The pilot of the Cessna 185 Floatplane, with five passengers on board, was making a water departure for a charter flight. The pilot positioned the floatplane for a take-off into a north easterly wind of 15 kts that was gusting to over 20 kts. The take-off path was over a sand spit, approximately 50 ft above the water level. To the north, and left of the take-off path, was a steep, rocky headland that rose to a height of approximately 300 ft above mean sea level.
The pilot reported that he had selected 20 degrees of flap and applied maximum power for take-off. The aircraft became airborne after a short run and the pilot climbed it at an indicated airspeed (IAS) of 70 kts. At about 200 ft the pilot reduced engine power to 25 inches of manifold pressure and 2,500 RPM. The pilot reported that just after he reduced power, the aircraft encountered turbulence and started to descend rapidly. He turned the aircraft left, away from the spit, with the intention of regaining altitude over the water before he attempted to cross the spit. However, the aircraft continued to descend, and the pilot decided to land straight ahead. The aircraft contacted the water and bounced, then ran aground on the beach and overturned.
The pilot reported that he exited through a window and instructed the passengers to evacuate quickly, as there was a possibility of fire. The passengers reported that they were entangled in their seat belts and had difficulty releasing the buckles. A small child was being held by a passenger and another passenger was temporarily restrained by clothing that became caught on the right control yoke.
The load chart for the flight showed that the aircraft was 31 kg below its maximum take-off weight. The pilot commenced the take-off with a take-off distance of approximately 1,100 m, which exceeded the minimum take-off distance of 1,000 m stipulated by the aircraft's flight manual. However, this take-off distance was less than the 1,300 m pilots were directed to use by the operator's Authorised Landing Area (ALA) register. The pilot reported that he had not used the full length available as previous take-offs that day, from the same point in lighter winds, had been uneventful. He considered that the increased headwind component would have improved the take-off performance and climb gradient of the aircraft.
The ALA register also stated that a north-easterly wind required a climb over the spit to avoid turbulence in the lee of the adjacent headland. Another warning in the operator's ALA survey report cautioned "Dumping will be encountered on the lee side of the headland especially in the north easterly winds".
A fact sheet on mountain wave turbulence that accompanied a recent ATSB report (Occurrence 200104092) involving mechanical turbulence stated, in part:
"Flowing air near the ground is forced up the windward side of any elevated barrier and then sinks down the leeward side. Air flowing at speeds greater than 20 kts produces seriously turbulent air and significant downdrafts on the leeward side."
That situation was referred to as "dumping" in the operator's ALA survey sheet. The pilot reported that he hadn't encountered severe "dumping" during any previous take-offs.
The fact sheet also stated, in part:
"In addition to generating turbulence that has demonstrated sufficient ferocity to significantly damage aircraft or lead to loss of aircraft control, the more prevailing danger to aircraft in the lower levels in Australia seems to be the effect on a aircraft's climb rate. General aviation aircraft rarely have performance capability sufficient to enable the pilot to overcome the effects of a severe downdraft generated by a mountain wave, or the turbulence or windshear generated by a rotor."
The Cessna 185 Pilot's Operating Handbook (POH) procedures for both normal take-off and short field take-off recommend that once clear of any obstacles, the pilot retract the wing flaps and select full throttle and 2,700 RPM. Operations manual data produced by the operator listed the climb power setting as 25 inches manifold pressure and 2,700 RPM, with a footnote that the information be used as a guide only and that the user refer to the POH and Flight Manual. The aircraft's flight manual did not provide guidance on take-off procedure or associated power settings. The climb power setting of 25 inches manifold pressure and 2,500 RPM, selected by the pilot when the aircraft reached approximately 200 ft, delivered only 81 per cent of the available power.
The pilot reported that he had been encouraged by the operator to reduce power as soon as possible after take-off as a noise reduction technique. The Chief Pilot stated that a power reduction early in the climb was demonstrated during training to reduce the noise impact and to reduce engine wear. The Chief Pilot also stated that, during training it was emphasised that power reductions should only be made when clear of obstacles and when terrain had been cleared. It was also stressed that when required, full power should be used, at the pilot's discretion.