Just another number
Waddington was right
By Dave Hirschman
Engine manufacturers have a notoriously hazy crystal ball. They can’t accurately predict when engines they make will need overhauls because each one is operated differently. The same model engine can be flown in a wide variety of aircraft, environments, and manners—yet the engine on a seaplane flown irregularly in the tropics is subject to the same one-size-fits-all time between overhauls as a trainer flown daily in the dry Southwest or a charter operator in the Arctic.
The military, airlines, and even some high-end general aviation firms learned long ago that manufacturer TBOs are folly. So, they perform maintenance based on condition rather than hours flown—and the improvements in fleet reliability and cost are both immediate and dramatic.
During World War II, for example, British scientist C.H. Waddington—a biologist by training—set out to fix the poor dispatch rate of Royal Air Force B–24 Liberator bombers on anti-submarine duty. In 1943, only half of the RAF’s 40 Liberators were mission ready at any given time.
Waddington and his team dug into the data and found that unscheduled maintenance increased sharply after scheduled maintenance, then steadily declined until the next scheduled maintenance event when it spiked again. Their counterintuitive yet inescapable conclusion was that maintenance introduced new problems that, if left alone, likely wouldn’t have occurred at all.
Scheduled maintenance, Waddington found, “tends to increase breakdowns, and this can only be because it is doing positive harm by disturbing a relatively satisfactory state of affairs.”
The so-called “Waddington effect” led to changes that improved the readiness of the RAF coastal fleet by 60 percent. Yet GA stubbornly clings to old, discredited, and unreliable engine overhaul schedules as if they came down from Mount Sinai.
Modern borescopes, oil analysis, and advanced computerized diagnostic tools allow today’s maintainers to determine engine health better than ever. Graphical engine analyzers provide a wealth of data that allows pilots to observe and share performance data in color-coded detail, and the data collected is an early warning system for failing components.
Replacing components and overhauling engines on condition doesn’t necessarily mean going beyond the manufacturer’s TBO. There’s a significant chance that an engine will show signs of stress well before it reaches any magic number—especially if it’s neglected, abused, stored in a corrosive place, or goes long periods between flights.
But an engine that’s flown regularly within normal limits, with modern lubricants, and is closely monitored may exceed the recommended TBO by a large margin.
A Lycoming O-360 in an Aviat Husky that flew almost daily on multi-hour pipeline patrol missions in the dry Mountain West was still going strong at 3,500 hours when the owner finally overhauled it. And even then, it showed no signs of ill health or impending failure.
Individual aircraft owners aren’t likely to know more about the broad spectrum of aircraft engines than manufacturers. But owners know their own engines intimately. They know how regularly their airplanes fly, how they’re stored, and how they’re operated and maintained. They’re also finely attuned to any changes in engine behavior. Is it getting more difficult to start? Does it consume more oil? Are the spark plugs more prone to fouling? Is a particular cylinder running hotter than the others? Or hotter than it used to? Pilots are stingy to a fault, and we don’t like spending more on maintenance than we have to. If there’s a way to save a buck, we’re on it.
But no one has more riding on the health of an aircraft engine than an owner/pilot. We take our families and friends flying, and they trust us to ensure their safety.
We want to know all we can about the condition of our engines because that’s what matters—not just flight hours. That’s not owner hubris. It’s good stewardship.
Replacing an engine solely on the basis of hours is like forcing pilots to retire on a particular birthday. Some may be worn out, and others are fit to continue. Numbers in isolation—whether TBO or age—reveal precious little. 
Past TBO? No-go
Time is not on your side
By Ian Wilder
Part 91 owner-operators love to believe that they know what’s best for their airplane. Oftentimes, I’m inclined to agree that the people who meticulously care for their airplane and spend all day, every day, around it are in the best position to make decisions about it. But not always.
Recent movements in engine maintenance have prompted some owners to exceed the engine manufacturers’ recommendations for time between overhauls (TBO), in some cases by a significant margin. Usually, there’s a good amount of alleged substance behind this reasoning: good preventive maintenance, continued engine oil analysis, borescope inspections, good compressions, and so on.
There’s a pessimistic view of the world that involves the FAA mandating manufacturers to release these figures, after which the manufacturers throw a wrench at a wall and say, “2,000 hours.” Some even believe it’s a liability thing, with the manufacturers covering their behinds by making an overly conservative estimate. It is a conservative estimate, yes, but conservatism rules this industry, and for good reason.
True, the first hours after an overhaul or installation of a new engine are statistically among the most dangerous, with a much higher risk of failure than an engine at 2,000 hours since major overhaul. But once the engine settles in, the risk settles with it.
Flying past TBO does not automatically create an increased risk of the engine quitting. But how far is too far? How often can you have the engine inspected? How often do you have the oil inspected? Hidden wear is very possible; it only takes one component to fail suddenly to give you a really unpleasant time, and that can sometimes develop between inspection intervals.
I will yield that this is a personal choice. Some people will do their due diligence and make the decision to exceed TBO, and more power to you. You know who won’t trust those people? Buyers. It takes meticulous record keeping to lead potential buyers to believe that you made a wise choice flying your engine 1,000 hours past TBO. Even then, some people simply will treat that engine as less valuable than the engine that was overhauled at the manufacturer-recommended interval.
It also becomes a “stacking” worry. One hour past TBO is stress-free, at least for Part 91. But with every passing hour, that worry might start to accelerate. The costs might too. Overhauling your engine while it’s in good shape is far from a euthanasia procedure. By that reasoning, changing your oil at 50 hours is bloodletting. An oil change is simply a preventive step to make sure your engine is still in good shape.
To address the elephant in the room: Yes, failures are most common right after an overhaul. But human error is hands down the biggest factor in that, and that risk is unrelated to the time on the engine.
And then there’s cost. Flying past TBO for most people is a false economy. You might believe that the extra hours you’re squeezing out of the engine will mean that you’ll have accumulated more hours before your overhaul, thus reducing the cost per hour of the work performed. It’s a seemingly strong argument, but that seems to forget that overhauling a perfectly capable engine is much cheaper than overhauling an engine that fried a cylinder because of some wear you weren’t even aware of.
The real liability isn’t the hours on the engine—it’s the pilot who thinks he knows better. You’re not Lycoming, you’re not Continental, and you’re (probably) not an A&P. You’re just Joe with an engine logbook and pilot certificate, convincing yourself you know better than the people who literally design and certify these engines. The problem isn’t the number stamped in the manual, it’s the arrogance of thinking you can outsmart it. Respect the expertise, respect the limits, and stop pretending you’re the exception. 
Related Articles
