SUBJECT - COLLISION AVOIDANCE PROCEDURES AT NON-CONTROLLED
AERODROMES
SAFETY DEFICIENCY
The current published procedures that place reliance on only two
radio broadcasts to facilitate alerted see-and-avoid between
aircraft at non-controlled aerodromes are inadequate for collision
avoidance.
FACTUAL INFORMATION
Related occurrences
Occurrence 9900970
The pilot of a Beechcraft Sundowner (Beechcraft) was taxiing at
Hoxton Park to begin his first solo navigation training exercise.
The pilot of a Pitts Special (Pitts) had earlier flown from
Bankstown to the training area and then to Hoxton Park, in order to
practice circuits and landings at Hoxton Park before returning to
Bankstown. The pilot of the Pitts reported that he transmitted a
broadcast on 127.0 MHz as he entered the circuit at Hoxton Park,
before reducing speed in order to maintain separation with a
preceding aircraft in the circuit.
After that aircraft had landed, the pilot of the Beechcraft taxied
onto the threshold of runway 34 and commenced to take off. In the
meantime, the Pitts was approaching the same runway. The pilot of
the Pitts reported that he had momentarily lowered the nose of the
aircraft on a number of occasions during the approach, in order to
check that the runway was clear before committing himself to a
landing. The pilot reported that he did not see any aircraft on the
runway or on the taxiway adjacent to the runway.
As the Beechcraft was accelerating during its take-off roll, the
Pitts landed on top of it. Both aircraft became entangled and
travelled approximately 100 m down the runway, before turning
sharply left and stopping. The pilot of the Beechcraft received
fatal injuries and both aircraft sustained substantial
damage.
An onsite investigation found that the radios of both aircraft were
tuned to the Hoxton Park Common Traffic Advisory Frequency (CTAF)
frequency (127.0 MHz) at the time of the accident. Each radio was
checked and found to be capable of normal operation. The
investigation team subsequently conducted a reconstruction of the
sequence of events leading up to the accident. That reconstruction
revealed that the Beechcraft would have been very difficult to
discern if it had been stopped on the threshold "piano keys" at the
time the pilot of the Pitts was scanning the runway.
Occurrence 9802022
A Piper Archer (Archer) collided with a Piper Tomahawk (Tomahawk)
at an altitude of about 1,200 ft as the Archer was tracking to
enter the crosswind leg for a landing on runway 34 at Hoxton Park
aerodrome. The collision occurred in fine and clear conditions,
about 1 NM to the east of the upwind end of the runway. Both
aircraft were being flown under the visual flight rules. The pilot
of the Archer was able to maintain control of his aircraft and make
a successful approach and landing on runway 34, although the nose
landing gear had been substantially damaged in the collision. The
aircraft stopped on the runway, resting on the collapsed nose
landing gear.
The Tomahawk was observed by witnesses to immediately spiral down
and crash into an unoccupied house in a suburban housing area. Both
occupants received fatal injuries. There were no injuries to
personnel on the ground.
The pilot of the Archer reported that he had tuned to the Hoxton
Park CTAF frequency at Bonnyrigg, 2 NM north-east of Hoxton Park,
and made an inbound broadcast. He reported that he made a further
broadcast as he was joining crosswind for runway 34. Just prior to
the collision, he had observed an aircraft turning onto crosswind
after departure from runway 34 and another aircraft on downwind.
The investigation was unable to determine the flightpath of the
Tomahawk nor was it able to determine if the pilot of the Tomahawk
had made the required CTAF broadcasts, as there was no automatic
recording of that frequency.
Common Traffic Advisory Frequency (CTAF) procedures
CTAFs are normally located outside controlled airspace and within 5
NM radius of a designated aerodrome up to and including 3,000 ft
above ground level (AGL), unless otherwise specified. Because of
its proximity to Bankstown aerodrome, and the lower limit of
controlled airspace above the aerodrome (2,000 ft), the Hoxton Park
CTAF is limited to an area of 2 NM radius up to and including 1,700
ft AGL. Pilots of aircraft operating within that airspace are
required to comply with CTAF radio communications procedures
published in Aeronautical Information Publication (AIP) ENR
3.1.
The carriage and use of radio at CTAF aerodromes is not mandatory,
but pilots of radio equipped aircraft are required to make
broadcasts on the CTAF. Pilots are only required to make one
broadcast when operating into a CTAF aerodrome (inbound), and
similarly only one broadcast when departing from a CTAF aerodrome
(taxiing). Due to the airspace constraints mentioned above, pilots
adhering to the minimum requirements of the published CTAF
procedures at Hoxton Park, are entering the circuit at the same
time that they make their inbound broadcast.
Adherence to CTAF procedures varies considerably from one aerodrome
to another, with some pilots limiting their radio broadcasts to the
minimum required whilst others appear to make a number of
broadcasts, announcing their location at a variety of key locations
within the CTAF. In addition, the degree to which pilots use the
radio to arrange self-separation in order to minimise the risk of
collision also varies between aerodromes and pilots. There have
been instances where pilots have been unable to establish the
position of other aircraft in CTAFs due to a significant amount of
radio traffic between other pilots attempting to arrange
self-separation.
Bankstown General Aviation Airspace Procedures
Bankstown aerodrome is located approximately 7 NM to the east of
Hoxton Park. The procedures to be followed at Bankstown aerodrome
are described in Enroute Supplement Australia. Pilots of aircraft
departing Bankstown who intend to remain outside controlled
airspace, are to change frequency when 5 NM from Bankstown. Many
pilots track to Hoxton Park from Bankstown for circuit and landing
practice. If the procedures described above were strictly followed,
pilots would be on the boundary of the Hoxton Park circuit
area/CTAF before they changed to the CTAF.
Some aircraft are equipped with dual radio systems which allow
pilots to maintain a listening watch on the Hoxton Park CTAF while
tuned to and operating on the Bankstown frequency; however, many
light general aviation aircraft are not so equipped. This means
that many pilots operating into the Hoxton Park CTAF may not have
any prior knowledge of traffic within the CTAF until they change to
that frequency to make an inbound broadcast. This also occurs to a
lesser degree with pilots tracking to Hoxton Park from other
locations, who have been operating on the area frequency. However,
in that circumstance there is greater latitude for pilots to change
to the Hoxton Park CTAF prior to reaching its boundary.
Limitations of unalerted see-and-avoid procedures
A previous BASI report on "The Limitations of See and Avoid" (1991)
stated the following:
"A traffic search in the absence of traffic information is less
likely to be successful than a search where traffic information has
been provided because knowing where to look greatly increases the
chance of sighting the traffic (Edwards and Harris 1972).
Field trials conducted by John Andrews found that in the absence of
a traffic alert, the probability of sighting a threat aircraft is
generally low until a short time before impact. Traffic alerts were
found to increase search effectiveness by a factor of eight. A
traffic alert from ATS or from a radio listening watch is likely to
be similarly effective (Andrews 1977, Andrews 1984, Andrews
1987)".
One of the recommendations of that report was that "pilots should
recognise that they cannot rely entirely on vision to avoid
collisions. Consequently, they should attempt to obtain all
available traffic information, whether from Air Traffic Services or
a listening watch, to enable them to conduct a directed traffic
search".
The Civil Aviation Safety Authority (CASA) proposed changes to
mandatory radio requirements for Class G terminal airspace
A summary of responses to a previous Notice of Proposed Rule Making
(NPRM) 9702RP was issued in January 1999. Included in that document
were a series of proposed changes to current radio requirements for
aircraft operating into aerodromes in Class G airspace. This
included both CTAF and Mandatory Broadcast Zone (MBZ) aerodromes.
In that document, CASA advised that apart from the mandatory
carriage and use of radio in MBZs, "procedures used in MBZs and
CTAFs are otherwise identical. Compliance with procedures is
generally believed to be higher in MBZs than in CTAFs, although
there is evidence to suggest that compliance rates are related to
traffic densities rather than the designation of the airfield". The
proposed amendments to the radio requirements will apply to any
licensed aerodrome and include a "more comprehensive list of
recommended calls which should be made when safety will be
enhanced". For a circuit entry approach, for example, up to seven
broadcasts at various locations have been recommended. The NPRM
recognises that the current list of broadcast requirements listed
in the AIP is not all-inclusive and that the use of a more
comprehensive list of recommended broadcasts would reduce
unnecessary dialogue between individual aircraft.
ANALYSIS
Related occurrences
In occurrence 990970, it appears likely that the pilot of the
Beechcraft was preoccupied with his preparations for departure for
his first solo navigation exercise. He may have broadcast his taxi
report before the Pitts pilot first tuned to the Hoxton Park CTAF.
Subsequently, the Beechcraft pilot may not have heard the "joining
crosswind" broadcast made by the Pitts pilot, or he may have heard
the broadcast but did not respond. It is also possible that he
believed that an aircraft that had landed (before the Pitts) was
the same aircraft that had reported on crosswind, and as such,
considered that the circuit was clear for him to take off.
Alternatively, the Beechcraft pilot may not have been tuned to the
Hoxton Park CTAF until after the Pitts pilot had made his crosswind
broadcast. In either case, by missing one critical broadcast, both
pilots were totally reliant on visual scanning in order to
determine each other's location. Without some prior indication of
the presence and location of aircraft in the circuit, the chances
of sighting each other were reduced. Had the pilot of the
Beechcraft broadcast that he was entering the runway for departure,
it is likely that the Pitts pilot would have been alerted to the
conflict and taken appropriate action.
Whilst the investigation was not able to determine what, if any,
CTAF broadcasts were made by the pilot of the Tomahawk in
occurrence 9802022, it is again possible that one of the pilots was
not tuned to the CTAF when the other pilot made a vital broadcast.
Even though the Archer pilot made an inbound broadcast at 2 NM from
the airfield, the Tomahawk pilot may have still been tuned to the
Bankstown frequency or the area frequency and not have been aware
of the position of the Archer. It is also possible that the pilot
of the Tomahawk had broadcast an inbound call before the Archer
pilot changed to the CTAF.
In either case, it appears that neither pilot had knowledge of the
other's presence until it was too late to avoid a collision. Had
both pilots been tuned to and/or maintaining a listening watch on
the Hoxton Park CTAF earlier in the sequence of events, the
collision may have been avoided. In addition, it may have been
prudent for both inbound aircraft to have remained at 1,700 ft
until overhead the aerodrome in order to gain an appreciation of
the circuit traffic before descending to circuit height.
CTAF procedures
Despite the correct use of prescribed procedures and the
application of sound airmanship by pilots, reliance on a single
radio broadcast for arrival and departure of aircraft at
non-controlled aerodromes appears to be inadequate for collision
avoidance. If one of these critical broadcasts is not heard, or the
significance of that broadcast is not appreciated, unalerted visual
scanning is the only defence available to a pilot to avoid a
potential collision. Effectively, a critical safety defence has
been removed, with a subsequent reduction in the level of
safety.
Whilst the proposed changes to mandatory radio requirements are
commendable and represent a potentially significant improvement in
the level of safety at non-controlled aerodromes, the listed
broadcasts have been referred to as "recommended", rather than
mandatory. Such advice will allow for significant differences,
which already exist in the level and type of radio usage at those
aerodromes. The potential exists for no change at all in the
current level and type of radio usage, and thus no change in the
level of safety.
The Bureau agrees that allowing pilots to use the radio for
self-separation as the primary means of alerted see and avoid can
lead to significant congestion of the radio frequency with the
potential for critical broadcasts to be missed. Similarly, at busy
aerodromes in particular, requiring pilots to make up to seven
radio broadcasts may create the same congestion. Some
rationalisation of this proposal, with the emphasis on mandatory
radio broadcasts for identified critical locations for departures
and arrivals, would be more appropriate. Other broadcasts could be
discretionary and only used by pilots if they considered them
necessary in the circumstances.
The Bureau also believes that the proposed mandatory radio
requirements should encompass both licensed aerodromes and any
unlicensed aerodrome into which fare-paying passenger services
operate. Such action would achieve a greater consistency in the use
of radio and would provide further opportunities for pilots to be
alerted to potential conflict situations.
Hoxton Park CTAF
The Bureau does not believe that recommending Hoxton Park be
upgraded to an MBZ would provide any significant safety benefit.
Evidence suggests that the carriage and use of radio by aircraft
operating at Hoxton Park is not dissimilar to aircraft operating at
MBZs. In addition, the airspace constraints of Hoxton Park would
not allow for any significant expansion of the dimensions of the
CTAF.
One area of concern at Hoxton Park is the limited opportunity for
pilots to change to the Hoxton Park CTAF after leaving adjacent
airspace, in particular, Bankstown. If pilots made an inbound
broadcast to Hoxton Park at the same time they passed the Bankstown
control zone boundary they would have, at best, 3 NM before they
reached the boundary of the Hoxton Park CTAF. As the Bankstown
procedures advise pilots to change frequency 3 NM from the control
zone boundary, it is likely that some pilots departing Bankstown
for Hoxton Park would not change to the Hoxton Park frequency and
make an inbound broadcast, until they were at or near the boundary
of the CTAF. Even if a listening watch were maintained from the
time the pilot departed the Bankstown zone, a limited opportunity
would be afforded to the pilot to gain a proper appreciation of the
circuit traffic at Hoxton Park.
