By Lisa Turner, EAA Lifetime 509911
This piece originally ran in Lisa’s Airworthy column in the September 2024 issue of EAA Sport Aviation magazine.
I left the FBO and walked out to the porch, where a few seats were left on the benches. It was a bright and crisp spring day. Watching the first flights of the morning get underway was always a pleasure — unless something went wrong.
Since I was between airplanes, I had become a watcher rather than a participant. I remember as a student pilot looking over at the crowd on the rustic old porch, worried about what the onlookers would think of my poor landings.
“That’s the last thing you should worry about,” my instructor said. “Pay attention to the airplane, not the people.”
I looked to my right to see the training flights getting started. I watched one student conducting a preflight on a Cessna 152. It brought back memories of grabbing the manual and the checklist and methodically moving around the aircraft. I had no idea what to look for back then. But even when I started flying my homebuilt and knew the routine by heart, I never stopped the habit of walking with the list.
The student opened the cockpit door and placed his bag inside. Then he began walking around the airplane, looking. No list. “I guess he is smarter than I am,” I thought. After one circuit of inspection, he went into the airplane and retrieved the fuel cup. He checked both tanks for contaminants. Then he looked in the wing tanks for fuel quantity and got into the airplane.
I was bothered by something. Not just the lack of a checklist, but something else I couldn’t put my finger on.
“He didn’t look at the prop,” I blurted.
“What?” said the man next to me.
“Sorry, I said out loud what I was thinking,” I said.
I turned to see Mike from the pilot shop on break.
“Oh, right. It’s as fun to watch the students on the ground as it is in the air.”
“Don’t forget, we were there ourselves,” I said, realizing it was all too easy to criticize when we had made the same mistakes when we were starting out.
We watched the red and white 152 head out to the run-up area. A few minutes later, we heard a muffled cracking noise from that end of the field.
“That was pretty loud,” Mike said.
Back came the 152, powering down the taxiway but shaking up a storm, looking like an unbalanced washing machine on tri-gear.
“No! He lost part of the prop! Shut it down, buddy. Shut it down!”
Mike ran inside to get on the radio. About 30 seconds later, the 152 shut down. Fire trucks whined past us toward the field.
The engine had been on the verge of coming out of the mount on shutdown, twisting and cracking the cowling. Four inches of prop tip were gone, and the windshield was cracked. The student, having exited the airplane as soon as it stopped, was nervously pacing 20 feet from the aircraft.
“Lucky,” Mike said. “Very lucky. If he’d gotten into the air …”
“Indeed,” I said. “But if he’d inspected the propeller, he might not have gotten into the airplane in the first place.”
When it comes to aircraft propellers, there are two areas that concern me as a pilot and mechanic. The first is a general failure to adequately inspect the propeller on preflights. The second is the failure of pilots and mechanics to fully understand the seriousness of prop strikes.
What Can Go Wrong?
The NTSB made news earlier this year when it released Safety Alert SA090 in April. The alert — “Aluminum Propeller Blades” — directs owners, operators, and pilots to inspect all areas of the propeller blade — including both sides of the blade and the leading edge — for damage such as nicks or corrosion.
This issuance got me thinking about propellers in general, and the requirement to keep them well maintained. Like tires, they may not receive the respect they deserve. If you consider that the propeller is one of the most stressed components on the aircraft, you realize that small amounts of damage can add up to severe consequences.
As noted in the FAA’s Advisory Circular 20-37E, Aircraft Propeller Maintenance, “During normal operation, 10 to 25 tons of centrifugal force pull the blades from the hub while the blades are bending and flexing due to thrust and torque loads and engine, aerodynamic and gyroscopic vibratory loads.”
These stresses occur every time you fly. Over time, fatigue cycles will amplify tiny cracks, nicks, pits, or areas of corrosion to the point where blades may fracture.
Although the safety alert talks about aluminum propellers, we see the same forces operating on wood and composite propellers.
How to Improve Preflight Inspections
If you were a blackbird on the fence post at the airport and could observe all the preflight inspections that go on every day, you might notice that the propeller and the tires seem to escape thorough examination. I can understand the tires, especially if they are encased in wheelpants, but you’d think the propeller would be easy to inspect. But that’s the problem; it’s right there in front of us, and it looks okay.
If you were taught to preflight by a pro, then you know that a prop inspection includes tactile, auditory, and visual information. Even a simple one-piece propeller will need a thorough inspection. Here are some tips.
Before your next preflight, pull out the prop log (or general log if you have all the info in one place) and the propeller manufacturer’s maintenance manual. Make sure your preflight list for that prop matches the inspection procedures the manufacturer recommends. Make note of any special inspections or overhaul intervals.
For your next preflight, do a self-audit. Look, feel, and listen. Carry a 10X magnifier with you. Are you doing the following?
- Inspection rule No. 1: Protect yourself from distraction. This is not easy. No matter how much we think we can multitask, something gets lost in the process. Before you begin your list, stop and clear your mind. Tell the little inside voice that continues jabbering to you about all sorts of things to be quiet.
- Check the prop for dirt, grease, and oil. If you find these, identify where they are coming from if you can. Walk behind the prop (what some people call the “back” of the prop, which is actually the “face” of the prop, or what you can see from the cockpit) and look at the surfaces of the blades, and anything else you can see from this side.
- Look for erosion and corrosion, bumps, gouges, nicks, and scratches.
- Feel along the leading and trailing edges for dents, nicks, gouges, and scratches. Use your fingertips to feel the metal/wood/composite.
- In wood or composite propeller blades, look for cracks or delamination on the blade surface and at blade edges. In wooden propellers, check the glue lines for de-bonding.
- Sight down edges to find deformations.
- Use your hands to lightly feel for looseness and twisting where there shouldn’t be any play (manufacturer’s checks).
- Check the spinner and bulkhead for security, missing fasteners, damage, and cracks. Cracks typically originate from the attachment screws.
If you have in-flight adjusting mechanisms, note the items that the manufacturer wants you to inspect, and how often. If you’re new to this, ask an A&P mechanic or experienced owner to review it with you.
Annuals or condition inspections should include the checklist and recommendations that the manufacturer gave you. You will start with the preflight routine and then go on to the additional items, such as tracking and balance. Visit a qualified prop shop for some of the more complex inspections and adjustments, particularly balancing.
Safety note: Don’t move the prop by hand when conducting your inspection unless it’s for a reason and you’ve checked that the ignition switch shows OFF. Assume that the engine can start whenever you move a propeller, even a little bit. Remember that the switch can say off, but the system might not be grounded (off) if the P-lead fails or is disconnected.
Did You Hit Something With the Prop?
For aircraft with the prop in front (“tractor”) versus behind the cockpit (a pusher), it’s the first part of the airplane that encounters surprise objects like hangar doors and tow bars, making prop strikes more common than you would think. Add to that rocks and grass at the side of the runway, or operating from grass or backcountry strips, and there’s plenty of opportunity to suffer damage.
AC 20-37E, 206.b.1 states, “Operators may not operate any aircraft after a propeller has been subjected to an impact without a thorough inspection by an appropriately rated person or repair station.”
I’ve found that pilots frequently make up their own definitions of “subjected to an impact.” I hear things like, “Oh, yeah, the prop just nicked the edge of the door,” when I see a half-inch of metal bent over on the tip.
I understand the reason for wiggling. Depending on how much prop damage you have, your airplane may require an engine inspection. Not analyzing what happened and taking the correct actions can mean an engine failure following the prop strike if you keep flying, sometimes immediately. Maintenance shops have found bent crankshafts, cracked cases, and broken gear teeth in the mix of problems.
Don’t risk an engine failure. Get a professional opinion if you suffer a prop strike, or if you run into water or grass producing a substantial drop (or stoppage) in engine rpm. Engine manufacturers are very clear about what constitutes a strike.
For more on these situations and definitions, see “Prop Strike,” EAA Sport Aviation, March 2018.
Lessons Learned
Use all your senses as you inspect your airplane. For the propeller in particular, feel the leading edges carefully, listen as you gently pull and push, look at both sides of the blades as well as the spinner, and listen to your “sixth” sense, which might tell you that something is off.
Many propeller failures are caused by inadequate preflight procedures. Most of these failures were not because the pilot purposefully skipped the propeller inspection but because they hadn’t been taught how to conduct a full inspection. Make a note to yourself now to find out all you can about the recommended inspections for your airplane’s propeller. Find and read the manufacturer’s manual for the prop you have. You might be surprised. Better to be surprised now than when you’re in the air.
Lisa Turner, EAA Lifetime 509911, is a manufacturing engineer, A&P mechanic, EAA technical counselor and flight advisor, and former designated airworthiness representative. She built a Pulsar XP, a Kolb Mark III, and half of a RotorWay Exec helicopter. Her book, Dream Take Flight, details her Pulsar flying adventures. Lisa loves hearing from readers. Write her at Lisa@DreamTakeFlight.com.