Understanding Piston Engine Overhauls

A review of the basics of piston engine overhauls and how to get the most out of your engine.

There comes a time in the life of every piston aircraft when its engines must be overhauled. There's no exception to this rule. Based on their TBO (Time Between Overhaul) ratings, every piston engine must be removed, taken apart, checked, serviced, have components repaired or replaced as needed, and then be reassembled and reinstalled by trained professionals in order to remain in service.

"Piston engine TBO is predominantly predetermined by their manufacturers, based on a combination of their hours of service and their age," explained Victor Sloan, president of Victor Aviation Service in Palo Alta, California. "Two thousand hours between overhauls is a pretty common requirement, assuming that the engine has been properly maintained in the interim. As well, Lycoming and TCM recommend that their engines be overhauled every 12 years, regardless of the number of hours on the engine."

"Each engine manufacturer has their own specific requirements, as far as what has to be done to an engine, in order to qualify as an engine overhaul," observed Bob Pasch, director of maintenance for Frederick Aviation in Frederick, Maryland (www.frederickaviation.com). Whatever is specified by engine OEMs such as Teledyne Continental Motors and Lycoming, "The bottom line is that a piston engine will be removed from the aircraft and shipped to an engine facility. Once there, the engine will be completely disassembled and assessed for its overall condition. Different components of the engine will also be checked to see whether they meet new tolerance standards, serviceable tolerance standards, or are unserviceable."

Fighting Back Against Wear-and-Tear

In a perfect world, piston engines wouldn't need overhauls. And if they did, their TBO periods would be far longer than 2000 hours.

But, aircraft engines function in a real world where the environment is far from friendly. In fact, "The environmental conditions that an engine is operated in play a major role in causing wear-and-tear, be it corrosive salt water air; a dry, dusty environment, or one where the climatic changes are dramatic," said Sloan. "In such conditions, the result can be excessive internal corrosion."

Short-term use is also no favor to piston engines. The reason: "Short term use results in increased moisture condensation within the engine," said Sloan. In other words, unless a piston engine is run for reasonable periods of time -- an average of 40 hours/month is recommended -- it will experience corrosion due to condensation that doesn't get evaporated off its parts due to heat.

Besides the external environment, piston engines experience a great deal of expansion and contraction due to the heat of internal combustion, and the cool-down after landing. The more metal is expanded and contracted, the more prone it is to wear and degradation. Add the metal-against-metal friction caused every time a piston engine fires up -- a problem that can be minimized by proper lubrication, but not eliminated -- and wear-and-tear is inevitable.

In this real world of heat, moisture, and dirt, piston engine components take a beating. "The most common parts that need replacement are cylinders," said Pasch. "Connecting rod bolts are another one of those items that need to be replaced." This is why regular overhauls are so important; not just to maximize engine life, but to ensure enough power for the safe and reliable operation of the aircraft in all flying conditions.

Extending Engine Life

Now more than ever, pilots and aircraft owners are trying to keep their piston engines flying as long as possible. "The reason is PMA aftermarket prices," said Bill Middlebrook. He's president of Penn Yan Aero in Penn Yan, New York, and the third generation of Middlebrooks to run the company since his grandfather founded it in 1945. "With the reduced cost of PMA'd replacement parts, it's cheaper for people to repair their engines rather than replacing them. This is why the general age of the piston engine fleet is much older than it used to be."

"I would agree that people are trying to get more and more life out of their engines," said Frederick Aviation's Pasch. "It's not unusual to see a 1960 Cherokee or `58 Cessna come into the shop for an engine overhaul."

So what does it take to keep a piston engine reaching its TBO, time after time? Those in the know agree that nothing beats frequent oil changes.

"Your engines aren't going to make their TBO targets if you don't do regular oil and oil filter changes," said Steve Staudt, Champion Aerospace's aviation marketing manager. "And don't kid yourself that you can get away with changing the filter every second time that you change your oil. The average filter is big enough to hold up to a quart of dirty oil. If you don't replace the filter, you're going to mix that dirty oil into your clean oil, which is not going to do your engine any good."

"Face it, when you're trying to maximize aircraft engine life, regular oil and filter changes are pretty cheap insurance," added Alex Feil, Champion Aerospace's general aviation product manager. "You don't want to be the person who has to tell the boss that they've just lost an expensive power plant, just because someone decided to skimp on oil and filter changes."

You won't get any argument on this point from engine overhaulers. "I religiously try to get my customers to do 35 hour oil changes, rather than 50," said Pasch. "It's like they say on those Fram oil filter commercials: `You can pay me now, or you can pay me later.'"

This said, there's more to extending engine life than fresh oil and filters. To get piston engines to their TBOs, it is vital to maintain them conscientiously, to monitor their performance constantly, and to do your best to hit that magic 40 hours/month flying time.

As well, it is important to understand that each engine has its own needs, and should be managed according to those needs. "Continental big block engines have to be babied," said Pasch. "Very few Continental engines make it to TBO without needing servicing."

"The majority of Lycoming engines make it to 2,000 hours," he added by way of contrast, "In fact, Lycoming has a TBO extension program. If you operate the engine at 40 hours a month up to TBO, you can extend the time to service by up to 400 hours. Lycoming realizes that an engine that gets this kind of use is more likely to stay within its advertised parameters." (When Frederick Aviation operated its own flight school) "we tried to make sure that each of the aircraft got as close to 40 hours of monthly use to extend their TBOs, as opposed to one aircraft getting 75 hours and another getting a few," Pasch said. It was a strategy that paid off in long engine life and reliability.

But how much of a difference do proper maintenance and overhauls make to an piston engine's lifespan? Well, Victor Aviation's Sloan credits exhaustive maintenance and precision engine overhauls with keeping legendary pilot Bob Hoover's twin-engine Shrike Commander flying for 20 years. "Using two Lycoming TIO-540 Turbo Charged engines that we overhauled -- hence the `Powered by Victor' painted on the engine cowlings, and now in the Smithsonian Air & Space Museum -- Bob Hoover did everything you weren't supposed to do to engines on a regular basis, including running at max power, then shutting them off and on while in flight," Sloan said. "Bob flew to TBO three times with those TIO-540s, problem free, which is a testimonial to how the combination of a well-overhauled engines and proper servicing of the aircraft can pay off."

Challenges of the Piston Engine Overhaul Trade

For engine overhaulers and MROs, getting their customers to do regular oil changes is just one of many challenges of the trade. Another is attracting enough business to stay alive. "People are flying less, mainly due to the cost of fuel," explained Tim Rogers, director of maintenance at TAC Air Fort Smith in Fort Smith, Arkansas. "A lot of places are running at $4-$5 a gallon, and when you're burning about 10 gallons an hour, that adds up quite a bit."

"All you can do in these times is just tough it out," said Rogers. "That's how we handle these problems in aviation."

For those companies who have the good fortune to be in busy markets, the problem may be finding and retaining quality mechanics. "Finding good help is always challenging," said Pasch. "My workload has increased 25-40 percent over the past five years, which puts a big demand on our people to keep up."

"Frederick Aviation is a very progressive company that puts a lot of emphasis on training and keeping good staff," he noted. "As a result, we have a very low turnover rate."

At Victor Aviation, the strategy is to grow their people. "We have no problem getting sharp technicians, because we train them in-house," said Sloan. "We begin by hiring journeyman machinists, then train them to become some of the finest aircraft technicians in the world. However, we have trouble from time to time getting engine manufacturers to send us parts when we need them."

Still, people like TAC Air's Rogers worries about where the next generation of piston engine technicians will come from. "I'd love to see something be done to attract the younger generation to flying," he mused. "When I was growing up, everyone wanted to fly. Now you don't hear that much anymore."

Rising fuel prices and the search for qualified staff and parts aren't the only challenges. "The biggest change we're dealing with is due to the introduction of PMA'd piston engines, such as those made by Superior Air Parts and ECI," said Middlebrook of Penn Yan Aero. "The experimental market is also huge, and there's been big changes with the advent of engine kits."

"The PMAs are absolutely starting to cut into the piston engine market," he added. "They have taken a significant share of the market, although the OEMs are staring to fight back on price." As such newcomers enter the market, Penn Yan and other engine overhaulers have to become expert in their systems as well, in order to keep up.

The Future

Despite rising fuel costs, changes in engine technology, and the ongoing search for new talent, those in the piston engine overhaul market have faith in their future. "The bottom line is that airplanes are still being sold and as some people sell, others buy," said Pasch. And even with the advent of Very Light Jets (VLJs), piston engines will still remain a staple of aviation. "There's an awful lot of people still flying Navajos and large twins," said Middlebrook. "As long as they are, we'll be there for them."