Asking for trouble

In college, one of my favorite courses was internal combustion engines taught by Prof. Groves. One semester, the main topic of the course was the differences between spark ignition (SI) and compression ignition (CI) engines. After taking the final test, on which we were to compare the advantages of the SI and CI engines, I stopped to talk to Groves, along with several other students. While we were talking in the hall, another student came up and asked, “What does SI and CI stand for?” I remember Groves just hitting his head against the wall and shaking his head.

I feel this way sometimes when someone asks me, “I was working on my plane recently and did not have any aviation grease, so I used some automotive wheel bearing grease. Is that OK?” Unfortunately, people jump to the conclusion that if it does the same job in an automobile as in an airplane, they should be the same. This logic may or may not work — and if it doesn’t, you can have serious problems.

The first problem is compatibility. Many greases will not mix with other greases because of differences in the chemical components. This can leave a mixture that is so thin that it will run out of the bearings or cause other problems. That is why all greases that are manufactured against a certain spec must be tested for compatibility with all of the other greases certified against that same spec.

Even if you use the incorrect product and seem to get away with it for a while, there is the problem of selling the plane. Will you remember to thoroughly wash out each and every bearing and replace it with the correct product before you sell your aircraft? And what if someone else does maintenance on your plane and they mistakenly use the correct grease? This is the stuff of which lawsuits are made.

Another problem with using uncertified grease is seal compatibility. In the non-aviation world, a manufacturer can change any material in their product whenever they choose. In aviation, all components are specified and any change must be STC’d or re-certified. For example, most of the rubber components in most aircraft are not compatible with ethanol fuel, even though all automotive components are. The same is true for grease seals. There are many aviation seals that are not compatible with some of the new technology greases and this can cause a problem.

There are many other reasons why a non-qualified grease should not be used. For example, many automotive types of grease do not have the cold temperature thickness requirements needed for aircraft. This can lead to wheels that do not turn well when you land, which can make landings, especially in a taildragger, a thrill ride.

So what do you do if you suspect or know that the incorrect grease was used? The best thing is to thoroughly clean and inspect the bearing, replace the seal and then repack with the correct grease. This may seem to be a lot of work, but it sure beats a thrill ride.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.

Comments

  1. Rodney Hoover says:

    Bill,
    Your car’s bearings are sealed up tight. A light GA plane’s wheel bearings are sealed with felt. They don’t seal out moisture and all the dirt that the rubber seals on your car’s wheels do. It’s not an issue with how much the wheel spins, but contaminates getting into the bearings. Drive your car through a big puddle of water, and the bearings won’t even know it. Taxi your plane through a big puddle of water and you won’t be able to move it the next day because the bearings are rusted in place.
    Sure can tell when the last A&P I/A didn’t do a full annual because the wheel bearings will have old, nasty grease in them. You need to inspect the bearings and races for corrosion at the annual, have to get the grease out of the way to do that anyhow.

  2. Bill Ferrigno says:

    Hi Ben,
    I don’t understand why wheel bearing grease must be changed at every annual. If the average plane does 50 takeoffs and 50 landings per year between annuals (and I think that’s stretching it), at 1000 feet per… that is only about 19 miles. My car is 13 years old and has 145,000 miles on it and the wheels turn fine. Am I off base here?
    Respectfully,
    Bill

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