Military Airlift

The C-17 military transport plane is being developed by McDonnell Douglas Corp. to carry military cargo into small, austere airfields. One of the key technologies being used on the C-17 is powered lift, which uses wing flaps to increase the aircraft's lift. GAO looked at whether the flap designed for the C-17 could withstand damage from heat and acoustic stress (vibrations caused by sound). Tests in February 1991 predicted that the flap would have to withstand temperatures in the 300-degree Fahrenheit range or twice the temperature for which the flap was designed. In actual flight tests, however, the temperature on the extended flap exceeded 400 degrees Fahrenheit. As a result, McDonnell Douglas is adding titanium and additional composite materials to the inboard flaps. Acoustic vibration tests in June 1990 on a five-foot section of the wing box produced internal cracking after five minutes of acoustic exposure, suggesting that the flap needed strengthening. A later test showed that cracks occurred after one hour of exposure to acoustic vibrations, and McDonnell Douglas has concluded that the flap's life expectancy would be only 400 hours. Temperature, which could also affect flap durability, was not considered. Although McDonnell Douglas has strengthened the flap further, it has not retested to demonstrate the life expectancy of the strengthened flap. An independent Air Force review team concluded that the initial structural design of the flap was unacceptable because the flap could not withstand the acoustic and temperature stresses expected for the required life of the aircraft. Because of these uncertainties, GAO believes that test plans should be established to determine whether the flap meets the contract service life requirement before the C-17 enters full-rate production.