The pilot carried out a normal circuit at his destination and obtained a gear green indication. A visual check in the engine cowl mirror also showed the nose gear to be in the fully extended position. The pilot also recalled seeing three greens on his gear check on final approach. Following a normal touchdown, the nose gear collapsed and the aircraft slid to a stop on the runway. The aircraft was subject to an identical occurrence at Kimba on 16 June 1988. The hydraulic power pack was found to be in a poor condition at overhaul. The poppet-valves and check-valves were worn. The nose gear is held in the down position by hydraulic pressure and by an over-centre lock. When the gear selector is placed in the gear down position, the down line is pressurized and the gear extends. When the pressure in the down line reaches system pressure the gear selector lever automatically returns to neutral and forms a hydraulic lock in the down line. However, with the worn valves in the power pack this pressure bled away leaving only the over-centre feature to hold the nose gear in the extended position. The Kimba strip has a rough surface. It is believed that the rough surface caused the over-centre mechanism to become unlocked and since the hydraulic pressure had bled away, the nose gear collapsed.
