Tuesday, December 07, 2010

On Condition

Aircraft engines must be maintained according to a manufacturer's published TBO (pronounced Tee-Bee-Owe) or Time Before Overhaul. An airplane engine is typically considered reliable for about the same duration as a car engine, perhaps two thousand hours of use, but when a car's engine wears out, most owners discard the car and engine.1 With an airplane, you overhaul the engine: returning it to factory specifications, replacing whatever parts are necessary to achieve that. It's expensive and time consuming, interrupting normal use for the airplane and , like any maintenance procedure, sometimes introducing new problems. Engine life is a bell curve, of course, so a few engines break down and require overhaul before their TBO and many are in fine condition after reaching TBO. For this reason, regulatory provisions are available to allow an operator to run an engine beyond the TBO, subject to various conditions.

I was amused by this line at the beginning of the Transport Canada description of the process: "The program was not being used as envisioned, for an extension to a useful life, but has been extended, beyond its intended purpose, to an overhaul avoidance program." Well duh! What would the purpose of extending an engine's useful life be if not avoiding overhauling it? It's not a humanitarian gesture to allow old engines to feel useful, or an environmental statement. Every decision made in an aviation environment is balancing safety against economics, and if you're cynical even the safety decisions are based on economic criteria: accidents are expensive.

As I read it, the on-condition program is a bet the operator makes that his pilots, AMEs and operating conditions are better than the manufacturer's assumptions and that therefore his engines don't have to be overhauled as soon as the manufacturer says. You have to make that bet for your whole fleet, and stick to it. There's nothing that specifically says you can't hedge your bets and decide to go on condition a few hundred hours from the end of the engine life, but once one aircraft is officially on condition, the others of the same type in the fleet must be too, regardless of the relative time on their engines. Once the "approved maintenance schedule" specifies that the fleet operates on condition, then all the relevant extra checks must be done.

The price of the bet the operator makes is:
a) the specified inspections, which I assume are slightly more thorough than the regular inspections of an engine, and
b) the risk that the engine fails an on condition inspection before TBO.
I think b) is not a great worry, because if an engine component were in a condition that it would fail such a check, then you'd be glad to have caught it in a routine inspection rather than through a catastrophic failure causing further damage and downtime.

The payoff of the bet is that you may be able to extend the engine life past the usual TBO. Experience with our fleet suggest that it's not necessarily a safe bet, because the engines seem to need major attention pretty close to TBO anyway. If the on condition inspections cost any appreciable time or money over the regular ones, it might not be a good bet for the long term. But it always seems like a good idea just as an overhaul is coming due on both engines of a reliable airplane that is needed online.

This afternoon I went out for a run with the new AME. He's fast. His pace quickly sends my heart rate out of my training zone, but just switch off my heart rate monitor and keep up. A fast run now and again is probably good for me. After that it's Pasta Tuesday at Boston Pizza, which we admit sadly has become the highlight of our week.

1. There are people who do overhaul automotive engines, and keep automobiles running rather than discarding them. This is much more common in the US than Canada, and evident as soon as you cross the border, because the mix of automobiles on the road immediately skews to older and more American makes. I'm guessing it's because the American cars manufactured twenty or thirty years ago were a lot more amenable to user-maintenance than the European and Japanese models that were already more common in Canada. American movies always show guys fixing up old cars, sometimes as a rite of passage. I think it's part of the culture.


Tom said...

Hi Aviatrix! Just a quick "HA!" from today's post. I misread your phonetic spelling of "TBO" as, "Tee-Bee-Ohh-Whee!!!"

Incorrect, I know, but feel free to use it if/when you get bored!

david said...

A quick extra note (which Aviatrix already knows): TBO, on-condition, etc. applies only to commercially-registered aircraft. Private aircraft owners monitor their cylinder compressions, plugs, metallic content in oil, etc. and decide when it's safe and economical to overhaul.

I know of no statistics that show there's any safety advantage to the commercial approach with piston engines -- in fact, I think I remember reading that the most dangerous time to operate an engine is just after overhaul, because so much can be wrong (I was lucky to spot a major fuel leak from a loose hose after my overhaul, in 2005).

A Squared said...

Canadians typically scrap their cars at 100,000 miles? (2000 hours @ 50 MPH, a very optimistic average speed)

Engine life is a bell curve, of course

Not really, actually it's more of a bathtub curve, or half of a bathtub with a sloping tail.

An engine is far more likely to fail in the first hundred hours after an overhaul than in the first hundred hours after passing TBO.

Here's an article which delves into engine reliability vs time in service, complete with reference to the data used to draw the conclusions.

The article concludes:

"There is no engineering basis for assuming a correlation between aviation piston-engine unreliability and high time in service."

Anonymous said...

My winter truck has over 5000 hours. It weeps and burns about one quart of oil in its 5000 mile oil change interval. It is about ten years old.

I've replaced the battery once when it failed to start on a cold night, and I replaced one out of six ignition coils after an intermittent miss. The PCM let me know exactly which coil had failed, minimizing downtime and uncertainty.

It runs at a lower average load than an airplane engine, but it also has to deal with vastly more transients and starts. You can replace the entire truck for the cost of a cut rate aircraft engine overhaul...a common choice given our destructive road deicing.

The transmission is likely to need overhaul long before the engine.

Paul Tomblin said...

As David says, private operators don't *have* to overhaul their engines at TBO. In the flying club, because our planes got frequent use and therefore never had time to rust, we used to get 3,000 hours out of engines pretty frequently.

As to why you don't see so many old cars in Canada, in Southern Ontario that's because they use so much road salt that our cars become porous before they're 10 years old. Not sure why you wouldn't see old cars in the parts of the country where it's too cold to use salt.

TgardnerH said...

I agree with Tom, there's definitely a lot of regional variation with car life-span... they seem to get pretty old here in Texas, as was the case with California, but in Massachusetts, cars only lived to about age 10.

The other thing about end-of-life care for engines that I don't see mentioned here is that, from my experience, as the parts wear they lose power. In a car who's ample power I abused as a 16 year old, I now feel lucky to make it up a steep hill. It's no big deal in a car, but it must be a safety hazard in an airplane. Perhaps TBO is partially intended as a way to prevent operators from using engines that, while perfectly reliable and still running smoothly, suffer from the same general malaise as my Dad's 1996 Volvo.

david said...

@TgardnerH: Aircraft owners keep the power up by cleaning and gapping the plugs and by replacing the cylinders on their engines when the compressions are too low, though they overreact on the second point: as mentioned in this article TCM tested an engine with the cylinders tricked out to give only 40/80 compression, and the engine still produced full rated horsepower. Certainly, replacing a 60/80 cylinder doesn't make sense unless there's something else wrong with it.

dpierce said...

Regarding car longevity in the US, weather plays a role, and "car culture" here is equally important. There is an enormous infrastructure here related to maintaining (improving, customizing, re-building, etc) cars. I couldn't begin to count all the shops I'm familiar with in just a 3 mile radius of me that deal with auto modification. Around racetracks (which themselves contribute to the culture) the population of car related business is even denser. Nevermind the magazines, and there's even a cable TV channel (Speed) that deals a greatly in auto restoration.

With that much infrastructure, economics come into play. I can buy a new car, or have my car's engine and transmission re-built for about $2k. That's an easy decision for a lot of people. Of course, at the 10 year mark, so many little things start to break down that the economics go backwards on you.

It's worth noting however, that aircraft are made to be repairable (nevermind your recent experiences with cable routing), while automobiles are increasingly designed to be disposable. Just changing headlights in some late model cars is an exercise in tearing down the entire front part of the car.