Prop Strike!

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When it happens, it is loud and scary. But prop strikes can range from minor to life-threatening, and your health and pocketbook depend on determining how much inspection and repair is needed.

Editor's Note: This article originally appeared in Cessna Owner and Pipers Magazines.

Maintenance

My wife, Kate, has been taking flying lessons on-and-off for a few years now. She's been taking every opportunity to practice, so was up and out at the airport bright and early one Sunday while yours truly slept in.

When the phone rang, I wasn't really very awake. "Hello," I said, more than half asleep.

"I just did major damage to the plane," a small voice said.

My first concern was to make sure that she was OK and nobody else was hurt. And nobody was. What happened to Kate is one of the most common minor ground problems private pilots encounter: She forgot to take the tow bar off, and when she tried to start the engine, it interrupted the prop. (See Kate's story on the sidebar below.)

I'm sure most readers have heard about hitting the tow bar. Stories abound of them being flung over hangars, and of airplanes being successfully started and taxied with one attached -- only to hit with a bang when the airplane lands. It's apparently so common that the Civil Air Patrol launched a major program to minimize such incidents. And, of course, the tow bar isn't the only thing the prop may strike: rocks, birds, and the runway (during gear-up landings) all come to mind.

Is That Bad?

So, if the prop strikes something, just how bad is it?

Pretty bad. McCauley Service Bulletin 176D defines a "Blade Strike" as "any impact or suspected impact of the rotating propeller upon such items as, but not limited to, the ground, tow bars, landing lights, carts, snow banks, hedges, etc.," and goes on to say that deciding whether or not you've actually had a blade strike is up to you as the aircraft operator.

If you've had one, though, SB 176D doesn't provide much leeway: "Any McCauley propeller experiencing a Blade Strike must be removed from the aircraft and completely overhauled by an FAA approved propeller repair station ..." (emphasis mine).

Why? In part -- as SB 176D notes -- because you can have internal damage to a prop (particularly a constant-speed prop) even if there's no visible damage. And it's not a simple matter of dressing or replacing the affected blade(s): SB 176 D goes on to say that the propeller hub itself must be scrapped if any single blade is damaged beyond repair limits.

Senesnich -- which manufactures only fixed-pitch props -- is a little more lenient; its Service Bulletin R-17 says, "Do not fly your aircraft under any circumstance before a thorough inspection by qualified personnel if the propeller has been subjected to impact." It's up to the inspector to decide if the prop must be removed and overhauled.

Do We HAVE To Open The Engine?

Well, that takes care of the propeller -- what about the engine? Up through the mid-1960s, it was common practice not to open the crankcase after a simple blade strike. Instead, a dial-indicator test was performed on the crankshaft end (basically, to see if the shaft was bent). This is now considered inadequate. Expensive as pulling the engine and opening the crankcase may be, consider that the alternative could be a catastrophic failure in flight (or during takeoff, when the engine is generating maximum power) at some point down the line ...

For exactly that reason, the engine manufacturers are unanimous: Any damage that requires repairing the prop also requires pulling the engine for inspection and overhaul.

Lycoming's Mandatory Service Bulletin 533A defines a prop strike as "Any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades ... [or] in which the propeller impacts a solid object which causes a drop in RPM and also requires structural repair of the propeller (incidents requiring only paint touch up are not included) ... [or] A sudden RPM drop while impacting water, tall grass, or similar non-solid medium, where propeller structural damage is not normally incurred ..." It goes on to say that in any of these cases "the safest procedure is to remove and disassemble the engine and completely inspect the reciprocating and rotating parts, including crankshaft gear and dowel parts. Any decision to operate an engine which was involved in a [prop strike] without such inspection must be the responsibility of the agency returning the aircraft to service."

Similarly, TCM's Service Bulletin SB96-11 defines "propeller strike incidents" in much the same way as Lycoming, and says that "Following any propeller strike a complete engine disassembly and inspection is mandatory and must be accomplished prior to further operation ..." It does include one small out: "For instances where the propeller is damaged by a small foreign object during operation, such as a small stone, inspection and repair must be accomplished in accordance with the propeller manufacturer's published instructions. Any time foreign object damage requires propeller removal for repairs other than minor dressing of the blades, the incident is considered a propeller strike and [requires a complete engine disassembly and inspection]."

I talked to Terry Horton, TCM's supervisor of customer service, about this. "What can happen is that you get a crack, which can lead to a crankshaft failure," he said. "You can also load the front main journal in the case to a condition that leads to a crack because of the load there; and you can get accessory gear damage. We got more strict because we found a series of situations where a guy has a strike, and checks it out according to the old rules and it looks OK; but we found some people were having problems down the line. We also had people trying to interpret -- what constitutes a prop strike? In essence, unless you have something that only requires a minor dressing of a prop, then you have to tear the engine down and overhaul it -- period. We made it a more comprehensive inspection than in the past, which catches more problems and makes it safer. If it requires you to remove and repair (or replace) a prop or blade, then you need to tear down the engine. It may seem like overkill, but it's not. We see it as a very serious safety issue for our users. We've had instances where the old-style tests were done, and looked fine -- and then, a couple of hundred hours down the line you'd have a crankshaft failure. If our history didn't show this, we'd never have changed our inspection criteria."

John Standiford, a mechanic with Ultimate Engine, agrees: "Anything that requires sending the prop to a prop shop -- finishing, filing, etc. -- is considered a prop strike. I've even seen a situation where someone hit a spinner with a hangar door, and the insurance company wrote it off as a prop strike. If it's up to me, and there's any question about it, I'd do an overhaul. Say you have a minor strike and it passes all the usual tests -- dial indicator, magnaflux, what have you. 300-400 hours later, it could fail after flexing in operation. It's just safer to do the overhaul and be done with it."

Not So Bad This Time

For me, the one small ray of hope in all this was the unanimous opinion of everyone I talked to -- starting with my own local mechanic -- that what happened to us didn't really constitute a true prop strike: There was no visible damage to the prop, which indicated that the engine never actually started: The prop was being swung only by the starter motor. But contrast that with the experience of a friend (and Mooney M20K-231 owner) who prefers to remain nameless:

"I made what looked like a normal landing; but something didn't feel right. I was a little long -- and I had some people on board who'd never been in a small plane before -- so I went around, landed, shut down, and went to close my flight plan. When I came out, one of my passengers said, 'Did you see the prop?' I looked -- and both blades were bent back at the tips. I didn't even realize it was damaged.

"I couldn't believe I'd actually flown it with a damaged prop. Suppose I'd had a vibration problem? I could have lost the engine!"

This happened during the return from a group flight to Mexico on a medical mission. I remember how pale the pilot quoted here looked when we caught up with him (my plane's a bit slower than his). Even his story, though, turned out to have a silver lining:

"I called my insurance company, USAIG. They asked who I wanted to do the repair. I really didn't know, but asked a mechanic at my home airport to do it. He got a ferry permit, dialed the crankshaft, installed a spare propeller, picked up the airplane, and brought it home. Then he pulled the engine and did a major overhaul, and replaced the prop. I traded up to a three-blade and had to pay the difference between that and a new two-blade out of pocket. Everything else was covered by my insurance, and my rates didn't go up. The airplane was in the shop for about one month total."

Now that's got to be the single best reason to carry hull insurance I can think of.

As for my own story -- we got lucky because the engine hadn't actually started when the prop contacted the tow bar; only the starter motor was operating, and it expended most of its energy pushing the nose wheel to the side. Our mechanic looked it over, and his verdict was two words: "Fly it."

My brother, an aeronautical engineer for the U.S. Air Force, suggested that we get an oil analysis done soon (I do one at every oil change) to see if any metal might turn up in the oil; but as I write this, it looks like we got off lightly.