During his pre-flight inspection, the pilot detected water in the fuel samples from the various drain points. Further samples were taken until no trace of water was evident. The subsequent flight of almost 90 minutes was uneventful, until the pilot selected full flap on final approach to land. At this point the engine lost all power and during the ensuing forced landing the aircraft collided with a fence post. Investigation revealed that the fuel caps were not providing adequate sealing, and a substantial amount of water remained in the fuel system. Prior to the flight the aircraft had been parked in the open for some days and considerable amounts of rain had fallen. The water in the fuel system had most probably been trapped in wrinkles in the left fuel bladder, and had entered the engine following the attitude change associated with selection of full flap. During the previous scheduled maintenance, a mandatory inspection and leak test of the fuel tank filler caps was not carried out. The owner/pilot had frequently found evidence of water during pre-flight inspections, but had not specifically instructed the maintenance organisation to investigate and rectify the problem.
