Aircraft Maintenance: Continental starter adapters

Predictability means that you can catch a mechanical issue before it progresses to failure. As a whole, GA scores fairly high on this one. If you’re proactive with your maintenance, almost any failing component will show symptoms before failure and few family vacations will be canceled because of maintenance surprises. That said, some systems are not quite as predictable as others. We experienced that firsthand during our recent SocialFlight summer adventure trip to Glacier National Park.

Continental’s big-bore, fire-breathing, six-cylinder engines are marvels of engineering. The IO-550B that powers our Bonanza A36 delivers 300 horsepower, pulling us through the sky at close to 180 knots on less than 15 gph. It’s been bulletproof for us, and we have flown over stretches of open ocean without worry based on our experience with the engine and the folks at the factory who support it. Almost everything on the engine is fairly straightforward…except for the starter system. The starter motor is mounted at a right angle to the engine at the rear of the engine, connected to the engine through a complex “clutch-like” mechanism known as the starter adapter. Starter adapters are utilized on Continental’s 300, 346, 360, 470, 520, and 550 engines.

When the electric starter motor spins, it turns a shaft on the starter adapter. Inside the starter adapter, the shaft is connected to a coil spring that (when the starter is spinning) grabs a drive shaft and cranks the engine. The design is a little like a Chinese finger trap: The spinning starter forces the spring to tighten around the shaft and, the harder it turns, the harder it grabs the shaft. As soon as the engine lights off and starts running faster than the starter, the spring tension is lost and the shaft spins free of the starter.

The system works well…until it doesn’t. After years of wear on the shaft and the spring, at some point the spring will simply not grab the shaft anymore. Unfortunately, this can happen with little advance warning. The metal-to-metal design of the system leaves very little “gray area” for partial failure. It either grabs or it doesn’t. The good news is that failure of a starter adapter is not a safety-of-flight issue. You can either start the engine or you can’t. The bad news is that your first sign of failure can also be your last sign of failure. Once you see it slip, you could have 10 more starts to go, or none.

In our case, I knew that we were on borrowed time and needed to get the ball rolling ASAP on a replacement part. Fortunately, I’d seen this very issue play out previously for other Bonanza owners and knew exactly whom to call. At our next fuel stop I called Aircraft Specialties Services in Tulsa, Oklahoma, a company that came highly recommended by owners and A&Ps alike. I’ve spent time at the company’s booth at EAA AirVenture and the Sun ‘n Fun Aerospace Expo learning about the engine components that the firm manufactures in-house, and so it was fortuitous that I was now in dire need of one of the adapters that I had been admiring in the show booth only months before.

I connected with Eric Anderson, the company’s director of operations, who immediately got the ball rolling on a replacement adapter while he explained the inner workings of the units, why they fail, and what Aircraft Specialties Services does to help stranded pilots and ensure that the adapters that leave their shop are as good as or better than new. Starter adapter overhaul includes all recommended inspections, including Alodining and gold cadmium plating the housings and external hardware. New seals, gaskets, and bearings are installed, and the shaft gears are reground and Magnafluxed. However, the key differentiator in Aircraft Specialties Services’ overhaul is the proprietary, FAA-approved heavy-duty clutch spring. Since the clutch spring is the weak point in the system, the company designed and certified its own custom springs for the application. In addition to being heavy duty, the firm made springs available in special sizes that extend the life of the adapters in cases when the shaft must be ground undersize to return it to service. As one would imagine, a shaft that has been ground smaller requires a smaller spring to maintain the same tolerances. Aircraft Specialties Services produces springs that are 30 and even 40 thousandths undersized to keep more starter adapters in reliable service in the field (and out of the “red tag” bin).

Since the starter adapter is connected to the engine’s accessory case, it can double as a “PTO” (power take off) to drive other accessories such as air conditioning compressors. This means that there are more than a few models of starter adapters out there, and some are more complex than others. And so, Anderson explained that the quality control team at the facility tests much more than just proper functioning of the starter mechanism itself. PTO-enabled units are thoroughly tested on test stands for oil flow rates and “spun up” to ensure that the oil scavenge pumps operate properly and are ready to drive the other components installed on the aircraft.

Above all else, Anderson stressed that Aircraft Specialties Services prides itself on rescuing stranded pilots. The company can overnight an adapter to just about anywhere, something it considers a routine service. In our case, we managed to get the additional four starts we needed to make it home. As soon as I landed, I made the final call to Anderson, and a replacement adapter was on its way.

Installation is fairly straightforward, consisting of removing the starter, followed by the adapter, which is mounted on four studs at the rear of the engine. On some aircraft, however, there isn’t enough space between the rear of the engine and the firewall to remove the adapter, so the engine itself has to be moved forward. Thankfully, that isn’t the case on Bonanzas and I had the units swapped out within a few hours.

If you find yourself stuck at a remote airport with a spinning starter, but not a spinning prop, my friend and fellow IA Mike Busch recommends one last-ditch technique to get yourself home: See if the FBO has a power cart available to connect to the aircraft for starting. The added boost of power from the cart can sometimes give you that little extra boost you need to get the failing adapter to “grab” just one last time. Failing that, I recommend that you try preheating the engine because we found that the adapter was slipping when the engine was cold, but not slipping while it was hot at our intermediate fuel stops. If neither technique works, rest assured that help is only a phone call away. You might be spending an unplanned night away, but companies such as Aircraft Specialties Services are ready to come to the rescue with exchange parts in stock and FedEx on speed dial. Until next time, I hope you and your families remain safe and healthy and wish you blue skies.