|President, Cessna Twins Spar Corporation|
Cessna Twins Spar Corporation
36 Columbia Street, Farmingdale, NY 11735-2606 Tel: 516-847-0002
February 23, 2005
Dear Ms. Grady,
Per your request, the following are CTScs comments regarding the spar cracks discovered in two Cessna 402C aircraft.
The recently discovered spar problems on two extremely high-time Cessna 402C aircraft used in commuter service indicate that high airframe time coupled with a high cycle, low altitude, turbulent air, severe service history can combine to produce structural problems. This is a conclusion that can be reached by common sense, of course. The fact that the spar failure occurred so far outside of the area predicted by Cessna's study brings that study and its conclusions under further question.
Cessna's proprietary methods and data are not available for peer review and many have questioned their findings as a matter of logic and of a conflict of interest. The Cessna study is apparently unable to predict spar cracking in the real world of real airplanes, as demonstrated by this extreme spar failure in an area not predicted by Cessna and that no cracks have been detected in the stations predicted by Cessna. Due to the potential safety of flight and resulting loss of life that could result from this spar issue, demonstrated by actual cracks in actual aircraft, CTSc suports the FAAs decision to issue this Emergency AD However, CTSc believes that this incident and the resultant AD further invalidate Cessna's already-questionable (not to mention unrevealed) data and conclusions.
CTSc suggests that, considering the potential for loss of life, and the economic impact of the original NPRM for Cessna 400-series wing spars, and the fact that that an AD has been suggested purely on a predictive model, rather than real-world experience, it is necessary to study this issue more thoroughly and expeditiously. This incident seems to indicate that Cessnas predictive model is thus far useless, which makes an expensive and probably dangerous repair seem even more inappropriate. CTSc believes that the FAA, in the interest of better knowledge of aging aircraft/fatigue issues with the Cessna 400 aircraft, should fund a neutral third party such as the National Institue for Aging Aircraft (NIAR) to conduct a Cessna 400-series study using matching wings and fuselage (rather than 402 wings with a very stiff 425 fuselage used by Cessna, which creates an invalid stress model) and non-proprietary data and methods so that legitimate data can be generated with regard to wing spar issues in 400 series Cessna aircraft. The NIAR recently acquired a full-scale structural testing lab which will hold an entire aircraft and provide real world fatigue testing rather than theoretical models. This is especially important considering the very public failure of Cessnas predictive model to provide legitimate real-world results.
Robert J. Vila
Cessna Twins Spar Corporation