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
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.
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
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.
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.
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
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
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
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
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  recognized
the importance of wire markers in preventing wire
The Obstacle Collision Avoidance System (OCAS)
consists of units located on utility and power line
towers and detects all air traffic entering a
predefined warning zone and activates warning lights
that illuminate the tower. The fact that the OCAS
does not require any installations in the
helicopters can make it attractive to helicopter
operators. It is also attractive to utilities in
spite of its cost ($50,000 per installation).
However, the lights on the utility towers can
normally be turned off. The OCAS is presently being
evaluated by the FAA and has potential to prevent