Almost lost prop

On 12 May 2011, I was out practicing acro for the upcoming IAC contest in Grenada, MS 20-22 May. On several previous flights I had noticed a very slight vibration and had thought it was a fouled plug, but the vibration change was not consistent when switching mags. The vibrations seemed to increase on this date during the practice between 2400 and 2600 rpm so I throttled back and returned to the airport. I also had noted some dark gray smudges coming from the front and sides of the cowling on previous flights. We had replaced a broken front cowling bracket two months prior. When I first saw the smudges, The A&P and I had noticed that the rubber baffling around the cowling bracket and starter ring was touching and worn through, and thought this was the source of the small amount of residue.

After this flight, as we were about to remove and check the spark plugs, the A&P noticed more of the powdery residue and stated this was not rubber, but metal powder. On further investigation we found the prop could be moved perpendicular to the crank. When the spinner was removed, this is exactly what we saw. Two bolts had stripped and all the safety wire had broken. Tach time 608 hours.



Wooden prop (manufacturer will not be named), 70 x 78 inch was installed December 2007 with approximately 400 hours on the tach from installation to this incident 12 May 2011. This prop replaced the original 68 X 74 prop by the same manufacturer after engine was overhauled Conditional inspection with torque of prop bolts checked at 22 ft-lbs, on 28 Feb 2011. Tach time 570 hours.

My A&P mechanic thinks that the long prop bolts may have bottomed out, and created a false torque value when the bolts were retorqued on the conditional inspection. The previous engine overhaul company thinks the 8 ? inch bolts through the spacer were a contributing factor when the aircraft is used extensively for aerobatics. He recommends shorter prop bolts and using a spacer that is bolted to the engine flange and the prop bolted to the spacer. The prop and all the components on the hub were damaged and are being replaced. The engine will be torn down, inspected, and damaged parts replaced.

I think that calls for a drink ... or twelve! Finding that would scare the bejeebus out of me. Glad nothing worse happened. The lesson I take from this is to pay attention to what my aircraft is telling me. I think I'll go listen to the -6A some more.

Safe on the ground

Thanks for sharing and glad you are safe on the gound!

Hi Willy,

Many thanks for sharing this with us in you very well written, concise and to the point post. I too am glad you are safe on the ground. One thing I will check when I get there, is the space between the bolt head and the flange when the bolt is fully bottomed out.

My father lost a prop on a champ years ago and landed safe in a cow pasture. They found the prop a couple years later on a pond dam. Still have the prop. He had nine lives broke a crankshaft on a agwagon on take off went into a cotton field and flipped knocked him unconscious when he woke up said he smelled fuel knocked side window out crawled out got about 100 yards with broke ribs plane exploded burnt to a crisp. He died in 2006 of a heart attack he had a log aviation career as chief pilot for forest service flying T34s I would love to here some of that old hanger flying again.If you know a old timer strike up some old tails before its to late.

I can't see your pictures because I'm at work and they have the site blocked so I am responding somewhat in the dark. When I flew a Long-EZ with a wooden prop, it was mandatory to re-check the torque every month or so, especially as the seasons changed due to variations in humidity. Sometimes, the bolts would be close to loose. Also overtightening is a constant concern due to the eventual crushing of the prop core.

I'm so glad you made it on the ground safe. Wood props have their place but they require constant care and feeding.

Torque

As Randy noted, a wooden prop requires frequent inspections. I used to fly a YMF-5 Waco that required a torque check on the prop every 25 hours.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

Bill, too bad this couldn't wait until after Grenada so you could use all that prize money to help fix your plane! But seriously, sorry about the situation, and glad nothing catastophic happened in flight. My RV had the spacer bolted to the flange, and shorter bolts through the prop and spacer only. My front crush plate also had a circular channel milled to the width of the propellor bolt heads so that the bolt heads would recess in the channel, and keep them from rotating. No safety wire needed, and none to break.

I retorque my Catto prop three - four times a year. On a trip to the east coast, I retorqued the prop on the ramp at Crystal River (humidity concern or perhaps ignorance)

John Clark said:

As Randy noted, a wooden prop requires frequent inspections. I used to fly a YMF-5 Waco that required a torque check on the prop every 25 hours.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

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Yup....all need to add to your condition inspection form....check propeller torque regardless of the manufacturer or material........I know it is a PITA for some to do this but well worth the $$$ and safety to do so.....

Belleville Washers

This is common on wood and wood/composite propellors on the EZs. The solution is to use a stack of Belleville washers in a very specific stack order and to torque the stack a fixed number of turns instead of to a torque value. Contact me off-line and I can provide the details and contact information.

WOW....

Yes Willy, thanks again for that report and using those eyeballs. A great lesson there for prop inspection. I like to think I pay close attention to the prop; but this will be in my mind from now on. Catching this on the ground is mucho better. A great lesson!!!

___________________________
Deal Fair
RV-4 (34CB)
George West, TX (8T6)

How much does vibration play a part in prop problems like this?

I made the investment three-four years ago of a Dyna-Vibe and it's been well worth it just for the smoothness. I do other people's planes all the time so it earns it's keep.

Mike

Side note -

Asside from his questionable treatment of a pretty wooden prop, Bill is a great guy, and a heck of a pilot. He finished well flying Sportsman last November at the Keystone IAC contest near Daytona. This spring, he finished second (in a field of eleven). He has done a great job adapting the RV performance characteristics to the maneuvers required, flying them better and safer than many of the guys/gals floundering about in Pitt's and Extras. He listens to comments from more experienced pilots and really works hard to "fly it right". Anybody interested in IAC aerobatics in an RV should pick Bills brain bare. He's a good example of an RV guy operating in "less charted" territory.

Forrest

piling on to the side notes

MNForrest said:

Asside from his questionable treatment of a pretty wooden prop, Bill is a great guy, and a heck of a pilot. He finished well flying Sportsman last November at the Keystone IAC contest near Daytona. This spring, he finished second (in a field of eleven). He has done a great job adapting the RV performance characteristics to the maneuvers required, flying them better and safer than many of the guys/gals floundering about in Pitt's and Extras. He listens to comments from more experienced pilots and really works hard to "fly it right". Anybody interested in IAC aerobatics in an RV should pick Bills brain bare. He's a good example of an RV guy operating in "less charted" territory.

Forrest

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According to the IAC contest results this Forrest fella is pretty good in his S-1S too. Did you fly Sebring a couple weeks ago? I attempted to CD the Carolina Boogie last weekend, but had to cancel due to Wx. Major bummer.

Be sure not to overlook the part about bolt grip length

Several posts seemed to gloss over the part about false torque indication caused by long bolts bottoming out. If they are marginal to start with, repeated torquing will eventually use up the threads and you will no longer be tightening down against the prop.

Bob Axsom

Prop me up...

Willy,

Nicely done. Been there, done that brother. Now you know.
Having removed and installed "several" wood props on RV's what Bob said above is key. Make sure the bolt grip length is correct. Also a crush plate should be there, I don't see one. That would make up the space difference for the bolt length. Dial the crank before you tear the engine down. Unless you shook it off the mounts, it should be OK.
My buddy JJ had a wood prop split apart inflight and made a nice forced landing at an airport. He said the vibration was so intense he thought it would break off the tail. The engine was fine and he went with a Sensy metal although he liked my Catto.

Most all the wood props will give you very long life with proper care and feeding. Having said all that, the composite props (Catto, MT, Hertzler, P-Tip) hold their torque longer. I still check them every oil change.

Smokey

WillyEyeBall said:

Conditional inspection with torque of prop bolts checked at 22 ft-lbs....

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Prop material is sugar maple, yellow birch, or something else?

My A&P mechanic thinks that the long prop bolts may have bottomed out, and created a false torque value when the bolts were retorqued on the conditional inspection.

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Very possible and easy to measure.

The previous engine overhaul company thinks the 8 ? inch bolts through the spacer were a contributing factor when the aircraft is used extensively for aerobatics.

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If installed correctly length doesn't matter. The bolts are clamping devices, nothing more. Engine torque is transmitted by static friction between the flange face and the rear face of the wood prop. If static friction fails the design backup is generally engineered sizing of the flange bushings. When they wallow out (and they will in short order) the prop bolts will see bending loads and fail....as you've seen.

Let's assume that if you put a prop bolt in a flange without the prop and loosely tighten it until it stops (bottoms out). The distance from the lug forward point to the bottom of the bolt head is 8.0 inches.

How much MORE than 8.0 inches should all the prop bulkheads, crush plate and prop be to ensure that the bolts do not bottom out?

1/8 inch ?

3/16 inch ?

Ron Lee said:

Let's assume that if you put a prop bolt in a flange without the prop and loosely tighten it until it stops (bottoms out). The distance from the lug forward point to the bottom of the bolt head is 8.0 inches. How much MORE than 8.0 inches should all the prop bulkheads, crush plate and prop be to ensure that the bolts do not bottom out? 1/8 inch ? 3/16 inch ?

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This should help Ron....written by Henry S. Rose, Chief Engineer, Sensenich, back in the good 'ole days:

DanH said:

This should help Ron....written by Henry S. Rose, Chief Engineer, Sensenich, back in the good 'ole days:

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Wow, super cool! I consider that, outside the box engineering simplicity that many of us have almost lost .

Thanks Dan. Looks like 0.10 inch should be adequate. I did measure everything when I converted from an Aymar Demuth prop to a Catto several years ago but it helps to have info like this.

sandifer said:

According to the IAC contest results this Forrest fella is pretty good in his S-1S too. Did you fly Sebring a couple weeks ago? I attempted to CD the Carolina Boogie last weekend, but had to cancel due to Wx. Major bummer.

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Yes, Sandifer - I flew Sebring. My first attempt at Intermediate, after all winter off. It was rough. Had to bail before the unknown to make it home for Mother's day, but I was just happy to get some time in the box in front of critical eyes.

Forrest

prop info and thanks

Would like to thank all the responses and valuable information on this incident. I'll let everyone know what we find when the engine is opened up, and the replacement components, including a new prop that we will install.

Bill McLean
RV-4 Slider

I have a question, is giving the prop a tug or two as a pre flight going to allow you to detect a prop that's starting to come loose? I'm assuming that if it's at the point where you can feel play, it's too late.

Steve

50 hour inspections

The rental plane I fly, a Diamond DA-20 C1, has mandatory 50 hour inspections to retorque the bolts on it's Sensenich wood prop. My ground instructor had his prop fall off during a flight, and was able to glide to an airport and land safely. As I recall his story, there was really very little warning before losing the prop. The engine suddenly seemed very rough - shaking so much that he shut it down. The next instant, the prop was gone and everything went quiet. He said it was really quite peaceful.

So during preflight I aways try to look beneath the edges of the spinner and yank on the prop to see if anything is amiss. So far no problems.

Nobody has yet mentioned humidity!

I didn't notice anyone talking about humidity. This is one of the major reasons for re-torquing a wood prop frequently. If the prop is installed when the humidity is low, the prop will swell as humidity increases resulting in the bolts being over-torqued. On the other hand if the prop is installed during high humidity, the prop will shrink as the air dries out and the bolts will actually become loose.
Wood props should be re-torqued more often in areas where there are significant changes in humidity.

Mel said:

On the other hand if the prop is installed during high humidity, the prop will shrink as the air dries out and the bolts will actually become loose. Wood props should be re-torqued more often in areas where there are significant changes in humidity.

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There may have been a prop departure in the west not too many years ago on a canard that was based on the east coast. Perhaps it was a humidity change issue.

BTW.....

..........When you re-torque a wood or other prop, you should first back every bolt off a bit, then re-torque them all in proper sequence. Then check tracking.

Don't just add torque to an already torqued bolt.

Also....

PCHunt said:

..........When you re-torque a wood or other prop, you should first back every bolt off a bit, then re-torque them all in proper sequence. Then check tracking.
Don't just add torque to an already torqued bolt.

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Don't re-torque immediately. Give the prop a little time to "relax" before re-torquing.

What is "a little time"?

An hour? Overnight?

I usually give mine several hours or overnight. No basis other than gut feeling.

Ron Lee said:

What is "a little time"?
An hour? Overnight?
I usually give mine several hours or overnight. No basis other than gut feeling.

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My preference is to give them several hours at the very least. Overnight is good.

I am not sure that I have seen guidance on humidity retorque as a function of exposure.

If going from a dry climate to a wet climate, where swelling may occur, after how many days in the humid climate should you retorque the prop.

I did it after about four days. No sound justification. Just preferred to be overcautious.

Almost lost prop follow up

After two months of down time for repairs, the first test flight with a new prop was a non event, but the grin factor almost overstressed the facial muscles. Boy, do I love these airplanes. As noted with other comments and incidents, the time between having any idea that something could be wrong and things going bad with a loose prop are pretty quick. We (my A&P mechanics, engine overhauler, prop manufacturers) are not certain a single cause can be determined, but the possibility of one or more of the bolts bottoming out, the 8 inch bolts through the spacer, and the 22 ft-pounds maximum torque recommended by the prop manufacturer were considered factors and possibilities. For the engine, the crank had to be replaced along with everything attached to the hub (starter ring, 2 ? inch extension, prop, crush plate, and spinner). After considering my type of flying and prop types, I decided to stick with wood, but go with an all maple one that has a proven record on aerobatic aircraft and the bolts are torqued to 35 ft-lbs for the prop. I have an extension that was recommended by the prop manufacturer that bolts to the hub, and the prop bolts to the extension. I?ve received conflicting opinions from highly experienced engineers and builders on whether the 8 inch bolts through the extension/spacer were a factor or not, but if the prop had been bolted to the extension, most likely the damage would have been limited to the extension and prop, and not the engine crank and hub. The sun is up, so I?m off to the airport to get my facial muscle exercise.