The Road to Longevity Has No Guardrails

The Aero Club I do the maintenance for has two Cessna 172 Skyhawks, both of “late” vintage (they’re “only” 30 years old), which means they have 24 volt electrical systems (most single engine light aircraft have 12 volt systems). I’d guess Cessna was trying to save weight in the wiring, letting voltage supply more of the load than current, allowing lighter gauge wires to be strung through 32 feet of wing and 24 feet of fuselage, and less heavy wires to the bus and to the starter as well. But the 24 volt battery is considerably larger and heavier than a 12 volt battery, which to my mind makes any weight saved in the wiring a wash. Whatever the reasons, the 24 volt systems add a degree of hassle to the maintenance in that we have to be careful when specifying electrical parts and instruments that we need the 24 volt versions of things whose default version is often 12 volts.

Two years ago we put a used starter on one of the Cessnas to replace a broken one. Our IA (FAA designated Inspector) had supplied the starter, assuring us it was a sound 24 volt starter he’d had in his hangar, saving us the considerable expense of buying a new starter. At the time I had recommended getting a new “flyweight” starter, built with new technology that halved the weight of the old starters (10 pounds instead of 20), but the Club said if we’ve got a serviceable used starter being practically given to us, why spend $500? It was a point I had to concede.

And for the past two years that used starter cranked the engine without balkiness or complaint for close to 800 hours of flight time. Then, a month ago, several Club members posted squawks that the starter wasn’t disengaging after the engine was running; they heard some kind of clicking noise. That symptom was more an indictment of the Bendix drive than the starter motor, so we got a new $200 Bendix drive and put it on. (The Bendix drive is a heavy pinion gear that engages a large ring gear behind the propeller; it’s flung forward to mesh with the ring gear when spun by the starter motor; once the engine is started the higher RPM of the running engine throws internal pawls clear by centrifugal force and the drive pinion retracts, preventing the engine from spinning the starter motor. There’s a safety ratchet in the drive such that if the pawls get stuck, and the pinion can’t retract, the safety ratchet slips, also preventing the starter motor from being turned by the engine).

After I’d pulled the starter and disassembled the drive from the motor, I spun the safety ratchet by hand for two of the Club members who’d squawked it, asking them if that was noise they’d heard. They said yes. That was good enough for me; I wrote down the starter’s Model No. stamped on a placard on the motor casing, went to Aviall and got a new drive and had the airplane back in the air with only two days downtime.

Three days ago the Club’s maintenance officer said he still heard that clicking noise. I was skeptical that the new Bendix drive had failed exactly the same way the old one had; the problem, if there was a problem, had to be something other than the drive. Not only that, but perhaps the problem, if there was a problem, had never been the drive all along, we’d made a needless replacement. Anyway, I said, if the starter was cranking the engine there really wasn’t any problem. All we can do is keep an eye on it; it didn’t make any sense to replace the new drive yet again, if the starter was working in all other respects satisfactorily.

The next day the Club’s chief pilot grounded the airplane. “My student tried to start the airplane and nothing happened,” she said.

Well, that was a problem one could finally get a bead on.

I went out with a meter and in half an hour confirmed the battery was strong at 25 volts (so it wasn’t a dead or weak battery), and with the solenoid engaged I was getting 25 volts right at the starter motor terminal (so it wasn’t the ignition switch or the solenoid, or even the wiring). The starter motor itself had died.

Well, two years on a used piece of equipment, after 800 hours of flight time, that’s really pretty good mileage, especially since it had only cost us my labor and a nominal price to the IA to attain. Too bad we’d probably replaced the Bendix drive needlessly, but diagnostics are always a cost benefit balance: spend too much time chasing down the real problem and you spend more than just eating it and replacing the entire suspect component. (That’s a lesson that should be taught starting in the third grade, but unfortunately doesn’t get taught at all).

So this afternoon I pulled the starter off and put it on the bench, then talked the maintenance officer into replacing it with the new flyweight starter I’d recommended two years ago. It’d be a few hundred more than putting on a rebuilt starter of the same ancient design, but the reliability of the new starters more than make up for their increased cost. No problem, he said, go for it. Tomorrow morning after a meeting with the FAA (concerning adding a rating to a license) I’ll drop by Aviall and pick it up.

Meanwhile, while our maintenance officer amused himself by getting under the panel and trying to find the directional gyro’s model number with a flashlight and a mirror (another problem – anytime an airplane has a problem it never has A problem, it has several), I idly looked at the dead starter lying on the bench. I called our IA.

“Hey, Stan,” I said, “I think I know why that starter failed. The one you gave us two years ago.”

“Oh, yeah?” he said. “What did you find, a flat spot on the commutator?”

“No,” I said. “Let me read you the placard. Prestolite Model No. MZ 4222. TWELVE Volts. It’s stamped on the placard. This is a 12 volt motor. It’s amazing it lasted as long as it did.”

“I’ll be damned,” he said.

And that, boys and girls, is why life will always be interesting and just a little dangerous. I had held that 20 pound starter in my hands twice struggling to get it on the airplane, had even written down the model number from its placard, and never saw that 12V right there in plain lettering. I assumed our IA was correct when he said it was a 24 volt starter – and one can’t spend life second guessing the IA, you’d never get anything done. You could say, as we ALWAYS say, with 20-20 hindsight, that we should have been more careful. But there’s reasonable caution, and there’s paralysis. So long as we prefer not to be paralyzed, we just have to accept getting tripped up from time to time, and recognize that the road to longevity might be paved with stones of diligence, but that some of its foundation is nothing more than ineffable good luck.