In aircraft maintenance, the line between preventative and unscheduled maintenance can often blur.
MAINTAINING YOUR AIRPLANE Servicing Cessna 172 Stuck Exhaust Valves A rough-running engine means it’s time to pop valve covers.
by Richard Scarbrough
R ecently, a GA pilot at Newnan-Coweta County Airport (KCCO) in Georgia pulled a Cessna 172 out of the hangar, ran through a quick preflight inspection, and yelled, “clear prop!” He fired up the aircraft, only to find the Con- tinental O-300-D engine started running rough. That was as far as it went. Mission aborted. The aircraft never left the ground. Aircraft Troubleshooting Cessna 172 owner Corey Sampson reached out to me to discuss the engine running rough and the dark side of maintaining an aircraft: unscheduled maintenance. The first step in the troubleshooting chain was to remove and replace (R&R) the spark plugs, as they were coming due soon. That didn’t help. The next step was to run the engine while idle, using a hand-held laser thermometer, and take the temperature of each cylinder one at a time. One by one, the temperature readout for each cylinder measured 150 to 160 de- grees Fahrenheit. The number 4 cylinder was 95 F. Cylinder number 5 was 159 F, and number 6 was back to around 95 F.
Sampson could now perform preventative maintenance by wearing his owner/ operator hat. Once troubleshooting leads beyond preventative maintenance, it is time to turn around the hat to the A&P side and begin unscheduled maintenance. Unscheduled Aircraft Maintenance There is a trick to relieving pressure off the valves to remove the rocker arm while keeping enough resistance to hold them in place to pop off the rotocoil, keepers (valve locks), and springs. It was time for the rope trick. Checking in with JD Kuti, president of Pinnacle Aircraft Engines in Silverhill, Alabama, I asked him to break down the procedure for me: • Feed rope into the spark plug hole. • Spin the prop until the rope holds the valve in place. • Use the spring compressor tool to remove the keepers and valve springs. I watched a mechanic on the internet try to pry off the valve keepers with a pocket knife. That looked like a good way to get cut to me. Back in my engine shop days, we just popped a magnet against the valve locks, and voilà, you set them free. Kuti mentioned a trick he has used in the past while servicing valve guides: “If you need to drop the valve into the cylinder, tie a piece of floss to the valve through
10 Aircraft For Sale | July 2024
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