By Carl Ziegler, A&P/IA

The sounds of summer: birds, kids in swimming pools, airplanes fly­ing overhead, the happy banter you hear on the com, and the sound of your three-blade McCauley propeller ripping into your Powertow tug that you left on the nose gear of your Rockwell 114 Commander after you pulled it out of the hangar. Sigh!

I hope this doesn’t sound familiar, but events like this are not uncom­mon. I was recently reminded of a similar incident, where a new owner put gas in the airplane and then ne­glected to secure the fuel cap, which is located on top of the fuselage just below the pylon. You guessed right! After engine start, where did the fuel cap go? On a pusher-engined airplane, everything ahead of the engine is possible FOD (foreign object debris) for the poor pro­peller. The three-blade MT propeller, which is composite, lost an inch or two on each blade, as it chopped into the free-flying fuel cap. Until you see the damage that one bird can do on a fan or an airliner, it’s difficult to figure out how one item can do so much damage to so many blades! Now if that wasn’t bad enough, I received a text later that this owner apparently decid­ed to fly the airplane to his home base with the three damaged propeller blades. I can only imagine the vibration of a 200-horse Lycoming sitting on a pylon spinning three damaged blades. Thankfully, he made it okay.

Here is the next $10,000 question: Will the aircraft owner have the required Lycoming airworthiness directive completed due to the prop strike or will he just go ahead and install a new MT propeller?

Several years back I was at my airport adjacent to my home in Minnesota. I heard the sound of a Citabria 8KCAB on a takeoff roll. I heard the initial power up, a short takeoff roll, and then the sound of the propeller digging into the dirt runway. I came around the corner of the hangar just in time to see the airplane stopped on the runway, but not inverted. Since I live relatively close to the airport I followed up to see if the required inspection would be accomplished on this engine. A few days later I noted the airplane was flying again, most obviously with a new propel­ler. There was no doubt in my mind that this one had a sudden stoppage and should have received a very good teardown of the accessory case to check for the crankshaft retaining bolt (AD 04-10-14 and Lycoming MSB 475C required). In addition, due to the hard prop strike, the accessories, such as the magnetos, should (but not required by the AD) have been inspected. As there are no mechanics on our local field, the maintenance was either completed late at night or not at all, other than swapping out the modified propeller.

I can’t imagine that there was any hanky-panky going on or any undocumented maintenance on this aircraft. I wonder where I’ve heard the term undocumented maintenance?

What Constitutes a Prop Strike

Lycoming, in their definition of prop strikes for the pur­pose of the AD, says:

(i) For the purposes of this AD, a propeller strike is defined as follows: (1) Any incident, whether or not the engine is operating, that requires repair to the propeller other than minor dressing of the blades. (2) Any incident during engine operation in which the propeller impacts a solid object that causes a drop in revolutions per minute (RPM) and also requires structural repair of the propeller (incidents requiring only paint touch-up are not included). This is not restricted to pro­peller strikes against the ground.

Furthermore;

1. 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 flight.

Let’s Take a Look

On the Lycoming engines in particular, it’s important to look at the crankshaft gear on the back end of the crankshaft. Specifically, examining the locking tab for the single retention bolt on the crankshaft gear. If you look at the accompanying picture of the gear, you will note that the locking tab is actually broken.

The one in the photo was on an experimental Lancair that suffered a prop strike on a bounced landing. Subsequently, the release of the lock assembly on the gear will lead to a release of the gear, creating havoc and certainly stoppage of the engine. There would probably be a noticeable crunching sound for a minute or two, then it would get very quiet. In addition, on this engine, we went through the rest of the accessories on IRAN (inspect repair as necessary) reassembled. This was not considered an overhaul, just an inspect and repair as necessary. We noted that the prop strike inspection had been accomplished in addition to the installation of a spanking new compact series Hartzell.

Now here is a kicker. Lycoming, in their wisdom, chose NOT to require complete teardowns and used SB 457C as the guiding reference for completion of gear inspection per the requirements of the AD. However, if you were to look deeper, you will see that Lycoming has further inspections defined by mandatory SB 533C (16 pages) which, if you have taken your engine to a sharp over­hauler, will be the guiding document for work and inspection.

The title of Lycoming MSB 533C sums it up:

Recommended Action for Sudden Engine Stoppage, Pro­peller/Rotor Strike or Loss of Propeller/Rotor Blade or TipMODELS AFFECTED: All Lycoming reciprocating aircraft enginesTIME OF COMPLIANCE: BEFORE FURTHER FLIGHT

It is not uncommon and I am aware of instances where me­chanics do not routinely remove accessories, especially the magnetos, for any kind of inspection with a prop strike. The shockwave transmitted to accessories on the aft part of the en­gine as well as gear-driven alternators mounted on many mod­els of Continental engines is significant. In cases of heavy strikes, it’s also possible to detune your counterweights on the inside of the engine. The inside of both Slick and Bendix magnetos are home to sets of nylon gears. A sudden stop (or intermittent hit) in particular, has a very high risk of cracking or shaving teeth off the nylon gears.

So why would you go through all the trouble of having these components removed when inspecting, per the Lycoming re­quirements, and NOT open them up and verify conditions? On Continental engines that have the alternators mounted on the front or rear of the case, it would be very foolish to not pull the alternator off and look at the drive coupling. Continental says, strikes warrant complete checks of all rotating parts. The alter­nator coupling on these is designed to absorb torsional vibra­tions during normal operations and does have a certain amount of give. These alternators are spinning at an order of magnitude faster than the crankshaft and the sudden stoppage or pulses to the drive couplings risk major damage to the couplings. Ignor­ing critical inspections on these components will pretty much guarantee winning the odds of sudden engine stoppage.

Don’t ever let someone tell you that just “dialing the crank­shaft” is the only thing you need to do. Apparently, Continental engines are known to throw crankshaft flanges after supposedly benign hits, and the crankshaft has dialed out within limits. With Lycoming, you have the AD to force someone to look at the components.

Other Items to Inspect

Engines are not the only items requiring inspection after prop strikes. A new Mooney owner found out the hard way that not all prop shops may be above board. After the accident, the investigators determined that the propeller bolts had failed in flight, allowing the prop to separate from the crankshaft; this ended up being the primary cause of the loss of oil. Of the six bolts that secured the propeller to the crankshaft, five bolts had broken under tension in cruise flight. The sixth bolt failed in shear during impact with the ground. The pilot had come about as close as you can to losing a propeller in the air.

Probable cause for the bolt failures: Stress fractures, caused during the preceding gear-up accident the previous October. Yes, the bolts were (apparently) reused, as hard to believe as it sounds. Although Hartzell requires that the subject bolts be re­placed after a prop strike, there seems to be some doubt as to whether it was actually done in this case.

A friend was recently looking for an RV-8 homebuilt aircraft. Over a period of several months, he fed me a constant stream of logbooks to review for possible candidates. The process of read­ing logbooks can really give you a dose of reality as to the levels of maintenance being performed in the aviation marketplace.

One aircraft he was interested in had very sketchy informa­tion on the engine. I was able to track through the logbooks that this engine originally was in a Mooney M20 series that came up a few feet short of the runway on landing. The cor­responding accident report I found indicated the airplane was totaled. Next, this engine (from the accident) was installed in a homebuilt aircraft of a very unique pusher design. It was only in this aircraft for a couple of years before it found its way into the RV-8 that my friend was examining. At no time in the his­tory of this engine being installed into TWO aircraft was there ever a statement indicating that this engine was torn down, inspected, and/or overhauled. I just shake my head in disbelief that this kind of lineage is out there for the unsuspecting, but I love this kind of sleuthing.

Remember, if your propeller hits something, comply with inspections mandated by regulations and go the extra mile to check for damage to auxiliary components.

Carl has close to 50 years of continuous experience working as an aircraft technician and 38 years as an IA. In addition to GA, he has acquired over 38 years of airline experience with Northwest and most recently with Delta, finishing his last 13 years of airline service as an aircraft inspector. He currently flies a 1976 Cessna 172N.