Airframe and Powerplant

Searching the SDRs for the sick, the sinister, and the surprising

Around annual inspection time, it's tempting to sit down and ponder what's likely to go wrong with the airplane this year. Experience is always an excellent guide, but for the new owner or for some just switching types, a thumbnail sketch of what's likely to break can help illuminate the annual. Naturally, the intent is to fully inspect the whole airplane, spinner to tiedown ring, but that's not always what happens. Most of us have heard hangar tales such as: "Old Henry's Comanche hasn't had its gear swung since it left Lock Haven."

Fortunately, there's a source for information of what breaks, called service difficulty reports (SDRs), a program run by the Federal Aviation Administration. Because it's a voluntary program, relatively few of the actual incidents make it in the books; the FAA estimates that SDRs account for about 10 percent of the real number of problems. What's more, the SDRs tend to reflect items that are either fairly extreme — like a cylinder blowing off in flight — or highly unusual. Some of the more mundane maladies, such as vacuum pumps with the half-life of Chevy Chase's talk show or landing lights good for a half-dozen touchdowns, don't warrant a mention because they are so common. Imperfect as it is, however, the SDR system represents a valuable tool, and perusing the list for your particular airplane could be time well spent.

In answering the overall question, "What's likely to break?" we found no fall-off-the-chair surprises, although a couple of problem types appear to be more common than generally thought. We decided to peruse a run of SDRs covering the past two years on two popular airplane types, the Piper PA-28 series (excluding the retractables) and the Cessna 210. These were chosen largely for their numbers, although a detailed run of SDRs for another type may well uncover somewhat different trends. Further, this sampling is not the final word for either of the airplane types, merely an exercise in illustration.

In general terms, it's not too difficult to guess what is likely to break. The most common items are those under the greatest stress. We're talking about engine components mostly, the cylinder assemblies in particular. This shouldn't surprise anyone, because the heat and pressure of combustion put the cylinders, pistons, valves, and valve gear under tremendous stresses. Moreover, the parts asked to do double-duty tend to suffer the most; take valve guides, for example, because they have to not only direct the valve, but also dissipate the heat from the combustion chamber. Given the temperatures involved, it's no wonder the exhaust valves were noted as failing far more often than were the intakes, which receive some form of cooling from the incoming fuel and air.

Components with great rotational stresses also show up frequently, like crankshafts and camshafts. There were a number of reports of crankshaft counterweights coming adrift, either ruining a bearing or forcing a crank fracture as a result.

In more specific terms, here's what we found. Starting with the Pipers, we viewed 278 individual reports. As mentioned, by far the sub- group with the greatest number of entries concerned the engine. Valve- train troubles, mostly spalled lifters and camshafts, showed up in eight reports; remember, this group includes the O-320-, O-360-, and O-540- series engines but not the O-320-H2AD engine, which has such a reputation for cam and lifter troubles. Times in service were all over the board in these cases. We spoke with mechanics about this and were told that valve- train troubles are high on the list of concerns. They suggest that use of the airplane, along with timely oil changes and proper winter preheat, will help keep valve-train problems at bay. Oil analysis is also highly recommended, to catch trouble before it gets to the point of stopping the engine and emptying the savings account.

Three crankshafts showed up broken; there was a report of one outright cylinder head and barrel separation and a piston failure. But what's most alarming is the number of cases of cylinder cracking in the report. Fully one third of the reports mentioned cracks in the exhaust port area and around spark-plug holes. In these cases, the cylinder had not yet separated, instead giving off nasty noises that alerted the pilot to something amiss under the cowling. About half of these cases were discovered during the annual inspection. Oil-pump troubles were indicated in four cases, two of which involved aluminum impellers failing; the FAA has issued an airworthiness directive (AD) to make sure aluminum and sintered-iron oil-pump gears will be removed from service; only steel gears are allowed.

The Pipers also showed up with 14 cases of magneto trouble, the vast majority of which involved weak or failed coils. The rest were a smattering of impulse coupling problems and failed breaker points. A total of 14 call-outs indicated propeller troubles, the most common being broken or cracked spinner bulkheads. Incorrect or improperly torqued hardware was mentioned in a handful of cases.

Staying on the engine beat, let's look at the Cessnas. Here we noted 324 SDRs for the 210, T210, and P210. Of those, 50 pointed to powerplant troubles. (This wasn't the highest single subset; the landing gear and hydraulic system racked up 62 SDRs.) Unlike with the Lycomings, the reports disclosed no mechanics finding cracks in the Continentals' cylinders during routine inspections. (This could well be an anomaly of our particular batch of SDRs, because it's pretty common knowledge to look for cracks in these cylinders. One mechanic we spoke to said that he pays special attention to high-time 360- and 520-series Continentals for cylinder cracking.) However, about 15 percent of the engine-related SDRs indicated in-flight cylinder separations, some causing an off-airport landing. An example: "Engine problems on takeoff...rough engine. Made a forced landing...nose gear torn out...inspection revealed [number] 1 cylinder separated 1.5 inches above base flange."

A nearly equal percentage of call-outs indicated case cracking is still a problem with the big Continentals. In addition, there were six instances of crankshaft or crank-gear failures, plus two cases of the aforementioned wayward counterweights. Valve troubles were reported in four cases, with a broken exhaust valve, a seat that had pulled out of the head, and worn guides. A handful of reports indicated piston failure through, according to the posting mechanics, excessive heat or detonation.

Magnetos showed up as maintenance culprits in 19 cases, which included melted distributor blocks, broken mounting bases, and fried coils. Perhaps because most of the 210 fleet is turbocharged (and many have pressurized magnetos), we should not be surprised by reports of internal mag corrosion and contamination. (Pressurized mags, even with in- line filters, tend to pick up moisture easily. Check those filters often.)

What is surprising is that only seven SDRs note turbocharger troubles. The turbo is one of the hardest-working mechanisms in the airplane, but it apparently gets sufficient care and inspection that few are left to fail outright. There are exceptions, however, like this one: "Mounting lug on turbine housing broken off. Turbine bearings worn out. Blades rubbing housing. Rear turbo support bracket holes elongated."

With the 210, you also get a constant-speed prop, and there were 22 call-outs for prop trouble. (Although here we run into one of the SDR traps. These reports are for just 10 airplanes; multiple problems often show up as multiple entries, so read carefully.) Mainly they reported corrosion of the blade shanks and hub inner surfaces.

So much for the firewall forward. The lessons here ought to be clear. Keep a sharp eye on the top-end components, looking particularly for cracks and signs of valve-train troubles. (The Lycoming valve wobble test ought to be second nature to the owners of blue engines.)

Next to all the commotion ahead of the firewall, the airframes are pretty quiet SDR-wise. In the Pipers, there were several reports of control group problems, including frayed cables and dry pulleys. The Centurions showed off a well-known problem with cracking elevator and horizontal stabilizer hardware and mounting structure. Also, both sets of airplanes had trouble keeping trim tabs and their internals from cracking.

Considering all the pounding they take, the Pipers' landing gear had relatively few reports, with the usual smattering of flat oleos and fast-wearing brakes. Owners of 210s, on the other hand, had much more fun with the undercarriage. A total of 60 reports in the SDR listing we used pointed to the landing gear or hydraulic system. These were well spread among the Centurion's complex gear system, involving the squat switch (which either wouldn't let the gear come up or make it go down), jamming of various up- and downlocks, tires rubbing on structure, faulty hydraulic power pack pressure switches, and broken ends of the hydraulic actuators. Two items showed up in greater numbers: Cracking of the saddles — which the main gear uses to transmit landing loads to the fuselage — and failures of hydraulic lines. Seems quite a number of the lines, some metal and some flexible, had either been left in too long or had come in contact with a moving component of the system. In the 210, when you lose the hydraulic circuit, it is sometimes impossible to lower the gear.

If there's a moral to it all, it's that 210 owners should spend the time necessary to maintain the gear, paying particular attention to the lines and to the overall rigging of the system. Mechanics familiar with the 210 say that they see a fair amount of poor or nonexistent maintenance on the gear system. It's a complex arrangement and contains subtleties often beyond the airframe and powerplant mechanic asked to work on a wide variety of brands. This is certainly not an indictment of the general practitioner, but it should also be clear that 210 owners ought to get painfully familiar with the system.

Auxiliary systems also show up in the SDRs. For the Pipers, we noted 14 cases of alternator problems, usually broken belts as a result of misalignment of the pulleys or simple old age, and, often, catastrophic structural failure of the alternator itself. There were 15 reports of alternator mayhem for the 210s, in much the same vein. Voltage regulators show up as taking early retirement fairly often, too. Vacuum pumps, though if judged by the collective pilot consciousness seem to fail all the time, showed up in just five cases for both airplane types. Maybe we're so used to them failing that technicians don't feel much need in filing an SDR.

What all this amounts to is awareness. It's quite possible that you could own a Centurion whose gear operated perfectly for several years and have one of the problems listed in the SDRs brewing. Likewise, you might think that the O-320's bulletproof reputation makes careful annual inspections almost superfluous, but the SDRs show that even the most durable devices break. Do this, then: Obtain an SDR listing for your airplane. Several vendors will provide them, as well as full AD listings, on diskette, CD-ROM, or through an on-line service. (AOPA offers SDRs through AOPA Online.) Take that information to the next annual. Finding a problem before it becomes a, well, problem could make it all worthwhile.