Manufacturers of 4,198 airline transport aircraft will be required to set new life limits for their products under new US Federal Aviation Administration rules.
As part of the widespread fatigue damage final rule, airframers of US-registered transport aircraft with take-off weights of 34,019kg (75,000lb) or more will have between 18 and 60 months to determine the number of flight cycles or hours that each type can accumulate before showing the effects of the problem.
Operators of affected aircraft will then have 30-72 months, depending on the model of aircraft, to adjust their maintenance programmes.
Once the new limit is written into a maintenance programme, operators will not be allowed to fly it beyond that point without an FAA extension.
All new aircraft certificated under Part 25 air transport certification rules will be required to have the limits.
The FAA defines widespread fatigue damage as "the simultaneous presence of cracks at multiple structural locations that are of sufficient size and density that the structure will no longer meet the residual strength requirements of Part 25".
The condition gained prominence in 1988 when an Aloha Airlines Boeing 737 lost a 5.5m (18ft)-long section of its upper fuselage at 24,000ft, killing one flight attendant.
The FAA published a preliminary rule on the topic in 2006 based on recommendations made by an aviation rulemaking advisory committee.
Unlike the preliminary rule, the final rule does not require certificate holders to evaluate widespread fatigue damage associated with most repairs, alterations and modifications to the baseline aircraft structure, except for cases where modifications were mandated by airworthiness directives.
FAA administrator Randy Babbitt calls the new rule "a comprehensive solution to ensure the structural safety of today's airliners and the airplanes of tomorrow" that will save the FAA $4.8 million over the next 20 years because it will not have to issue ADs to address individual cases of fatigue damage as they are found.
Other benefits not considered in the financial analysis include accidents avoided and a "longer economic life" for the aircraft.
The agency estimates that the new rule will cause one airplane from today's fleet to be retired because of its reaching its widespread fatigue damage life limit in the 20-year analysis period.
"The retirement of this one airplane will result in costs of approximately $3.8 million, with a present value of approximately $3.6 million," says the agency, adding that cost impacts to manufacturers to implement the rule would be "minimal".
Source: Flight International
