Forced/precautionary landing

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.

The pilot of the Schweitzer 269CB helicopter reported that during cruise on a repositioning ferry flight, the helicopter's engine misfired and then stopped completely so he completed a power off autorotation to a nearby dam wall. The pilot, the sole occupant, received no injuries and the helicopter was not damaged.

Troubleshooting by maintenance personnel isolated the problem to the engine's carburettor. The carburettor was replaced and the engine test run. No other anomalies were noted and the helicopter was flown back to the maintenance base.

A technical disassembly of the Precision Airmotive carburettor, model HA-6, part number 10-6030, serial number 75060303 was initiated. The examination included fuel flow level checks. During the checks, the throttle linkage was exercised through its full range from idle to full power. Throttle operation was smooth and unhindered. Several rapid accelerations were then carried out to observe the operation of the accelerator pump. No fuel was observed to flow from the accelerator pump discharge tube, positioned in the carburettor venturi, during these tests. Further examination revealed that the accelerator pump plunger mechanism was disconnected at the upper circlip attachment point on the plunger.

In addition, the accelerator pump/economiser linkages were found to have severe wear on the cam lobe face corresponding to the full throttle position and on the linkage plate that the camshaft lobe acted upon. The linkage plate also had severe wear on the cam lobe-mating surface and on the opposite non-cam lobe contact surface. That indicated that the plate had been reversed at some point during its operational life.

Carburettor background

At the time of the occurrence, the carburettor had accumulated 1,668.2 hours time in service (TIS). The carburettor had been removed from the engine at 1,154.7 hours TIS for a discrepancy reported as suspected low engine power. The repair documentation noted an air-metering pin adjustment defect. Following readjustment, the carburettor was bench tested and returned to service.

At 794.4 hours TIS, the carburettor had been removed from the engine, for a discrepancy reported as leaking fuel when in the full lean/cut-off position. The repair documentation noted that a new float needle and seat were fitted. The carburettor was bench tested and returned to service.

The pilot of the Bell Jet Ranger reported that while cruising at about 2,500 ft AGL, the rotor RPM indication decreased to zero. He initiated auto-rotation immediately by lowering the collective pitch. The controls were very stiff, however all engine parameters were normal. The helicopter descended at higher than normal speed and landed heavily. The right skid crosstube support failed and the aircraft rolled onto its side. Both occupants vacated the wreckage with minor injuries.

The pilot reported that he did not hear the low rotor alarm and did not notice the low rotor RPM caution light illuminate.

Investigation determined that the drive spline and coupling to the hydraulic pump and rotor tachometer generator were devoid of lubrication and worn to the degree that drive to both units had failed. This resulted in the loss of hydraulic power to the controls and the loss of the main rotor speed indication.

The low rotor RPM sensor which provides a signal to activate the low rotor alarm and the low rotor RPM caution light was tested and functioned normally. However, it is likely that the pilot did not notice the low rotor RPM warnings, due to his immediate initiation of an auto-rotation, and the ambient light conditions. The warning horn is cancelled by a cut-out switch on the collective pitch handle when the handle is lowered as in the case of an auto-rotation. As the helicopter was tracking south at the time, the bright sun from the northeast would have made illumination of the low rotor RPM caution light difficult to distinguish.

Investigation of the maintenance requirements for the helicopter found that, prior to January 1998, the hydraulic pump drive splines were required to be lubricated by hand every 1200 hours or twelve months, whichever came first, or under extreme operating conditions, every 300 hours. In January 1998 the manufacturer issued a revised manual with a changed requirement for the Bell 206B III model to lubricate the hydraulic pump drive splines every 300 hours with no calendar requirement.

The accident helicopter had only accumulated about 90 hours time in service since January 1998 and 217 hours since the initial issue of its Australian certificate of airworthiness in 1990.

The Robinson R22 helicopter was sent to Fossil Downs Station by the operator to conduct a small mustering assignment. As the operator's pilot was relatively inexperienced and not qualified to conduct mustering operations, the helicopter was fitted with dual controls so that an experienced and qualified pilot who was on-site could conduct the flying while the operator's pilot occupied the other seat. However, after the aircraft arrived at the station, the on-site pilot requested that the operator's pilot transport two passengers from the station to Fitzroy Crossing. Although the operator's pilot had insufficient hours to conduct mustering, he held a commercial pilot's licence and was qualified to carry passengers.

The pilot transported the first passenger from the helicopter landing site without incident. However, during the second departure, at about 15 ft and just as the helicopter was achieving translational lift, it sank back towards the ground. When the pilot increased the collective pitch in an attempt to regain the required departure profile, the low rotor RPM warning horn sounded and the rate of descent increased. The pilot reported that he checked that the throttle was fully open but the main rotor RPM continued to decay. The helicopter landed heavily and the main rotor blades clipped a tree. The pilot reported that as soon as it landed, he shut the engine down. The helicopter was extensively damaged but neither occupant was injured. The pilot reported that he flew the second flight's take-off into wind along a similar path to that flown during the previous passenger flight.

The maintenance organisation that repaired the helicopter reported that no mechanical fault was found that would have contributed to the accident. The accident was not subject to an on-site investigation by the Bureau of Air Safety Investigation.

Weather conditions

The pilot reported that the ambient temperature was about 37 degrees Celsius. He also reported that the humidity was high and increasing as storms were developing in the area. The wind was averaging about 15 kts from the south-east and gusting. The density altitude at the site, without factoring the relative humidity, was calculated to be about 3,000 ft.

The helicopter landing site

The pilot reported that he was using a southerly departure from the site to align with the general wind direction. The pilot reported that in the southerly direction, the helicopter landing site had an available length of about 50 to 60 m from the departure point with about 3 m high bushes at the departure end. There was also a small fence running east to west about 40 m from the departure point.

Helicopter performance

The helicopter weighed close to its maximum all up weight of 622 kgs. The helicopter's flight manual indicated that the calculated maximum weight to hover out of ground effect in nil wind, was about 605 kgs. The pilot did not consult the helicopter's flight manual for likely power requirements and power availability; nor did he conduct a power check prior to arriving at the helicopter landing site in order to ascertain the actual power available. While the Robinson R22 Flight Manual provided hover performance data, it did not contain performance data related to the expected climb performance of the helicopter during take-off, or in forward flight. The operator's operations manual did not provide guidance regarding power margins. There was no requirement under the existing regulations for information or guidance related to required power margins for departure or climb performance to be provided by either the helicopter's manufacturer or operator.

The pilot reported that the power setting required to hover the helicopter in ground effect was about 23 inches manifold air pressure, which was approximately the placarded limit manifold air pressure of 23.5 inches. The pilot could not recall the power indication during the accident take-off.

Ambient wind conditions can have significant and differing effects on a tail rotor equipped helicopter's performance. Engine power is delivered to a transmission system, which drives the main and tail rotors The power required to drive the transmission system is determined primarily by the amount of drag being produced by the rotors and the power available is determined by the power output of the engine(s). The difference between the power available and power required is known as the power margin. If the power required to drive transmission exceeds the amount of power available from the engine, then the main and tail rotor speed will decay, or droop. When the speed of the main rotor droops significantly, the main rotor loses lift and the helicopter descends. Wind blowing over a main rotor provides translational lift that can significantly reduce the power required to drive the helicopter's transmission system. Wind may also assist a helicopter to maintain heading, which also reduces the load demand on the transmission and therefore reduces the power required to drive the transmission. Conversely, a wind from an adverse direction may increase the load demand on the transmission and, in turn, the power required from the engine. Therefore, the wind may cause a net effect which, depending on its strength and direction, will reduce or increase the power required for a tail rotor equipped helicopter to maintain flight.

The density of air is affected by a number of factors including its moisture content. Relative humidity is the ratio of the amount of moisture in the air to the amount it is capable of absorbing at a given temperature. The greatest decrease in air density (increase in density altitude) due to moisture content will be at a high temperature. In general, as the density altitude increases, helicopter rotor and piston engine performance decrease. The performance data provided in the R22 helicopter's flight manual is only valid for nil-wind conditions and does not account for the adverse effects of high relative humidity.

Although the Civil Aviation Orders (CAOs) specify minimum performance requirements for single and multi-engine aeroplanes, there are no minimum performance criteria specified for helicopters.

Pilot Experience

The pilot had about 173 hours flying experience, of which about 83 hours were in command. All his flying experience had been gained on the Robinson R22 helicopter. Prior to joining the operator, he had flown about 14 hours during the 15 months since gaining his commercial (helicopter) pilot's licence in September 1997. He had been employed by the operator for less than one month and had accumulated just over 36 flying hours in that time. His initial training was conducted at several helicopter training schools in Queensland, and he reported that during the training, he had not experienced the helicopter being close to limits of power or practiced rejected departures and had not previously experienced main rotor RPM droop. He also reported that at the time of the accident, he was unaware of how to recover from a low rotor RPM condition. There were no available records related to his initial flying training, however, the pilot reported that he considered the training to be adequate. The operator conducted a proficiency check on the pilot about three weeks prior to the accident. The pilot's performance during the check was rated as satisfactory. About a month after the accident, the pilot attended a Robinson Safety Course where he flew with an experienced R22 helicopter instructor. The instructor reported that the pilot demonstrated an inappropriate take-off technique and that he required remedial instruction.

The Civil Aviation Orders impose a minimum requirement of 100 hrs as pilot-in-command before a pilot may conduct mustering operations. The Civil Aviation Regulations specify that a pilot may obtain a commercial (helicopter) pilot's licence, under certain circumstances, after a minimum total of 105 flying hours of which at least 35 hours are as pilot-in-command.

The company's operations manual required pilots to obtain authorisation from the Chief Pilot or a person nominated by the Chief Pilot before conducting any flights. The pilot reported that the Chief Pilot told him that the on-site pilot was "in charge". When he was requested to conduct the passenger flights, the pilot believed that the on-site pilot had the appropriate authority.

The pilot of the Cessna 208B reported that, after a normal departure from Badu Island, he established the aircraft in cruise at about 5,000 ft for the flight to Saibai Island. The engine had performed normally up to that time. Shortly after becoming established in the cruise, the pilot heard a muffled bang from the engine compartment. On checking the engine instruments, he saw that the gas generator RPM (Ng) had stabilised at about 52% and the turbine temperature was at about 700 degrees C. (The maximum allowable temperature was 740 degrees C.) The pilot initially thought that these symptoms may have indicated an engine fire, so he shut down the engine and feathered the propeller. However, appropriate checks indicated that there was no fire. During these observations, the pilot had turned the aircraft towards Badu Island.

The pilot then attempted two engine starts, neither of which was successful. On each occasion, Ng stabilised at 17%, fuel flow was 110 lb/h, and engine light-off occurred at 850 degrees C. Eventually, Ng stagnated at 42-43% and the propeller unfeathered, but there was no indication of engine torque.

The pilot transmitted a distress call after the first start attempt. After the second attempt, he decided to concentrate on flying the aircraft and realised that his best option was to try to land on a narrow beach on the northern side of Badu Island. He was able to achieve this without any damage to the aircraft.

The aircraft was powered by a Pratt and Whitney Canada PT6A turboprop engine. Examination of the engine revealed that a compressor turbine (CT) blade had failed at about mid-span. The liberated section of blade had struck two adjacent blades, causing them to break. Metallurgical examination of the remaining section of the failed CT blade indicated that the fracture line passed through the centre of a deep sulphidation pocket. Impact damage was observed on all the remaining CT blades. Detailed examination of these blades revealed a total of seven blades that exhibited some degree of cratering due to sulphidation. Inspection of the compressor section of the engine also revealed significant corrosion consistent with operation in a salt-laden atmosphere.

"Sulphidation" refers to the reaction of sulphur containing compounds with metallic components that have been exposed to a hot gaseous environment. Components of gas-turbine engines located in the hot gas path, such as blades and vanes, are exposed to sulphidation during normal operation. Corrosive sulphates are formed during the combustion process from sulphur in the fuel and sodium and potassium salts present in the fuel and air, in particular the air in marine environments. If the accumulations of sulphur-containing salts are not removed from the surfaces of the turbine blades and vanes, the protective oxide coating will be attacked and the underlying alloy rapidly corroded.

The PT6A engine maintenance manual required desalination water-washes to be applied to both the compressor and the turbine after the last flight each day. The operator was conducting compressor washes prior to the first flight of each day, using the compressor wash ring installed on the engine. A special wash tube assembly tool for installation into the gas generator igniter boss, to enable wash solution to be introduced directly to the first-stage turbine blades, was available from the engine manufacturer. The operator was not using this tool. As a result, effective washing of the turbine blades was not achieved, allowing salt deposits to build.

The engine was fitted with an Engine Condition Trend Monitoring (ECTM) system which recorded a number of key engine parameters such as fuel flow, inlet turbine temperature, torque, and Ng. Use of this system allowed operators to delete a fixed-time hot section inspection (HSI) requirement in favour of basing the HSI interval on the results of engine condition trend monitoring. Use of the ECTM system was acceptable to the Civil Aviation Safety Authority (CASA), provided the procedures were in accordance with CASA Airworthiness Advisory Circular 6-29 `PWC PT6A Series Engines HSI Policy' 5/98. Investigation revealed that the operator was not complying fully with all the requirements of the circular.