AAIB Bulletin No: 1/2002

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Cessna 421C, N6315X AAIB Bulletin No: 1/2002 Ref: EW/G2001/07/26 Category: 1.2 Aircraft Type and Registration: Cessna 421C, N6315X No & Type of Engines: 2 Continental GTSIO 520-N piston engines Year of Manufacture: 1980 Date & Time (UTC): 24 July 2001 at 1435 hrs Location: Oxford Airport Type of Flight: Private Persons on Board: Crew - 1 Passengers - 1 Injuries: Crew - None Passengers - None Nature of Damage: Damage to right propeller, engine, wing lower skin and flap Commander's Licence: Airline Transport Pilots Licence (Frozen) Commander's Age: 25 years Commander's Flying Experience: 730 hours (of which 5 were on type) Last 90 days - 70 hours Last 28 days - 3 hours Information Source: Aircraft Accident Report Form submitted by the pilot Introduction In January 1998 this aircraft had been involved in an overrun incident at Denham Aerodrome in which the right engine and propeller, wing, flap, aileron, tailplane and right landing gear strut had all been damaged (ref: AAIB Bulletin 5/98). Subsequent repair of the aircraft had been carried out by a company at Kidlington Airport and had involved replacement of the right wing and a strip examination of the right main landing gear (MLG) hydraulic actuator/sidestay unit. Following satisfactory functional checks of the aircraft systems, including operation of the landing gear, the aircraft had undergone power and taxi checks prior to its first test flight following these repairs. During these checks, all three landing gear lights had operated satisfactorily and indicated 'green' with the gear down and locked. History of the flight After a normal take off, with an observer on board, the landing gear was retracted. However, as the aircraft climbed through 600 feet agl, the airspeed indication began to reduce to zero. The pilot also noticed that there was no indication on the vertical speed indicator and the altitude indication remained at 600 feet, despite the fact that the aircraft was still climbing. The pilot continued in a right turn and requested a priority landing. The landing gear was lowered during the downwind checks, but only the nose and left landing gear green lights indicated; the right main landing gear red light illuminated. With reference to the Flight Manual, the pilot tried all of the related emergency procedures for lowering the landing gear including yawing the aircraft, applying positive 'g' and operating the landing gear 'blow down' system. However, these actions were not successful and after recycling the landing gear several times the aircraft was twice flown past the control tower for visual inspection. The pilot was informed by the controller that all three landing gears appeared to be fully down. With the red light still illuminated for the right landing gear the pilot made an approach to land on Runway 20, with the Airport Fire Service in attendance. The aircraft touched down gently on both main wheels and the pilot then immediately shut off the fuel, closed down the engines and feathered both propellers. However, during the final part of the roll out the right main landing gear leg slowly retracted, damaging the right propeller and right wing as they contacted the runway, and causing the aircraft to veer off the right side of the runway onto the grass. No injuries were sustained and there was no fire. The pilot shut down all systems before he and the observer evacuated the cabin. Initial examination of the aircraft indicated that the right MLG locking actuator, which also performs the function of a sidestay for the leg, appeared to have failed to lock into the down position. These actuators contain an internal mechanical locking device which is designed to lock the actuators in their extended positions. Previous strip inspection of the right main landing gear actuator The particular actuator fitted to the right MLG of this aircraft was the same unit which had been fitted at the time of the aircraft's previous accident in 1998. Because the eye end bearing had sheared off this actuator at that time, the company which had carried out the required repairs on the aircraft at Kidlington Airport had decided to completely strip the right actuator in order to assess its condition and check the straightness of the ram. However, having strip-inspected the actuator components and determined that they all appeared satisfactory, the actuator was reassembled in accordance with the relevant Cessna Component Maintenance Manual. It was then function tested on a test rig and subsequently installed on the aircraft. Satisfactory landing gear retraction tests were then performed. Landing gear tests after this accident Following this accident, the aircraft was placed on jacks and the landing gear system was operated, with AAIB participation. During several operating cycles of the landing gear system, it was observed that all three landing gears would consistently lock down, with three green lights illuminated. However, with no hydraulic power applied to the system, the right MLG actuator consistently failed to lock down when pulled into position by hand and the right landing gear cockpit indication remained at red. The left MLG, however, could be consistently and positively pulled into the locked position by hand. Actuator description The MLG hydraulic actuator was of conventional design with a ram/piston operating inside a hydraulic cylinder. Fitted inside the head of the cylinder was a locking mechanism, which was designed to lock the ram in the extended (gear down) position. Hydraulic pressure was not required for the lock to engage, this function being accomplished by an internal spring. Hydraulic pressure was required, however, to disengage the lock. A diagram of the locking mechanism, drawn with the ram at mid stroke and the lock disengaged, is shown in Figure 1a, and with the ram extended and the lock engaged in Figure 1b. The lock function is achieved at the ram extended position by four bronze locking segments