Prop Strike Damage

Prop Strike Damage

I make a very stupid, costly mistake today.  I pulled my 172N out of my hangar using a manual tow bar.  After doing a few things around the hangar, I hopped in and began to taxi to the fuel tank without remembering to remove the tow bar.  When I crossed over a little dip in the taxi way my prop struck the tow bar.  I'm wondering if any of you guys have an idea of the damage.  The prop is bent a little, and has a half moon shaped indention on the leading edge that is about 1/4 inch deep.  I'm wondering if I'm only gonna be out a prop, or if the engine will need to be torn down.  The tow bar was made out of thin tubing and I was only turning about 900 rpm, but it was still quite a bang.  Any ideas and/or advice (other than be more careful next time) is appreciated.

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Re: Prop Strike Damage

There is no law... However, the engine companies service letters say if the prop was stopped by the strike, a full teardown inspection is required. If the prop didn't suffer a sudden stop, I don't think it is required. BOB

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Re: Prop Strike Damage

I've seen a couple of props that have hit the ground...pavement and sorta "Q-tipped" them and the engines were deemed to be just fine.  The propstrike didn't stop the engine. 
I'd check with a reputable prop shop...they may be able to repair your prop and advise the likelyhood of engine damage.

Michael

I wont mention any names...but, I've seen a takeoff & landing with the towbar attached...without incident...
Imagine the adrenal rush if that tow bar flipped up and hit the prop in flight!!!

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Re: Prop Strike Damage

Whether or not the prop was ever completely stopped is hard to say.  The tow bar was made out of very light weight material, and it wasn't a real solid hit (compared to ground etc.) because the nose wheel was able to turn when the prop struck the bar.  I guess it is possible the prop could have stopped for a split second on impact, but from the cockpit the engine seemed to never break stride.

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Re: Prop Strike Damage

When I was learning a student did that and the piece of tow bar ended up embedded in the building. Thank god nobody was hurt. If the tow bar was aluminum like this was, the school just dressed the prop and put it back in service.

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Re: Prop Strike Damage

I did just what you described about 4 years ago.  Ended up with a nick about 1/4" deep in the tip.  While my mechanic didn't feel comfortable filing it out, we had the propeller repaired and placed back in service.  My mechanic didn't think the engine needed to be torn down and I respected his 35 years of experience, I've had about 225 hours of trouble free service since.  Don't feel bad, little things like this happen.

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Re: Prop Strike Damage

I plan to have a mechanic look at my plane tomorrow.  I know that something will have to be done about the prop, but from what I'm hearing the engine is a tough call to make.  Myself and most other people I talk to seem to think there is probably nothing wrong with the engine, but I have always made it a point not to gamble on flying.  To make it even tougher to think about an engine tear down is the fact that this is a factory new engine with about 30 hours on it.  Do any of you have advice on how to proceed if I do decide to tear it down?  Should I send the engine to someone like PennYan, or just let a good a&p call the shots on the engine health?  The prop is another issue.  Do I get mine rebuilt or just buy new?  I know little about how well rebuilt props usually perform.  Some names of a few companies who are good with problems like mine (prop and engine tear down?) would be appreciated.

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Re: Prop Strike Damage

John:
   If the engine didn't stop, there is no likely reason to have the engine inspected, but do have the flange checked with a dial indicator when you have the prop off.  A prop re-profiling might be in the hundreds, not thousands. If you are bound determined to have a "prop strike teardown" call your insurance company. the shops around here charge about 8 grand for a prop strike tear down inspection, not counting R&R. If you have a fixed pitch prop, buy a new one, if it's a CS have the hub checked and replace the damaged blade, either way it's at least several grand for the prop. So the total might run 12 to 15 grand for a "Full Monty" prop strike deal.
   BOB

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Re: Prop Strike Damage

it's very likely that a prop shop will make your prop like new.  They can fix a ding that size.  I'd think that changing the pitch on a prop would be a bigger operation than what yours needs.
Your prop whacked a whimpy little tube with one blade.  I doubt that your engine even noticed it.  A good prop person should be able to tell you what kind, if any, of damage your engine may have incurred.
The nearest prop shop to me is agout 2 hrs away...they pick up and deliver as part of their service. 
Also mechanic that had delt with similar situations should be able to give you a good opinion.
From your description, I don't think I be worrying just yet on how to finance a teardown. (that's pretty much the same thing as an overhaul...)

Good luck...don't worry yet..
Michael

ps...several years ago, some guy prop started his Champ.  He had way too much throttle and the tail tiedown snapped.  His prop gobbled up the horizontal & vertical stablizer of the plane in front of his.  He damaged 3 other planes in the process...his engine was fine.

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Re: Prop Strike Damage

John,
90% of the aircraft that we repair have had prop strikes (gear up landings). About 50% of TCM cranks reject, and probably 25% of Lycoming cranks reject after a solid strike. TCM and Lycoming both have mandatory services bulletins addressing engine sudden stoppage. Both basically say that and strike that causes engine suuden stoppage OR requires the prop or a prop blade to be replaced, warrants an engine TDI.
Now, if you are operating your aircraft Part 91, you may choose to completely ignore these SB's. The FAA does not have an AD, nor any other regulatory requirements for engine TDI for part 91 aircraft.
I always do the TDI; the insurance companies want it done. They don't want the liability should the crank later fail. But, if it were my personal plane, and the strike was as you have described, I would do the dial runout, and liquid penetrant inspection to the crankshaft flange. Then a couple more frequent oil changes, cut the filter open.
If you decide to do the TDI, CALL PennYan, make sure that you have the TDI performed at a facility they approve, and ask how it will affect the waranty.

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Re: Prop Strike Damage

Del,

What's the dial runout check?

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Re: Prop Strike Damage

It is a run-out check of the crankshaft. I doubt your little towbar could bend the crankshaft but that is what they would be checking for during this process.

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Re: Prop Strike Damage

I appreciate all the info and advice you guys have given me.  I haven't called my insurance company yet to see what they want me to do because I dread seeing  my rates jump.  If the insurance company doesn't get too crazy with my premiums, I will probably go ahead and send my engine off for inspection.  I was thinking either PennYann or Mattituck.  It is painful to do this after I just got my plane back from the 180 hp conversion that took around 1 1/2 months to complete.  I made several posts concerning this plane just after the conversion was complete.  My cht's were high and are still higher than what I prefer.  My hottest cyclinder still lingers in the low 400's after break in.  I've checked everything myself and many others can think of with no cure.  The only thing I haven't done that may be a possible cure is change the carb.  If I tear down the engine, is there anything that can easily be changed or checked that may solve or identify my high cht problem?  Many people tell me that I am being too much of a perfectionist and that my cht's are fine, but 4 out of 5 a&p's say they don't like to see cht's as high as mine.  I'm sorry to keep opening a new can of worms with each post, but it sure makes the decision making process a lot easier when there are several people bringing things to light.

Thanks,
John

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Re: Prop Strike Damage

John, There is a Airworthiness Directive 91-14-22 pertaining to prop strikes for Lycoming engines.  The AD also references a mandatory service bulletin put out by Lycoming (MSB 533A) which covers prop strikes.  However the AD specifically mentions certain engines that do not apply to the AD, for ex. O-320-H series engines.  Your insurance company may be able to pick up the cost of the engine TDI and prop overhaul/replacement.  I know exactly how easy this can happen, cause it happened to me after towing aircraft out of hanger.  After engine start up i realized the tow bar was still hooked up to nose gear.  I was fortunate that the prop did not hit it!  Have your mechanic research to see if this AD affects you.

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Re: Prop Strike Damage

John, You do what you have to do - but if that were my 172, I would carefully remove the propeller and take it to a qualified prop shop and have it fixed.

As for that engine teardown... Don't think so.. If you attacked a huge non-moveable object than Yes.. but your engine wasn't even aware of that little towbar. That towbar and prop absorbed the beating - I have seen engines hit tougher stuff than that and the teardown revealed nothing.. (actually a VW bug)

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Re: Prop Strike Damage

Barry,
The dial runout check is one of the inspections we do after a prop strike to determine if the crankshaft flange (where the prop bolts on), is bent, and if so, how much. This is done with the prop removed. A dial indicator is attached to a strategic location on the engine case, with the indicator resting against the aft vertical portion of the crank flange. Top plugs removed, engine cranked over while you watch the indicator.
This is something we do to every aircraft that has had a strike, before we ferry the aircraft here for repair. Some FSDO's require it, and some never mention it as a condition to obtaining a ferry permit. Occationally, we'll run into one of those few FAA inspectors who's parents were never married; the kind that'll stand over you shoulder to make sure your doing it right, even if he's never seen it done before.

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Re: Prop Strike Damage

Del,

First of all thanks for the very complete answer!

Besides the prop, it makes sense the crank shaft may be the first indication of a problem.  Since the crank shaft is the point that is being inhibited from turning while the engine continues to put force on it.  The shaft is restrainded by the bearings, its housing and motor mounts/supports.  I presume what you are saying is that these are considered much stronger and that the weak link is always the shaft?

John,

I'm asking Del and others just to understand the dynamics and aspects of a prop strike which I have no experience with.  Don't take it to mean that your tow bar caused any of these to even be bothered. I don't want to minimize this but you sound very concerned & I didn't want to add to it.  Based on your discription though it doesn't sound like it was even close.  Hell, I with my feeble human hands, can turn the wheels and have knocked my tow bar off all the time just pulling the airplane.  The damage done to the prop seems to be due to the speed of the prop hitting a small minimally restrained mass.   A small pebble can dent the prop! Aren't they made out of aluminum?

"but from the cockpit the engine seemed to never break stride"

You even said you heard a loud noise but never metioned that you felt anything while sitting in the airplane.  The tow bar wasn't restrained and gave way.  An indication of potential damage, I would think, is that the plane would probably have been shaken violently as the energy would be transferred through the engine mounts to the plane's structure. 

Good post and very informative.

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Re: Prop Strike Damage

John,
First, my disclaimer: Do what you have to do.
Now, my opinion: I agree completely with Barry.  You have to use common sense.  The prop is alum., a soft metal--easily dented.  The alum. acted as a shock absorber when it dented.  It absorbed the blow for the time it took to accelerate the tow bar from stationary to moving like a bat out of hell smile .  It was this dent that saved your crankshaft and other parts from destructive torsion.  If you hit a rock that dented your prop equally, would you tear the engine down?  I doubt it.  The most I would do is a test of the prop hub to see if it's cracked or bent (I seriously doubt either), and of course get the prop fixed.
Good luck!
Bob

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Re: Prop Strike Damage

I've never seen what I believe to be so much bad/mis-information.  Continental and Lycoming have a very simple test to determine if the engine should get a prop-strike/tear-down inspection.  Both have mandatory service bulletins addressing this issue.
  Here's the rule:  If the prop must be removed in order to repair it, then the engine must be disassembled and inspected for internal damage.  NOT check the prop flange for run-out.  (An old cheater-mechanic's trick that doesn't tell you squat.  If the prop flange is bent sufficiently to be read on a dial indicator there's definitely good reason to reject the engine, but unfortunately plenty of damage can occur internally without bending the flange.)
  John, don't practice the false economy so many are suggesting for you.  Surely you have insurance.  This event is EXACTLY WHY you'be been paying insurance premiums.  This event is not even an "in motion" event, since you weren't taxying for purposes of flight.  Some insurance policies won't even make you pay a deductible in this case.  You should immediately report it to your insurance agent, then select the best shop in the area to do the removal, and ship it to the best engine shop you can find to do the teardown.  All this, and the prop repair/replacement will be paid for by the insurance.  Don't worry about your premiums going up over it.  This is such a small claim relative to the big ones that cost lives your insurance company worries about, they'll not likely raise your premiums a single penny over it.  This exact same event happened to a good friend of mine, and he got a new engine, new prop, new governor, and new spinner, along with lots of other added value.
   Or you can take all the bad advice, do little or nothing, trying to keep this quiet, and then take a huge loss on your airplane when you try to sell it.  It doesn't take an Einstein to figure out a prop repair without an engine teardown means you didn't take care of it properly, and any subsequent owner will be risking big bucks and possibly injury because of your poor decision.   Wait until someone gets hurt over it, and see what your liability is.
  My 2 cents.

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Re: Prop Strike Damage

George,
Would an insurance company replace the engine, paid in full, or pro-rate the existing engine based on remaining hours and cut you a check for the difference?

TIA
Jim J

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Re: Prop Strike Damage

Jim J.,
They'll prorate the prop, but not the engine. The propellor is damaged, and must be repaired, or replaced, no exceptions. The manufacturer has a recommended TBO, so the insurance company will use this TBO figure and their established average cost of an overhaul to calculate the amount their going to pay for the prop. The engine is a different story. Because there is NOT a requirement to do a TDI, and some owners/mechanics may choose to not perform the TDI, you can bet that there have been and would be bad descisions made in the field that could result in engine failure later on, and very likely a much larger claim. As the information in the next post states, there are many variables to consider with a prop strike, and I believe that insurance companies would not want to even consider getting involved in deciding whether to pay or not to pay for a TDI. For this reason, they pay the full cost of a TDI. I've included engine TDI on every bid that I've submitted, that had prop strike, regardless of severity, with no questions ask. They pay to avoid any future liability.

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Re: Prop Strike Damage

John,
More info for your descision concerning your prop strike.

(cut/paste)

"AVCO LYCOMING WILLIAMSPORT DIVISIONAVCO CORPORATION
SERVICE LETTER
Service Letter No. L163B
December 23, 1977
(Supersedes Service Letter No. L163A)
TO: All Owners and Operators of Avco Lycoming Opposed Series Aircraft Engines.
SUBJECT: Recommendations Regarding Accidental Engine Stoppage, Loss of Propeller Blade or Tip.
On numerous occasions we have been consulted about recommendations on whe'ther to continue using the aircraft engine that has been involved in the separation of the propeller blade from the hub, the loss of a propeller blade tip or sudden stoppage following accidental propeller damage.
Conditions which surround accidents are many and varied; therefore the circumstances of the accident can not, in our opinion, be used to predict the extent of the damage to the engine or assure its future reliability.
Therefore, in matters of this kind, the severity of the damage to the propeller and suddenness of the stoppage must be two factors on which to base judgement as to whether inspection of the engine is required. For example, it is generally accepted that minor propeller damage will not cause hidden internal damage and the engine can be continued in service with reasonable assurance of trouble free operation.
Nevertheless, Avco Lycoming must take the position that in the case of a sudden engine stoppage, loss of propeller blade or tip, the safest procedure is to remove and disassemblies the engine and completely inspect the reciprocating parts. Any decision to operate an engine which was involved in a sudden stoppage, loss of propeller blade or tip without such an inspection must be the responsibility of the agency returning the aircraft to service.
NOTE
Revision "B" revises text and adds recommendations on propeller blade. 
WILLIAMSPORT, PENNSYLVANIA 17701
Member of GAMA
General Aviation
Manufacturers Association"

Now the Feds veiw:

"TO PREVENT LOOSENING OR FAILURE OF THE CRANKSHAFT GEAR RETAINING BOLT, WHICH MAY CAUSE SUDDEN ENGINE FAILURE
91-14-22
TEXTRON LYCOMING
Amendment 39-6916
Docket No.  No. 89-ANE-10
Applicability:  Textron Lycoming (formerly Avco Lycoming) direct drive piston aircraft engines (except:  O-320-H, O-360-E, LO-360-E, TO-360-E, LTO-360-E and TIO-541 series engines).
Compliance: Required at each engine overhaul, after a propeller strike, sudden stoppage, or whenever gear train repair is required.  A propeller strike for the purpose of this AD, is defined as a sudden engine stoppage, or loss of a propeller blade or tip."

John, if you did not have sudden stoppage, and did not loose a blade or tip of a blade, this AD is not applicable to your situation.
You may make your decision based on the recommendations givin in the Lycoming Service Letter, which leaves the decision up to the mechanic returning the aircraft to service.

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Re: Prop Strike Damage

George,
Maybe I should have made myself a little clearer. I'm not saying that we perform a penetrant inspection and dial the crank in lieu of a TDI. This is however the best indicator of the condition of the crank, that we use prior to a one time ferry flight back home where the TDI is then performed. Lycoming does have procedures and tolerances for the dialing the crank, and I'll try to find that document, just to satisfy your curiousity.

Thanks for your educated opinion, and entertaining slant on this subject. You sure know how to keep us on our toes.

Del

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Re: Prop Strike Damage

George. .
First Question... Does a Mandatory Service Bulletin mandate compliance?

Second - Did the Engine actually STOP. (by hitting the towbar that was able to be disengaged?

Third - Won't the Engine Manufactures always go toward the TDI if they are asked to print  an opinion?

I know that the TDI is the only 100% sure way of disclosing damage - but, would be disassembling and reskinning high time airframes also be a 100% good idea?. Why was the guidance from the FAA so foggy on that one?

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Re: Prop Strike Damage

As usual, George has brought some good advice to the table.  Thanks for straigtening us out!

How do they determine the extent of internal damage during a TDI? 

Volumetric exams of all drive components?
Visual inspection techniques? 
Measurement tolerances?

It's seems to me that a 100% inspection would cost as much as a new engine?  Is that the concensus or is it the point?

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