Effective Mar. 14, U.S. operators of Boeing 747-200B, 747-300, 747-400, 747-400D and 747-400F aircraft will be required to inspect and take required action for the aluminum strut side skin and corrosion-resistant steel (CRES) outer spring beam support fitting every 500 flight cycles.
The FAA airworthiness directive, published Feb. 7, found that cracking in the strut side skin and spring beam support fitting “could result in the failure of the outer spring beam support fitting, which could cause separation of a strut and engine from the airplane during flight.”
The issued notice of proposed rulemaking (NPRM), published June 21, 2016, proposed repetitive high-frequency eddy current (HFEC) inspections and open-hole HFEC inspection for the cracking, corrective actions and a fastener installation modification.
The final rule estimates 320 aircraft will be affected. Estimated costs include $340 per inspection cycle for labor and $98,064 for modification, including up to 490 work-hours and $56,414 in parts per aircraft.
The NPRM was met with several comments, including a request for revision from KLM Royal Dutch Airlines to change the flight-cycle requirement from 500 flight cycles to 1,250. KLM suggested the time to complete the repetitive inspections would be a “large burden.”
The FAA disagreed with the revision, stating that 500 flight cycles was the maximum an aircraft could go without inspection for an acceptable level of safety, based on damage-tolerance analysis from Boeing.
The FAA did, however, mention Boeing’s request to reduce the inspection work-hours estimate from 291 work-hours to 4, per new information included in the Boeing Service Bulletin Information Notice published Dec. 23, 2015. This results in a reduction of $24,395 from the estimated labor cost.
Additional comments include a request from Boeing, Delta Air Lines and KLM for additional clarification of service information, which resulted in partial revision to the AD. Boeing also requested clarification of the unsafe condition statement, and the FAA agreed.
Lastly, Delta requested clarification for how certain parts may be used for terminating action, noting that certain parts mentioned only referred to aircraft with the engine model General Electric CF6-80C2. The FAA agreed and revised to include part numbers for aircraft with CF6-80 and Pratt & Whitney 4000 engines installed.
According to Aviation Week’s Fleet Discovery, there could be up to 85 aircraft within the affected Boeing 747 fleet with PW4056 engine models.