Wirestrike Feature

Heli-Ops · September 4, 2004

This is from the latest issue of my mag, thought it might be good reading material for all members.

Heli Ops

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‘Getting Wired’

Wire strikes are one of the more likely, if not, Number One causes of helicopter accidents. They have become so commonplace, that many helicopter pilots categorise themselves as those ‘who have had a wire strike’, or ‘those who are going to’. It is scary to think the helicopter industry has progressed to a point where wire strikes are almost expected, but what can be done about this hazard?

The statistics tell the story. Of the 1,534 helicopter accidents in the US between 1996 and 2003, 89 were attributed to wire strikes, with over 30% resulting in fatalities. Australian data reflect a similar picture, with 38% of the 122 wire strikes recorded over the past ten years, involving helicopters*. Significantly, 38% of all these wire strikes were fatal.

A Minefield of Wires

Wires are found anywhere and everywhere – along and over reservoirs, roads, rivers, ridge-lines, and even in the country. Not only are they everywhere, there are many types of wires – telephone pole wires, high tension power lines, including unexpected guide wires from pole to ground which are usually perpendicular to the rest of the wires, flying fox wires across ravines and gorges, and ground guide wires attached to antennae.

Inevitably it is the aircraft that operate low-level that are most at danger of colliding with wires, such as helicopters doing agricultural work, filming and EMS during outfield landings. The cost is enormous in airframes, lives, and damage to power company infrastructure.

Whose Responsibility?

As seems to be the way in aviation, pilots are often attributed with the sole responsibility for these accidents, on the spurious assumption that pilots can ‘see and avoid’ wires. Relying on un-alerted ‘see and avoid’ as the only defence, has been shown to be an extremely poor method against mid-air collision. Around the world, utility, power generation and telephone companies have largely ignored their responsibilities in reducing the chances of wire strikes – conveniently blaming pilots. In the United States however, the law presently sees the responsibility for wire strikes as more equally shared between the pilot and the wire owner.

The reality is that wires and the towers that support them are difficult and sometimes impossible to see, even if the general location of the wire is known. Complicating the matter further, is the pressure brought to bear by the environmental lobby, pushing utility companies to make the wires and supporting infrastructure even less conspicuous. High visibility markers on the lines would certainly help.

Eye Problems

The human visual system is better at detecting moving rather than stationary targets, and the physical limitations of the human eye are such, that even the most careful search won’t guarantee that wires can be sighted. Research shows that the eye begins to lose its visual acuity at around three degrees off-centre. Beyond ten degrees off-centre, the visual acuity falls away significantly. Unless a pilot is looking straight at a wire it is unlikely to be seen, because a significant proportion of the view may be masked by the eye’s blind spot (where the optic nerve attaches to the eyeball). Or the eyes may focus at an inappropriate distance, due to the effect of an obstruction, or due to empty field myopia (absence of visual cues) – when the eyes focus at a resting distance of around half a meter. An object which is smaller than the eye’s acuity threshold is also unlikely to be detected.

Unfortunately, helicopters are often flown low-level on high workload tasks, increasing the stress on pilots. A pilot’s functional visual field contracts under stressful conditions or increased workload, and the resulting ‘tunnel vision’ further reduces the chance that a wire will be seen in peripheral vision.

Contrast, or rather the lack of contrast, will certainly lessen the chances of a wire being seen. Compounding the problem is that wires become less visible with age. For example, copper oxidizes and turns pale green – becoming almost impossible to see against rural backgrounds or in cloudy conditions. Aluminium wire turns pale grey over time – again impossible to see in low contrast conditions.

Considering the physical limitations of the human eye, the odds are stacked against the pilot seeing wires un-alerted, and even if the pilot is aware of a wire’s presence, fatigue, inattention or task focus can lead to a wire strike of a known wire. A pilot’s ability to see wires is also adversely affected by dirty windscreens, light conditions, the obscuring effects of terrain, and changes in visual perspective that occur during climb and descent. In addition, accurately judging the helicopter’s distance from unmarked wires is nearly impossible. Wires are also difficult to see from certain angles and against some backgrounds, colours and textures. Sometimes a wire will only be seen or be obvious from the ground or beneath/level with the wire. Tell-tale signs such as a line of poles, or a cleared line through the foliage can also be missed.

Causes

Accident reports demonstrate the factors involved with wire strikes. A recent Australian Transport Safety Bureau (ATSB) report stated that late in the afternoon the pilot of a Bell Long Ranger 206L-1 was returning from an agricultural crop-spraying task, and while transiting a ridgeline, the helicopter collided with wires and impacted the ground. The pilot was fatally injured. Apparently, the helicopter had approached the ridgeline at a very low height and struck two, three-strand lightweight, high-tensile steel wires of a powerline supplying a repeater site. A wire strike protection system (WSPS) was not fitted to the helicopter. The maximum height of the wires was 31.5 meters (103 ft) for the upper wire and 30.1 meters (99 ft) for the lower. The wires were not shown on the relevant charts, and high visibility devices were not attached. This accident reflects the dangers of helicopters not being equipped with a wire protection device, transiting unnecessarily at a low height, and crossing a ridgeline (where wires can often be strung). The lines were also unmarked.

In another accident, the pilot of a Robinson R22 struck wires after he did a reconnaissance of the proposed landing site by making a descending right turn from 500 ft. During the final approach, the helicopter struck the upper conductor of a dual-conductor spur line, running at approximate right angles to a main transmission power line. The helicopter struck the spur line two bays from the main line – about mid-span at a height of about 8.4 m (27.5 ft). The helicopter was destroyed by the impact and the pilot sustained serious injuries, while the passenger was fatally injured.

This accident occurred early in the morning and there was some low-level cloud. In this accident, the pilot had not conducted the prescribed reconnaissance procedure, and according to the ATSB, he would have had a higher probability of seeing the spur line if he had done so. The flat, (low contrast) ambient lighting was also probably a factor.

In a US NTSB report, the pilot of an Enstrom was flying down the Grand River at about 21 meters (70 ft), late in the afternoon and towards the sun. The helicopter struck power transmission wires strung across the river. The low sun angle and the low-level flight down the river were factors.

Avoiding Wires

The danger height for wire strikes appears to be from around 10 meters above the ground (telephone or electricity wires), to over 100 meters for high-tension power lines. Flying fox wires can appear anywhere, anytime, and at heights commensurate with the surrounding terrain. These are hardly ever marked on a map, and do not have significant supporting structures, or have high visibility markers attached.

The first and most significant defence against a wire strike is to stay out of the environment. If there is no reason to be low, then the aircraft should be flown at a height that will avoid the wire environment. The problem is that much of a helicopter’s usefulness is when it is down low, or landing in an outfield.

Part II on next post...

Reconnaissance & Precautions

Although property owners or ground crews may supply information on known wires, pilots should participate in all aspects of the reconnaissance, including from the ground, if possible. In addition to reviewing charts and talking with pilots who are familiar with the area, a pilot who is about to begin low-altitude operations should first conduct a reconnaissance flight at a higher, safe altitude. A reconnaissance for wires should take in a variety of angles to improve the chances of seeing the wires or their supporting infrastructure.

Complacency in the environment plays a significant factor in colliding with wires. Although the ground crew or property owners will usually tell the pilot about wires on the property, the pilot needs to be concerned about the ones not seen or forgotten about. Wire maps are good tools but should not be relied upon, because the position of wires can change, new ones appear, or are missed in a wire survey. Constant vigilance is needed – an overhead reconnaissance is extremely valuable, but even these can fail to detect wires such as those with support poles hidden in trees.

Many pilots have hit known wires. In the agricultural environment, known wires are often struck late in a sortie. Fatigue and complacency appears to play a role here. Pilots should have regular breaks, eat and drink properly to reduce the effects of fatigue, and avoid letting their guard down towards the end of a flight.

It is easy to become target-fixated when looking at a wire, and the distance from a wire should never be judged by looking at the wire in isolation. The use of a reference feature like a nearby tree should be considered.

EMS helicopter operations regularly operate at the critical wire environment heights, including around unfamiliar off-airport locations, and therefore face significant wire hazards. Often they have little time to flight-plan so they need to be especially aware. They should always be on the lookout and use triangular sighting, especially if crewmembers are in the cabin. They should be looking out to the forward and rear at 45° angles.

On approach to landing or during the reconnaissance-phase of a landing site, crewmembers should be watching for any elevated structures, or gaps in trees along the intended flight path. If there are a series of high towers or poles, pilots should always fly over one of the structures, as opposed to between them. This assures clearances, and avoids the near-invisible web of wires that may exist below. Crews should be careful though, to consider the existence of higher structures and wires in the vicinity of high-tension lines and towers already seen. Approaches should be slow and steep, and crews need to be cognisant of additional smaller telephone and electrical poles and wires that may have been unseen during the initial approach or reconnaissance. Crews should continue their scans during the entire approach, without fixating on any one object.

When departing the area, these same guidelines need to be considered. Pilots should concentrate on making a safe departure, and pay attention to the surrounding area for evidence that may indicate the existence of wires, including houses, barns, pump sheds, poles etc. Often there are spectators, media, family or friends on hand for the departure. Crews must maintain their professionalism, and avoid any impromptu desire to show off. Where possible, if the area above the helicopter has been positively cleared, a towering or vertical departure is advisable if power margins permit (especially twin-engine helicopters).

Helping the pilot

In some instances, spherical wire markers and wire-detection devices may help pilots to notice, and then remain aware of wires.

Spherical coloured markers can be mounted on power lines, communications lines, and guy wires at airports or helicopter approach areas, and at locations where wires cross over rivers and gorges. Some spherical markers used on electrical power lines are designed to glow as a result of the power line’s electrical field; other spherical markers are patterned for improved visibility, or equipped with flashing lights.

Detection Systems

In recent years, aircraft-installed wire-detection systems have been developed, including the Safe Flight Instrument Corp’s Powerline Detection System, that senses the electromagnetic fields developed by live electrical power lines, and emits an audible alert through the aircraft’s audio system and illuminates a red warning light in the cockpit.

The Hellas system, developed by Dornier, a subsidiary of EADS, uses eye-safe laser radar to scan the environment for wires and other flight obstacles, to provide optical signals and acoustic signals to warn pilots about their presence.

A system that was never designed to provide wire avoidance is Honeywell’s Enhanced Ground-Proximity Warning System (EGPWS). During flight tests where the locations of power lines were included in a computer database, the system delivered warnings of approximately 30 seconds to pilots, that their helicopters were approaching lines. Although the system is capable of delivering the warnings, its database lacks the required detail for all wires – but the database is being developed.

The Obstacle Collision Avoidance System (OCAS) is an idea of two former Royal Norwegian Air Force fighter pilots. The Civil Aviation Administration of Norway, the Air Force, and Stattnett SF, Norway’s utility company, support the work.

The system generates low power L-band radar signals from transmitters on mast-mounted antennae, near obstructions such as power lines or towers. It activates ground lights and warnings when a collision looms, and it then illuminates the obstruction, using any lighting near these structures. If the lights don’t convince the pilot to change course, the system broadcasts an aural warning on all aeronautical VHF frequencies at low power – to keep the broadcast from propagating beyond the immediate hazard area. The system also logs data on the encounter for later analysis.

When Avoidance Fails

When avoidance hasn’t worked, there’s the Wire Strike Protection System (WSPS) that is installed on the front of many military and some civil helicopters. WSPS was developed for the US Army and was designed to reduce the chances of the helicopter controls becoming entangled with a wire, with the ensuing catastrophic failure of the rotor system. The WSPS diverts the wire into the cutter blade assemblies affixed to the top and bottom of the frontal area of the aircraft, and these will usually cut the wire. The WSPS system protects 90% of the frontal area of the helicopter, and reduces the hazard from most wire strikes. With the WSPS, the pilot has a 95% chance of surviving a single wire strike. The odds of survival decrease as the number of wires increases. Two wires will reduce the chances to 75%, three wires to 50%, and four wires to about 25%. WSPS is not suitable for all helicopters though.

Wires are killers – and it is up to helicopter pilots to avoid them. Experience has shown that the best methods of reducing the risks associated with flying near wires are education, using valid procedures, and pilots being ever vigilant.

SIDE BAR:

Helicopter Accidents in the US

1996 and 2003

1,534 helicopter accidents recorded

89 (5.8%) attributed to wire strikes

1996 and 2000

30% of helicopter wire strike accidents resulted in at least one fatality

18% resulted in serious injuries.

14% of these accidents occurred at night;

57% of the night accidents were fatal

Data from the Helicopter Association International (HAI)

Helicopter Accidents in Australia*