KOXR Mooney blade failure

[A64Pilot]

5 minutes ago, MikeOH said:

THAT does not sound like fun! But seems all the rotating ‘stuff’ was still in balance in your situation. I’m just thinking missing a large chunk of prop is going to be much worse. BWTHDIK

It is. Sometimes essentially ripping the engine off its mounts. The attached report from memory rendered the aircraft unable to maintain altitude because the engine was mostly ripped loose and hanging down or sideways maybe but whatever it created a large amount of drag.

 https://asn.flightsafety.org/wikibase/36677

The cause of this prop failure was traced to “reactionless mode” and I think this high profile accident is what caused it to be addressed, essentially reactionless mode is a harmonic that causes extreme stress and therefore blade fatigue, what’s so bad about this mode is when it occurring there is no vibration felt, it’s actually very smooth, hence the name reactionless, but it’s why yiu hear some Pratt four and five bladed aircraft with such high ground idle speeds so high they sound like Garrett’s.

I dealt with it by having the MVP-50T flash yellow if you were in the range instead of turning the idle up real high.

https://hartzellprop.com/FAA/NE-06-13.pdf

[EricJ]

48 minutes ago, bluehighwayflyer said:

Didn’t they used to chain or strap Formula One racer’s engines to their mounts, just in case?  I think I remember that.  Maybe they still do.  

Drag racers used to do that sometimes, and offroad people as well.   Sometimes it was just to limit engine travel relative to the chassis due to torque.

[Hank]

53 minutes ago, bluehighwayflyer said:

Didn’t they used to chain or strap Formula One racer’s engines to their mounts, just in case?  I think I remember that.  Maybe they still do.  

I've heard of airboats doing that, but wasn't sure if it was due to operational reasons or theft prevention.

[EricJ]

11 hours ago, MikeOH said:

THAT does not sound like fun! But seems all the rotating ‘stuff’ was still in balance in your situation. I’m just thinking missing a large chunk of prop is going to be much worse. BWTHDIK

Both create severe vibrations, both stress the crank and the case and the mounts and a bunch of other stuff.   Yes, they're different, but everything is. 
 

[Pinecone]

10 hours ago, bluehighwayflyer said:

Didn’t they used to chain or strap Formula One racer’s engines to their mounts, just in case?  I think I remember that.  Maybe they still do.  

Yes.

FYI for others, he is talking about Formula One AIR racers.  Not F1 cars.

How to go over 200 MPH on an O-200

https://en.wikipedia.org/wiki/Formula_One_Air_Racing

[A64Pilot]

On 3/11/2025 at 11:32 PM, EricJ said:

Drag racers used to do that sometimes, and offroad people as well.   Sometimes it was just to limit engine travel relative to the chassis due to torque.

Believe it or not but a few of us did on long distance cruising sailboats. Reasoning was that God forbid but the weather was so bad that the boat rolled you didn’t want the engine coming loose in the cabin.

What could you chain it to in an airplane though? It’s not just the rubber that breaks on an aircraft, it’s the tubular engine mount itself. 

We did it only on the left side of drag cars if memory serves because torque pulls the L side up.

[DXB]

This anxiety-provoking thread makes me wonder if I should have formally overhauled my prop rather than IRAN’d/resealed it recently.  It’s a 12 year old Top Prop with ~1500hrs that was slinging a modest amount of grease on one blade. It had a few nicks and chips over the years.  The engine was getting overhauled so it was a good time to do it.  They made it look brand new for less than half the cost of a prop overhaul, and it runs very smooth now even before dynamic balancing.   But I wonder if I would hsve gotten much more peace of mind from the careful dye penetrant testing that’s included with overhaul…

…and I would love to know what the metallurgy of this failure was….

[takair]

I wonder if this STC possibly missed some harmonic resonance of this combination airframe (Porsche legacy) , engine, prop?  Is this exact combination on many other airframes? 

I would be curious to hear from a metallurgist if and how calendar time in service can play into this type of fatigue failure. I don’t believe it does. Formal NDT may have caught it, but it seems to me that flight time in service is a better indicator of cycle related stress than calendar time. Even then it seems like there is a missing link to this failure. 

I also have a theory that the annual lube interval and amount adds up to an overfilled and leaking hub in about 12 years.  I would again be curious how many, if any, leaking props come back underlubricated. Hard to say what the alternative is since it is hard to determine that there is adequate grease through the hub, but I am coming to my own conclusion that the leakage and 12 year interval may not be entirely coincidental and may not be purely due to the orings. I wish there was a better way to asses grease condition and dispersal than to just add grease as the various iterations of the service manual have suggested. 

Edit:  I am venting my frustrations of my Hartzell props with regard to the grease. Likely not appropriate for this thread…apologies. 

[Shadrach]

On 3/10/2025 at 7:32 AM, DXB said:

I imagine this requires crank  inspection like a prop strike?  The asymmetric loads must be enormous 

It might require a prop inspection, but I’d like to see the data behind that requirement.  If you think about it, there are many asymmetric loads within a reciprocating engine. Think about the asymmetric loads on a piston pin given the piston is reversing direction every 180°.  It’s pretty amazing to consider how much pressure it takes to disrupt the hydrodynamic plane.  I think the motor mounts are toast. I’d be curious to see how the firewall fared.

 

this is a long video, but it’s very well done and conveys a lot of information that most of us don’t think about in terms of the physics of multi cylinder, reciprocating engines.

 

 

[M20F]

This is EXACTLY why I won’t use G100UL!!!

[Coachella Bravo]

23 hours ago, M20F said:

This is EXACTLY why I won’t use G100UL!!!

You mean TODAY you won't use G100UL

[M20F]

19 minutes ago, Coachella Bravo said:

You mean TODAY you won't use G100UL

Give me lead or give me death!

[Pinecone]

On 3/15/2025 at 9:25 AM, DXB said:

This anxiety-provoking thread makes me wonder if I should have formally overhauled my prop rather than IRAN’d/resealed it recently.  It’s a 12 year old Top Prop with ~1500hrs that was slinging a modest amount of grease on one blade. It had a few nicks and chips over the years.  The engine was getting overhauled so it was a good time to do it.  They made it look brand new for less than half the cost of a prop overhaul, and it runs very smooth now even before dynamic balancing.   But I wonder if I would hsve gotten much more peace of mind from the careful dye penetrant testing that’s included with overhaul…

…and I would love to know what the metallurgy of this failure was….

The downside of having your prop overhauled is that they MUST grind the blades to reprofile.  Do that a few times and your blades will be too thin and need to be replaced.  Or you get a new prop.

[IvanP]

On 3/15/2025 at 9:55 AM, M20F said:

This is EXACTLY why I won’t use G100UL!!!

You may not have that choice for very long if the rabid environuts have their way. 

[Vance Harral]

1 hour ago, Pinecone said:

The downside of having your prop overhauled is that they MUST grind the blades to reprofile.  Do that a few times and your blades will be too thin and need to be replaced.  Or you get a new prop.

This is a real concern, but like much of aviation lore, I think it's oversold vs. other concerns.

Story time: we bought our M20F in 2004, with the Hartzell HC-C2YK-1BF common to that vintage.  The prop was installed in 1991, and had never been overhauled at the time of our purchase.  In the past 21 years we've had it overhauled twice, including the dreaded blade profiling.  Once in 2006 when we installed a new "B" suffix hub to eliminate the prop AD.  Again in 2016 because it was slinging grease, and the well-recommended local shop that allowed us to avoid shipping to other parts of the country declined to IRAN a prop with a little over a decade since the last overhaul.  We're now at 34 years and counting on this set of prop blades, with two overhauls, and perhaps a third coming up in the next few years if we follow suit.

The last time we had the prop overhauled, I asked the shop how many re-profiles they thought the blades had left in them.  The answer was "definitely one more, likely two, maybe three if your really lucky".  So we're on track to get 45+ years out of this set of blades (and I might be dead by then), despite ignoring advice to "never get a full overhaul if you don't absolutely have to".

If we'd had the prop overhauled every 6 years as the manufacturer recommends, we might be at limits on the blades now; but maybe not - a competent shop only removes the minimum material required to re-establish the profile, and the less operational time on the prop (which correlates somewhat with calendar time), the less material removed.  But even if we were forced into a new set of blades today, that would amortize the cost over 34 years, which is longer than most people own an airplane in the first place.  Looks like our F7666 blades retail for about $9K apiece these days, $18K for a set.  Nobody wants to write a check for $18K, of course; but in our case that would work out to about $500/year.  Not exactly break-the-bank money in terms of multi-decade aircraft expense planning.

None of this is to say that IRAN-instead-of-overhaul is a bad idea, and I acknowledge there are probably some incompetent shops out there that take way more material off a blade during overhaul than is necessary.  But re-profiling is done for a reason, as are other operations required in a full overhaul, and those things do actually have nonzero value.  Equally important, understand that if you have a prop that hasn't had been completely overhauled in a long time, a subset of prop shops - including some good ones - simply aren't going to IRAN it for "policy reasons" (that was the chief driver of our 2nd overhaul - the local shop was really convenient, competitively priced, and well-regarded, but they just weren't going to IRAN a 10-year old prop).  There are plenty of shops with more liberal policies, of course, but they may not be in your local area and/or have the best rates, so that complicates the decision making.

Bottom line: sure, get an IRAN and avoid profiling if you can, you won't get any criticism from me.  But blade profiling is not the only concern, and it's not dumb to opt for a full overhaul.

[EricJ]

Another thing that affects the longevity of a blade is how often and how much it gets dressed by an A&P to remove nicks due to rock or other debris strikes.    A common practice these days is to remove a nick by filing the entire leading edge down (instead of just locally dressing the nick), and then removing an equal amount from the other blade(s) to "maintain balance".   A brand-new C182 that I help with and also fly had a lead fail in one of the mag harnesses, and the shop that came to fix it ran it up on the ramp at full throttle to try to clear what they initially thought was a fouled plug.   It sucked up a rock and put a big ding in one of the blades.    Instead of just locally dressing the affected blade they filed all three blades down.   This was on an airplane with only a couple hundred hours total time.   I was...disappointed, but this is not the first time I've seen that done.    It'll certainly affect the blade profile of all blades when that is done.   If that is done whenever any blade gets a nick, all of the blades will wind up with reduced service life.

 

[Pinecone]

I think the idea is, don't automatically do an overhaul if an IRAN will work.

Work with your prop shop.

[201Steve]

On 3/15/2025 at 9:25 AM, DXB said:

But I wonder if I would hsve gotten much more peace of mind from the careful dye penetrant testing that’s included with overhaul…

If it’s already paint stripped, I can’t see a good reason why they wouldn’t (or at least on request) do that test as standard. It’s pretty easy to work with. Spray, wait, wipe, spray, wait, wipe. 

[A64Pilot]

22 minutes ago, 201Steve said:

If it’s already paint stripped, I can’t see a good reason why they wouldn’t (or at least on request) do that test as standard. It’s pretty easy to work with. Spray, wait, wipe, spray, wait, wipe. 

It’s also not necessarily real definitive, the defect must be present at the surface for dye penetrant to discover a crack, then of course the test is only valid for the time the test was done, unless your willing to strip and rest yearly or whatever the interval the experts determine is less than the time between a discoverable crack and failure is. I couldn’t even hazard a guess.

A better test would be eddy current as it will find sub surface defects, and I think doesn’t require paint stripping, but don’t quote me on that

My SWAG is this prop had an inclusion in it, and that was the defect that the crack propagated from, or possibly if it had an avoid range that the pilot ignored it, in short I hope this was a one in 100,000 kind of thing and won’t effect the series. If it was an inclusion they will find that pretty quick.

 

Edited March 18 by A64Pilot

[201Steve]

2 hours ago, A64Pilot said:

If it was an inclusion they will find that pretty quick

How does one determine the existence of an inclusion? Just curious

[dkkim73]

36 minutes ago, 201Steve said:

How does one determine the existence of an inclusion? Just curious

Eddy current inspection 

[Jim F]

On 3/11/2025 at 7:42 PM, bluehighwayflyer said:

Didn’t they used to chain or strap Formula One racer’s engines to their mounts, just in case?  I think I remember that.  Maybe they still do.  

YES, Air racing Formula 1 requires a cable between the cylinders bases to the engine mount.  Back in the 80s a blade failed, and the engine tore loos from the mounts.  The engine was hanging from the hoses.  Without the weight of the engine, you are no longer controllable.

Jim

[M20F]

19 hours ago, Vance Harral said:

This is a real concern, but like much of aviation lore, I think it's oversold vs. other concerns.

100% this, it’s a signature, it’s a game.  
 
Play it one way and it costs you $10,000’s.  Play it the other way and it costs you a blade separation.  
 
Play the middle. 

[A64Pilot]

15 hours ago, 201Steve said:

How does one determine the existence of an inclusion? Just curious

It will be immediately apparent upon visual inspection, usually. Never say always. I think rarely a scanning electron microscope is needed to see the beach marks, but that’s above my pay grade.

Classic fatigue leaves witness marks called beach marks and they radiate from where the crack started. Not sure why they are called beach marks, maybe because they look like wave marks on a beach, maybe?

https://www.quadco.engineering/en/know-how/material-fatigue-how-can-it-be-recognized.htm

You find an inclusion prior to failure by NDT, often either eddy current or x-ray if we are talking aluminum, other methods for composites and magnetic steel although eddy current works fine for steels like 4130 etc.

Edited March 19 by A64Pilot