hub has a design flaw and is
offering a beefed up hub to avoid the inspection.

Where did they admit this? Improving a design is not the same as
admitting that there is a design flaw in the previous version. I'm not
defending them...just pointing that out.

The letter I received said that the current design is subject to blade
separation and that the new design addresses this.

2) The new hub deal requires you have servicable blades. They will not
be able to put 30 year old blades on a new hub. Figure $6K to replace
blades and hub.

Since when does age have anything to do with it? If the blades meet
serviceable limits (various physical criteria), they'll be fine. I've
already been told that.

If the prop does not have enough metal left, it is illegal to
reinstall it. I'm assuming if your prop is 30 years old enough metal
has been removed during facing and O/H's that there will not be
enough. If not, good for you.

-Robert

I just spoke with 1 prop shop that said the rejection rate for blades
of those replacing just the hub under this AD is about 75%. Those guys
pay extra because the shop had to verify them (vs just ordering an
entire prop and R&R'ing)

-Robert

"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."

Dynamic prop balancing won't reduce prop stresses very much. It will
reduce the vibration transmitted to the engine and eventually to the
airframe via the mounts & cowl etc, but not the hub or blade stress.

Prop blade and hub fatigue is caused by the constant torsional
pounding of the engine combining with a torsionally resonant
crankshaft and propeller blade system. On some engines there is a
dynamic absorber at the back that substantially reduces the resonant
buildup of crank and propeller stress, but many versions of the 4
cylinder IO-360 Lycoming engine don't have these absorbers. Instead
they placard the tachometer and expect the operator to not run at
certain subharmonics of the crankshaft-propeller torsional resonance.

To understand how this torsional vibration mode operates, you must
imagine being an observer sitting on the spinner while the engine is
running. When this mode is excited, you would see the prop tips
oscillate to-and-fro while the rear of the crankshaft oscillated fro-
and-to. There is a lot of high frequency (about 220 Hz) torque going
through the prop hub and that's what this is all about.

Most mechanics don't understand how the dynamic absorbers (they are
not counterweights) on the crankshaft if included, are supposed to
operate.

"nrp" wrote in message ups.com...
:
: Most mechanics don't understand how the dynamic absorbers (they are
: not counterweights) on the crankshaft if included, are supposed to
: operate.
:

Elaborate, please...

Elaborate, please...

Check your PM. It is a big file.