How To Avoid Prop Damage On A Gravel Airstrip & What To Do If You Are Missing An Airframe Maintenance Logbook

by Pete Schoeninger
Published in Midwest Flyer – October/November 2019 Issue

Q: What trends do you see in used airplane values?

A: As I see it, I think the market is fairly strong, led by airplanes commonly used for training, especially late model Skyhawks. New airplane sales have picked up a bit in the first 6 months of this year. But older airplanes that are neglected and haven’t been flying, and haven’t had annual inspections recently, are decreasing in value. I suspect this because as owners face increasing costs of bringing them back to life, and making them ADS-B compliant, they are electing to scrap them out.

Q: What does tire size mean, and what about ply rating requirements?

A: The most common general aviation tire size is 6:00-6. The 6:00 is the width of the tire in inches, and the 6 is the rim diameter in inches. Occasionally you will see a number followed by an x, followed by the standard width and rim size. This is NOT usually shown on real common tires which are on many general aviation airplanes. One example would be the tire used on a Cessna 172RG and 182RG, which is 15×6:00-6. The 15×6:00-6 is a smaller diameter tire than most 6:00-6 tires, but retains the common 6-inch width and 6-inch rim opening, and is made to retract and fit into the small wheel well of a 172RG and the 182RG.

The ply rating, usually 4 or 6 or 8, is shown on the side wall of the tire. The higher the ply rating, the more weight the tire can carry. For instance, a 1975 Cessna 172 uses a 6:00-6 tire with 4-ply rating, but the heavier 1975 C182 uses a 6:00-6 tire with 6-ply rating. The newest C172, which is a little heavier than the old ones, also require a 6-ply tire. You often can use a higher ply rating tire than required, but not lower. Check with your mechanic to make sure.

Q: I visited the New Holstein, Wisconsin airport for the Super Cub function the week before Oshkosh 2019. Lots of the owners I talked with had what they called a “3 X 3” landing gear conversion. What in the world is a 3 X 3 landing gear?

A: The 3 x 3 landing gear moves the wheels 3 inches forward and has 3-inch longer gear legs. This allows the airplane to sit on the ground at a higher angle of attack, allowing liftoff at a slightly slower speed, and gives needed clearance for bigger props. Some Cubs are modified with bigger engines and bigger props and bigger tires, all which move the empty C.G. forward a little. Moving the gear forward a little should reduce the chances of a nose over. The 3 x 3 landing gear and big tires combine to allow higher angle of attack takeoffs and landings, but come at the expense of reduced visibility over the nose while landing and taxiing. The big tires slow cruising speed because they have more drag. They also will roll over obstacles found on off-airport landing areas, like small rocks, uneven terrain, airplane salesmen, etc.

Q: My buddy says the only difference between the 1962 Cessna 172 and 1962 Cessna 175 is that engine RPMs are increased and a geared prop was added to develop more power. Is that true?

A: No, they are different airplanes. The 1962 C172 is approved by the FAA on type certificate 3A12. The 1962 C175, the Hawk XP, and the 172RG, are all certified on FAA type certificate 3A17. While the airplanes appear cosmetically similar, there are substantial differences in fuel capacity, engines, gross weights, control travel, etc.

Q: l am looking to buy a friend’s 1967 Bonanza. The second airframe maintenance logbook which would cover airframe maintenance from 1976 through 1985 is missing. All other records seem normal. Our local shop, which maintains the airplane, says it is in good condition. How concerned should I be about this missing logbook?

A: Any missing airframe maintenance logbooks is some detriment to value, but there is no set amount of decrease…each situation is unique. If there are good engine records covering the years missing, and airframe logs showing routine annual inspections and maintenance, the impact on value would be relatively small. But if both the airframe and engine logs for 8-10 years are missing, that is more of a concern. You can, and should, spend ten bucks on a CD from the FAA on your prospective Bonanza purchase, which should show your major repair and alteration forms (337) to find major incidences during the missing airframe log periods. Do an internet search for FAA CD.

Q: I recently purchased a 1978 Piper Warrior. During taxi the airplane occasionally wiggles a little. I took it to the local FBO shop and asked their mechanic to check nose steering linkage. After about an hour he pronounced the nose linkage and rigging as all OK, but my Warrior still wiggles sometimes. Now what?

A: I think you may have the Cherokee Waltz, and a failure to communicate, too! Had you told your mechanic your symptoms, he probably would have first jacked up one side of the airplane, let some air out of the oleo, to check if the main gear wobbled a little before checking nose linkage and rigging. If the main gear wobbles a little, the gear probably needs some maintenance to include removing, cleaning and inspecting the scissors; checking shims; and re-greasing. Once again, a failure to communicate between owner and his mechanic may have cost you some money. (Thanks to A+P/IA Leon Rinke of Redgranite, Wisconsin, for help on this question.)

Q: Six weeks ago, while on vacation in Alaska, I hired a local pilot with a Skyhawk to take me for a ride to a couple of remote strips. On takeoff from a gravel strip, the pilot pushed the throttle in, only about halfway until we were going perhaps 25 mph or so, then he added full power. We forgot to ask him why he did that, so I am asking you now?

A: A propeller on a stationary or slow-moving airplane turning up full power on a gravel surface will suck up some gravel into the prop, causing prop damage. If you wait until you have some speed before going to full rpm, most of the gravel will blow up behind your prop.

Q: This summer, I went for my biennial flight review with a CFI who is new to me. It was a warm day. I used the checklist to include checking mags, and then pulled on carb heat for a couple of seconds, and noted the expected RPM drop. At that point the CFI suggested I pull carb heat on again, leave it on for 30 seconds, which I did. I was surprised that the engine coughed a little, then RPMs actually picked up a little.

A: You made a mistake that I have seen frequently. Your carb heat check should not only make sure the carb heat is working, but also check that you have not accumulated carb ice during taxi for takeoff, which is what you had. Some CFIs recommend on very humid warm days to leave the carb heat on from the end of your runup to the beginning of your takeoff roll, to prevent further accumulation. Some airplane designers locate the carb heat control in their airplanes so that you can turn it on or off with your thumb when you advance power. Some will argue that you might ingest a bit of unfiltered air during this brief period. But if you ask any engine if it would rather inhale a few whiffs of unfiltered air vs inhaling carb ice, you can guess what the answer is.

EDITOR’S NOTE: Pete Schoeninger appraises airplanes for estates, divorces, and partnership buyouts. He is a 40-year general aviation veteran, starting out as a line technician as a teenager, advancing through the ranks to become the co-owner and manager of a fixed base operation, and manager of an airport in a major metropolitan community. For aircraft appraisals, contact Pete at or call 262-533-3056 (

DISCLAIMER: The information contained in this column is the expressed opinion of the author only, and readers are advised to seek the advice of others, and refer to aircraft owner manuals, manufacturer recommendations, the Federal Aviation Regulations, FAA Aeronautical Information Manual and instructional materials for guidance on aeronautical matters.

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