Graham Warwick/ATLANTA
GATX/AIRLOG HOPES to gain US Federal Aviation Administration approval for a modification to its Boeing 747 freighter conversion by mid-1996. An initial attempt to obtain relief with an airworthiness directive (AD) limiting gross weight failed, and the company is conducting additional structural analysis.
The AD limits the gross weight of 747s with side cargo doors installed by GATX until the company can substantiate the loads used in design of the freighter conversion. The limitation has essentially grounded ten aircraft with four airlines, as they cannot be operated economically at the lower weight.
GATX/Airlog president Rick Hatton says that the AD requires the loads used in design of the cargo-door installation to be substantiated by finite-element analysis. Only two companies have FAA-approved finite-element models, he says: Boeing and Israel Aircraft Industries (IAI), both competitors of GATX in the 747 freighter conversion market.
GATX produced a modification kit based on a "quick and dirty" analysis conducted at IAI in December 1995, using surrogate structure, but this was not approved by the FAA, admits Hatton. The company has now returned to IAI to conduct a more-detailed analysis using actual structure. This is expected to reveal areas requiring strengthening and will result in production of kit enabling operators to comply with the AD.
"We are at the mercy of one of our competitors," Hatton laments, while acknowledging that loads had not been substantiated during the design of the cargo-door installation. "Theoretically, the design is deficient, but practically there is no evidence that it is," he says, citing a lack of structural problems in 83,000h and 8,000 cycles accumulated by the ten aircraft.
Source: Flight International
