The Aging Aircraft Laboratory at Wichita State University’s National Institute for Aviation Research recently completed a research project titled “Destructive Evaluation of Aging General Aviation Airplanes” sponsored by the Federal Aviation Administration.   The project, which began in September 2002, involved the destructive inspection and evaluation of three aged commuter service airplanes, a 1969 Cessna 402A, a 1979 Cessna 402C and a 1975 Piper Navajo Chieftain.

The research objective was to determine if potential continuing airworthiness problems exist for the general aviation fleet due to the aging process. The project provides insight into the condition of a typical aged airplane, raises the awareness of aging concern within the general aviation community and provides guidance for additional maintenance and inspection. Upon completion, researchers determined that the general condition of an aircraft structure and its systems is directly impacted by the usage and operating environment of the aircraft.

The three airplanes were chosen because they are representative of the small airplane commuter fleet. The Cessna 402A was primarily used for flying Grand Canyon tours, which is considered a severe turbulence flight environment, while the 402C was primarily used for short island flights along the East Coast, a more corrosive environment and a higher operational usage. The Navajo had numerous owners, and it was operated for service periods of more than three years each in the cold-climates states of Alaska and Maryland and hot-climate states of Arizona and Las Vegas.

Initially, each aircraft underwent a complete non-destructive inspection in which researchers conducted a survey of aircraft maintenance records, routine visual inspections and supplemental non-destructive evaluations.

Each aircraft then underwent complete disassembly. This phase involves inspection of system components, intrusive wiring visual inspections and circuit breaker testing, structural assessment utilizing alternative non-destructive inspection techniques, detailed disassembly of major aircraft sections and microscopic examination of critical and suspect areas.

Upon completion of the teardown evaluation of the three aircraft researchers found that the general condition of the aircraft structure and systems is directly impacted by the usage and operating environment of the aircraft. The 402A aircraft had cracking problems only in selected areas, such as the wing auxiliary spars and nose gear steering bell crank. Only a few locations on the 402A had moderate to severe corrosion, including the forward skin lap joint on the right stub wing and the wing attachment fittings.

The 402C aircraft had a significantly larger number of areas with cracking, such as the carry-thru structure, an angle bracket on the front carry-thru structure, the left stub wing outboard rib, wing auxiliary spars and the engine beam structure. The 402C aircraft also had many areas with moderate to severe corrosion, including stub wing and wing attachment fittings, right fuselage channel, wing front and rear spars, wing auxiliary spars and front and rear spars on the horizontal and vertical stabilizers.

After a thorough teardown examination on the Piper Navajo, multiple cracks, corrosion, wear and damage were identified and characterized on the left and right wings, fuselage and horizontal and vertical stabilizers. Light to light-moderate scattered corrosion was observed scattered throughout the airframe, and numerous areas of moderate to severe corrosion were found on many airframe components.

Even though the three aircraft were comparable in age and flight hours, the Cessna 402A and Piper Navajo aircraft were older than the 402C, and were flown in a severe flight environment with increased turbulence due to low-level flights in a desert climate. A general observance was that the condition of the Cessna 402A was better than the Cessna 402C and Piper Navajo due to higher incidences of cracking and corrosion as a result of increased usage and corrosive operating environments on the latter two.

While the routine visual inspections were not comprehensive or intrusive enough to identify all the areas of concern found on the three aircraft, supplemental inspections, service bulletins and airworthiness directives address areas of concern due to aging effects that were discovered in the teardown evaluations. For this reason a major recommendation from this program is that maintenance inspection programs for general aviation airplanes include supplemental inspections based on either service history or damage tolerance analysis. These supplemental inspections should be more comprehensive and intrusive, and they should be performed at a latter stage in the service life of the aircraft. Researchers also recommend that teardown evaluations be expanded to other models of general aviation airplanes that lack existing supplemental inspections, and that results from all teardown evaluation projects be used to develop generic recommendations for maintenance inspection practices and FAA regulations regarding aging effects of general aviation airplanes.