Rene Bubberman

Rene Bubberman

Well Known Member
Hi RV-ators,

Please give me your opinion on these two cases:

Case 1: We bought a RV-7 which was wrecked in a nose-over landing accident. When it happened the engine was idling, the prop was a MT wooden 3-blade which desintegrated directly on impact and the runway was soft grass. I want to have the engine checked for shockload just to be sure but it will cost me about 4000 dollar. Some people say that it is not really neccesarry, that the chances on damage to the engine are allmost zero given the circumstances during the incident and that it is even bad for the engine to take it apart after only 30 (!) hours.
Case 2: Someone I know hit the concrete at full power (!) with a 2-blade metal prop during a taxi/power check incident. The prop was bent at the tips and will be replaced but the engine will not be tested for shockload.

I'm a bit confused: I would expect that the higher the momentum (velocity at the tips, arm, weight and strength of the prop), the higher the load on the crankshaft and the chances on damage will be. Other people say that a higher momentum will bend/splinter the prop more easily thereby at once removing the momentum on the engine and the crankshaft. What do you think? Am I too careful, wasting money and probably making a good engine worse? How about the other plane?

Please give me your opinion! Thanks!
 
As far as Lycoming is concerned ANY reduction of rpm caused by the propeller striking ANYTHING, even grass, is cause for a teardown.
The most common failure found on a Lycoming is the bolt that retains the gear on the rear of the crankshaft. Any sudden stoppage or reduction of rpm can damage this bolt.

In my opinion, whoever operates the engine in your "case 2" scenario does not have a bright outlook on life!
 
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My opinion which is solely based on conversation with friends of friends whose brother-in-laws is married to the sister of an A&P?. I must agree completely with Mel.

However based on personal experience, I had an engine (not Lycoming) that had a prop strike on the runway (probably at idle) with composite prop and the prop was totally destroyed. Many told me that the engine is probably is fine but I choose to have it checked out as I just wanted to have a peace of mind. The engine was checked to be OK.
 
prop strike

I got an IO360-A1B6 that had a prop strike. The gear on the Acro sport it was on collapsed on landing. The engine was at idle. The prop flange dialed .003. Everything in the accy case was fine. I had planned to fly it as is, but my wings were back ordered, so what the heck, I split the case and sent in the crank, rods and cam.

The Crank was cracked 1/3 of the way around at the thrust face. I gotta go with Lycomings reccomendation at this point.
 
I also agree with Mel.
The potential for damage has less to do with the power setting than it has with the rate of deceleration. Their is a lot of rotational inertia in the magnetos, accessory gears and other accessories that are being driven by them. This is what causes problems with the gear on the back of the crank shaft.
I know of an engine that had a sudden stoppage of a windmilling prop in a forced landing that was caused by fuel starvation. The engine was not running (not even at idle, it had quit). The propeller was not very badly damaged so a teardown inspection was not done. Sometime later after the engine was put back into service, during an oil change when the oil drain plug was lowered after unscrewing it, there was a small rectangular shaped piece of metal about 1/4" on all sides laying in the recess of the plug.

It ended up being one of the two drive lugs on the oil pump shaft, that engage in the two notches on the back of that same gear that is on the aft end of the crankshaft.
On closer inspection (and after the engine was disassembled) it was easy to see that the lug had initially cracked about 1/3 of the way through and then a fatigue crack continued the rest of the way from engine operation.

So even the oil pump has enough rotational inertia to cause damage even if the engine is not producing power.

Many people skip the tear down and get away with it. In fact the majority of engines that get teardown inspections, no problems are found.

You need to ask yourself if the gamble is worth it. I know for me I would never feel good out in the middle of no where over the mountains if I had not checked it out.

BTW my current airplane was also rebuilt from a wreck. Having the engine inspected wasn't a difficult choice because the crankshaft was bent and needed to be replaced. Even if it hadn't been, with the knowledge of some of the strange things I have seen, I would not have thought twice about spending the $4000 (which is a pretty standard price by the way).
 
It seems every discussion of a prop strike always gets around to...........

MT prop that snapped easily
The engine idling
The surface Istruck was soft or grass
Flange runout was within limits

As mitigating factors that suggest engine teardown and inspection is not necessary.

When I suffered engine failure and gear collapse at Rebel's Bluff I struck a soft turf runway with my MT (wood/composite ) Prop. The prop was just windmilling (the engine was generating no power). All 3 blades snapped easily at the hub. Flange runout was just fine but I would never have trusted the engine in flight again, though it was tempting. I sold the engine as a core fully disclosing the prop strike. The new owner had it dissassembled and rebuilt. He said when they opened the accerssory section it looked like a grenade had gone off in it. The crank, rods and rest of the engine were fine but any thing attached to the crank through a gear on the aft side was toast. While it was not a Lycoming it demonstrates the point that no matter the prop or lack of severity of the strike nor the concentricity of the flange, significant damage can occur from the sudden deceleration.

Dissassembly and inspection is expensive but the consequences of not doing so could be deadly and infinitely more expensive.
 
My TMXIO540 had a bad day in the mud. MT composite 3 blade prop destroyed still slightly under power. Disassembled the engine to find the bolt at the cam drive gear spun, the dowel broken off and the gear itself had a nice little crack in it. Fortunately for me, everything checked dimensionally within new limits, and miraculously the crank face was NOT damaged by the gear/bolt/dowel damage. None the less, I sent all the guts to Aircraft Specialties for removal of the gear bolt and complete check out. Money well spent.

Yeah, a tear down and inspection can be expensive, but even if your flange dials .000, there still might be bad things going on back there at that gear. Or elsewhere.
 
Prop & Spinner Strike

The replies in this thread seem to focus on prop strikes, but the original post indicates this engine was involved in a nose-over, meaning that it wasn't just the prop that was struck. The spinner also likely saw some form of impact. IMHO, since the crankshafts in our engines have hardened surfaces, the right kind of impact can result in a crack without necessarily bending the crank. Also, any time more than one blade has been struck, it's possible that the crank bent on the strike of the first blade, then straightened on the strike of the second. All of this means that simply dialing a crank to see if it is still within spec WRT straightness will not tell you if there is a crack as a result of the strike. An engine tear down and inspection is cheap insurance compared to the potential risk of life by trying to save some money.

Skylor
RV-8 QB
Final Assembly
N808SJ
 
It is like a poker hand....you never know which you you will be dealt with...

I will go with the reccomended tear down inspaction since I value my life more than any chances I can take.
 
Rene...

As Skylor hints, I am not sure it is the best treatment for an engine to be turning, whilst the flywheel supports ~1500lb of aeroplane and people moving perpendicularly at 50(+?)K across soft ground :eek:

In the UK I don't think there would be a choice legally... in this case I cannot see one morally either ;) To not shockload strip/test that engine is playing with peoples' lives :mad:

Please give me your opinion!
Click to expand...
You did ask ;)

Andy
 
...well do ya?

DocThrock said:
Yeah, a tear down and inspection can be expensive, but even if your flange dials .000, there still might be bad things going on back there at that gear. Or elsewhere.
Click to expand...

Or, in the words of Dirty Harry, ?You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya punk?? :rolleyes:

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
Dirty Harry is a wise man ;)

Thanks for your input. It supports my decision to have the "case 1" engine tested, that I am not TOO careful and that it is money well spent.
I also learnt a lot about what goes wrong inside an engine (and what to look for!), thanks for that.
But now, to understand: what does more damage to an aero engine: case 1 (landing, idling, wooden 3-blade prop, grass runway) or case 2 (stationary, full power, metal 2-blade prop, concrete), and WHY?

all the best,
 
It depends on what engine it is. As an A&P I have been involved with a few prop strikes. From airplanes bellying in to flipping over. An engine that has crankshaft dampners will need to be torn down. The dampner bushings get damaged and then it throws the dampner. The 0235's bend the crank at the prop flange. The normal 320's and 360's keep on running. I think now however is the legality of the issue. There is an AD on all Lycoming prop strikes and that makes it a manditory inspection I think, weather it is experimental or not. So no matter what my experiance has been Lycoming has had more and is taking the cautious route. The AD/SB also says to replace the crank dowel pin.
 
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