Synopsis
During an approach to Sumburgh Airport, the flight crew of a Jetstream 4100 (registration G-MAJW) received primary and standby landing gear indications that the nose landing gear (NLG) was not down and locked. The crew aborted the approach, elected to return to Aberdeen Airport, and after unsuccessful troubleshooting declared a MAYDAY. The aircraft landed safely at 1235 hrs, and all occupants evacuated via overwing exits after the aircraft stopped on the runway. A subsequent ground inspection confirmed the nose landing gear was down and locked, but the primary indication wiring harness had failed and the standby indication microswitch was out of position.
History of the Flight
The aircraft departed Aberdeen Airport on an uneventful charter flight. At 1038 hrs, while on approach to Runway 09 at Sumburgh, the crew selected landing gear down. The primary landing gear status indicator showed a red light for the nose landing gear (NLG) and green for the main landing gear (MLG). The standby indicator showed green for the MLG but no light for the NLG, and shortly afterward the selector lever indicator illuminated red. The crew aborted the approach and consulted Emergency and Abnormal Checklist Card 48, 'Landing Gear Not Locked Down.' They verified no loss of electrical or hydraulic power, reset landing gear circuit breakers, and recycled the landing gear. When the gear was retracted, both systems indicated all gears up and locked. The crew declared a PAN and returned to Aberdeen at 1052 hrs with the gear retracted.
At 1126 hrs, the aircraft entered a hold above Aberdeen. The crew performed the emergency extension procedure from Card 48 and conducted small manoeuvres at normal and maximum speeds to release the NLG, but the green indication did not appear. At 1138 hrs, they declared a MAYDAY and prepared for landing using Card 49, 'Nose Landing Gear Not Locked Down.' The crew made two passes over the runway at 3,000 ft and 1,500 ft. Visual confirmation indicated the NLG was down, but the downlock status could not be assessed. The crew evaluated the risk of an NLG collapse as extremely likely and decided to evacuate passengers via overwing exits after landing, briefing cabin crew, passengers, and ATC.
The aircraft landed at 1235 hrs with the NLG remaining down and stopped on the runway. The evacuation command was issued by cabin crew immediately after stop, but they noticed propellers still turning and instructed passengers to wait. Once propellers stopped, subsequent commands were issued by both flight and cabin crews. All passengers and crew evacuated without injuries and were bussed to the terminal.
Aircraft Examination
Post-incident inspection revealed damage to the overwing emergency exits from ground contact during evacuation. A systems check identified a fault in the primary NLG downlock microswitch harness and the NLG standby microswitch was found out of position.
Primary Downlock Microswitch: The harness had been fitted during scheduled maintenance on 6 May 2016 and had completed 2,291 flight cycles. No visual defects were apparent, but movement between the wiring harness and microswitch indicated damaged and broken wires. The harness was to the latest modification standard with protective boots and sheathing in good condition and adequate moisture-inhibiting compound. In response, the operator introduced a 600-flight hour repetitive inspection of both indication systems; initial fleet-wide inspections found no further harness failures.
Standby Downlock Microswitch: The NLG standby microswitch was serviceable, but a gap existed between the plunger and downlock pin, preventing operation when the gear was down and locked. This condition could have resulted from incorrect rigging at the last maintenance interval, in-service movement of the support bracket, or a combination. The operator performed a one-off fleet inspection of standby microswitch positions. Of 12 aircraft inspected, five were correctly positioned, five were marginally out of position with no operational effect, and two were only just engaged with the downlock pin. The inspection also highlighted an issue with the Aircraft Maintenance Manual (AMM) task 'Adjust the NLG downlock microswitch,' which stated it was applicable to both primary and standby microswitches but only referred to selector panel indicators. The manufacturer issued a Technical Operation Response to clarify the AMM task and is revising the AMM.
Emergency and Abnormal Checklists
During the incident, the flight crew used Emergency and Abnormal Checklists (E&AC) to troubleshoot. The operator customized the manufacturer's checklists. The investigation identified two instances where sections of one card were duplicated on another with information changed or missing. On Card 48, the emergency extension procedure was reproduced but missing several notes present on Card 47, including warnings about hydraulic fluid loss and cautions about nosewheel steering availability. On Card 49 and Card 96, engine shutdown actions were required 'on touchdown' for Card 49 but 'after aircraft stop' for Card 96; fire extinguisher use was detailed after stopping on Card 96 but not before. The crew had sufficient time to study all related cards and were aware of the additional notes.