Recently I was talking to an engine rebuilder about oil change intervals for aircraft engines. He related an experience with a plane owner who was having him do an annual inspection on his aircraft.
A problem came up when the mechanic listed an oil change as part of the inspection, even though the aircraft had been flown just 12 hours in the last 12 months.
The owner insisted it did not need an oil change since the manufacturer recommends an oil change only every 50 hours. He completely disregarded the other part of the manufacturer’s recommendation: “Every 50 hours or four months, whichever occurs first.”
There is no good answer for what the oil change interval should be for a super low usage aircraft, like this one that is only flown 12 hours in a year.
I would not expect this owner to change the oil every four hours. But going four years without an oil change will probably result in a ruined camshaft and other problems.
The real problem here is that this is not a rare occurrence in general aviation.
The latest edition of Lycoming Service Letter 270 states that after the initial transition 50 hour oil change, subsequent oil changes should be every 50 hours, but can be extended to 100 hours if unleaded fuel is used. There is an asterisk to a note that adds “or every four months, whichever occurs first.”
So, the 100 hour recommendation would only apply to aircraft flown 300 hours or more a year.
The reasoning behind Lycoming’s recommendation is that when flying on unleaded fuel, the oil does not get lead particles so the owner should be able to go longer between oil changes.
While I know that Rotax engines do have a lead sludging problem, most Lycoming and Continental engines do not.
For example, the University of Illinois Institute of Aviation flight school has run its aircraft with 100 hour oil changes using 100LL for many years. Flight school officials noted they did not have any significant problems, but they did stipulate that the aircraft had to be flown at least 30 hours a month.
In the U.S., the average private plane only flies about 100 hours a year. Many of these pilots ignore the four-month recommendation and use the 50-hour limit for their oil changes.
Lycoming’s service letter should stress the four months oil change interval is more important than the number of hours.
The main lubricant-related failures come from rust and corrosion on camshafts and lifters. This is mainly seen on low usage aircraft that sit idle for extended periods of time.
When airplanes sit for an extended amount of time, surface rust occurs on the cam and the lifters, which is rubbed off and ends up in the oil. Known as rust rouge, it acts like a lapping compound that leads to higher wear and possible failure.
The lead in fuel does not have a significant affect on this process. Therefore, going to an unleaded fuel will not significantly affect the wear rate of the engine. The language in the service letter implies that unleaded fuel will reduce the wear in the engine. The only way this will happen is from poor maintenance leading to oil system plugging.
I understand that aircraft flown over 300 hours a year can go to 100 hours between oil changes when they use unleaded fuel. But they could also do 100 hours between oil changes with 100LL. The secret here is to fly the airplane around 30 hours a month.
Engine manufacturers need to stress the need for high usage of airplanes, not just 100 hours between oil changes.
I have been stressing the importance of changing oil every four months — and not just every 50 hours — for many years.
But many pilots just look at the hours flown as the only criteria for when to change the oil. Then they have problems with their engines.
I know everyone hopes that going to unleaded fuel will cure all of the problems in general aviation. But I do not think it will change that much in the lubricants area.
As long as there are pilots flying only 12 hours a year, there is not going to be full TBO life for all aircraft.