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Maintenance error leads to in-flight oil leak

By NASA · September 26, 2023 ·

This is an excerpt from a report made to the Aviation Safety Reporting System. The narrative is written by the pilot, rather than FAA or NTSB officials. To maintain anonymity, many details, such as aircraft model or airport, are often scrubbed from the reports.

I was VFR on-top on an IFR flight plan from ZZZ direct to ZZZ1. My Mooney M-20 T Predator had two cylinders replaced at an aircraft maintenance shop about 5 tachometer hours previously. The engine oil level was measured normal (7 quarts) before the flight.

At about the state line, I noticed fluctuations of the engine oil pressure indication. This was concerning, so I made a 180 turn and informed ATC that I was returning to my departure airport.

I then observed the oil pressure gradually decrease toward the red line.

I determined that ZZZ2 was the closest airport, and informed ATC that I was diverting there. Fortunately my engine kept running without high temperatures and I landed at ZZZ2 safely.

Exiting the airplane, I saw a lot of oil behind the left side of the cowling.

After calling flight service to inform ATC of my landing, I contacted a local mechanic, and removed the engine cowling.

The mechanic found loose rocker-cover screws on the recently-replaced cylinders, one of which was missing the exhaust valve rocker cover, causing an oil leak. Only a small quantity of oil remained in the engine.

Obviously, more careful torque-checking of the screws by the mechanics should have been done.

I was very lucky to have made the airport before oil starvation and engine seizure, which could have occurred minutes later.

In retrospect, I should have immediately diverted to the nearest airport at the first indication of an anomalous oil pressure reading, instead of several minutes later.

My current awareness of how this incident could have ended badly will shorten this response time, should I be unfortunate enough to have a similar experience again.

Primary Problem: Aircraft

ACN: 1990965

About NASA

NASA's Aviation Safety Reporting System (ASRS) captures confidential reports, analyzes the resulting aviation safety data, and disseminates vital information to the aviation community.

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Comments

  1. Eric T says

    September 30, 2023 at 6:22 pm

    It is very important to re-check torque values after a maintenance action such as this after the first couple flights, better yet after the first flight. Rocker covers are notorious for loose hardware, we check them every time the cowlings are off at our shops. Any major maintenance action should be followed up after a short run, as gaskets compress and hardware loosens in short order. ET

  2. Harold Altland says

    September 29, 2023 at 1:19 am

    I work on a car from time to time I make short trips to the store and back.To shake out any problems. I am a bed side pilot never would I consider taking off without going up and down the runway and recheck everything on the engine . Then a take off and back to check engine that should be standard practice??

  3. Harold Altland says

    September 29, 2023 at 1:10 am

    Most aircraft fixtures have safety wire? I worked in a instrument repair shop and we put safety wire on everything?

  4. Kert H Kerner says

    September 28, 2023 at 10:27 am

    👍

  5. Jerome says

    September 27, 2023 at 9:42 pm

    It’s fortunate that the pilot did not become fixed on the oil pressure fluctuation, and continued flying the aircraft. I remember being over Lake Mineral Wells Dam, and getting fixed on a radio transmitter not working, and becoming so distracted that I failed to secure map compass, cigs & lighter, before making an acute bank, and it all went out the bubble.
    Fortunately I was able to use receiver as a direction finder, and got to stage field with 10 min of reserve.

  6. Pete D says

    September 27, 2023 at 9:14 am

    Depending on the engine and type of rocker gaskets used, many require re-torquing of the screws after being run up or flown for a short period of time. This doesn’t mean they have a high torque, they are relatively low torque. Over torquing can damage the gaskets too.

    • JimH in CA says

      September 27, 2023 at 6:49 pm

      I replaced the orange silicone gaskets because they can’t be tightened without squeezing the silicone and the screw torque relaxes and did not prevent leaks.
      So, I went back to solid cork gaskets and I add ‘fuel lube’ to both sides, which seals it from oil leaks.
      What is also important is that the valve covers have flat mating surfaces, which may mean a bit of ‘hammer and dolly’ work.
      Then I make sure I have external tooth lock washer on the screws.
      So, no oil leaks on my 6 cylinder GO-300.

  7. Barry Pilot says

    September 27, 2023 at 7:04 am

    I believe he [or she] was facing oil exhaustion, not oil starvation.

  8. Robert West says

    September 27, 2023 at 4:39 am

    A Good Mechanic Is Hard To Find!

  9. Robert west says

    September 27, 2023 at 4:31 am

    Aircraft owners and pilots need to be mindful that maintenance Errors create significant hazard and must act accordingly the most likely time for an aircraft to suffer mechanical problem is on the first flight after maintenance be thoroughly Skeptical anytime Aircraft comes out of the shop your preflight and post flight maintenance test flight or last line of defense against maintenance errors.

    • Dennis Jones says

      September 28, 2023 at 7:05 am

      I’m going to call this a half story. What your getting is only one side of the story. As an A&P I’ve been on the other side. These cheap Pilots want you to reuse gaskets they don’t want to spend money on extra oil that is needed to break in two new cylinders. I’m going to say that he lost more oil due to the new cylinders than any gasket leaking. All engines that have had cylinders replaced should be flown locally no less than 5 hours after completion of work to ensure the break-in of the Rings has occurred. Excessive oil consumption will occur on new cylinders especially if break-in oil is not used. The aircraft should have been filled with at least a quart over full for a long trip and then extra oil should be carried along with the aircraft. But as you notice, we were told none of this during the story. I’d love to get to talk to the mechanic to see his side.

      • Roger C Thompson says

        September 28, 2023 at 8:20 pm

        I’m not an aircraft mechanic…but in the automotive world cylinder break in won’t use anywhere NEAR 7 quarts…

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