Two Dreaded Words

What a prop strike can do to your aircraft

November 1, 2007

the most are "prop strike." The thought of the damage and cost involved makes all of us shudder. But what is a prop strike and what exactly are the ramifications?

My investigation started with my need for another engine for my Cessna 210. The one that came with the airplane had passed TBO (time between overhauls) but its logbooks didn't go back to when it came from the factory. They were legitimate, having been initiated after an inspection, but that doesn't help when selling the airplane. So my question was, should I overhaul the present engine or look for one with good logs?

The decision was made for me by an advertisement in Trade-A-Plane. The engine for sale was the model I needed, with about 1,000 hours since a factory overhaul. Unfortunately, it had experienced a prop strike. I took the ad to an engine overhaul shop, Corona Cylinder and Engine Overhaul in Corona, California, and had the owner, Joe Anderson, call the person selling the engine. It turned out to be a salvage shop and the owner confirmed that the logs for the engine were available, and the crank had been dialed, and found to be OK. Dialing checks the run-out, which is the amount of movement at the flange when the crank is rotated. Although not a guarantee of any kind, the lack of movement showed that utter destruction had not occurred.

According to the salvage yard, the prop strike came about after an out-of-fuel landing on a road in Louisiana. During the subsequent takeoff, the airplane's right wing hit the mirror of a semitruck, which veered the airplane into a fire truck and eventually a ditch. The accident was recorded on video and available on the Internet. I watched the video and figured the pilot had reduced power before hitting the ditch. I bought the engine and started my quest to determine what I absolutely had to do to get the airplane airworthy. I wouldn't skimp on safety, but if I could save a few dollars on cosmetics, I sure would.

What's a prop strike?

To start my investigation, I wanted the official definition of a prop strike.

Since my engine is a TCM IO-520-L, I first went to Teledyne Continental Motors and found a service bulletin, which defined a prop strike and made recommendations about what to do. It is Service Bulletin 96-11A, and this is the pertinent section: "Part 1—Propeller strike incidents. A propeller strike is: (1) any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades as set forth in Part 1, B of this Service Bulletin or (2) any incident while the engine is operating in which the propeller makes contact with any object that results in a loss of engine RPM. Propeller strikes against the ground or any object can cause engine and component damage even though the propeller may continue to rotate. This damage can result in catastrophic engine failure."

Airworthiness Directive 2004-10-14 is applicable to Lycoming engines that experienced a prop strike. It pretty much echoes the TCM service bulletin but adds two other situations. One, "A sudden rpm drop while impacting water, tall grass, or similar yielding medium, where propeller damage is not normally incurred." And two, "The preceding definitions include situations where an aircraft is stationary and the landing gear collapses causing one or more blades to be substantially bent, or where a hangar door (or other object) strikes the propeller blade. These cases should be handled as sudden stoppages because of potentially severe side loading on the crankshaft flange, front bearing, and seal."

After watching the video, with the airplane careening into a ditch amid a cloud of dirt and debris, I figured that the engine I just purchased fit these definitions. Now I had to determine the consequences.

Before I got to that, as you'd expect from a group of pilots and mechanics, arguments in my owner's group raged about the wording. For example, how much propeller filing constitutes "minor dressing?" And what's a "solid object?" Recently, a transient aircraft hit a rubber traffic cone with the prop and kept going, taking off and disappearing into the western sky. In another incident a prop hit a small rock, which had been dislodged and thrown back by a larger aircraft. The prop had a good-size dent in it, prompting the pilot to remove the prop and send it in for repair. Should he also have performed the obligatory engine inspections?

My group also argued about the propeller hitting grass and water. How much grass and how deep should the water be, and how sudden is "sudden," and what constitutes an rpm drop? Does 500 rpm fit that bill? How about 50 rpm? Plus, unless you were watching the tach, how would you know? Sure, some of these arguments are specious, but the point is, there's a lot of gray area, and this can cause many incidents to be ignored. You can bet the fellow who hit the traffic cone never made a logbook entry unless the engine had to be repaired. Unfortunately, it may be the next owner who discovers any hidden damage.

Mandatory post-strike actions

Now that I had a definition, what would I have to do about such an event? After defining a prop strike, TCM's SB 96-11A states:

"A. Propeller strike inspections. Following any propeller strike complete disassembly and inspection of all rotating engine components is mandatory and must be accomplished prior to further operation. Inspect all engine accessories in accordance with the manufacturer's instructions."

TCM's SB 97-6 specifies the mandatory items that must be replaced when the engine is torn down. I won't list them all here, but some of the more expensive ones are: hydraulic valve lifters, connecting rod bearings, the crankshaft's main and thrust bearings, all needle, ball, and roller bearings, exhaust valves, valve springs, piston pins, pistons, crankcase through bolts, all the hardware, seals, gaskets, O-rings, and lots of other small stuff.

There are other things that have to be checked, including the magnetos, propeller governor, and fuel system. Basically anything that moves is subject to visual and other forms of nondestructive testing.

The concern, of course, is the prospect of hidden damage, which can cause, and has caused, subsequent engine failures. Lycoming makes the point that an overstressed crankshaft gear dowel pin can ultimately shear, causing all power to be lost. Lycoming has reports of ground strikes, which have resulted in overstressed connecting rod bolts, that failed later. This is one failure no one wants to experience. If you can imagine a loose rod banging around inside your engine, you know it's bad.

Expert opinions from the field

Service bulletins are one thing, but the real information comes from the people working in the field. But let me start this section with a caveat: There is no black and white, and the subject under discussion is the grayest of the gray areas. The opinions differ with each individual, and what maintenance technicians would do personally is quite often at odds with their professional opinions. That said, you'll understand that this article isn't going to state, or even hint at, what you should do in this situation. I spoke with several shop owners and asked their opinion.

On the record, owners said that if someone came into their shop and said he or she had an aircraft with a prop strike, they would follow the procedures outlined above to the letter.

Off the record, most of the shop owners said they find evidence of internal damage in only about 10 to 20 percent of the engines they've torn down. They all agreed that if it were their aircraft, they would think long and hard about doing a teardown and inspection. Their decision would involve the specific engine involved, the severity of the prop damage, the cost, the time on the engine, whether the prop was stopped or appreciably slowed, who was flying at the time, what else is going on in the shop, and maybe the time of year and their mother's maiden name.

Anderson, owner of Corona Cylinder and Engine Overhaul, said the cost of a teardown and inspection, for a small four-cylinder engine like the O-360 or -320, is about $4,000. There's also the cost of inspection of the accessories, which is extra. If you need someone to remove and reinstall the engine, that's another $1,800. A full overhaul will cost about $12,500 plus accessories. The $8,500 differential is not insignificant. As for whether the inspection should be done, Anderson said he couldn't answer for anyone but himself; however, he wouldn't hesitate if it were his engine.

John Schwaner, author of the Sky Ranch Engineering Manual, a must have for all those interested in aircraft engines, and owner of Sacramento Sky Ranch, in Sacramento, California, said the quandary is, "How do you know if something is good or not? Maybe it is and maybe it isn't." Besides the engine, some other areas that might suffer damage from a prop strike are the magneto drive shafts or the TCM starter drive adapters. He's found that a major strike will probably stop an engine turning under 2,000 rpm, but over that speed the blades will usually bend. On TCM engines with the geared alternators up front, he has seen the mounting flange and the gear drive bolts break during a prop strike.

As for dialing the flange, he said TCM nitrides the crank so checking the run-out of the flange is a waste of time. The flange will break before it bends. On Lycoming engines, which are not nitrided, the flange can be bent. Also, on prop strikes involving frontal impact, the crank can be pushed back into the engine, cracking the case at the slinger ring, and this may not be visible.

Ken Tunnel of Lycon Aircraft Engines in Visalia, California, an engine facility used by a number of owners of show airplanes, said, "There's no way you can tell if there's any damage regardless of the circumstances. What looks like light damage may sometimes result in something broken or highly stressed inside, while a major collision shows nothing wrong after it's been torn down." He used a football analogy of a player who can be brutally tackled and get right up, then trip over a bucket and break his leg.

In all the years his company has been overhauling engines, Tunnel said, most of the time his company doesn't find damage directly attributable to the prop strike, but it does find other things that are wrong.

Talking about whether to have the engine overhauled or just inspected, Tunnel said it can be penciled out. How much more time before overhaul is remaining on the engine? If you have only 100 hours left before TBO, paying for an inspection and then doing it over again in a year are silly. But if there's 1,000 hours or more left on the engine, you're probably better off with just an inspection.

How to protect yourself

I alluded to the potential of buying an airplane that suffered a prop strike without the required inspections being completed. Removing and reinstalling a propeller take just a few hours, and the prop shops don't care who brings the job through the door. We like to think that those we deal with are honest, however, there's a chance of impropiety by a seller, so when buying an airplane, be careful.

Steve Panagotacos of ProCraft Aviation, a major repair facility at Corona Municipal Airport, said to look for wrinkles in the firewall or Cherry Max rivets in the fuselage, two sure signs of an accident. He also said prospective owners should review the logs to see if the propeller has been changed. If it has, and there's no indication in the engine logs about a teardown and inspection, it's time for a long explanation from the seller.

Another thing to look for in the logs is a prolonged period of inactivity, such as years between annual inspections and oil changes. This could indicate a gear-up landing, or other source of damage that took time to fix. Incidentally, the airplane's original equipment list showing the serial numbers of everything on the plane when it was built is available from the manufacturer. Panagotacos recently paid $66 for the list of a 1956 Cessna 182, which is in his shop.

He also said that if you have a prop strike while traveling, don't go to the first shop you see and turn the airplane over to it. Secure the airplane and figure that it will be out of commission for a few months. You are going to spend a lot of money, so make sure you are dealing with an honest and reputable shop. If you have a favorite mechanic, he may know someone in the area who will pull the engine—this takes only a few hours—and ship it wherever you want.

Schwaner made the point that more than a few people have been told their engine's crank is bad, but it just so happens that the shop has the one you need in stock. In fact, the customers were sold their own crank. If possible, he suggests, mark the flange somehow, or take a photo of the scratches and nicks so you can compare it to the "new" one later.

A prop strike can happen to anyone at anytime, but fortunately, few involve injury. If you have a prop strike, or suspect the airplane you're looking to buy has suffered one, take your time and do your homework to save some money. Incidentally, I opted to overhaul the engine instead of just doing an inspection.

Steve Whitson, AOPA 593652, of Perris, California, has more than 3,000 hours and owns a 1977 Cessna 210.