The email was to the point and dispassionate, but it told a tale of woe:
“I am seeking some assistance regarding wing lower spar cap corrosion for Aero Commander 560E:
“Attached are the images of exfoliation corrosion found on the LH wing forward lower spar cap between wing stations 201 and 219.66. The corrosion was a result of the previous owner having left an inspection panel removed for a length of time which resulted in a bird nesting in the wing and causing corrosion to the spar cap. Can you please give guidance to a repair solution.
“Our intent is to ferry the aircraft to a suitable maintenance facility. Your advice is greatly appreciated.”
The photos attached to the email were visual proof of the very expensive damage that was a consequence of a small but significant act of careless neglect. Unfortunately, damaging corrosion resulting from animal droppings or moisture is a not-uncommon occurrence, and it’s not always due to overt carelessness such as not bothering to reattach an inspection panel or plate.
Storing an airplane outside for more than a brief period invites problems with birds, rodents, and other wildlife that are resourceful in finding ways to get inside the airframe and nest. When Service Bulletin 237 was issued, calling for an inspection of engine mount-beam support straps on top of the upper wing skins, we had several reports from technicians who found birds’ nests in the affected area. One owner applied for an extension of the inspection compliance time, but was denied when an FAA inspector took a look at the airplane and discovered that the engine nacelles were full of birds’ nests.
Nesting, defecating, urinating wildlife is not the only corrosion-inducing problem. Moisture can make its way inside the airframe where it can be difficult to detect and eliminate. We’ve heard of mushrooms found growing behind the couch in the rear of the passenger cabin. If mushrooms, which love a dark and dank environment, can prosper in the cabin, corrosion should have an easy time of it.
One potential source of moisture contamination is the picture window in the passenger cabin of 690/A/B/C Twin Commanders. The windows are installed from the inside, and are designed to attain the maximum seal when pressurized from the inside. When ambient pressure is equal inside and outside the cabin, as is the case when the airplane is on the ground with engines off, moisture can seep past the window seals and trickle down the fuselage to the floor. If the nose wheel strut is properly inflated so that the airplane sits slightly nose high, that moisture can eventually make its way under the carpeting to the rear of the cabin and puddle against the aft pressure bulkhead. Undetected, it can be the catalyst to damaging corrosion.
The passenger cabin is not the only area of the airplane subject to undetected corrosion due to moisture contamination. A Twin Commander Aircraft Maintenance Alert issued September 2, 2011 addresses potential exfoliation corrosion on the skin beneath the dorsal fin on certain serial number 690A/B/C/D and 695A model Twin Commanders. The corrosion results from years of undetected moisture contamination beneath the dorsal fin. Such corrosion can lead to very expensive replacement of upper aft fuselage skins.
The moral of the story? Store your airplane inside, if at all possible. Use engine nacelle and exhaust stack plugs, and plugs or other devices to block openings to the internal airframe. Do thorough preflights with an eye toward animal and moisture contamination, and have the airplane thoroughly and professionally inspected at regular intervals. Most important, fly it often. Airplanes are made to be flown, and tales of woe can afflict an airplane that isn’t allowed to perform its basic function often enough.
Geoffrey Pence is Customer Service Manager for Twin Commander Aircraft.
