Delta Air Lines Flight 1092

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.

Verbatim NTSB analysis and probable cause from the NTSB dataset