United Air Lines Flight 634

Crew statements and recorded flight data indicate the flight was normal from the departure from Chicago until the first officer selected gear down while on final approach to runway 4R. There were no relevant open maintenance items on the airplane, and all applicable airworthiness directives had been complied with. After the landing gear was selected down during approach to runway 4R, the flight crew observed a warning indicating that one or more of the main landing gear (MLG) was not down and locked and one or more gear doors were not closed. The crew executed a go-around and cycled the landing gear handle in accordance with the electronic checklist. After cycling the landing gear, the warning remained and still indicated that one or more of the MLG was not down and locked and one or more gear doors were not closed. The flight crew coordinated with ATC for delaying vectors and continued to follow the abnormal procedures checklists in an attempt to extend the MLG. The United Airlines A320 Landing Gear Unsafe Indication After Extension checklist referred to the Landing Gear Gravity Extension checklist, which concluded that if the extension was unsuccessful the user was referred back to the Landing Gear Unsafe Indication After Extension. Although the flight crew reported that the checklists were confusing, they were able to accomplish all of the specified actions on the Landing Gear - Partial Gear or Gear Up Landing checklist in an attempt to rectify the MLG problem and prepare for landing. The first officer attempted to get assistance from United's maintenance control, but a difficulty with radio communications prevented effective assistance. After the incident, United amended their checklists for clarity.

After completing the checklist items without success, the flight crew coordinated with ATC for a partial gear-up landing on runway 4L at EWR. During approach, the crew discussed when the engines should be shut down. The Landing Gear - Partial Gear or Gear Up Landing checklist specified that if one MLG indicated abnormal both engines should be shut down "at touchdown", and if both MLG indicated abnormal the engines should be shut down "...in the flare, before touchdown." Just prior to touchdown, the Captain acknowledged the correct checklist instruction, but called for the engines to be shut down prior to touchdown. Although not contributory to the landing gear issue, shutting down the engines, and the concurrent loss of electrical generators without the auxiliary power unit running, would result in a loss of various capabilities including nosewheel steering, anti-skid, and some spoiler capability. The airplane's speed at this time would be too low to provide electrical power from the Ram Air Turbine as well. Operation of the fuel cut-off switches and loss of electrical power resulted in the flight data recorder and cockpit voice recorder shutting down prior to touchdown. The loss of power did not affect the cabin emergency lighting.

Post incident examination of the airplane revealed that the right MLG door actuator had failed to extend the door to its fully open position. The partially open door obstructed the right MLG from extending to its down and locked position. The outboard tire of the right MLG was found resting on the top of the door assembly.

Examination of the door actuator revealed that some of its internal components had failed, one of them producing large quantities of fragmented metallic debris that collected in one location between the piston and the cylinder with smaller particles being embedded in the piston seals. Testing of the actuator revealed that it took more pressure than normal to initiate extension and retraction and it is considered likely that, under the right conditions, the debris might prevent the piston from moving in the cylinder. Although the majority of the debris remained within the cylinder, smaller particles were able to migrate through the actuator's retract line, coming to rest in the retract restrictor and its mounting block. This debris likely restricted the flow of fluid through the actuator which would reduce the speed of extension and retraction, allowing the main gear tire to contact the door and preventing further motion of the gear and door.

During the evacuation, the 1R door did not fully open and only 1/8 of the 1R emergency evacuation slide dropped from the pack after the 1R door was opened. Prior to the flight the purser said she felt resistance when she armed the 1R door. Review of the photographs taken of the 1R slide following the incident, revealed that the 1R evacuation slide remained jammed inside its pack, and the girt was taught between the pack and the girt bar, preventing the 1R door from fully opening. Further, no anomalies were found with the 1R power assist actuator. According to the slide manufacturer, improper stowage of the girt bar in the floor brackets and/or improper packing of the evacuation slide can result in excessive bulk at the bottom of the slide pack and may impact the orientation of the slide pack at the initial stages of the deployment sequence. Because no mechanical deficiencies were found with the door, slide or girt bar, the likely reason for the lack of deployment of the 1R slide was improper packing of the 1R slide. The loss of availability of the 1R door and slide did not negatively impact the evacuation.

Verbatim NTSB analysis and probable cause from the NTSB dataset