DAVID LEARMOUNT / LONDON

FAA proposes structural integrity regulations for large transport aircraft

Manufacturers and operators of US-registered airliners face proposed rulemaking by the Federal Aviation Administration that would update existing regulations for determining the acceptable life of an airframe.

Aloha disaster W445
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The 1988 Aloha Airlines disaster spawned original ageing aircraft programme

Accumulating knowledge about the nature of what the FAA calls widespread fatigue damage (WFD) has led the agency to consult industry about drawing up more accurate rules to assess the projected life of new aircraft and monitoring the safe life of ageing aircraft, including repairs and modifications to them.

Two categories of aircraft structural degradation

Widespread fatigue damage can take two basic forms as defined by the US Federal Aviation Administration: multiple site damage (MSD), which is “the simultaneous presence of fatigue cracks...in the same structural element such as a large skin panel”; and multiple element damage (MED), which is “the simultaneous presence of fatigue cracks in similar, adjacent structural elements such as frames and stringers”. An example of combined MSD and MED would be crack damage to window surround structures, or involving rib-to-skin structures, the FAA says.

The proposed rule would affect all US-registered new or in-service transport category aircraft with a maximum take-off weight in excess of 34,000kg (75,000lb). Operators would be required to incorporate the operational limits – when they have been developed – into maintenance programmes, says the FAA.

WFD is the natural result of cyclical stresses that occur in normal operations on all parts of an airframe, leading to gradual structural weakening. “WFD is the simultaneous presence of cracks at multiple structural locations that are of sufficient size and density such that the structure will no longer meet the residual strength requirements,” says the FAA. “WFD results from many cracks that are generally too small to be reliably detected using existing inspection methods,” it adds.

The notice of proposed rulemaking (NPRM) suggests the existing system of defining the life of an aircraft, known as a “design service goal”, should be replaced by a compulsory “initial operating limit” (IOL), beyond which an operator may not use the aircraft without the approval of an extension.

Both the existing and the proposed limits are quantified in flight cycles. Under the WFD system, repairs and modifications would also be given operational limits that would have to be taken into account for inspection and maintenance purposes. The FAA says this will have a total cost over 20 years of $360 million, of which about 10% will be faced by manufacturers and the rest by operators.

Offsetting that, there may be an expected benefit of about $83 million resulting from the early detection of conditions that would be cheaper to correct than if they resulted in accidents or the need for unscheduled repairs, according to the agency.

The proposed WFD programme is an updating and tightening of the parameters in existing regulations empowered by the Ageing Aircraft Safety Act passed in 1991 by Congress. The Act followed the failure in the air of an Aloha Airlines Boeing 737-200 upper fuselage structure in April 1988. The new rule could see IOLs established by December 2007.

Source: Flight International

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