Wirestrike

The helicopter was spraying chemical over a banana crop when it struck a powerline and heavily impacted the ground. The helicopter caught fire after hitting the ground.

FACTUAL INFORMATION

The pilot had positioned the helicopter at "Fernleigh" homestead on the previous Friday and had not flown the helicopter again until the day of the accident. Flying commenced at 0645, with the helicopter being flown to "Bunburra" property to treat serrated tussocks. Operations ceased there at about 0945, and the helicopter was flown back to "Fernleigh". Further flights were carried out until 1200, when flying stopped for a lunch break. Each treatment flight took approximately 10 minutes, with the helicopter returning to reload with chemical.

After the pilot and loader/driver had finished lunch, they refuelled the helicopter. Two further flights were completed before the helicopter took off again at 1305. This was to have been the last flight in the treatment area before moving to a new location. The pilot had advised the loader/driver that during his return from the treatment area, he would inspect for regrowth another area he had previously treated. Before take-off, the property owner reminded the pilot of the presence of power cables in the area that he was going to inspect. The map location of the cables was not reviewed by the pilot. When the helicopter had not returned by about 1315, the loader/driver and property owner became concerned. Their initial search failed to find the helicopter, but shortly after, the owner of "Belmedie", an adjoining property, advised them that it had crashed.

The helicopter had struck power cables at a height of 26 m whilst tracking in an easterly direction at a calculated airspeed of 45 kts. The power cables were located in the area that the pilot had planned to inspect for regrowth. There were no witnesses to the actual flight path of the helicopter preceding the collision. A witness at "Belmedie" had heard and glimpsed the helicopter near the homestead. Weather conditions at the time of the accident were fine.

The accident site was located on the western side of an open area of rising ground. On both sides of the flight path lay heavily wooded hills. Obstructions to the flight path were two sets of diverging (approximately 46 degrees) 22,000-volt power cables, strung from a pole on the top of a hill to the left of the flight path. The first set, with a span of 224 m, went to the "Belmedie" homestead. The second set, with a span of 428 m, went to a pole on rising ground across and to the right of the flight path.

An engineering examination of the helicopter did not find any anomalies or defects that may have contributed to the accident. Damage to the helicopter structure was consistent with the main and tail rotors having come into contact with power cables. The main rotor blades had initially contacted the power cables, followed by the tail rotor. The cables then passed between the tail-rotor gearbox and the pitch change links around the tail-rotor drive shaft, before breaking. The dynamics of the cable strike resulted in the main rotor slicing off the tail boom forward of the tail-rotor gearbox. Other damage sustained by the helicopter was consistent with severe forces generated during the subsequent ground impact sequence. No emergency locator transmitter (ELT) was fitted.

ANALYSIS

The approach of the helicopter close to the homestead, en route to the inspection site, was consistent with a practice followed by pilots to locate and avoid power cables running to building sites. It would appear that, once having located the power cables to the homestead, the pilot continued towards the area to be inspected. The poles which supported the 428-m cable, which the helicopter struck, were located in heavily wooded areas. The cables could not have been easily seen, due to poor background contrast.

Initial contact with the power cables was by the helicopter main rotor blades, followed by the tail rotor, before the cables broke. The dynamics of the cable impact resulted in the main rotor slicing off the tail boom. As a result, the pilot experienced loss of control as the helicopter was now without directional control, coupled with a significant forward shift of the centre of gravity.

It is likely the pilot either forgot or did not see the second set of cables before colliding with them.

SIGNIFICANT FACTORS

1. The pilot did not adequately establish the location of the power cables prior to the flight.
2. The nature of the terrain in the vicinity of the power cables inhibited the capacity of the pilot to see them.
3. Flight control of the helicopter was lost when the tail boom and tail rotor gear box were severed from the helicopter.

FACTUAL INFORMATION

Sequence of events

The pilot had been spraying cane fields near the accident site. The aircraft was observed flying towards an airstrip about 1 km away where a spray hopper was positioned on a trailer. At about 0650 hours, the aircraft was seen flying straight and level towards the airstrip. The aircraft was then seen to strike powerlines, roll inverted and impact the ground. The aircraft came to rest inverted. The aircraft had impacted two high-voltage powerlines approximately 17 m above ground level.

Damage to aircraft

The fuselage forward of the wing leading edge was destroyed and the engine was forced rearwards into the hopper. The main cockpit structure was intact, and the seat and harness were undamaged. The windscreen was broken but the windscreen frame was intact. The instrument panel was badly damaged by impact from the pilot. The fuselage rear of the cockpit area was creased but the tailplane and fin were intact, suffering only minor damage. The right wing outboard leading-edge section was severed by powerline contact. The aircraft severed two wires. The top aerial earth wire consisted of three strands of 2.55mm aluminium conductor wound around four strands of 2.5mm steel conductor, and a 66-kilovolt cable consisted of 30 strands of 2.55mm aluminium conductor wound around seven strands of 2.5mm steel conductor.

Meteorological information

The weather was fine at the time of the accident, with a light south-easterly wind.

Wreckage examination

There were no mechanical defects discovered with the airframe or the engine which may have led to the development of the accident. The seat harness was found undone immediately after the accident and did not exhibit any signs of stress as would have been expected after such impact.

Tests and research

Considerable research and testing was carried out to ascertain if there was a possibility of the seat harness being released by impact forces. Testing of the harness and buckle was conducted at Crash lab, the Roads and Traffic Authority of New South Wales test and research centre, which specialises in dynamic testing of safety harness.

Initial examination of the harness found there were no signs of severe loading on the buckle, adjusters, anchors or webbing. However, load bearing marks on the webbing of the lap and shoulder sections of the harness were evident following dynamic testing of the harness.

Dynamic testing of the aircraft safety harness was conducted to test the overall integrity of the harness and also to investigate the possibility of the over-centre buckle being inadvertently released by the occupant's abdominal region during a dynamic impact. The dynamic test demonstrated that the harness was able to restrain the anthropomorphic test dummy without evidence of abnormal distortion, separation or damage to the harness webbing and components. The test also demonstrated that the over-centre buckle did not release during dynamic impact, when correctly latched.

The testing indicated that the aircraft safety harness was not correctly fastened at impact. The possibilities were, therefore, that the harness was not worn at all by the pilot, that the harness was worn but the over-centre buckle was not correctly latched (in other words, the detent mechanism was not fully engaged), or that the buckle was inadvertently unlocked during the flight or accident sequence by clothing or arm contact with the latch.

ANALYSIS

Given that the powerline was the largest and most obvious in the area, it is difficult to understand why the pilot failed to avoid it. The possibility of the low angle of the sun momentarily dazzling the pilot was considered but this is unlikely due to the aircraft being in a right turn and heading in a south-easterly direction shortly before impact. Some witnesses described seeing what appeared to be puffs of smoke from the engine seconds before the aircraft hit the powerline. The aircraft was fitted with a smoke generator so the pilot could assess the wind drift. This could occasionally emit puffs of smoke due to small quantities of oil leaking into the system.  The possibility of an engine malfunction distracting the pilot at a crucial moment and diverting his attention to inside the cockpit cannot be discounted; however, there was no evidence found to support this theory.

Examination and testing of the seat harness led to the conclusion that the harness was not fastened at impact. The harness was not fitted with an inertia reel. The possibility that the pilot momentarily undid the harness to retrieve an item from the cockpit floor was considered. However, this is unlikely as the floor cannot be reached even with the harness undone.

SIGNIFICANT FACTORS

1. The aircraft struck a powerline, causing loss of control.
2. The pilot's seat harness was unfastened prior to or during the impact sequence for reasons unknown.

SAFETY ACTION

Action by the Civil Aviation Safety Authority

The Civil Aviation Safety Authority had previously issued Airworthiness Directive (AD) Restraint 7 Amendment 2 for this safety harness buckle [MS22013 (ASG)] in response to industry concerns about inadvertent buckle release during aerobatics.

Action by the Bureau of Air Safety Investigation

As a result of this occurrence, the Bureau of Air Safety Investigation issued Safety Advisory Notice (SAN) 960153 to the Civil Aviation Safety Authority. The safety deficiency identified was:

"AD restraint 7 Amendment 2 for safety harness buckle MS22013 (ASG) refers to utility and acrobatic category aircraft only.  The AD should address all categories of operations."

Factual Information

On the morning of the day of the accident, an employee of the property owner accompanied the pilot of the helicopter on an inspection flight of the crops to be sprayed and of the powerlines considered hazardous to the operation. Three areas were to be treated, ranging in size from 5ha to 40ha.

After treatment of the two larger areas was completed, the pilot began spraying a 5ha potato crop. The helicopter was subsequently seen manoeuvring to the west of the crop. Because it had not been seen manoeuvring in this area before, witnesses believed that the pilot was probably positioning the helicopter for a clean-up run. Shortly after, while proceeding in a southerly direction in a level flight attitude, the landing skids struck a powerline suspended across a gully.  The helicopter pitched forward violently, causing the main rotor to sever the tail boom. The aircraft then tumbled to the ground, coming to rest inverted, before a fierce fire broke out.

The pilot was rescued from the burning wreckage and given first aid until medical and rescue personnel arrived. However, he had sustained burns to 90% of his body and died 17 days after the accident.

The powerline was aligned in a north-west/south-east direction. The helicopter had struck a 450m span

approximately 16 m above the ground. During the inspection flight, the pilot had reportedly been made aware of the powerline. Soon after his rescue, the pilot indicated that he was unaware of what the aircraft had struck. Visibility at the time of the accident was good. However, the powerline was extremely difficult to see in the bright sunlight because the pilot's perspective was such that the powerline was below the horizon and tended to merge into the background terrain. The length of the wire span and the location of the supporting poles in relation to the treatment area made it unlikely that the pilot would have been alerted to the existence of the powerline.

Because of his injuries, the pilot was unable to provide further information concerning the circumstances of the accident.

Significant Factors

The following factors were considered relevant to the development of the accident:

1. It is most probable that the pilot forgot about the location of the powerline.
2. In the prevailing conditions the powerline would have been extremely difficult to see from the helicopter.

The helicopter had been hired to spray noxious weeds on steep, hilly terrain and had sprayed several local properties in the two days prior to the accident.  On the day of the accident, the pilot began his preparations at 0410 local time but did not begin spraying until 1030 because of fog in the treatment areas.  He then sprayed three sites before arriving at about midday over the property where the accident occurred.

The pilot conducted an aerial inspection before commencing to spray a very steep rocky area at the northern end of the property.  He systematically flew about ten short spray runs north of the powerline then crossed to the southern side of the powerline and flew two spray runs over a small paddock. Ground witnesses then observed the helicopter flying north at about 100 ft towards a previously treated area. They became very concerned that it was flying towards the powerline at about the same height as the wires.  One witness used hand signals in an attempt to prompt the pilot to climb but the aircraft struck the powerline. It pitched steeply nose-down and began breaking up before impacting the ground, inverted, about 70 m beyond the powerline, then rolled 15 m before coming to rest. There was no fire.

At the time of the accident, the temperature was about 24 degrees Celsius, there was a light breeze but no cloud or turbulence, and visibility was at least 20 km.

The wreckage was subsequently examined by engineers.  Evidence was found in the engine compressor and the combustion chamber to confirm that power was still being produced at ground impact.  No pre-existing faults were found with the aircraft which may have contributed to the accident.

Wire strike marks on the helicopter showed that it first contacted the powerline with the forward right fuselage at about cabin floor level.  Two wires then slid down the chin and snagged on the right spray boom which then separated from the aircraft.  The helicopter pitched nose down so severely that the tail boom, along with much of the airframe directly above the engine, was severed by the main rotor blades, one of which detached from the aircraft.

The helicopter carried fuel sufficient for the flight and was within its approved centre of gravity and gross weight limits at the time of the accident.

The pilot was endorsed on the Hughes 396HS helicopter and held an Agricultural Rating Class 2.  His total agricultural flying experience was 1079 hours. He had been provided with detailed maps of the treatment areas.

The pilot was seen wearing a crash helmet minutes before the accident, but it came off during the accident sequence. Damage to the seat belt inertia reel housing was consistent with the pilot wearing the full harness at ground impact.

The helicopter was equipped with a survival beacon which did not transmit a distress signal because it had not been either armed or switched on by the pilot.

The powerlines did not carry markers on the wires. Treatment areas were either side of the powerline and not far apart so the pilot should have been aware of the powerline even though it traversed the valley with a span of 478 metres between poles. However, due to poor contrast between the powerline and the terrain, the pilot probably found it difficult to detect the two wires in time to avoid them. It could not be determined if the pilot applied an appropriate method of identifying the position of the wires from the air before he began spraying.

VH-YEA was not fitted with a wire-strike protection system (WSPS).  The Hughes 500 may be fitted with a WSPS as an optional extra. A standard helicopter WSPS includes one wire-cutter fitted forward on the roof of the cabin and a second cutter forward on the belly, plus devices to guide the wires into the cutters.  VH-YEA was fitted with a Simplex agricultural spray kit which included a belly tank, pressure pump and boom.  When fitted, this particular model Simplex tank protruded so far forward that there was not enough available space for a lower wire-cutter to be installed on the fuselage.  Other helicopter spray tanks are available which, when installed, allow space for both cutters to be fitted.

Had an approved WSPS been fitted to VH-YEA, the lower cutter would probably have severed both wires and the helicopter may have received minor wire-strike damage.

Significant Factors

The following factors were considered relevant to the development of the accident:

1. the powerline was probably difficult to detect due to a lack of contrast with the background terrain; and
2. the helicopter was not fitted with wire-strike protection equipment.

Safety Action

Helicopters are not specifically designed for agricultural work, unlike most modern agricultural aeroplanes which come with re-enforced cabin and wire deflectors/cutters. Helicopters have been adapted for agricultural operations and have approved spray kits or spreaders attached. However, most helicopters used for agricultural operations do not have added crashworthiness built into their cockpits; nor do they have WSPS fitted.

WSPS have been developed and approved for several helicopter types, mostly as a result of low-level military roles. However, rescue operators, fire bombers, medical retrieval helicopters and particularly agricultural helicopters are often in the low-level environment where powerlines exist.

Analysis of Bureau records indicate that, wire-strikes account for about 9% of helicopter accidents in Australia. Since 1984 there have been 73 reported occurrences of wire strikes by helicopters. Of these approximately 50% may have benefited by having an approved WSPS fitted, including 12 occurrences that resulted in fatalities. It is probable that had a WSPS been fitted to this helicopter, the accident would not have occurred.

Recommendation R950120

The Bureau of Air Safety Investigation recommends that the Civil Aviation Authority:

1. require the fitment of approved wire-strike protection system kits for all helicopters engaged in low flying activities for which a kit exists; and,
2. that only agricultural spray kits compatible with wire-strike protection systems be approved for fitment to these helicopters.

The helicopter was being crewed by the pilot and the property owner.  They were assisting stockmen to herd cattle 

from one paddock to another.  The fence line contained a number of gates through which the cattle were being moved. Normally the cattle were taken through a gate well to the south of the accident site, but on this occasion the other gates were being used as well.  One of these gates was under a high-tension powerline (with steel towers) and the other was at the corner of the paddock at its junction with a fence running along a road.

Witnesses reported that the helicopter had been working at the head of the mob for a short time to recover some strays before making a left turn to move around to the rear of the mob.  The helicopter was moving towards the road when it struck the wire.  Witnesses reported that some cracks were heard, and the pilot seemed to have the aircraft under control again before it broke apart about 6 ft above the ground.  It caught fire and burned.  The stockmen were able to get the pilot away from the fire but were unable to save the other occupant.

The powerline struck was a single wire earth return line about 7 m high and running parallel to the road fence about 6 m inside the paddock.  Investigation at the scene determined that the powerline was struck by the front brace on the left skid.  This caused the tail to be raised into the rotor disc, whereupon it was severed by at least two blades, producing the cracks reported by the witnesses. The anticlockwise yawing tendency produced by the wire on the left skid was countered by the clockwise yawing tendency of the rotor torque, and the helicopter ran along the wire for about 23 m before freeing itself.  As it came free the wire progressed toward the leading edge of the skid, eventually breaking off the forward end of the skid.

Initial impact marks on the ground were produced by the forward end of the right skid which broke off. The subsequent marks were produced by the remainder of the right skid.  The marks indicated that the helicopter was probably yawing rapidly anticlockwise at the time.  The main rotor mast and fuel tanks then broke away from the main body of the helicopter. Evidence indicated that a fire erupted after the helicopter collided with the ground. There was no sign of fire prior to ground contact.

No evidence of any pre-existing defects was found.

Although the pilot was unable to remember the accident after the collision with the wire, he could recall noticing the wire just prior to colliding with it.  He was aware of the wire's presence, having worked in the area on a number of previous occasions.  On this pass through the area, he forgot about it.

In regard to aerial stock mustering operations, Civil Aviation Order 29.10 para 5.2 states, in part: 'During aerial stock mustering operations a pilot shall not carry more than one other person, and that person must be essential to the successful conduct of the operation'.  An examination of the injuries to people involved in mustering accidents in the ten years to the time of this accident was conducted.  Seven pilots and three passengers had received fatal injuries, while 13 pilots and two passengers had received serious injuries.  A further eight pilots and eight passengers had received minor injuries.  Of the 132 people involved in mustering accidents, 87 were pilots and 45 were passengers.  The most effective way to minimise the injury rate to passengers is considered to be an educational campaign intended to make graziers more aware of the dangers involved in aerial stock mustering operations.

Significant Factors

1. The pilot forgot about the presence of a power line.
2. The tail boom of the helicopter was severed after collision with the powerline.
3. The pilot was unable to maintain directional control of the helicopter.

Safety Action

The Bureau of Air Safety Investigation is preparing an article concerning the potential hazards of carrying passengers during aerial stock mustering operations. This will be featured in a forthcoming issue of the BASI magazine 'Asia Pacific Air Safety'. The Bureau also proposes to distribute the article to aviation and rural industry publishers for wider dissemination.

The pilot was having a dam constructed on his property which is adjacent to the Oakey airfield. Witnesses working on the dam site reported that the pilot had indicated that he would conduct a low fly-past over the dam after taking off from Oakey.

The aircraft was observed to fly at low level across the dam and commence a climb before striking powerlines located on the property. After the mainwheels contacted the wires, the aircraft impacted the ground inverted. The pilot received fatal injuries.

The main purpose for investigating air safety occurrences is to prevent aircraft accidents by establishing what, how and why the occurrence took place, and determining what the occurrence reveals about the safety health of the aviation system.

Such information is used to make recommendations aimed at reducing or eliminating the probability of a repetition of the same type of occurrence, and where appropriate, to increase the safety of the overall system.

To produce effective recommendations, the information collected, and the conclusions reached must be analysed in a way that reveals the relationships between the individuals involved in the occurrence, and the design and characteristics of the systems within which those individuals operate.

The helicopter had been hired by the pilot to take family members on scenic flights in the local area. It was observed operating at low level between Tacoma and the F3 freeway during the afternoon. During its fourth flight it was observed flying low over the F3 freeway on a southerly heading then make a rapid 180 degree turn onto a northerly heading which it maintained for a short time. The helicopter then turned to the east and descended into a shallow depression where it collided with high tension power lines some 47 feet above ground level.

The high tension power lines span a depression on the eastern side of the F3 freeway. The wires are suspended between poles which are masked by trees on their western sides and are hidden from view when approached from the west at low level. The wire spans blend with the background vegetation. As the helicopter was approaching the wires from the west at low altitude it is probable that the pilot did not see the wires in time to avoid collision. The reason why the pilot was operating at that height at the time of the accident was not determined.

Investigation did not reveal any mechanical deficiencies with the helicopter which may have contributed to the accident.

Significant Factors

• The helicopter was operating at a very low height.
• The high-tension power lines are difficult to see when approached from the west at low level.

The aircraft was conducting cotton spraying operations on a 61 hectare field. The pilot stated that he had completed numerous swath runs, using a human marker for guidance, when it became necessary to fly beneath power lines converging at the southern corner of the field. He then completed a pass under the power lines in a south easterly direction and carried out an extended procedure turn to facilitate the next pass under the wires. He said he could then see the marker moving off to the next swath position.

The aircraft was positioned to pass beneath the powerlines at a speed of about 100 knots with a power pole to the right and the marker to the left, and moving left. As the pilot passed the power pole he heard a loud thudding noise, associated with a left yaw. The pilot said that he thought at the time he had clipped the power pole, but as he regained control of the aircraft, he noticed the left wing was damaged. Shortly after, he noticed the marker lying in an irrigation ditch, near where she had been standing. The marker was killed as a result of being struck by the aircraft.