Even if a wire can be seen, a pilot’s ability to judge its position accurately may be reduced by a number of factors. For example, ambient temperature can change the location of the wire by causing the wire to sag or tighten, and windy conditions may cause sagging wires to be blown about. In addition, the ability to judge distance correctly can be distorted by optical illusions.

The ability to identify the presence of power lines can be facilitated by objects and landmarks on the ground. Buildings such as houses and sheds are likely to have power connected through above ground wires. Roads may also provide a convenient path for power lines. Furthermore, supporting poles may offer clues as to wire direction and height.  

By identifying at least two poles, a pilot may be able to gauge the path of the wire. Insulators attached to the poles run in the same direction as the wire and may also assist in identifying the number of wires and their direction. The orientation of the insulators could indicate whether the wire continues in the same direction or turns a corner. 

Although poles provide pilots with one of the most reliable indicators of the presence of wires, the poles themselves are not always easy to see. Wooden poles, in particular, can be easily camouflaged by the landscape or hidden by foliage and trees. Since poles are typically used by pilots to alert them to the presence of a wire run, the concealment of poles may increase the risk of a wire strike. 

Risk mitigation strategies associated with low-level flying rely heavily on the level of situational awareness maintained by the pilot. Strategies used to establish and maintain adequate situational awareness include reading the physical structure indicators (i.e. orientation of insulators, presence of bucked arms and sighting two or more poles), self-discipline, preflight briefing, pre-flight reconnaissance and observation, memory and awareness, appropriate flying techniques, maintenance of a good visual scan and consideration of weather factors. 

To assist pilots in the detection of wires, a number of non-human strategies have been developed, these include wire markers and owners/operators of private airfields and strips should consider working with the utility suppliers who operate wires in the vicinity of their airfield to identify those wires that pose a hazard and, where possible, incorporate some form of markers. Where this is not possible, the location and nature of wires in the vicinity should be detailed in an airfield/airstrip pilot briefing that should be available to both resident and visiting pilots. 

In September 2008 the FAA released a report “Safety Study of Wire Strike Devices Installed on Civil and Military Helicopters” The objective of the research was to conduct a study on wire strike accidents of civil and military helicopters between 1994 and 2004 to establish trends, assess the potential of existing technology for reducing wire strike accidents, and to recommend solutions that could substantially reduce the number of wire strike accidents.  

Trends in accidents were established for both military and civil wire strike accidents. The age group and experience profiles of the pilots involved in civil helicopter wire strike accidents were found to be similar to those found in an earlier study. Devices available for warning pilots about the proximity of wires are described and their relative merits assessed. Recommendations are made for reducing the number of helicopter wire strike accidents.  

In Summary - Wire strike accidents involving United States military and civil helicopters for the period 1994-2004 were analyzed using military and the National Transportation Safety Board databases.  

The objective of the research was to conduct a study on wire strike accidents of civil and military helicopters between 1994 and 2004 to establish trends, assess the potential of existing technology for reducing wire strike accidents, and to recommend solutions that could substantially reduce the number of wire strike accidents. Trends in accidents were established for both military and civil wire strike accidents.  

The age group and experience profiles of the pilots involved in civil helicopter wire strike accidents were found to be similar to those found in an earlier study. Devices available for warning pilots about the proximity of wires are described and their relative merits assessed. Recommendations are made for reducing the number of helicopter wire strike accidents.  

The wire strike accident rate to United States (U.S.) civil helicopters accounted for approximately 5% of all accidents from about 1963 to the present. In spite of the excellent reports on the effectiveness of wire cutters in U.S. Army helicopters, no detailed study has been carried out on these accidents and the potential of the currently available devices to reduce the accidents. 

Helicopter wire strikes have been a matter of concern for both civil and military helicopters. Devices to protect the occupants in case of wire strikes have been available for some years. Systems that warn the pilots on the proximity of wires have also been developed. In spite of these developments, wire strikes continue to account for about 5% of all civil and military helicopter accidents. 

Studies show that wire strikes continue to be a matter of concern because helicopters need to operate at low altitudes, wires are difficult to observe, and the background combined with sunlight can obscure wires. Also, not all helicopters can be equipped with wire cutters, and some of the recently developed devices that warn a pilot of potential wire strikes are very expensive. 

In the year 2003, the number of accidents for fixed-wing aircraft was slightly above 0.3 accidents per 100,000 hours. Thus, the rate of accidents per 100,000 hours for fixed-wing aircraft is about 3% of the accident rate for helicopters.  

Between the years 1994 and 2004, there were 124 wire strike accidents in which 41 were fatal accidents. These accidents resulted in 65 fatalities, 45 serious injuries, and 42 minor injuries. The average age of the pilots involved in the wire strike accidents was 43.5 years, and the average rotorcraft flying experience of the pilots was about 4000 hours.  

The NTSB determined that the probable causes for most of the accidents are one or more of the following reasons: inadequate visual lookout (38 accidents), failure to maintain sufficient clearance with the obstacle (59 accidents), failure to maintain proper altitude (9 accidents). Other reasons (for 19 accidents) include improper judgment (e.g., decision to continue flying visual flight rules (VFR) under instrument flight rule (IFR) conditions), inadequate preflight planning, failure to see and avoid wires, intentional buzzing (low altitude flight), and selection of unsuitable area for landing.  

Warning and protection systems 

The Powerline Detector system (Safe Flight Instrumentation Corporation) senses the electromagnetic fields surrounding power lines. This detector only senses active power lines and the range of detection depends on the electrical power in the lines. The system will not detect other types of wires such as guy wires, telephone lines, and nonactive power lines. Further, the pilot is not alerted to the direction of the power lines with reference to the aircraft.  

Devices that use lasers or radar to scan the surroundings for the presence of obstructions are Obstacle Awareness System (OASys) (Amphitech System), Laser Obstacle Awareness System (LOAS) (Goodrich Sensor System), and Dornier’s Helicopter Laser Radar (HELLAS) (European Aeronautics and Space Company (EADS) Dornier). They are comparatively heavy (between 35 and 60 lb) and expensive (more than $100,000). Thus, they are too heavy and quite expensive for most civil helicopters.  

Honeywell Enhanced Ground Proximity Warning System (EGPWS) can warn pilots about obstacles that are over 30 ft AGL using a global positioning system (GPS) and the database maintained by Honeywell Aerospace. The EGPWS weighs about 4 lb but costs about $45,000, which makes the system too expensive for most civil helicopters.  

Developing less expensive devices that can be fitted to light helicopters, such as the Robinson R22, can be helpful to about 75% of the helicopters.  

Spherical markers mounted on wires will help to make the wires more visible and reduce wire strikes. The Marking of Wires and Towers (MOWAT) Work Group [9] recognized the importance of wire markers in preventing wire strikes.