Collision with terrain

FACTUAL INFORMATION

The AS350 helicopter was engaged in a controlled burn of a logging coupe. A drip torch was slung underneath the helicopter. It dripped ignited napalm gel on command to initiate burning on the ground. The drip torch weighed 250 kg when fully loaded with napalm.

The pilot advised that while at 50 kts and 200 ft over cleared ground, the helicopter suffered a hydraulic failure. It banked violently to the right and the hydraulic warning light began to flash but the loud warning horn never activated. He turned the hydraulics off using the switch on the collective lever and regained level flight. The helicopter then banked to the right but not as severely. He tried to electrically release the drip torch from the cargo hook but it failed to detach. He then tried the mechanical release but the load still failed to detach. At the request of the pilot, the crewman on board also tried to mechanically release the drip-torch, but still the load would not release. For four minutes, the pilot flew the helicopter back to the airstrip under control, hydraulics off, with the hope of releasing the sling load and performing a run-on landing. He flew one circuit without being able to release the sling load. Then at about 40 kts and 200 ft AGL, the helicopter banked sharply left, pitched nose down, yawed left and entered a left spiral from which the pilot could not recover before ground contact. A crewman watching from the ground said that the helicopter banked smartly to the left and nosed down almost vertical to the ground with the drip torch swinging wildly underneath. Finally the helicopter landed on top of the drip torch. The main rotor and the tail rotor blades impacted the ground but the helicopter did not roll over.

After the accident it was discovered that the main rotors had struck the tail boom and the tail rotor drive shaft was severed. It has not been determined whether the drive shaft was severed in flight during an unusual manoeuvre or when the helicopter landed on top of the drip torch.

The load-ring attaching the drip torch cable to the cargo hook was oval in shape. The client provided the load-ring as part of the drip torch equipment. The ring had been manufactured to suit a slightly different type of cargo hook which may also be fitted to an AS350. Prior to being used operationally, inflight trials had been conducted with the load-ring to ensure it was not prone to dynamic roll out from the hook. The ring was in good condition and had been in service successfully for a considerable time. After the accident it was discovered that it was possible for the load ring to lodge itself partially around the cargo hook housing under negative "G" conditions. Once lodged around the housing, the rim of the only weld on the ring could snag on the housing. In this condition, neither the electrical nor the mechanical release system could detach the snagged load-ring.

The cable between the load-ring and the drip torch was not long enough to enable the pilot to land beside the drip torch with its cable still attached to the cargo hook. The existing cable length had proven to be successful during normal operations; its length minimised the potential pendulum effect of the load. If the helicopter pilot had been able to hover safely, hydraulics on, and rest the load on the ground, a crewman could have stood under the helicopter, de-snagged the load ring from the housing, and manually released the load.

With various combinations of helicopter cargo hooks and sling gear equipment, problems in not being able to release a slung load, or inadvertent load release, have been rare. Dimensions for the primary attachment ring to the cargo hook are contained in the AS350 flight manual supplement for the cargo hook. The load ring in use in this accident was larger than the dimensions described in the supplement.

Subsequent inspection of the hydraulic pump, its drive belt, the hydraulic servos, accumulators, and flight controls, found no significant fault with the mechanical aspects of the hydraulic system. No hydraulic fluid leaks were found. No fault was found with the hydraulic solenoid and switches. However several wires were found to be broken, mostly as a result of the damage sustained during the landing. The wire which controlled the hydraulic warning horn was found to be corroded and broken; this broken wire de-activated the warning horn. A wire to the hydraulic on/off solenoid was found to be broken but it could not be determined whether or not this wire was broken before the accident.

The company chief pilot had previously checked out the pilot and found him to be competent coping with hydraulic emergencies; this included successful hovering of the AS350 with hydraulics off. Weather was not a contributing factor in the accident. The helicopter was within its centre of gravity and weight limits.

ANALYSIS

The load ring was larger than allowed by the flight manual supplement. The way that the load-ring caught on the cargo hook housing was most unusual. Momentary negative "G" must have occurred in flight to have lifted the load ring high enough to snag.

Despite a thorough examination of relevant components no definite cause of the hydraulic failure has been ascertained. The problems encountered in flight by the pilot might have resulted from a wiring problem causing an uncommanded intermittent on/off situation with the hydraulics. The pilot may have experienced heavy, non servo assisted controls suddenly followed by servo assisted light control forces. This could partially account for the unusual inflight attitudes experienced by the pilot. The drip torch swinging wildly underneath the helicopter probably exacerbated the pilot's ability to control the helicopter.

SAFETY ACTION

The operator advised that load rings have since been replaced to prevent snagging. The operator has adopted a system of colour-coding load rings and cargo hooks to ensure that wrong size rings are not used on the variety of cargo hooks fitted to different helicopters owned by the company.

The provider of the drip torch has removed the oval load-rings from service and is working closely with the helicopter operator to ensure that sling equipment is compatible with the cargo hook fitted to helicopters used in controlled burning.

The student was approaching first solo standard, having completed some four hours of incident free circuits prior to a dual session of take-offs and landings. During the first take-off from runway 11C, just prior to rotation speed, the student applied full back stick and 'froze' on the controls. The instructor was unable to regain control before the Tomahawk lifted off and stalled at a height of about 30 ft. The aircraft struck the runway under full power, incurring substantial damage. Both the student and instructor were uninjured

The pilot was carrying out spraying operations and had already spread two hopper loads of spray medium onto a cotton crop. Weather conditions were fine with clear skies and light winds from the north-north-east. The hopper was again filled, and two swathe runs were completed. The pilot reported that on the third swathe run, he left the pull-up manoeuvre to clear trees too late, and the aircraft's propeller collided with branches.

The aircraft initially gained about 100 ft following the collision, however, it then commenced to lose height. The pilot quickly realised that the impact had caused damage resulting in a loss of power and/or thrust. In an attempt to improve the aircraft's climb performance, he selected the dump valve to the fully open position and jettisoned the remaining hopper contents. This action failed to arrest the sink rate of the aircraft and it subsequently stalled into trees and collided with the ground. Whilst the pilot escaped with minor bruising, the aircraft was consumed by fire.

The pilot reported that he had obtained a weather briefing from the Bureau of Meteorology at Cairns, prior to the flight. This information including early morning observations from Mareeba Airport. After selecting a launch site to the south-east of Mareeba, Winds in the area were again assessed using a weather balloon. The departure and flight were uneventful until north-east of Mareeba, when the pilot became aware that the surface wind was a southerly, gusting between 10 and 15 kts.

During the approach to land, at about 90 ft above ground level, the balloon was effected by a wind rotor near some trees. The rotor distorted the canopy and caused it to lose heat. As a result a high descent rate developed. The pilot estimated that about 50 ft of height was lost before he was able to arrest the descent using the burner. During the latter part of the descent, the basket and lower flying wires struck the branches of a tree, causing damage to lower panels of the canopy. A fast landing then ensued. After landing, the canopy was further damage when it became entangled in a barbed wire fence.

The pilot reported that a rain shower was experienced some minutes after the landing. He believed that the unusual localised wind during this accident was due to the approaching rain.

A pilot/aircraft engineer who later assessed the aircraft for recovery purposes reported that the grass was about 60 cm long. The pilot later advised that when he applied full power for take-off, the manifold pressure gauge was indicating 26 inches and tachometer was indicating 2,600 RPM.

As he initiated forward flight the helicopter began to sink and loose main rotor RPM, which he was unable to regain in the time available by lowering the collective control to reduce the main rotor blade pitch angle. He turned the helicopter through 90 degrees to the right, which would have reduced the tail rotor blade pitch angle, but it kept on descending without any rotor RPM increase. He then turned left and tried to find enough room through the trees to afford a recovery, but the main rotor struck a tree about 20ft above ground level and the helicopter rolled to the right before impacting the ground. The pilot and his two passengers evacuated with minor injuries.

The helicopter was 73kg below its maximum take-off weight at departure, and within its longitudinal and lateral balance limits. The temperature was about 28 degrees Celsius and a very high humidity, with the density altitude approximately 2,000ft. In these conditions the helicopter’s performance would have been reduced, but it still had sufficient engine power at take-off to effect a vertical climb, which would have reached its limit when the helicopter was hovering out of ground effect at the top of the climb.

As he changed from the hover to forward flight, the pilot probably increased collective pitch slightly to overcome a loss of height as the cyclic control was moved forward.  This would have started the rotor RPM decay as no more engine power was available to maintain it. There was insufficient height above the treetops for the helicopter to settle and allow time for the rotor RPM to recover when the collective control was reduced, or to obtain airspeed by diving to provide translational lift.

The pilot reported a nil wind condition, but thought he may have experienced a slight tailwind during the climb. In either condition, with the engine power available, he would have had difficulty hovering out of ground effect, then initiating forward flight without some form of translational lift being available, such as a headwind or by diving.

The pilot had planned to conduct a series of tourist flights from the helipad at the rear of the Lodge. A 20 m diameter pad had recently been cleared for the operation. The pad was covered with short grass however, it was surrounded by grass up to 0.5 m high. A shrub-like weed was also woven into the tall grass. Only one passenger was carried on the first of the tourist flights and the helicopter performed as expected. On the second tourist flight, this time with two passengers on board, the pilot flew the helicopter to a 1 m hover and then departed towards the east. As it moved through translational-lift the helicopter descended and the right skid entered the 0.5 m-high grass around the helipad. The grass retarded the helicopter, and the leading edge of the skid made contact with the ground. The helicopter started to roll to the right. The pilot attempted to stop the roll by reducing power. When this was unsuccessful, he increased power to the maximum and attempted to fly out of the roll. This was also unsuccessful, and the helicopter ended up rolling onto its right side.

The helicopter was loaded to just under its maximum weight. The pilot misjudged the amount of excess power that would be available under the prevailing conditions, and he was unable to keep the helicopter clear of the grass during the departure. The scrub-like weed provided sufficient retardation for the right skid to dig into the ground starting a roll to the right. The pilot's description indicates the helicopter had probably entered a condition known as dynamic rollover (Aviation Safety Digest No. 95, 1975, pg. 25) before he started recovery action. As a result, his actions did not work.

The pilot was conducting a swath run parallel to a power line, when the tip of one of the main rotor blade made contact with a power pole. The helicopter was subsequently landed without further incident. This occurrence had not been reported.

The pilot reported that he was conducting mustering operations with a second person on board. While operating at about 50 ft above ground level and at 15 kts indicated airspeed the pilot realized that the helicopter was moving downwind. He immediately increased power, however the main rotor over-pitched, causing the helicopter to land heavily. The wind was from the south-east at 5 kts.

The Air Tractor 502 was conducting spraying operations late at night in fine conditions, with scattered cloud, no moon, and a barely discernible horizon.  After levelling at approximately 200 ft from a pull up and procedure turn, the pilot reported that his attention was drawn to another aircraft which was spraying a nearby property.  Whilst the pilot was watching the other aircraft the Air Tractor struck the ground at an estimated airspeed of 95 knots.

Wreckage distribution and pilot evidence indicated that, following the initial impact, the aircraft became airborne for a further 200 metres, with no engine or electrical power, before again striking the ground and sliding to a stop. The pilot, who had been wearing a full harness, escaped without injury.  The aircraft was destroyed.