Background
In
1991 ATSB's predecessor (BASI) published a research report titled
Limitations of the See-and-Avoid
Principle. This report concluded that 'the see-and-avoid
principle in the absence of traffic alerts is subject to serious
limitations'. Unalerted see and avoid has a 'limited place as a
last resort means of traffic separation at low closing speeds', and
is 'completely unsuitable as a primary traffic separation method
for scheduled services'.
Nevertheless, operations in a number of types of airspace
currently require the application of see-and-avoid techniques by
the pilots of both visual flight rules and instrument flight rules
aircraft operations. In areas such as mandatory broadcast zones,
pilots should be assisted by radio calls from all other aircraft to
provide an 'alerted' see-and-avoid environment. However, the final
level of protection is provided by pilots being able to see
potentially dangerous traffic in time to take avoidance action.
The report highlighted the fact that 'many of the limitations of
see-and-avoid are associated with physical limits and human
perception' and encouraged pilots to be 'made aware of the
limitations of the see-and-avoid procedure, particularly the
factors which can reduce a pilot's effective visual field'.
Each year ATSB investigates incidents where aircraft have come
perilously close whilst operating in weather conditions well above
the visual meteorological conditions minima. Some of these
incidents occur in the circuit area, where pilots should have had
an acute awareness of the position of all traffic at all times.
Incidents also occur where aircraft were established in an en-route
cruise. Given that there indeed is a lot of sky out there, there is
often an understandable tendency during the cruise to be less
assiduous in maintaining a lookout. The following paragraphs
address the issue of detecting other aircraft during an en-route
cruise by examining some of the problems of lookout or visual