This PBY-5 based on Johnston Island, was on a search for survivors, of a crashed Army B-24 enroute from Oahu to Johnston Island. Upon returning to base at 20:27, it was seem to commence a normal approach at an altitude of approximately one-thousand feet, at which time it suddenly nosed over, lost altitude very rapidly and crashed into water, bursting into flames upon impact. The night was very dark with no moon. This cause is undetermined but the following possibilities are mentioned: (1) pilot did not get corrected altitude settings before attempting his landing and flew into the water without realizing he was so near the surface. (another pilot stated that his altimeter read 200-ft plus when he landed just an hour or so before crash), (2) collision with large birds which are numerous in the area and are known to fly at night, (3)engine failure, (4)fire in air just prior to crash. Pilot was considered competent with out restrictions for any assignment in the PBY-5 type aircraft.