United Airlines Flight 924

United Airlines flight 924 received an engine indicating and crew alerting system (EICAS) message regarding the main landing gear configuration after takeoff from Dulles International Airport (IAD). The flight returned to IAD and landed without incident. Post landing inspection revealed damage to the forward trunnion bearing and main gear beam. The flight crew reported that the departure sequence was normal. When the first officer retracted the landing gear, the flight crew received a “gear disagree” EICAS message. The flight crew ran the appropriate quick reference handbook (QRH) checklists to correct the issue, but the gear disagree message remained illuminated. The captain reported that they discussed the issue with dispatch and maintenance for guidance, and they decided to return to IAD for an overweight landing. During the approach into IAD, the normal gear extension did not work, and the crew performed an alternate gear extension per the QRH checklist. The captain reported that the left main and nose gear position indicators had green lights, but the right main gear indicator light was not illuminated. A “right side brace” caution message then appeared on the EICAS, and the captain initiated a go-around. The captain contacted dispatch again to discuss a course of action. The crew attempted to troubleshoot the landing gear issue with the assistance of engineering and maintenance crews on the ground but were unable to rectify the situation. The group agreed to have the aircraft conduct a flyby of IAD tower to visibly check the condition of the landing gear. IAD tower controllers confirmed that the right main gear was down but could not confirm if it was in a locked position. The decision was made to have the aircraft circle the airport to burn fuel to lighten the aircraft for landing. The passengers and flight attendants were prepared for the emergency landing and a possible evacuation. The flight crew stated that after running the appropriate QRH checklists, the captain performed the touchdown first on the left main landing gear, then the nose gear and then the right main landing gear. The crew stated the rollout was smooth and slow, and they avoided any abrupt actions. The airport fire rescue crew reported that there was no fire or smoke. The passengers, flight attendants and flight crew exited the airplane via airport mobile transport busses without injury. The airplane was examined by the operator after landing. The inspection found the right main landing gear trunnion, (see Figure 1) which is the upper end of the landing gear and attaches to the wing structure to support the airplane on the ground, and facilitates rotation of the gear on retraction, had fractured on the forward end. As a result of the fracture of the forward trunnion attachment, the rear trunnion became the main point of support for the right landing gear. The increased load on the rear main gear beam support damaged the rear trunnion bearing, bushing, and caused an overload crack in a web of the main gear beam. Additionally, the loss of the forward trunnion support allowed upper components of the right main landing gear to shift and damage some secondary structure and panels.

Figure1. A graphic depicting the main landing gear with forward trunnion highlighted. (Source: United Airlines)

The operator and Boeing conducted a metallurgical examination of the forward trunnion failure and identified an origin location (see Figure 2). The examination determined the fracture was initiated by stress corrosion cracking from pits that were present on the cross-pin hole. The crack continued to propagate by stress crack corrosion until the final fracture occurred by ductile separation. Further testing and material sampling of the origin in the cross-pin hole found that corrosion had not been completely removed at the last overhaul, and chrome finish was applied in the cross-pin hole instead of a cadmium and primer finish as specified in the component maintenance manual. As a result of this accident, United Airlines performed inspections on select 767-300 airplanes that had been overhauled at the same facility as N676UA with no similar findings.

Figure 2. Pictures showing crack in trunnion at the cross-pin hole and resulting trunnion failure. (Source: United Airlines)

Verbatim NTSB analysis and probable cause from the NTSB dataset