Advanced age plus severe stress put firefighting aircraft at risk of structural failure, and the maintenance and inspection programs currently in effect are inadequate, the NTSB said in safety recommendations issued on Friday. The board determined that fatigue cracking was the probable cause in three air-tanker accidents involving in-flight wing separations during firefighting operations. Inadequate fatigue-detection procedures were cited as a factor in all three crashes -- a June 2002 crash of a C-130 in Walker, Calif.; the July 2002 crash of a P4Y-2 in Estes Park, Colo.; and the August 1994 crash of a C-130 in Pearblossom, Calif. Eight crew members were killed in the three crashes. "We hope the release of these reports will raise operator awareness of the unique problems that affect these specialized aircraft, and the importance of a thorough maintenance program to detect safety issues and prevent accidents," NTSB Chairman Ellen Engleman-Conners said in a news release. The board recommended that the Department of Agriculture and the Department of Interior develop maintenance and inspection programs for firefighting aircraft that include consideration of the airplane's original design, age and operational stresses, as well as engineering evaluations to predict and prevent fatigue cracking. The board also recommended that the Department of Agriculture and Department of Interior hire personnel with aviation engineering and maintenance expertise to oversee the new maintenance programs. The initial investigation of the Pearblossom accident in 1994 resulted in a different probable cause. However, a recent review of the accident information, including the examination of wreckage that was not recovered in the initial investigation, revealed previously undiscovered evidence of fatigue fracturing in right-side, center-wing fragments that supported revising the probable cause, the NTSB said. The board issued its safety recommendation letters to the Department of Agriculture, the Department of Interior and the FAA.