This is a weird one, but it happened just the same — which means it could happen to you. A friend of mine had a nice, 1979 Cessna 172 on the flight line for rent. The care and upkeep of the plane was meticulous — all the way down to the 100-hour inspections. The interior was in good shape, the engine ran strong and had good compressions, and the airport was just busy enough to keep it flying, but not make it impossible to rent.
SOUNDS PERFECT, DOESN’T IT? Still, some weird stuff turns up in life that can befuddle everyone once in a while, and this is a perfect example. The local A&P had just completed the required items for the 100-hour inspection. The plane had been working fine when it was originally brought in for the inspection, and there were no squawks with its performance.
The A&P did the usual inspection items. He had changed the oil, inspected the airframe, pulled, cleaned and replaced the plugs, checked the engine timing… etc. The final item on the inspection was a ground runup to look for leaks and problems, prior to turning the plane back over to the owners.
AFTER ADDING A SINGLE SHOT OF PRIMER, the mechanic flicked the ignition switch, expecting the engine to spin a few times and jump into life. After three turns of the prop, the A&P was already worried — the engine had not caught, or even shown signs that it was going to. After nearly 30 seconds of cranking, the mechanic took his hand off the ignition, and started to think of what it could be.
TRACING HIS FINGERPRINTS, (figuratively) he concluded that it had to be the spark plugs. If the spark plugs weren’t firing for some reason, the plane would have problems. He pulled all the spark plugs, but left them connected to their respective magnetos, and got the plane’s owner to crank the engine while he carefully watched. Now the problem was the lack of one — every plug had a good spark!
At this point, everyone was pretty confused. The spark plugs were the only ignition component that was touched during the inspection, apart from the plug wires. The mechanic had just checked the plugs again (he checked them with the tester prior to reinstalling them in the engine the first time), and they were working fine. He triple checked the plug gaps, and confirmed they were in tolerance. What in the heck was going on here?
The mechanic at this point removed a few of the spark plugs from the engine, and took a good, hard look at them. He checked the connection on the top of the plug for damage or corrosion, and found none. He inspected the electrode for shorts or traces that would cause the spark plug to misfire, but found nothing. He looked for cracks anywhere on the plug, and found zippo!
FRUSTRATION SETS IN. This didn’t make sense. The plugs worked fine — outside of the engine, but seemed to fail *inside* the engine. As a final test, the mechanic bit the bullet and installed a new set of plugs. After another half-shot of primer, the mechanic turned it to the START position and the engine immediately started… and ran smoothly. The only thing that had changed was the spark plugs — you know, the ones that had worked with the magnetos on the plane, and on the spark plug tester on the bench!
LESSONS IN BLACK MAGIC: The mechanic did a good job of troubleshooting, and when all the evidence pointed toward the spark plugs, he puzzled over it, but did the right thing. He replaced the spark plugs to address the most likely symptom. The engine sprang back to life, and he was a hero with the owner (even with the extra price of the spark plugs added into the bill).
In the end, we can only speculate about what was going on inside the engine, but something wasn’t right with the spark plugs. Perhaps the plugs electrically shorted and failed in such a way so as to not work in the airplane, but to work outside the engine. We’ll never know… though I anticipate I may hear a few guesses from our readership, here.
BOTTOM LINE: The lesson here to the average pilot is simple: When the problem eludes your troubleshooting, figure out what the most probable cause of the problem is and TREAT THE MOST PROBABLE SYMPTOMS FIRST — AND COMPLETELY. By using this approach (when all others fail) you will have the best chance of solving the problem for the least amount of money.