Recommendation

SUBJECT



Powerline markings.



OCCURRENCE SUMMARY



The aircraft, a PA 38-112 Tomahawk, was on final approach for a
landing at Torquay (Vic.) ALA when it struck powerlines. The
aircraft suffered significant damage from the powerlines and
further damage from the impact with the ground. The pilot and his
passenger (also a pilot) were not harmed.



ANALYSIS



The aircraft collided with two strands of 3 X 12 gauge (2.75-mm)
galvanised steel powerlines, grey in colour. The pole height was 12
m and the line dropped to 6.55 m above ground level at its lowest
point. The lines ran perpendicular to the runway direction, a short
distance outside the fence marking the end of the runway surface.
The line was marked by orange marine buoys of 200-mm diameter that
had faded on the upper surface to a dull white. This combination of
grey powerlines and faded markers made the lines virtually
indistinguishable from the dry, light brown surface, especially in
the prevailing bright but overcast conditions. The electricity
company had conducted a ground inspection of the markers every 3
years but did not have a requirement to check the upper surface for
fading. At the time of the accident, the markers had been installed
for approximately 8 years.



Many aerodromes and landing areas around Australia are situated
near buildings or homesteads, and many of these have power supplied
to them by overhead powerlines. Many of these lines have been
marked prior to the publishing of the relevant standard in 1991,
hence a wide variety of markings have been used, many of which are
not effective in ensuring clear visibility from the air.



Australian Standard 3891.1 recommends that any powerlines which
infringe the transitional slope or the approach and take-off slope
of an aerodrome shall be marked by a minimum of three spherical
markers of at least 600-mm diameter placed less than 30 m apart.
One of the markers should be off- white and the other colours
should be chosen to provide maximum contrast with the ground. The
other recommended colours are canary yellow and signal red, with
international orange available for circumstances requiring an
unusual contrast. The markers should also be made of a material
that is resistant to deterioration caused by ultraviolet light. The
document containing the full details of this standard is available
from Standards Australia. Comparing the installed markers with the
Australian standard, the marine buoys installed were a third of the
diameter required and they had lost their colour on the upper
surface.



There is a good chance that this accident would not have occurred
had the powerlines been marked to meet the Australian standard. If
the standard had been followed there would have been at least three
markers on the line, each of which presented over eight times more
visible surface than the installed markers. In addition, the
brightly coloured material would have provided a strong contrast
with the background.



SAFETY DEFICIENCY



Powerlines in the vicinity of some aerodromes and landing areas are
not marked sufficiently to be easily visible in all
circumstances.