SUBJECT
Maximisation of the use of "separation assurance" techniques in
conjunction with "alerted see-and-avoid" procedures by flight crews
of aircraft operating in Class G airspace.
DEFINITIONS
"Alerted see-and-avoid" is a procedure where flight crew, having
been alerted (usually by radio communications) to the existence and
approximate location of other traffic in their immediate vicinity,
seek to sight and avoid colliding with those known aircraft.
"Unalerted see-and-avoid" is a procedure where flight crew, who
have no specific knowledge of other aircraft in their immediate
vicinity, rely solely on their ability to physically sight and
avoid colliding with aircraft that may be in their immediate
vicinity.
"Separation assurance" techniques involve positive separation by
mutual arrangement of a vertical or lateral distance that will
ensure separation will be maintained, even in the event of a
subsequent radio failure or when the crews of either aircraft do
not see the conflicting traffic.
OCCURRENCE SUMMARY
The crew of an Instrument Flight Rules (IFR) regular public
transport (RPT) De Havilland Dash 8 (Dash 8) aircraft chose to
proceed from controlled airspace, on descent to non-controlled
airspace, using "alerted see-and-avoid" procedures. The crew had
been provided with traffic information on an IFR RPT Embraer
Brasilia aircraft which was proceeding in the opposite direction.
Because of frequency congestion, there was no communication between
the crews until after the Dash 8 passed underneath the
Brasilia.
In another occurrence, the pilot of an IFR Cessna 310 (C310)
aircraft descended in non-controlled airspace using "unalerted
see-and-avoid" procedures and came in close proximity to an IFR
Cessna 402 (C402). The pilot of the C402 estimated that the two
aircraft had passed with approximately 200 ft lateral separation
and at the same level. There had been no communication between the
pilots of the two aircraft as they were operating on different
frequencies (one on air traffic control and the other on the Common
Traffic Advisory Frequency).
The crew of an IFR RPT Dash 8 aircraft, on descent to Taree in
non-controlled airspace using "alerted see-and-avoid" procedures,
was delayed in obtaining traffic information from flight service
due to the number of aircraft using the area frequency. After
receiving and analysing the information, the crew discontinued the
descent at 10,000 ft due to a slower aircraft ahead of them which
was maintaining 9,000 ft. Had the crew of the Dash 8 experienced a
further delay in obtaining the traffic information, their continued
descent would have possibly resulted in a proximity
occurrence.
SAFETY DEFICIENCY
Flight crews of aircraft are not establishing communications prior
to entering Class G airspace in sufficient time to arrange positive
separation. They are placing an over reliance on their ability to
see conflicting traffic in adequate time to manoeuvre and avoid
other aircraft.
ANALYSIS
Current situation
As a result of the investigations of these incidents, two regional
airlines have amended their flight operations manuals to emphasise
the use of "separation assurance" in conjunction with "alerted
see-and-avoid" procedures. Crews are required to establish vertical
or lateral separation with conflicting traffic. A third regional
airline is also considering similarly amending their flight
operations manual.
A survey of chief pilots from five other low-capacity airlines
revealed that only one company had published "separation assurance"
techniques for self-separation of aircraft in Class G airspace.
Three of these airlines had unwritten procedures that were common
practice within their companies while the last airline only used
the "alerted see-and-avoid" procedure.
The Aeronautical Information Publication (AIP) details procedures
to enable flight crews to be alerted to possible conflicting
traffic either by ATS operators or by communications on the
appropriate area or Mandatory Broadcast Zone (MBZ)/Common Traffic
Advisory Frequency (CTAF) frequency. However, the AIP does not
require flight crews to use "separation assurance". Consequently,
they would be totally reliant on their ability to sight the other
aircraft in adequate time to safely implement avoiding
action.
Operational technique
Generally, crews of IFR flights which are required to leave or
descend from controlled airspace into non-controlled airspace
expect that they will be able to establish timely communications
with crews of conflicting aircraft. Having established
communications, the crews can then arrange separation if necessary,
prior to the flight leaving the protection provided by controlled
airspace. However, this process does not account for those periods
when the Class G airspace area frequency is so busy that
communications cannot be established with any potential conflicting
traffic.
The options for the crew of the aircraft entering Class G airspace
are either to request an amended clearance to remain in controlled
airspace until communications are established, or to continue into
Class G airspace under "see-and-avoid" procedures. In the latter
case, potentially conflicting traffic could remain "unalerted" with
no assurance of separation.
Use of "alerted see-and-avoid" procedures
Evidence indicates that crews are relying on the "alerted
see-and-avoid" procedure as the prime means for separation from
conflicting traffic in non-controlled airspace. However, a crew's
ability to visually acquire other traffic is limited by a number of
factors.
The problems with human visual perception are well documented and
have been the subject of considerable study. The BASI research
report "Limitations of the See-and-Avoid Principle", published in
1991, related specifically to "unalerted see-and-avoid" procedures
and concluded that "the most effective response to the many flaws
of see-and-avoid is to minimise the reliance on see-and-avoid in
Australian airspace". An article in the Airline Pilot magazine of
October 1988 stated "Recent research shows that the concept of
see-and-avoid is more seriously flawed than anyone, except perhaps
pilots, suspected". Similarly, H E Mathinson of the US Airline
Pilot's Association wrote in an article that "the see-and-avoid
concept is an outmoded, antiquated system of traffic
separation".
A recent Flight Safety Foundation article described research which
analysed visual acquisition and the probability of detection of
aircraft. This research found that "the probability of target
detection was quite low in most cases".
Procedure guidance
The Aeronautical Information Publication (AIP) NCTL 51.1 describes
the actions to be taken by the crew of an aircraft descending into
Class G airspace. However, this does not provide particular
guidance in relation to the actions required when the area
frequency is congested.
Conversely, the AIP CTL 21.1.6 advises pilots to consider aircraft
performance and the possibility of frequency congestion when
determining the time to make a request for a clearance to enter
controlled airspace. Also, AIP CTL 21.1.7 advises crews of dual
VHF-fitted aircraft to monitor the area frequency during the
clearance request.
Similar guidance in the AIP NCTL section should cover the actions
that may be required prior to entering Class G airspace. The
establishment of communications, prior to leaving controlled
airspace, with the flight crew of aircraft operating in Class G
airspace, is an essential precursor to establishing "separation
assurance".
Having established communications with potential conflicting
traffic, the flight crews should then arrange to have appropriate
vertical or lateral separation applied between their
aircraft.
Conclusion
The greater use of larger and faster aircraft for RPT flights in
non-controlled airspace increases the need for the adoption and use
of "separation assurance" techniques in conjunction with "alerted
see-and-avoid" procedures by all flight crews.
Although, the inclusion of "separation assurance" techniques in
airline flight operations manuals would address the deficiency to
some extent, procedures for other IFR and the majority of Visual
Flight Rules (VFR) flights are also essential. Many pilots may
require guidance and training on "separation assurance"
techniques.
"Alerted see-and-avoid" procedures used in conjunction with
"separation assurance" techniques provide a fail-safe method of
self-separation which enhances safety.
