Thursday, December 2, 2010

Maintenance: Engine "Infant Mortality"

N6338F - Engine maintenance
Contributed by C. Howitt, Plane Captain N6338F

The engine in one of our C172s was replaced just last year. Since the club has put just shy of 300 hours on the engine. We had been told that the risk of failure in a brand new engine was often higher than a well-cared-for engine at or beyond TBO. We are discovering first hand why this is the case, how to spot the issues early, and get the engine serviced before catastrophic failure occurs. This is a detailed description of the current MX issues we are experiencing. For new members, we hope this lends insight on how maintenance is handled in the club---one of the club's greatest strengths is our emphasis on the safe operation of the fleet.

The last two oil samples for 38F taken at the recurrent 50hr oil change showed elevated levels of iron.

The elements in the oil sample are quantified in parts per million (PPM). The universal average for iron wear for this type and age of engine is 27 (PPM). On the last analysis we had increased levels of  55 (PPM) an upward trend and more than double the universal average.

Iron wear comes from Cylinders, rotating shafts, the valve train, and any steel part sharing the oil. During Engine Break-in this figure is typically high as the cylinder rings ‘seat’ themselves. However we were well past the seating phase. This was something different.

The 50hr oil change encompasses more than just the oil analysis. In fact we should refer to it as the 50hr Inspection since it is a mini-inspection. It gives us a chance to check over most of the common points. One of the checks included is the compression test. The compression test was good (They have been consistently good with the new engine) so the problem was not likely with the cylinders, piston or piston rings - more likely to be coming from the camshaft, lifters or followers or valve train.

For this 50hr Inspection we took the plane over to C&W to get a more thorough examination of the oil filter. No metal had been seen in the filter in the last two oil changes. However this doe not mean metal was not present. The metal shards get embedded deep in the filter and are often invisible to the eye. Even if you manage to see a few it is difficult to gauge how many are present. ZV was in the shop at LDJ and we wanted to have a fresh set of eyes looking at the situation. Over at C&W they ran a stick magnet test over the oil filter...This picture below is the actual stick magnet test result a lot of metal....!
Sean at C&W explained that the long grey slithers, similar to iron filings are indicative of wear from the Followers or ‘Lifters’.

As the Camshaft rotates and the tear shaped Camshaft Lobes ‘lift’ the Camshaft Follower.
The metal wear is thought to be coming from contact with the Camshaft Lobe and the Camshaft Followers. This can cause ‘pitting’ on the Camshaft lobe. The picture below is not from our engine but shows what Camshaft Pitting looks like:
Pitting is not normal for any engine and is considered to be damage. Because of the amount of metal found we contacted Penn Yan for comment. On seeing the pictures of the metal they asked for the engine back ASAP.

The oil report for this oil change was now showing 66(PPM). This was the comment from Blackstone Labs:
“It's getting more and more difficult to write off the elevated iron as non-problematic. This time, the oil run was a bit shorter, though iron increased quite a bit. Based on the average wear rate of iron (ppm/hour), we would have expected it to read about 35 ppm for this sample. If this engine has been seeing a lot of use at high power settings that could explain some iron, otherwise it may show wear at cylinders or steel shafts, so let's keep an eye on things.
Monitor compressions and watch the oil filter for metal just to be safe. Try an average (40 hour) oil run.”
We had the engine taken out the next day, Friday and on a truck to Penn Yan on the Monday. They were able to inspect the day after Thanks giving:
“We have the engine apart and our thoughts were correct the camshaft part number LW-18840 and/or the lifters part number AEL72877 have failed. We have not yet determined what caused the camshaft or lifters to fail, all eight lifters and the camshaft were new at the overhaul less than one year and 278 hours ago, five out of the eight lifters show signs of pitting and/or failure and one of the camshaft lobes is severely worn.“
The engine is still under warranty and Penn Yan have agreed to change out our Camshaft for new. Interestingly they noted a very slight amount of corrosion on the Camshaft. Corrosion is usually the result of the plane not being flown enough. The average for our planes is around 200hrs, 38F has 287 hrs so this is clearly not the case. When asked about the corrosion Penn Yan noted that there is always some small signs of corrosion and most shops use this as grounds for denial of warranty!

This is clearly a problem with bad parts and a further bone of contention so far is that they are only partially covering the cost of engine removal and Installation.

However repair of the Camshaft, would have taken several weeks so the exchange is good news and hopefully we will have 38F back on line before the Holidays as planned.

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