N955AT

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-pene...