The Federal Aviation Administration ( FAA) has finalized its airworthiness directive (AD) for certain Boeing 757-200, 757-200CB, and 757-200PF following a crack growth analysis of the upper frame.
According to the regulator, a crack growth analysis prompted it to issue a notice of proposed rulemaking (NPRM) in July, which resulted in the AD, which was finalized and published on December 9.
“The NPRM was prompted by a crack growth analysis, which indicated that current inspections are not adequate to detect cracks in certain sections of the upper frame at the frame splice between certain stringers before a single frame fails.”
As a result, the FAA proposed mandatory inspection of records for existing repairs, repetitive crack inspections of the upper frame at the frame splices between certain stringers in certain sections, and on-condition actions.
Prior to the NPRM, the regulator published a supplemental NPRM (SNPRM), which proposed revised repetitive inspection intervals for 757-200s converted to freighters by Aviation Partners Boeing (APB) with a supplemental type certificate (STC) ST01518SE.
Boeing, FedEx, European Air Transport Leipzig (EATL, operating aircraft on behalf of DHL), and United Airlines provided suggestions to adjust the NPRM, with Boeing supporting the directive without any changes.
FedEx said that its 757-200s had been modified by VT Mobile Aerospace Engineering (VT MAE), requesting the FAA to implement shorter repeat intervals for its aircraft.
The FAA declined to adjust the directive, saying that the cargo airline did not justify and/or provide enough data to support its request.
EATL and United Airlines expressed concerns about the mandatory compliance times, with the latter arguing that they could create “an undue burden on the operators.”
“United added that the FAA did not provide details regarding how it determined requiring a reduction in compliance by a factor of two. United understood that a reduction is necessary because APB does not yet have an approved service bulletin to provide its compliance requirements.”
EATL said that the directive included substantial penalties for aircraft equipped with winglets and modified by the aforementioned APB STC.
While Boeing and APB typically work together to align impacts generated by the installation of winglets, the AD would prevent aircraft with winglets, which were modified according to the STC, from scheduling the inspections required by this AD during base maintenance events.
“The FAA stated the reduction in compliance by a factor of two is to allow for APB to complete its evaluation of airplanes with APB STC ST01518SE installed. The FAA determined this is a conservative and appropriate compliance time for this AD.”
The FAA explained that it issued the directive to address cracking at the upper frames common to the splice at stringers S-13 to S-14, which could interact with fuselage skin cracking at the stringer S-14 splice in Sections 43 and 46.
“The unsafe condition, if not addressed, could result in the inability of a principal structural element to sustain limit loads, and could adversely affect the structural integrity of the airplane.”
According to the FAA’s estimates, there are 456 757-200s affected by the directive, which will mandate a general visual inspection (GVI) and repetitive inspections, which have to be done either every 3,000 or 6,000 flight cycles (FC). The latter will take up to 267 work hours, per the US regulator.
Ch-aviation data showed that Delta Air Lines and United Airlines have 147 757-200 aircraft in their fleets.
The 757-200s are either active, stored, or in maintenance, with the average fleet age of the type at the two airlines being 28 and 27.9, respectively.
