Event DCA23FA339

Factual narrative

HISTORY OF FLIGHTOn June 28, 2023, about 0858 eastern daylight time (EDT), Delta Air Lines flight 1092, a Boeing 717-200, sustained substantial damage when the nose landing gear did not extend before landing at Charlotte Douglass International Airport (CLT), Charlotte, North Carolina. The 104 passengers and crew evacuated with no injuries. The flight was operating as a Title 14 Code of Federal Regulations Part 121 scheduled domestic passenger flight from Atlanta, Georgia, to CLT. The flight crew reported that when the airplane was about 2,000 feet AGL, the first officer (FO) lowered the landing gear handle and observed the nose wheel unsafe condition light illuminate. The unsafe condition was confirmed in the configuration page of the electronic instrument system (EIS). A go-around was initiated to troubleshoot and complete the applicable checklists. A manual gear extension was attempted without success. Delta Air Lines’ Atlanta flight control was notified via Aircraft Communication Addressing and Reporting System (ACARS), an emergency was declared with air traffic control (ATC) and the flight proceeded inbound on the ILS 36L approach. As the airplane approached 300 feet AGL, ATC notified the flight crew that the nose wheel was not visible, and a second go-around was initiated. In an effort to lower the nose wheel, multiple normal and manual landing gear extension attempts were made to no avail and the decision was made to proceed with the landing. The airplane touched down about 1,400 feet from the runway’s threshold and the nose was lowered onto the runway at about 80 knots. The airplane stopped just before taxiway W7 and CLT airport aircraft rescue and firefighting (ARFF) sprayed the nose wheel area with water. The flight crew conducted the shutdown and evacuation checklists. After the fire chief verified that the area was safe, an evacuation was performed through the two forward entry doors with the passengers utilizing emergency slides. A view of the airplane in its final resting position is shown in figure 1. Figure 1. View of airplane in final resting position. (Source: Photo provided by FAA) An on scene postaccident examination of the nose landing gear system revealed a fractured upper lock link. Due to the fractured upper lock link, the lower lock link was free to swing down to a vertical position and contact the nose landing gear assembly and thereby restrict its movement. The fractured lock link was removed and sent to the NTSB’s Materials Laboratory for further examination. Figure 2. Displaced lower lock link contacting nose gear assembly in foreground. Note the forward section of the fractured upper lock link was removed from the lower lock link during recovery. The aft section of the fractured upper lock link is in the background. View looking down and aft. AIRCRAFT INFORMATIONThe upper lock link was originally manufactured as part number (P/N) 3911464-505. Prior to original delivery McDonnell Douglass Engineering Order (EO) 3914464, revision Z, dated December 2001, was incorporated. Per the EO instructions, the upper lock link was reidentified as P/N 3914464-509. P/N 3914464-509, serial number (S/N) 0007 was delivered to fuselage number 5051 in February 2001. In June/July 2009 the link was removed from the original airplane to have service bulletin (SB) 717-32-002 dated December 2001, incorporated. According to maintenance records, the gear including the upper lock link was sent to IAI for overhaul. The IAI airworthiness directive (AD) and SB listing showed that SB 717-32-002 was accomplished on the upper lock link P/N 3914464-509V. The “V” was added after the part number per SB instructions to indicate compliance with the SB. The link had been in service for 17,313 flight cycles at SB accomplishment. P/N 3914464-509V, S/N 0007 was then installed on aircraft S/N 55076. The link remained in service for an additional 7,365 flight cycles, when it was removed with the landing gear for landing gear overhaul. After overhaul, the link was installed on N955AT in March 2014 and remained on the airplane until the accident. AIRPORT INFORMATIONThe upper lock link was originally manufactured as part number (P/N) 3911464-505. Prior to original delivery McDonnell Douglass Engineering Order (EO) 3914464, revision Z, dated December 2001, was incorporated. Per the EO instructions, the upper lock link was reidentified as P/N 3914464-509. P/N 3914464-509, serial number (S/N) 0007 was delivered to fuselage number 5051 in February 2001. In June/July 2009 the link was removed from the original airplane to have service bulletin (SB) 717-32-002 dated December 2001, incorporated. According to maintenance records, the gear including the upper lock link was sent to IAI for overhaul. The IAI airworthiness directive (AD) and SB listing showed that SB 717-32-002 was accomplished on the upper lock link P/N 3914464-509V. The “V” was added after the part number per SB instructions to indicate compliance with the SB. The link had been in service for 17,313 flight cycles at SB accomplishment. P/N 3914464-509V, S/N 0007 was then installed on aircraft S/N 55076. The link remained in service for an additional 7,365 flight cycles, when it was removed with the landing gear for landing gear overhaul. After overhaul, the link was installed on N955AT in March 2014 and remained on the airplane until the accident. WRECKAGE AND IMPACT INFORMATIONPostaccident examination found that the upper lock link had fractured into two pieces about mid-span, between 7.2 and 7.9 inches from the forward lug. A relatively shiny and flat thumbnail shaped region approximately 1.07 inches wide by 0.42-inch deep was observed emanating from the lower surface with visible curved crack arrest (beach) marks, consistent with a progressive cracking mechanism. Outside of the thumbnail shaped region, the fracture surface was rougher and exhibited a generally matte appearance. The lower surface of the part adjacent to the fracture origin area exhibited horizontal and vertical scratch features near the fracture surface and under the paint layer. Figure 3. A schematic showing the position of the upper lock link component, highlighted in red, with the nose landing gear in the extended position. (Source: Boeing Service Bulletin 717-32-002) The part was cleaned, and the paint was stripped chemically from the part’s surface using a methylene chloride-based paint remover. A non-destructive fluorescent penetrant inspection was performed on the lower surface of the part and no relevant linear cracklike indications were observed. Linear indications observed were attributable to visible scratch features on the part surface. A section cut was performed on the forward piece of the upper lock link about 0.6 inch below the fracture surface to facilitate closer examination. The fracture origin area was observed on the lower surface of the part near a parting line feature. A darker thumbnail shaped region approximately 0.26 inch wide by 0.10-inch deep was observed emanating from the fracture origin area. Several crack arrest (beach) marks were observed within this region. A larger thumbnail shaped region that was lighter and shinier in appearance extended outward and terminated at an approximate overall size of 1.07 inches wide and 0.42 inch deep. Several crack arrest (beach) marks were observed in this region with prominent ones indicated by black arrows. Closer examination of the fracture surface using scanning electron microscopy revealed fatigue striations within the thumbnail shaped region. The portion of the fracture surface outside of this region was relatively rough and matt in appearance with some curved progression marks and shear lip features observed at the free edges of the part. The overall appearance of the fracture surface was consistent with fatigue fracture originating at the lower surface and propagating under cyclic tensile or bending loads with an overstress dominated fracture mode at final separation. Figure 4. A digital microscope image of the cleaned fracture surface on the forward piece of the upper lock link. The fracture origin area is indicated by a yellow bracket with progressive crack growth direction indicated by white arrows. A ratchet mark was observed as indicated by the blue arrow. The curved yellow line highlights the boundary of progressive crack growth with several curved arrest (beach) marks indicated by black arrows. Curved arrest marks observed outside of the progressive region are indicated by red arrows. ADDITIONAL INFORMATIONBoeing/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 of the forged upper lock link. The bulletin provided instructions for eddy current inspection of the nose landing gear upper lock link assembly and modification/reidentification or installation of a new/modified assembly. The upper lock link examined was listed as an affected part in this bulletin and was marked with a “V” following the part dash number, which indicated the Service Bulletin procedures had been performed, which included a task to “verify the surface finish is 125 RMS smooth and free from any transverse manufacturing machining marks and modify as required.” Safety Actions As a result of the investigation Boeing performed the following safety actions: o Notified operators of the Boeing 717 airplanes of this accident. o Recommended that Boeing 717 operators perform a high frequency eddy current inspection of all in-service nose landing gear upper lock links in addition to any spares in inventory. o Requested the results of the high frequency eddy current inspections from the operators. o Revised the Boeing 717 Overhaul Manual 32-21-2, in July 2024, to require that at the next and subsequent overhauls upper lock links be stripped of paint, the parting plane area be fluorescent-penetrant inspected for surface imperfections (scratches, nicks, and toolmarks), and high frequency eddy current inspections be performed on top and bottom sides of the link to check for existing cracks. o Initiated a new Alert Service Bulletin 717-32A0043, effective for all 717 airplanes, to recommend an on-aircraft initial and repetitive high frequency eddy current inspections on in-service upper lock links. As a result of the investigation Delta Air Line performed the following safety actions: o Completed an inspection of all upper lock links installed on their fleet of Boeing 717 airplanes and all spare lock links in their inventory with no reported crack findings. As a result of the investigation the remaining operators of Boeing 717 airplanes performed the following safety actions: o Completed a fleet inspection of Boeing 717 airplanes upper lock links in January 2024 with no reported crack findings. FLIGHT RECORDERSThe NTSB Vehicle Recorder Division received a Honeywell solid state flight data recorder (FDR) (Model 4700) and a Honeywell cockpit voice recorder (CVR) (Model 6022). The recorders were in good condition and the data was extracted normally from the recorders. The flight data recording contained approximately 27 hours of data. The event flight was the last flight of the recording, and its duration was approximately 1 hour and 11 minutes. The cockpit voice recorder recorded a minimum of 120 minutes of digital audio stored on solid state memory modules. Four channels were recorded: one channel for each flight crew member, one channel for a cockpit observer, and one channel for the cockpit area microphone. All four channels had good to excellent quality audio and a summary was created of the audio associated with the entirety of the accident flight. TESTS AND RESEARCHTo determine how long the fatigue crack had been present, a quantitative fatigue crack growth rate analysis was performed over the fatigue crack fracture surface to estimate the number of cycles to failure by counting striations at intervals along the crack depth. As previously noted, the thumbnail crack exhibited striations at high magnification, consistent with fatigue crack propagation. A series of 35 locations, progressing along the crack depth direction, were imaged using a field emission scanning electron microscope (SEM) and stage positions were recorded to correlate crack growth rate and crack depth. Prominent fatigue striations were counted and averaged along these increments with minor striations excluded from the analysis. Fatigue striations were not resolvable within 0.043 inch of the fracture origin and the maximum fatigue crack depth was measured to be 0.417 inch. The estimated number of major fatigue striations was calculated to be 39,059. According to maintenance records, the upper lock link component had accumulated 41,257 flight cycles. Engineers from the aircraft manufacturer indicated that the lock link component experiences 2 major loads and several minor loads per flight cycle (extension and retraction of the nose landing gear), with the extension load being greater due to the gravitational force. The surface adjacent to the fracture surface near the origin area showed evidence of vertical and horizontal scratch features near the parting line. The fracture plane at the origin was oriented parallel to a horizontal scratch feature just below the fracture surface. Surface roughness profile (Rq) measurements were performed longitudinally and transversely along the parting line and the lower surface away from the parting line. Roughness measured over a 0.03-inch sampling length on the lower surface was 59-66 microinches root mean square (RMS) compared to 185-508 microinches RMS on the parting line. Roughness measured over a 0.1-inch sampling length was performed in the longitudinal direction and resulted in values of 86 microinches RMS and 347 microinches RMS for the lower surface and parting line, respectively. Delta Air Lines flight 1092 was about 2,000 ft above ground level (AGL) when the flight crew lowered the landing gear handle and observed the nose wheel unsafe condition light illuminate. The unsafe condition was confirmed, and a go-around was initiated to troubleshoot and complete the applicable checklists. Multiple normal and manual landing gear extension attempts were made to no avail, and the flight crew conducted an emergency nose gear up landing. After completing the shutdown and evacuation checklists and verifying the area was safe, an evacuation was performed through the forward entry doors utilizing the emergency slides. A postaccident examination revealed that the nose landing gear upper lock link failed from a fatigue fracture. Fatigue cracks initiated along scratch features which were observed on the lower surface at the parting line of the forged aluminum alloy component. The appearance of the scratch features was consistent with tool marks such as from filing or grinding operations. These scratch features likely acted as stress concentration areas for crack initiation. Once the fatigue cracks had propagated through approximately one-third of the material cross section, the upper lock link fractured in tensile or upward-bending overstress. Visible crack arrest marks indicated the overstress fracture may have occurred over several load cycles. Boeing/McDonnell Douglas issued Service Bulletin 717-32-0002 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 of the forged component. The bulletin provided instructions for eddy current inspection of the nose landing gear upper lock link assembly and modification/reidentification or installation of a new/modified assembly. The upper lock link examined was listed as an affected part in this bulletin and was marked with a “V” following the part dash number, which indicated the Service Bulletin procedures had been performed, which included a task to “verify the surface finish is 125 RMS smooth and free from any transverse manufacturing machining marks and modify as required.” A 125 RMS (root mean square) surface finish refers to a roughness average (Rq) of 125 microinches (µin) or approximately 3.2 micrometers (µm). It means the RMS average height of the discontinuities on a surface that make it 'rough' is 124 µin.  According to maintenance records, the upper lock link was delivered in February 2001 and installed on a Boeing 717 airplane. The link was removed in June/July of 2009 from the original airplane and service bulletin (SB) 717-32-002 was completed by Israel Aerospace Industries (IAI). A “V” was added to the part number per SB instructions to indicate compliance with the SB. The link remained in service for an additional 7,365 flight cycles, when it was removed with the landing gear for landing gear overhaul. After overhaul, the link was installed on N955AT in March 2014 and remained on the airplane until the accident. A fatigue crack growth analysis was performed by counting major striation features across the fracture surface and estimated 39,059 cycles to failure which agrees well with the maintenance records of 41,257 total flight cycles accumulated on the upper lock link. Boeing indicated that the upper lock link experiences two major loads (extension/retraction) and several minor loads per flight cycle with the extension load being higher due to the gravitational force component. Assuming major fatigue striations correspond to the extension load, fatigue cracking likely initiated early in the part’s lifecycle, and the scratches were present in the as manufactured condition, therefore it is likely that the service bulletin was not adequately complied with by IAI. Some pitting consistent with environmental attack was observed on the adjacent lower surface near the fracture origin, which may have contributed to the increased stress concentrations caused by the scratches. However, this could also be attributed to environmental/chemical exposure after the accident and/or chemical paint stripping that was performed to analyze the fracture. No other cracks were found on the lower surface parting line by fluorescent penetrant inspection. In addition, no material defects or property deficiencies were observed aside from the scratches along the forging parting line. Source: NTSB Aviation Accident Database Retrieved: 2026-02-12

NTSB Findings

Hierarchical cause / factor breakdown from the FAA bulk avdata database. Each finding tagged C (Cause) or F (Factor).

• — Aircraft-Aircraft systems-Landing gear system-Gear extension and retract sys-Failure
• — Aircraft-Aircraft systems-Landing gear system-(general)-Fatigue/wear/corrosion
• — Personnel issues-Task performance-Maintenance-Modification/alteration-Other
• — Organizational issues-Support/oversight/monitoring-Oversight-Oversight of personnel-Maintenance provider