A ploy to sell more engines?

Q: While TBO is recommended as 2,000 hours or 12 years, whichever comes first, the brokers trying to sell me a used plane always belittle that second limitation, chalking it up to Lycoming wanting to sell more rebuilds. Can you clear the air on this one?


A: The text of Lycoming Service Instruction 1009AT, which addresses “Recommended Time Between Overhaul Periods,” has always been a controversial issue. While it lists the recommended hours for each engine, it also includes a rather profound caveat that has become a point of discussion by many: “All engines that do not accumulate the hourly period of time between overhauls specified in this publication are recommended to be overhauled in the 12th year.”

While it was generally thought to be a ploy by the engine manufacturers to sell more parts or factory overhauled or rebuilt engines, this was not the case.

Both Lycoming and Continental offer factory exchange engines as an alternative to a field overhauled engine at competitive prices. This has generated many engines being sold on an exchange basis, while providing both manufacturers with a realistic learning curve about their engines that have been in service under various conditions and times over a period of years.

One of the first things they became aware of was the noticeable differences in the condition of these return engines. It became quite apparent that the engines that had been exposed to extended periods of inactivity showed more severe internal corrosion. The extended periods of inactivity were confirmed by a review of the engine logbooks.

Another thing that became obvious was that those engines having infrequent oil and filter changes seemed to add insult to injury where corrosion was concerned. There is no doubt this could have an impact on the overall reliability of the engines.

Where does this leave us as far as an aircraft that has low operating hours but is several years old? The 12-year recommendation does have merit, but there may be some options on an individual case-by-case basis.

If I were looking to purchase a plane that was 15 years old, had never been overhauled and only had 600 to 800 hours on it, there are some important things I’d be looking for before making any decisions:

1. Review the engine logbook, specifically looking for any unusual entries relating to maintenance or operation.

2. Has the engine had regular oil and filter changes and is there a mention of oil consumption?

3. Is there a record of engine oil analysis being conducted on a regular basis? One oil analysis does not tell you anything. They should be conducted over a period of time.

4. Is there a record of hot differential compression checks conducted at each annual inspection or other regular scheduled maintenance events? This is another thing where you must record several readings to establish a trend.

5. Take a close look inside each cylinder using a borescope. I’d insist this be one of the requirements of any pre-purchase inspection. Should this inspection reveal indications of rust on the cylinder walls you may have some decisions to make. If, and only if, the rust appears to be light and only in a few small areas, you may want to roll the dice and continue operating the engine. I have seen where slight rust has healed over nicely and no further issues were noted. The key here is that the rust pits do not appear to have embedded deeply into the cylinder wall. This may be a difficult decision to make simply by viewing these areas through the borescope, so I’d suggest you pick the cylinder with the worse indications of corrosion and remove it for further inspection. If the corrosion is into the cylinder walls, this may be cleaned up by honing the cylinder. New piston rings must be installed and straight mineral base oil should be used until the rings have seated, which usually happens within the first 25 hours of operation.

If there is heavier corrosion inside the cylinder and you choose to continue operating the engine, you may end up causing severe damage. As the piston goes up and down the barrel, it scrapes the corrosion off the cylinder wall and it migrates to the oil, where it passes over components throughout the engine, oil hoses and oil cooler. One of the first things you may notice is an increase in oil consumption as the ring wear increases from the rough surfaces of the cylinder walls. The material being carried throughout the engine also will become embedded in bearing surfaces, camshaft lobes, and tappet faces, making these components scrap when the engine goes to overhaul.

Nothing is as simple as it appears, which requires us to use logic and common sense, so ask as many questions as you can and always check the logbooks and maintenance records closely.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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