AAIB Bulletin: 6/2010

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7 © Crown copyright 2010 AAIB Bulletin: 6/2010 N1FY EW/G2010/01/04 ACCIDENT Aircraft Type and Registration: Cessna 421C Golden Eagle, N1FY No & Type of Engines: 2 Continental Motors Corp GTSIO-520 piston engines Year of Manufacture: 1981 Date & Time (UTC): 25 January 2010 at 1358 hrs Location: Kemble Airfield, Gloucestershire Type of Flight: Private Persons on Board: Crew - 2 Passengers - None Injuries: Crew - None Passengers - N/A Nature of Damage: Damage to the underside of the nose and to both propellers Commander’s Licence: Airline Transport Pilot’s Licence Commander’s Age: 47 years Commander’s Flying Experience: 7,000 hours (of which 500 were on type) Last 90 days - 70 hours Last 28 days - 25 hours Information Source: Aircraft Accident Report Form submitted by the pilot and subsequent enquiries by the AAIB three green lights illuminated in the cockpit to indicate that the gear was ‘down-and-locked’. The Kemble FISO (Flight Information Service Officer) on duty at the time was watching N1FY’s approach, through Binoculars, and he observed that the aircraft’s landing gears appeared to be in the down position before landing. At touchdown, the nose gear collapsed causing the aircraft to slide along the runway on its nose until it came to a halt just to the left of the centre line. The pilot and passenger were not injured in the accident and they exited the aircraft through the rear door of the aircraft. The fire service were in attendance in less than one minute. Synopsis Following an uneventful approach into Kemble Airfield, the aircraft’s nose gear collapsed on touchdown despite all three green ‘down-and-locked’ lights being illuminated in the cockpit. The aircraft suffered damage to the underside of the nose and to both propellers, but both occupants were uninjured and they exited the aircraft normally. Subsequent engineering analysis revealed a corroded downlock microswitch on the nose gear actuator. History of the flight The pilot was flying a visual circuit at Kemble Airfield. Upon selecting GEAR DOWN, the gear was felt to cycle and 8 © Crown copyright 2010 AAIB Bulletin: 6/2010 N1FY EW/G2010/01/04 The pilot stated that even after the nose gear had collapsed, the three green ‘down-and-locked’ lights in the cockpit were still illuminated. Landing gear operation description The landing gear on the Cessna 421C is a fully retractable tricycle landing gear consisting of a nose gear and a main gear located aft of each engine nacelle. The normal extension and retraction of the landing gear is by a hydraulic actuator at each gear. For normal extension of the gear, hydraulic pressure generated by engine driven hydraulic pumps is routed to a hydraulic unlock actuator at each gear uplock hook. When the hydraulic actuator has reached the full unlock position, fluid is routed to the gear actuator to extend the gear. Downlock microswitches are located on each gear actuator and cut off the hydraulic pressure to the actuators when all three switches close to indicate that the gears are ‘down-and-locked’. These switches also provide the status of the landing gear to several aircraft systems including the landing gear position indicators in the cockpit, the hydraulic gear extension/retraction system and the ‘landing gear unsafe’ aural alert warning system. Mounted in the instrument panel are three landing gear indicator lights that illuminate green when the respective gears are ‘down-and-locked’. An ‘in transit’ light also illuminates when any gear is in an unlocked position. The aircraft is equipped with an aural alert system to alert the pilot if the gear is unsafe when the throttles are retarded or the flaps are configured for landing. This system is disabled when all downlock microswitches are in a position that indicate all gears are ‘down-and-locked’. In the event of a loss of hydraulic pressure, emergency gear extension is available by means of an air bottle blow-down system actuated by an emergency control handle located in the cockpit. Engineering examination The repair agency for the aircraft conducted an engineering examination of the nose landing gear and ascertained that the landing gear failure occurred as a result of the nose gear downlock not being engaged. They also discovered that the nose landing gear downlock microswitch was stuck in the closed position due to corrosion. In normal operation when GEAR DOWN is selected, the nose gear leg will usually lock down before the main gear legs. However, in certain circumstances, if the main gear legs lock down first, and the nose gear downlock microswitch has failed in the closed position, the hydraulic pressure will be removed and the system will give a nose gear ‘down-and-locked’ indication irrespective of the gear actuator position. The result would be an aeroplane configured for landing with the main gears ‘down-and-locked’, and the nose gear partially extended but not extended by a sufficient amount as to engage the mechanical downlock latch. Related service information In 1989, following identification of a potential problem with the landing gear downlock microswitches, the aircraft manufacturer issued a Service Newsletter (SNL 89-3), which was subsequently mandated by a CAA Airworthiness Directive (AD 002-02-90). The Service Newsletter instructions were to seal the downlock microswitches on affected aircraft to prevent moisture ingress and possible corrosion, with subsequent repetitive inspections and the reapplication of sealant 9 © Crown copyright 2010 AAIB Bulletin: 6/2010 N1FY EW/G2010/01/04 when necessary. AD 002-02-90 was cancelled in 2003 when the European Aviation Safety Agency took over responsibility for Airworthiness Directives from the National Airworthiness Authorities. Maintenance history The maintenance organisation for the aircraft confirmed that the sealant and regular inspection of the gear actuators and downlock microswitches