The ‘Minor’ Oil Leak

Oil leaks and airplanes seem to go together like butter and potatoes, but not all oil leaks are benign… as I found out, first-hand. Pushed by the engine driven pump that provides pressure to your bearings and wear components, oil can and will leak out through the smallest of gaps. Whether you put on the best gaskets in the industry, or seal the engine, or steam clean your powerplant, the occasional oil leak seems to turn up and soil your engine compartment.

It looked to be coming from the tach drive cable. Never mind the fact that the cable had never leaked before, we sent our mechanic in after it. After removing the old seal and replacing it, the leak stopped — for a few hours — then it started up again, in earnest.

By this point, my mechanic was sure that we needed to pull the tach drive accessory housing off the back of the engine, and for that, we needed to take the plane to a good engine shop. I selected one nearby, and brought the plane in for repairs. I also asked them to change the oil, and to check my oil filter for debris.

Several days later, the shop called. They had found a “booger” of metal, which had become stuck on the tach drive shaft. That material caused an imbalance in the shaft, which caused it to run in an eccentric (read: egg shaped) orbit. This, of course, deformed the oil seal, and caused the problem… or so we thought.

The day came to pick up the plane. The shop flew it out to my home field, and I jumped in and flew it back to the shop to drop off their ferry pilot. Imagine my surprise when I checked to see if the leak was fixed and found my engine compartment was as wet as ever after the repairs.

Important: Regardless of how good the shop is, make sure that the problem you grounded your plane for is repaired. It would have been easy for me to jump in the plane and fly back, only to find the leak. By checking it before I left, I found the situation before it could add even more danger to my flying life.

The shop manager was clearly unhappy with the shop, as he called out a shop tech with dye penetrant “developer” spray, which was a white material. His intent was to look for a crack in the engine case, which would show up as a red line because the oil would activate the developer.

It looked like a good plan, but the results it yielded were quite surprising. As the shop technician sprayed down the back side of the engine, a crack clearly developed on the accessory pulley. The shop tech went to grab his tools, since the cracked pulley would need to be changed regardless of what else he found.

When the shop tech grabbed the pulley, it moved in his hands! The pulley was connected to the starter drive shaft, a thick shaft on the back of the motor that allows the starter to connect with the engine — and this was the source of my leak! THE STARTER DRIVE HAD SOME WEEKS EARLIER EXPERIENCED A CATASTROPHIC FAILURE OF ITS NOSE BEARING!

Then came the recriminations. The shop tech made a disturbing statement. “Oh, you’re brave to fly a plane like this,” he said. I reminded him that the shop owner was the one who had returned the plane to me, with the logbooks signed off as airworthy. I also inquired as to the results of my oil filter inspection. The shop tech insisted the filter was fine, but I persisted and asked to see it. Unfortunately, the oil filter was nowhere to be found… until we looked in a nearby trash can, where my filter was sitting uncut and clearly disposed of. When the filter was cut open and the pleats inspected, metal was found!

The starter drive has two bearings — a small needle bearing inside the engine, and a larger, roller bearing at the output of the shaft to the pulley. The larger bearing had failed, which allowed the slotted shaft to come in contact with the engine case. This in turn machined the engine case slightly, sending soft aluminum particles into the engine oil pump, which in turn sent them through the filter and possibly into the engine!

It would be anyone’s guess as to the cause of the failure. Perhaps it was an over zealous mechanic, who overtightened the generator drive belt, or perhaps it was the engine approaching TBO, but the starter drive had failed.

There were several missed cues on this event that would have kept the plane on the ground and allowed full troubleshooting. As the owner, I should have caught the tiny strands of stuff that was coming out of the starter drive assembly, and flagged them for the shop to inspect. The material looked like fabric, and turned out to be the nose seal of the starter drive, which had become the new bearing.

The shop should have found those fibers when they removed the lower accessory case… and should have followed my request to cut and inspect the oil filter. Had they done either, they would have identified the material and probed further, instead of accepting the easy answer and returning the plane to service.

Nobody got hurt this time — but only by luck. If the starter drive had another 10 to 100 hours in flight in this condition, it could have torn through the engine case and caused a catastrophic loss of oil pressure. This would have caused me to shut down the engine, if I caught the change before the engine shut itself down…

THE BOTTOM LINE: Have your mechanic actively look for and correct that leak whenever something is leaking oil in the engine compartment. If an oil leak develops that is suspected of coming from the accessory case, make sure you have the oil filter pulled and inspected for metal. INSIST on seeing the filter after it’s been cut, to make sure you understand what the shop is seeing… or didn’t see.