History of Flight
On June 28, 2023, at about 0858 eastern daylight time, Delta Air Lines flight 1092, a Boeing 717-200, operated as a scheduled domestic passenger flight from Atlanta, Georgia, to Charlotte Douglas International Airport (CLT). When the airplane was about 2,000 feet above ground level, the first officer lowered the landing gear handle and observed the nose wheel unsafe condition light illuminate, confirmed on the electronic instrument system configuration page. A go-around was initiated to troubleshoot and perform checklists. A manual gear extension was attempted without success.
Delta Air Lines' Atlanta flight control was notified via ACARS, an emergency was declared with air traffic control (ATC), and the flight approached on the ILS 36L approach. At about 300 feet AGL, ATC advised that the nose wheel was not visible, prompting a second go-around. Multiple normal and manual gear extension attempts failed, and the crew decided to proceed with the landing. The airplane touched down about 1,400 feet from the runway threshold, and the nose was lowered onto the runway at about 80 knots. The airplane stopped just before taxiway W7. Airport ARFF sprayed the nose wheel area with water. The crew conducted shutdown and evacuation checklists, and after the fire chief verified safety, passengers evacuated via emergency slides from the two forward entry doors. The 104 passengers and crew evacuated with no injuries.
Aircraft Information
The aircraft, registration N955AT, was a Boeing 717-200. The nose landing gear upper lock link, part number 3914464-509V, serial number 0007, had been manufactured originally as P/N 3911464-505. Before delivery, McDonnell Douglas Engineering Order 3914464, revision Z (December 2001) was incorporated, reidentifying it as P/N 3914464-509. The link was delivered to fuselage number 5051 in February 2001. In June/July 2009, it was removed to incorporate Service Bulletin 717-32-002 (December 2001). According to maintenance records, the gear, including the upper lock link, was sent to IAI for overhaul, and the SB was accomplished, adding a "V" to the part number. At SB accomplishment, the link had 17,313 flight cycles. It was then installed on aircraft serial number 55076, accumulating 7,365 more cycles, then removed for landing gear overhaul. In March 2014, it was installed on N955AT and remained until the accident.
Wreckage and Impact Information
Postaccident examination revealed the upper lock link had fractured into two pieces about mid-span, between 7.2 and 7.9 inches from the forward lug. A shiny, flat thumbnail-shaped region approximately 1.07 inches wide by 0.42 inch deep was observed on the lower surface, with curved crack arrest marks consistent with progressive cracking. Outside this region, the fracture surface was rougher and matte. The lower surface adjacent to the fracture origin had horizontal and vertical scratch features under the paint. The part was cleaned and paint-stripped; a fluorescent penetrant inspection showed no relevant linear crack indications, though linear indications from visible scratches were present. A section cut about 0.6 inch below the fracture surface revealed a darker thumbnail region (0.26 x 0.10 inch) at the origin, with several crack arrest marks. A larger lighter region extended to 1.07 x 0.42 inch. Scanning electron microscopy revealed fatigue striations in the thumbnail region. The overall fracture appearance was consistent with fatigue fracture originating at the lower surface under cyclic tensile or bending loads, with overstress fracture at final separation.
Tests and Research
A quantitative fatigue crack growth rate analysis examined 35 locations along the crack depth using field emission SEM. Fatigue striations were counted, with an estimated 39,059 major striations. The upper lock link had accumulated 41,257 flight cycles. The aircraft manufacturer noted the lock link experiences two major loads per flight cycle (extension and retraction), with extension load greater. Surface roughness measurements on the lower surface showed 59-66 microinches RMS (longitudinal) and 86 microinches RMS (transverse), while the parting line measured 185-508 microinches RMS and 347 microinches RMS, respectively.
Additional Information
Boeing/McDonnell Douglas issued Service Bulletin 717-32-002 on December 4, 2001, based on a reported upper lock link fracture due to tool marks and/or rough surface finish across the parting plane. The bulletin instructed eddy current inspection and modification/reidentification. The examined link was marked with a "V" indicating SB procedures were performed, including verification of surface finish to 125 RMS and freedom from transverse machining marks.
Safety actions by Boeing included notifying operators of the accident, recommending high frequency eddy current inspection of all in-service and spare upper lock links, requesting inspection results, revising the Overhaul Manual (July 2024) to require stripping paint and fluorescent-penetrant inspection of the parting plane area and high frequency eddy current inspections, and initiating Alert Service Bulletin 717-32A0043 for on-aircraft initial and repetitive high frequency eddy current inspections. Delta Air Lines completed inspections of all upper lock links on their fleet and spares with no crack findings. Remaining operators of Boeing 717 airplanes completed fleet inspections in January 2024 with no reported crack findings.