The Savvy Aviator #44: Making Metal
Recently, I heard from an aircraft owner who had just discovered something worrisome during an owner-assisted annual:
Mike, I attended your Savvy Owner Seminar in Denver last year, and have had an "interesting" development with my Cessna TR182. It has a turbocharged, Lycoming O-540 engine with about 1500 hours. The engine runs strong. I have two years of oil analysis data with the same lab. I have changed the oil about every 50 hours over the last two years.
During the annual earlier this month, I found fine, whisker-like, ferrous-metal particles in the oil filter. There was just enough to cover the end of a quarter-inch magnetic pick-up. I showed this to my mechanic, who did not think it was significant.
I just received my oil analysis report, and it shows a doubling of iron from 39.2 to 94.2 parts per million. The lab recommends that I resample in 15 to 20 hours.
I have a few questions for you:
- What is the threshold of concern? Is this just normal wear in an engine with this time?
- Should I follow the lab's advice and monitor the wear trend? At what point do I take additional action? What if the next lab report shows another doubling of iron, and/or the quantity of metal in the oil filter increases?
- Is it a time to pull a jug and look around?
- At what point do I overhaul the engine?
Let me start by saying that this owner was wise to ask for a second opinion at this point. That's always a smart move any time you have a question or concern about the advice you're getting, particularly if safety or big bucks are at stake.
My principal worry about this Lycoming engine is that it might have one or more cam lobes or lifters that are starting to spall (see photo at right). If that's true, then it won't be long before the engine will need to be torn down for cam and lifter replacement.
Published TBO is 2000 hours for this engine. If a teardown becomes necessary at 75% of TBO, the owner would most likely elect to do a major overhaul or exchange the engine for a factory rebuilt.
But is it time for the owner to panic and ground the airplane or pull the engine? No, not yet.
Lycoming has issued a number of service bulletins offering guidance on how to respond to metal found during an oil filter inspection, and its recommendations have been revised several times. The latest guidance appears in Lycoming Service Instruction 1492C dated July 14, 2000, which has the unlikely title of "Piston Pin Plug Wear Inspection" but actually discusses the much broader subject of oil-filter inspection. Here's what SI-1492C has to say on the subject:
Evidence of metal contamination found in the filter element or screen requires further examination to determine the cause. Below is a list of recommended actions based on the appearance and approximate quantity of particles.
- 5 or fewer small (1/16 inch diameter or less) pieces of metal -- place aircraft back in service and check oil filter or screen at next scheduled oil change/oil filter replacement.
- 10 to 20 small (1/16 inch diameter or less) pieces of shiny flake-like, nonmagnetic, or 10 or fewer short hair-like pieces of magnetic material -- place engine back in service and again check oil filter or screen in 25 hours.
- 20 to 40 small pieces as in step b. -- place the aircraft back in service and check oil filter or screen at the next 10 hours.
- As in step b., but larger amount, such as 45-60 small pieces -- change filter or clean screen, drain oil, and refill. Run engine on ground for 20-30 minutes. Inspect filter/screen. If clean, fly aircraft for 1 to 2 hours and again inspect filter/screen. If clean, inspect filter/screen after 10 hours of flight time.
NOTE: In items e. through j. below, the engine should be removed from service until the source of the metal is determined and corrective maintenance has been accomplished.
- Pieces of metal ranging in size of broken lead pencil point or greater. Remove suction (sump) screen to check for pieces of metal that may have fallen into the sump. In any event, ground aircraft and conduct investigation. A mixture of magnetic and nonmagnetic material in this case often times means valve or ring and piston failure. Removing bottom spark plugs usually reveals the offending cylinder.
- Nonmagnetic plating averaging approximately 1/16 inch in diameter; may have copperish tint. Quantity found -- 1/4 teaspoonful or more; ground aircraft and investigate.
- Same as in step b. but may be slightly larger in size and minus copperish tint. On direct drive engines, propeller action may be impaired. Ground aircraft and investigate.
- Nonmagnetic metal brass or copperish colored. Resembles coarse sand in consistency. Quantity of 1/4 teaspoonful or more -- ground aircraft and investigate.
- Anytime metal is found in the amount of 1/2 teaspoonful or more, it is justification for engine removal.
- If any single or several pieces of magnetic or nonmagnetic metal larger than previously mentioned are found, ground aircraft.
Incidentally, to the best of my knowledge, TCM has never published specific guidance on this subject, but Lycoming's advice strikes me as a pretty sensible approach for any piston aircraft engine, regardless of make or model.
It seems pretty clear that the quantity of ferrous metal reported by the owner is small enough to be covered by paragraphs b. or c. of Lycoming SI-1492C. Ten or fewer short, hair-like pieces of magnetic material calls for flying and re-checking the oil filter in 25 hours. If more ferrous metal is found -- up to 40 short hair-like pieces -- then the re-check interval should be shortened to 10 hours.
Lycoming does not advise grounding the engine unless the filter contains large pieces of metal (the size of a broken lead pencil point or greater), or numerous small pieces totaling 1/4 to 1/2 teaspoonful or more. That's a lot of metal -- see the photo at right for what 1/4 teaspoonful of ferrous metal looks like.
What about the oil analysis report? Lycoming addresses this subject in SI-1492C as well.
For turbocharged engines (like the one in this owner's Cessna TR182), it says that the engine should be grounded for further investigation if iron exceeds 130 ppm or aluminum exceeds 40 ppm. For normally-aspirated engines, the corresponding thresholds are 100 ppm of iron and 30 ppm of aluminum.
Looking at the oil analysis report on the Cessna TR182 engine (shown at right), it's clear that neither of these thresholds have been reached yet. Notice also that this same engine had a similar iron spike two years ago, but recovered nicely. The moral of the story is that one high reading does not establish a trend.
This might be the start of a cam lobe and lifter coming apart, or it might be a transient anomaly that resolves itself. It won't take long to find out which it is, because cam and lifter destruction tends to proceed quite rapidly -- typically 50 to 100 hours from first symptoms to an obviously unairworthy engine.
If a cam lobe and lifter are coming apart, the next oil change will reveal sharply increased metal in the filter and higher iron in the oil analysis. If that happens, the owner will know with reasonable certainty that it's teardown time.
Is The Engine Safe To Fly?
Is it risky to fly this TR182 for another 25 or 50 hours? No, I don't think so. Although cam lobe destruction usually progresses fairly rapidly, it will not cause a catastrophic in-flight failure or do anything that would make the airplane fall out of the sky.
If the cam lobe deteriorates far enough, it could cause a small loss of power and perhaps a small increase in roughness. If the airplane has a digital engine monitor, the owner might notice a gradual decrease in EGT and CHT on one or two cylinders (because the intake valves are not opening quite as far as they should, so the cylinders can't "inhale" quite as well as they should.
If the next oil filter inspection and oil analysis indicate that things are getting worse, the owner can be fairly sure there's a cam problem, and he'd probably be wise to tear down the engine sooner rather than later.
But I certainly wouldn't panic and ground the airplane right now. The metal in the filter and the iron spike in the oil analysis could be a transient event that might resolve itself. It's happened before on this engine, and with a bit of luck it'll happen again.
See you next month.
Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.