FLIGHT SAFETY FOUNDATION Aviation Mechanics Bulletin SEPTEMBER–OCTOBER 1998 F-117A Accident during Air Show Flyover Caused by Omission of Fasteners in Wing-support Structure FLIGHT SAFETY FOUNDATION Aviation Mechanics Bulletin Dedicated to the aviation mechanic whose knowledge, craftsmanship and integrity form the core of air safety. Robert A. Feeler, editorial coordinator September–October 1998 Vol. 46 No. 5 F-117A Accident during Air Show Flyover Caused by Omission of Fasteners in Wing-support Structure.......................................................... 1 Maintenance Alert ...................................................................................... 9 News & Tips ............................................................................................... 13 AVIATION MECHANICS BULLETIN Copyright © 1998 FLIGHT SAFETY FOUNDATION INC. 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In keeping with FSF’s independent and nonpartisan mission to disseminate objective safety information, Foundation publications solicit credible contributions that foster thought-provoking discussion of aviation safety issues. If you have an article proposal, a completed manuscript or a technical paper that may be appropriate for Aviation Mechanics Bulletin, please contact the director of publications. Flight Safety Foundation assumes no responsibility for submitted material. The publications staff reserves the right to edit all published submissions. The Foundation buys all rights to published manuscripts. Payment is made to authors upon publication. Contact the Publications Department for more information. F-117A Accident during Air Show Flyover Caused by Omission of Fasteners in Wing-support Structure Bart J. Crotty Aviation Consultant On Sept. 14, 1997, the pilot of a Lock- Force said that no one on the ground heed Martin F-117A felt an abnormal was seriously injured; news media re- vibration when he initiated a climb ports said that 12 people suffered during a flyover at 700 feet and 380 minor injuries. knots at an air show at Martin State Airport near Baltimore, Maryland, The Air Force said, in its investiga- U.S. The U.S. Air Force stealth fight- tion report, that the accident was er then abruptly rolled to the left and caused by the omission of four fas- pitched nose-up. teners, which resulted in failure of the brackets that attach a wing-support A large section of the left wing sepa- structure to ribs in the left wing. rated from the airplane and struck the ground between the runway and a The aluminum wing-support structure, taxiway. The nose section also sepa- informally called a Brooklyn Bridge, rated from the airplane. The pilot measures 18 inches by 5 inches by 5 ejected, and the airplane descended inches (45.7 centimeters by 12.7 cen- and struck the ground in a residential timeters by 12.7 centimeters). The area. structure spans the outboard-elevon integrated-servo-actuator (ISA) bay. The airplane was destroyed, and three The structure braces the wing and houses were damaged by fire. The serves as a platform for attachment of pilot sustained minor injuries. The Air the outboard-elevon ISA. (An elevon is a primary control surface that com- rigidity of the left wing and allowed bines the functions of an aileron and the support structure and the out- an elevator.) board-elevon ISA to move within the outboard-elevon ISA bay. This al- Figure 1 shows the wing-support struc- lowed uncommanded movements of ture. Figure 2, page 3 shows the loca- the elevon to begin as the airplane tion of the wing-support structure. began to climb during the air show flyover. The Air Force said that four Hi-Lok fasteners (bolts) were omitted from The intensity of the elevon oscilla- one of the support structure’s attach- tions increased and resulted in ment brackets. Omission of the fas- structural stress that ultimately teners resulted in reduced structural caused the wing to fail and separate Lockheed Martin F-117A Accident, Sept. 14, 1997 Left-wing Support Structure Top Cap Location of Four Missing Fasteners D OAR AFT OUTB Source: U.S. Air Force Figure 1 2 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • SEPTEMBER–OCTOBER 1998 about 2.5 feet (0.76 meter) inboard had been performed between 1992 of the outboard elevon, at the no. 11 and 1996 on the support structures in wing-rib position. both wings and the outboard-elevon ISA in the left wing. The accident airplane was built in 1983 and had accumulated 2,585 The wing-support structures in the flight hours. The Air Force said that F-117 fleet had a history of mainte- the pilot’s preflight inspection on the nance problems that began in 1988. day of the accident had revealed no The problems were caused by loose discrepancies. A review of mainte- Taper-Lok and Hi-Lok fasteners; nance conducted during the previous elongated bolt holes; and cracked 30 days (14 items) revealed nothing support “T” (T-shaped) brackets. that could have been involved in the accident. Two T brackets, four “L” (L-shaped) brackets and several fasteners affix A further review of the airplane’s each wing-support structure to the no. maintenance history by the Air Force 14 wing rib and the no. 15 wing rib showed that substantial modifications in the ISA bay. Fasteners also affix Lockheed Martin F-117A Accident, Sept. 14, 1997 Left Wing Inboard Elevon Outboard Elevon Location of Wing-support Structure Outboard Elevon Integrated Servo-actuator Source: U.S. Air Force Figure 2 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • SEPTEMBER–OCTOBER 1998 3 steel caps to the top and bottom of staffing situation had on the events each support structure. leading to the accident. During a scheduled (no. 3 phase) in- The Air Force found no record in the spection in January 1996, the support computer database (the core automat- structures in both wings of the acci- ed maintenance system [CAMS]) of dent airplane were found to flex up the removal, repair, reinstallation and and down. The aircraft manufacturer inspection of the wing-support struc- recommended removal and repair of tures by Air Force technicians. A the structures. The manufacturer pro- record had been made on paper, but vided on-site technical assistance and had been discarded; such paperwork tools, and Air Force structural-repair is not required to be retained beyond personnel removed, repaired and re- three months. installed the structures at an Air Force field maintenance facility. Maintenance personnel from at least four different Air Force technical The repairs consisted of stripping groups, an Air Force engineering tech- paint; resizing holes; conducting non- nical service representative and the destructive inspections; installing aircraft manufacturer’s representative new, oversized fasteners; and repaint- had been involved in the removal, re- ing. The repairs required two weeks pair, reinstallation and inspection of to complete. the wing-support structures. The Air Force said that when the Structural repair personnel had in- structural-repair personnel reinstalled stalled the wing-support structure in the wing-support structure in the left the left wing. The aircraft’s crew chief wing, they apparently failed to install or wing phase-inspection personnel four Hi-Lok fasteners on the upper then had installed the outboard- inboard L bracket that attaches the elevon ISA and the steel cap. With structure to the no. 14 rib. the cap installed, the wing-support structure and its rib attachments no The Air Force said that two people in longer were visible. key maintenance-management posi- tions at the field maintenance facili- Air Force aircraft are maintained, in ty were in transition (that is, they had part, according to procedures in spe- received new job assignments) and cific technical orders (TOs). There that no one had been assigned to an- was no TO for removal, repair and other key maintenance-management reinstallation of the wing-support position at the facility. The Air Force structure. Therefore, submittal of did not say what effect, if any, this a maintenance-assistance request 4 FLIGHT SAFETY FOUNDATION • AVIATION MECHANICS BULLETIN • SEPTEMBER–OCTOBER 1998 (MAR) by the field maintenance fa- two technicians conducted the in- cility and approval of the request by spection, but Air Force investigators appropriate Air Force authorities discovered that the inspection actu- were required before the work was ally was conducted by three other begun. technicians. The Air Force said that it found no The inspection required removing MAR or written record of approval the top steel caps, but not the wing- for the work, although some docu- support structures. The three techni- ments indicated that verbal approval cians who removed the steel cap from for the work had been given.