Manufacturers of 4,198 airline transport aircraft will be required to set new life limits for their products under new rules to be issued by the US FAA on 15 November.
As part of the widespread fatigue damage (WFD) final rule, airframers of US-registered transport aircraft with takeoff weight 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.
"Once manufacturers establish these limits, operators of affected aircraft must incorporate them into their maintenance programs within 30 to 72 months, depending on the model of aircraft," says the FAA. "After the limit is in the maintenance program, operators cannot fly the aircraft beyond that point unless the FAA approves an extension of the limit."
All new aircraft certified under Part 25 air transport certification rules will be required to have the limits.
The FAA defines WFD 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".
WFD 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 (ARAC). Unlike the preliminary rule, the final rule does not require certificate holders to evaluate WFD associated with most repairs, alterations and modifications to the baseline aircraft structure, except for cases where modifications were mandated by airworthiness directives (ADs).
"We've addressed the problem of aging aircraft with numerous targeted regulations and 100 airworthiness directives over the years," says FAA administrator Randy Babbitt. "This rule is a comprehensive solution to ensure the structural safety of today's airliners and the airplanes of tomorrow."
In terms of benefits, the FAA estimates it will save $4.8 million over the next 20 years by not having to issue ADs to address individual cases of WFD 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 WFD 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 were "found to be minimal".
Source: Air Transport Intelligence news
