Development of a Powerline Detector
Paul Levine
Project Engineer
Safe Flight Instrument Corporation
White Plains, NY
plevine@safeflight.com
Abstract
A major cause of helicopter accidents is wire strike, and most of these involve contact with
electrical transmission lines. The need for additional identification and protection against this
hazard is identified in this brief paper, and a means developed to accomplish this protection is
described. Of the various methods that could be used to detect power lines in flight, the direct
detection of their radiated power line frequency field appeared to be a practical low cost option.
Several of the characteristics of the field were investigated, and some of the airborne
measurements made are described. Some of the practical aspects of detecting the extremely low
frequency radio signal with regard to interference, antennas and receiving equipment are
explored. The choice of a particular audio and visual alert to the pilot is described, and a brief
description of some of the operational tests is given.
Introduction
Safe Flight Instrument Corporation has always reacted to aircraft accident statistics with
innovative hardware solutions. One of these was the first practical stall warning system. Several
years ago, during a brain storming session, the boss held up a small transistor radio and
exclaimed “Power lines are a hazard to helicopter flight. This thing makes noise when I’m near
power lines. Can we use it to develop some sort of detector that will keep our helicopter away
from them?” That was the start of a project. The development of the Power Line Detector
required some inspiration, a great deal of innovation, and plenty of experimentation before a
practical device was developed. This is an account of some of the steps in the process.
Product
Statistics show that flight into power lines is a real aviation hazard. For example, a recent search
of the NTSB aviation accident files for helicopters on the internet involving the terms “Power
Lines” and “Transmission Lines” resulted in 363 entries. A recent army safety data report from
the US Army Safety Center recorded 92 wire strikes between 1984 and 2004. (Figure 1) These
wire strike accidents occur under varying circumstances.
Figure 1. Army Safety Data 1984-2004
A/UH-1
A/MH-6
AH-64
CH-47
OH-58
M/UH-60
#
18
4
19
6
26
19
Fatal
13
3
6
4
11
1
WIRE STRIKE
Injury
Damage
4
$8,949,806
$506,227
$45,284,545
$13,936,712
14
$49,267,058
25
$30,338,865
Injury
$3,947,510
$2,350,000
$6,600,720
$2,740,000
$10,839,235
$1,431,480
TOTAL
$12,897,316
$2,856,227
$51,885,265
$16,676,712
$60,106,293
$31,770,345
Average
$716,518
$714,057
$2,730,803
$2,779,452
$2,311,781
$1,672,123
The helicopter pilot may just be unfamiliar with the area and local power line hazards. Against
some backgrounds such as desert country, the lines are almost invisible, and there are no ground
indications of their presence. Night, or low lighting, or bad weather contributes to this loss of
visibility. Pilot workload, such as the confusion and the urgency of the situation in a roadside
evacuation, might cause a pilot to forget about those nearby power lines behind the helicopter.
Several methods have been used to detect power lines, each with advantages and disadvantages.
Safe Flight considered some of them.
Visual Identification: A pilot is usually aware that he is flying in an environment that contains
power lines. The human eye and mind is very good in discriminating particular patterns of
interest against a busy background. Although visual contact is, and should be the ultimate
method of avoiding power lines, at times during a period of low visibility, or low contrast, or
distraction, it may fail.
Comparison with present position with charted data: Means have been developed to compare the
charted position of power lines with the present aircraft position. These require charts or data
bases that are accurate and up to date and airborne position determining devices such as GPS.
Such data bases are difficult to maintain, and smaller local power lines may not even be charted.
This method does have the advantage that the lines need not be powered for detection.
Laser Illumination: This means of detection uses a laser to scan the airspace ahead of the
helicopter, and analyses the pattern of reflections received back at the detector to differentiate
between wires and other objects. The device is heavy, complex, expensive, and is dependant on a
very good algorithm for the differentiation. This method also does not need the lines to be
powered for detection.
Heat Detection (Infra red): When power lines transmit energy, inevitably some of it is lost as
heat due to the electrical resistance of the conductors. An infra red detector scans the area ahead
of the helicopter and looks for a heat pattern that may be representative of wires. While this does
not require an illumination source like the laser detector, it has some of the same disadvantages,
and also requires that the power line must be excited.
Electromagnetic Field Detection: All excited power transmission lines generate an electric field.
If the power line field can be differentiated from interfering fields, a relatively simple,
inexpensive device may be used to detect it. This method will not detect unexcited power lines.
Based on the complexity and cost of the detector, we chose to detect electromagnetic fields from
the power line using basic radio techniques. Various signals are emitted from a power line and a
choice had to be made as to which of these would be most practical to detect.
Some of the detectable signals are radio noise, and the electrical and magnetic components of the
radiated power frequency field. Each could be used as the basis of a detector.
The type of noise heard on an AM car radio as you drive under a high power electrical
transmission line is caused by the radio noise generated by the line. It is caused by minor defects
which cause leakage, arcing and corona all of which are undesirable in the operation of the line.
The power companies attempt to minimize them by the design and maintenance of the line.
These are highly variable with weather conditions and the physical condition of the line, and may
be absent on lower power lines. For that reason we thought those signals inappropriate for
power line detection.
The power flowing in a transmission line generates an electrical field at the power line
frequency. This extraordinarily low frequency radio signal has both a magnetic and an electrical
component. Either one could be used to detect the presence of the field. We decided to build a
system to detect the electrical component based on the following considerations.
Antennas for the magnetic component involve very low impedances and are complex to
manufacture. Interfering magnetic fields generated within the helicopter are not attenuated by its
structure and may find there way to the antenna.
Although antennas for the electrical component of the field involve very high impedances at
power line frequencies, they are simple to manufacture. Existing antennas may be used without
modification. At these frequencies the body of the helicopter provides a very efficient shield
against internally generated electrical fields.
Once we decided to detect the electrical component of the field it needed to be characterized.
Much of the data required is unavailable and had to be determined on an experimental basis
To our advantage excited power lines radiate a substantial detectable field, and power lines on
larger more hazardous structures generate stronger fields. Although a few specialized power
lines throughout the world operate at other unconventional frequencies and even at DC, the usual
frequency is precisely 50 or 60 Hz. This allows very narrow tuned receivers to be used.
The distance from which a power line is detected has a direct relationship to the strength of the
field. Most of the work available is related to models of the fields on the ground, and good
models of the radio field at 60 hertz were unavailable.
Power companies attempt to balance their transmission lines electrically and geometrically
specifically to reduce external fields. These represent an energy loss to the company. The field
available for detection is only the residual field.
The terrain over which the line passes and its geometry affect the field strength and the ground
characteristics and elevation changes reflect and absorb electrical energy in unexpected ways.
Similarly airborne equipment for measuring these fields is unavailable, as this equipment often
needed a ground reference. At these frequencies a helicopter body is just not big enough to serve
as that reference.
Since a good analysis of the field strength was not available, a spherical antenna field strength
meter was developed and tested to explore it. Physical problems related to mounting it outside
the aircraft were not solved and ultimately other methods were used to design the detector.
The spectrum of the field from a power line consists of its fundamental frequency and several
harmonics. To investigate these, a prototype Extra Low Frequency radio receiver was built
feeding a laptop PC spectrum analyzer program and these were flown in our helicopter. The
power line detector was derived from this receiver.
The evolution of the receiver proceeded as follows.
Based on the spectrum measurements made (Figure 2), the electric field generated by a power
line indicated strong third and fifth harmonics. It was proposed that the receiver accept these
harmonics and that the total signal would add to the fundamental component resulting in a more
detectable signal than detecting the fundamental itself. This would require a receiver with an
easily manufacturable low-pass filter response.
Figure 2. Frequency vs. Amplitude
Further airborne spectrum measurements were made in the helicopter showed that equipment
electrical noise (generators, motors, power supplies and the transponder) and induced
electrostatic blade noises often fell in the harmonic range of the power line field.
A very narrow band filter response was therefore chosen for the required response. The
disadvantage of such filters is that they are difficult to build and that separate filters are needed
for 50 Hz and 60 Hz power line. Further design work is continuing to address this.
The maximum sensitivity of the receiver is a compromise between the minimum distance
required for a warning and those noise signals generated within and external to the helicopter
which might cause extraneous warnings. Ultimately a control was added to enable the pilot to
decrease the sensitivity in areas with high incidence of low level 60 Hz fields (i.e. near the
ground in highly populated areas) thus avoiding excessive nuisance warnings.
Although the antenna used with the power line detector can be a conventional aircraft
communication antenna, it operates in a highly unconventional manner. Antennas for radio
waves generated at 60 Hz that can be used on a helicopter are electrically extremely short. The
antenna theory usually applied in normal radio applications does not apply. In a 60 Hz field, the
electrical wavelength is 5,000 Km or 3107 miles. Aircraft antennas are often a quarter wave
length long. It would be kind of difficult to put a conventional quarter wave length antenna for
that frequency on a helicopter (!).
Short antennas are often characterized with an electrical ground plane. The body of a helicopter
is too small to serve as an effective electrical ground plane for the antenna and the helicopter
body becomes an important part of the antenna. The theory for this type of antenna is sparse and
led to some unexpected results with respect to the antenna placement.
Because the antenna is so short with respect to the wavelength of the field to be measured, the
voltage along its entire length is essentially constant. In conventional antenna theory the phase of
the field, which is due to the velocity of the radio wave along the length of the antenna, is
important.
For short antennas, the antenna can be considered to be a small capacitor connected to a source
equal to the field strength multiplied by the effective height of the antenna. A quarter wave
aircraft communications antenna (about one-half meter in length) represents an impedance of
over 800 megohms at 60 Hz. which must be matched to the receiver.
Because of the long wavelength of the signal, the position of the antenna on the helicopter leads
to some unexpected results. Since the helicopter body is not a conventional ground plane, but
part of a dipole-like antenna in free space, the entire body of the aircraft is an important part of
the antenna. Mounting the antenna on the front of the aircraft will not necessarily provide the
most sensitivity in that direction.
In the development of the field strength measuring equipment mentioned previously, we have
demonstrated very directional antennas for 60 Hz fields, but we have not yet developed a
practical antenna for use on a helicopter.
Since the helicopter might approach a power line in any orientation, the use of a directional
antenna with, say, high sensitivity in the forward direction, might be questionable. The field
intensity and direction around a power line varies with the terrain. The detected field does not
necessarily continuously increase but may dip and peak as the distance from line decreases. The
direction of the field will generally increase as the power line is approached but might not come
from a constant direction.
We have recommended using a common aircraft VHF communications antenna with the power
line detector. These are readily available and are qualified for use on helicopters. The antenna
must, however, be coupled to the power line detector receiver with the supplied impedance
converter. (Figure 3)
Figure 3. Safe Flight’s Powerline Detection System Installation in a Bell 206
Two means of alerting the pilot to the proximity of a power line were designed into this unit, an
audio signal to enable the pilot to remain heads up, and a redundant visual signal.
The warning sound was chosen to be unlike any other warning sound in the helicopter. It is
pulsed like Geiger counter, reminiscent of the static sounds in an AM radio near power lines. A
sense of urgency indicating the nearness to the power line is implemented by an increase in pulse
frequency as the field strength increases. A muting control is provided to avoid interference with
other audio inputs and an annunciator indicates that the audio has been muted. When the control
is activated a second time, or when power is reapplied to the power line detector, it returns to the
un-muted condition. As a backup to the audio warning, a warning lamp within the pilot’s scan is
illuminated when the field strength exceeds a selected value even when audio is muted.(Figure 4)
Figure 4. Pilot Muting the Audible Warning
An indication to the pilot that a system is working properly is essential in a system that might
activate rarely during a flight. This indication is designed to be used in a preflight check.
An input was designed which, when activated, injects a power line frequency signal is into the
receiver. If the power line detector is operating properly, the audio is not muted, and the
sensitivity control is turned up, the audio warning should be heard and the warning annunciator
will be seen.
A panel mounted power indicator panel shows that the power line detector is powered up and
ready to warn. This is normally activated by the power line detector circuit breaker.
The operation of the power line detector was confirmed by numerous flights near local power
lines. The operational range was determined during development by maneuvering a helicopter
near various types of power lines. The range was initially estimated visually using airspeed and
time calculations. Later, the GPS position of the helicopter when the warning was first actuated
was automatically plotted. (Figure 5) shows a measured range of about 3000 to 5000 feet from a
particular powerline.
Figure 5. Powerline Detector Test
A great deal of time was spent isolating and compensating for the low frequency signals
generated by helicopter equipment and the helicopter it self. Noise sources from the generator to
the transponder, and noise generated by the rotor blades themselves needed to be dealt with.
EMI tests were performed according to DO-160 during the qualification of the product and the
receiver was made insensitive to the types of radio interference likely in aircraft operation.
It is important to note that the Power Line Detector was developed as an aid to the pilot of a
helicopter in the avoidance of such wires, and not as an absolute means of indicating their
presence. It cannot detect wires that are un-powered such as deactivated power lines or support
cables. As with any device, it does not substitute for continued pilot vigilance in the avoidance
of power lines.
Conclusion
The concepts outlined in this paper have been used to produce a successful commercial power
line detector. Hopefully it will save a few lives by alerting a helicopter pilot to the presence of
that almost invisible or forgotten wire.