The Case of the Missing Propeller

Checkpoints: Diagnosis.

7

I received an interesting phone call a few weeks ago regarding an RV-6A that was in the process of being towed to our shop. It had recently been purchased in California and flown back to Falcon Field a day earlier. It was here for a final training flight for the new owner, who was going to fly home to Auburn, Alabama, once the lesson was completed. Obviously, a long cross-country trip doesn’t allow many opportunities for air work, such as stalls and engine-out maneuvers, so this was the opportunity to complete the training.During the flight from California, the pilots had noticed engine vibrations at varying rpm, usually most noticeable at startup and shutdown, but they seemed to smooth out at 2400 rpm. During the training flight, the vibrations got noticeably worse, and the decision was made to return to the airport ASAP. That was an excellent decision, as upon landing the propeller departed the aircraft!

I had left for the day, but my son, Nick, was still there. He remarked that he heard it making awful noises on final approach, sounds that he had never heard from an airplane. He lost sight of the aircraft behind some trees as it touched down, but another customer saw the propeller depart the aircraft. Luckily, the throttle was immediately reduced, and the aircraft landed without any further issues.

I couldn’t wait to get back to the shop in the morning to investigate, all the while thinking about how lucky they had been on the trip home. Surprisingly, the composite prop and spinner showed minor damage, and I was happy that the spinner was still attached as it meant the bolts should still be there.

All of the bolts remained captive inside the spinner, allowing good assessment of the cause. Three of the bolts were sheared, and three were stripped of their threads (left). They had been safety wired. It was clear that at one time the composite prop had been installed without a crush plate, which is mandatory for wood or composite propellers. Here you can see the damage caused by the bolt washers when tightened directly against the prop hub (right).

What Happened?

My initial thinking was that the prop bolts had not been properly torqued when it was installed. There is no logbook entry for the composite propeller installation. At some point we know there was a metal Sensenich propeller installed. Twenty hours prior to the sale, there is a logbook entry stating that a Sam James cowl was installed, along with a metal propeller crush plate “for W&B.” Allowing for the 10-hour flight since purchase, it had been flying for about 20 hours with the new prop prior to the sale. Every accident has a chain, and this one started here.

All the prop bolts were still inside the spinner. The safety wires had all been broken, and three of the bolts were sheared, with the remaining three all having been stripped of their threads. The aluminum spacer had severe fretting in multiple places, which is caused from the propeller being loose. The drive lugs on the spacer had “oblong” holes in the propeller. Clearly, this propeller had been loose for quite some time.

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The propeller had been loose for some time, as evidenced by the severe fretting on the spacer front face and the oblong holes in the propeller. The fretting is caused by the aluminum spinner backplate rubbing back and forth against the spacer during the engine power pulses.

Careful inspection of the front of the propeller showed evidence of washer indentations into the propeller hub, indicating that the propeller had been installed at one time without a crush plate. Wooden and composite propellers require a crush plate! Keep in mind that a metal propeller had been removed, so there was no crush plate. I could also see that the forward spinner bulkhead was deformed from the bolts and washers, plus it was not an entirely flat bulkhead as is common on RVs. It had a little ridge on it, which I think came into play to set up the eventual failure.

A 7-inch crush plate was eventually installed, but it should have been a 6-inch crush plate. The larger diameter of the 7-inch crush plate, coupled with the bulkhead deformation, did not allow it to fully seat against the forward spinner bulkhead, so even with proper bolt torque, the propeller was never really tight against the engine spacer adapter.

The installed crush plate for “W&B” was central to the failure. The problem here is that it was a 7-inch diameter crush plate, while the propeller needed a 6-inch crush plate. This 7-inch crush plate now sat on that ridgeline I mentioned, keeping it slightly above the propeller. Even with proper torque applied to the mounting bolts, there was no way this crush plate was going to fit tight against the propeller hub and properly secure the propeller to the flange. After 30 hours, it all finally let go.

The prop bolt on the bottom has been stripped of its threads. Propeller bolts are supposed to have rolled threads like the bolt on the top, not cut threads. Rolled threads are up to 35% stronger.

The Takeaway

There’s a big lesson here for new owners, whether you built the airplane or not. Yes, one of the attractions to amateur-built aviation is that we get to do our own maintenance and modifications without a lot of oversight. But we must remind ourselves that with that freedom also comes responsibility. Some changes and modifications have the potential to cause severe injury or death, and perhaps it’s wise to get some help or advice occasionally. Luckily, nobody got hurt here, but it is still going to be expensive as Lycoming has a mandatory engine teardown for propeller separation. The teardown and rebuild, along with labor to remove and install, will be close to $15,000.

I have some concerns as to the quality of the bolts. The middle F911 bolt is the short bolt attaching the spacer to the engine flange. The logo, threads and overall appearance look to be of high quality. The longer prop bolts were of poorer quality in terms of logo, metal plating and color.

I also have some concerns with the bolts that were used. They are F911 bolts, such as are used in the auto racing circuits. The shorter bolts holding the spacer to the engine flange appear to be high-quality F911 bolts. The longer propeller bolts look to be cheap, even though they are stamped quite poorly with the F911 logo, and the plating is a different color and is flaking off. There is some noise out there that there are bogus F911 bolts being made in China. Propeller bolts usually have rolled threads, not cut threads. Rolled threads are much stronger, up to 35%. Look at the picture and judge for yourself.

I advocate that changing a propeller on an aircraft should be one of those times when having an A&P take a good close look at it would be a wise decision. In this case, a careful inspection of the parts that were removed would have indicated that they should not have been reused. If not an A&P, then at least find someone who has lots of experience. Single-engine aircraft have many components in them that are critical to the safe outcome of the flight.

This is one of those times that enlisting the services of an experienced person would have kept the fun factor alive much longer. Luckily, the pilots didn’t suffer any injuries.

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Vic is a commercial pilot, CFII with ASMEL/ASES ratings, an A&P/IA, DAR, and EAA technical advisor and flight counselor. Passionately involved in aviation for over 40 years, he has built 11 aircraft and logged over 10,000 hours in 72 different kinds of aircraft. Vic volunteers as a Young Eagle pilot and Angel Flight pilot. He chairs the EAA Homebuilt Council and is a member of EAA’s board of directors. He also has his own sport aviation business called Base Leg Aviation.

7 COMMENTS

  1. A friend noted a ‘rattle’ as he tapped the spinner, as he was pre-flighting his PA-28. He and an A&P removed the spinner and noted that the front spinner mounting plate had 3 large cracks, going through the prop bolt holes. They removed the prop bolts and noted that the cracked edges of the mounting plate had caused fretting corrosion of the prop face , deeper than the allowed limits.
    The result was a red tagged prop and now , $5,000+ for a new prop and spinner mounting plates, and a couple of months waiting for the new prop to be delivered.

  2. For composite or wood props, in addition to the required crush plate, I would recommend using Belleville washers under the prop bolts. That’s become SOP in the canard community after a prop departed a Cozy at altitude, resulting in a successful engine-out landing at a desert airport. Some engineer-pilots have worked out the exact part numbers and torque schedule.

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