Labs-designed inspections help Forest Service get grounded tankers flying

The US Forest Service (USFS) reactivated the first two of many large firefighting air tankers last week using an improved aircraft inspection and certification program designed at Sandia’s Air-worthiness Assurance Center (AANC). The large air tankers — including P-3 Orions, DC-4s, DC-6s, and DC-7s, and P-2V Neptunes — are owned and flown by private companies under a contract administered by the Forest Service and used by several firefighting agencies. The planes were grounded after two fatal accidents involving C-130A and PB4Y air tankers last summer. Following findings of the Blue Ribbon Panel on Aerial Firefighting commissioned by the Forest Service and the Bureau of Land Management (BLM), USFS and BLM officials had pledged that none of the large air tankers on contract would be returned to service until an enhanced inspection program was in place. On Monday last week, using an enhanced inspection program recommended by Sandia for Lockheed P-3 Orions, the USFS certified that a contractor had met all inspection requirements for two P-3s and returned them to service. Five more P-3s are undergoing certification procedures now. Sandia is nearing completion of a similar inspection and certification process for the DC-4, DC-6, and DC-7 classes of tankers. The new procedures should be available next week to contractors flying those aircraft. Development of the improved inspection process for the Lockheed P-2V is under way. The C-130A and PB4Y aircraft are no longer being used by the agencies for aerial firefighting. "Until we can acquire newer aircraft down the road, this partnership with Sandia is a definite step towards safer operations with the current contract fleet," said Tony Kern, Forest Service Assistant Fire Director for Aviation. Administrative and technology changes As part of the three-year program funded by the Forest Service, Sandia, in consultation with the Federal Aviation Administration (FAA), is evaluating each contractor’s procedures for maintaining and inspecting their air tankers and is developing recommendations for enhanced procedures that would improve the contractors’ abilities to find and repair cracks and other structural problems before they pose a threat to aircraft and their crews, says Dick Perry, Manager of Airworthiness Assurance Dept. 6252. Although the final causal report has not yet been released from the National Transportation Safety Board, "fatigue cracks" growing unnoticed in the aluminum components of the center wing box are thought to be among the causes of last summer’s fatal C-130A accident. Among Sandia’s recommendations will be administrative changes to formalize and improve maintenance and inspection procedures, says Dick. Sandia also will encourage use of advanced nondestructive inspection (NDI) technologies that can identify flaws that are hidden from view or are too small to be detected by visual inspection, he says. One such technique, called eddy-current inspection, detects sizes and locations of subsurface cracks by sensing disturbances in magnetic fields as a hand-held scanning device is applied to the metal. Another NDI technique, called ultrasonic inspection, detects flaws by monitoring sound waves as they pass through materials.

Measuring stresses

In the past, inspections of in-service aircraft were accomplished primarily by experienced personnel performing visual inspections of critical parts for cracks and other defects. "Our initial objective is to use modern inspection technology and what we know about the aging of aircraft and materials and get the large air tankers safely back in service as soon as possible for the 2003 fire season," says Dick. In addition, Sandia will evaluate data gathered this summer from several air tankers instrumented with sensors in an effort to characterize stresses on the planes’ airframes in the unique flight environments the aircraft encounter. "They fly at low altitudes in mountainous areas under windy conditions," says Dick. "This turbulence causes stresses on the aircraft structure, like driving down a bumpy road in a car. "We need to know what kind of workout they get and how these loads differ from other aircraft so we can understand and control the effects of these stresses," he says. Based on the data, Sandia also will recommend long-term inspection procedures for each class of aircraft, including determining how often each aircraft needs to be inspected to ensure cracks don’t have time to grow into structural defects.

Tolerance limits

Many firefighting tankers operated by contractors are retired military or commercial transport planes designed and built decades ago. The DC-4s, for example, served in WWII as C-54 transport aircraft. These large air tankers are considered national resources and are assigned to assist in initial attack on wildfires in different jurisdictions throughout the country, says Rose Davis of the National Interagency Fire Center in Boise, Idaho. Although firefighting aircraft today are flown within their design tolerance limits according to strict guidelines for pilots, it is important to keep a close eye on how the normal aging processes and, in some cases, many hours of flying time are affecting the plane’s structural members, says Dick. Many of the NDI techniques used for aircraft inspection were developed or refined at the AANC near the Albuquerque Sunport, managed and staffed by Sandia for the FAA. The AANC’s primary role is to develop improved inspection and maintenance techniques that safely extend the service lives of aging commercial airliners.