By David Jack Kenny
Some pilots have mechanical expertise so broad and deep that they can accurately size up an aircraft offered for sale with no outside assistance. For a few, it might even be reasonable to buy one sight unseen, knowing they can make any needed repairs. For the rest of us, of course, a good pre-purchase inspection is essential, not just to avoid unplanned expenses but to identify potential hazards of which the seller might not even be aware. Locating the right inspector for the aircraft warrants taking some care even when it’s a well-known model whose manufacturer provided detailed inspection checklists. If the aircraft is unusual or those checklists don’t exist, it’s that much more crucial to find someone with the experience to know where to expect the unexpected.
On Nov. 14, 2009, a Sonex homebuilt crashed about a mile north of the field while maneuvering for an emergency landing at the Burnet Municipal Airport in Texas. The nose-first impact crumpled the cabin and killed the 81-year-old airline transport pilot. After flying in to an Experimental Aircraft Association chapter meeting at a private airport, he’d departed in train with a friend flying another Sonex. They maintained a loose formation past Burnet, the friend’s home base, but about five minutes after they parted ways he radioed that he was having problems and was returning to Burnet to land. His last transmission after declaring an emergency was that “Something is really wrong with the airplane.” Several witnesses saw the Sonex flying low across a field before it banked to avoid a row of trees and the nose dropped in an apparent stall. There was no discernible engine noise.
The pilot was rated for multiengine land and seaplanes, single-engine airplanes, and gliders, and held type ratings for several models. Friends described him as healthy and alert. He’d bought the Sonex almost two years earlier. The pilot’s initial application for insurance cited 12,000 hours of total flight experience but no time in the make and model. His logbook wasn’t found, but the total airframe time noted during the airplane’s annual condition inspection the month before suggests that he’d flown it at least 70 hours since making the purchase.
Investigators found no internal failures in the engine, but the carburetor jet needle was gummed up with an unknown substance, preventing it from metering the fuel mixture correctly. The airplane had two fuel tanks made of hard plastic, and the fuel selector was set to the forward tank. The finger strainer at the outlet of this tank was almost completely blocked by bits of a white substance; the same material was found flaking away from the tank’s walls and bottom. Chemical tests showed it to be a polyurethane compound sold as a fuel-tank sealant, and while the product’s packaging didn’t specifically advise against using it in plastic tanks, the manufacturer told investigators that it wasn’t intended for that application.
The airplane had been completed in 2004. Its builder recalled that the forward tank had begun to leak during its first 10 to 20 hours of operation; he’d applied the sealant at that time, and flown it another hundred hours since. The mechanic who’d done the last condition inspection found no sign of a fuel leak and no foreign material in the main fuel line or carburetor float bowl, but said that the normal inspection procedure didn’t include removing the finger screens.
However, Sonex’s installation manual advised “replacing the tank as the only acceptable repair” for a leak. A 2008 revision noted that the tank material “resists most chemicals, including adhesives” and that “there are no known adhesives or chemicals that will satisfactorily repair a leak.” A subsequent update specifically advised against using sealants of this type, as they “do not adhere to the plastic tank and will plug and contaminate your fuel system.”
That last warning was added four months after the accident. Even the earlier caution came out a good three years after the sealant had been applied. Sonex did post the update on its website, but the owner of a completed airplane might not have been in the habit of rechecking construction techniques; as long as the repair seemed to be working, the natural inclination would have been to leave well enough alone. Perhaps a detailed inspection by someone intimately familiar with Sonex airplanes would have identified the problem before its potential for trouble was realized.