Sequence of events
A Grob Twin Astir two-seat glider, was being towed aloft by a
Piper Pawnee aircraft for a passenger flight from Waikerie SA. The
pilot was seated in the rear of the tandem cockpit. Several other
gliders were airborne in the area, including a single-seat Grob
Astir CS that had been launched from Waikerie just over an hour
earlier for a local flight.
The pilot of the Twin Astir reported that the flight had been
normal following the take-off from runway 26. The tow combination
had turned left shortly after becoming airborne and continued
climbing in a wide arc. Three km south of the airfield, as the
towing combination was climbing through approximately 1,500 ft on a
northwesterly heading, he saw a single-seat Astir CS glider near
his right wingtip. He estimated that it was 8 to 10 metres away, on
a converging course with the towing combination, and travelling at
a slightly faster speed. He expected the pilot of the Astir CS to
turn right, away from the towing combination. When this did not
occur he broadcast a warning "Break right, break right" on the
common traffic advisory frequency (CTAF) in an attempt to warn the
other pilot. However, when he saw the Astir CS commence a gentle
turn left, and a collision seemed imminent, he released the
towline. He estimated the time from when he first saw the Astir CS
until towline release was 4 to 5 seconds.
The passenger in the front seat of the Twin Astir recalled
seeing another glider out to the left and above the towing
combination shortly before the collision occurred. It was flying in
the opposite direction but not close enough to cause him concern
about a risk of collision. His attention was then drawn to the
glider off the right wingtip when he heard the pilot broadcast the
radio warning.
The Astir CS struck the towline approximately midway between the
Pawnee and the Twin Astir, catapulting it into collision with the
Pawnee. The collision dislodged the tail of the glider and damaged
the left wing of the Pawnee. Witnesses at the airfield, who heard
the sound of the collision, saw the Astir CS descend almost
vertically. They described the Pawnee's descent as a spin or
spiral, completing at least one and a half rotations to the left
before it disappeared from view. The pilot of the Twin Astir took
avoiding action and returned safely to Waikerie. The wreckage of
the Pawnee and the Astir CS were found in a field about 3 km south
of the airfield. Neither occupant of the two aircraft survived the
collision and subsequent ground impact. The investigation did not
find any pre existing defect with either aircraft that could have
contributed to the collision.
Witnesses reported the weather was clear apart from isolated
patches of rain to the north of the aerodrome. Visibility outside
the rain areas was unlimited. Sun glare was reported not to have
been a problem at that time of the day because of the high sun
angle. Turbulence was light in weak thermal convection.
Pilot experience and background
The pilot of the Astir CS was a Japanese national who had
arrived in Waikerie a week earlier to further his gliding
experience and qualifications. He had received his initial training
to solo standard in Japan. In the week before the accident he had
received further training at Waikerie during which he had made 14
instructional and familiarisation flights. His instructor reported
that he had found it necessary to emphasise the need for the pilot
to keep a continuous traffic lookout during those flights. The
pilot of the Astir CS was issued with a Gliding Federation of
Australia (GFA) validation and allowed to make solo flights. He
had, as part of his training, undertaken a briefing and oral test
required for the use of glider radio equipment on the CTAF. His
examiner for the test was also a Japanese national employed as a
gliding instructor by the gliding organisation. The instructor
reported that the accident pilot's understanding of the English
language would probably not have been sufficient to understand the
significance of the unaddressed "Break right, break right" warning
broadcast by the pilot of the Twin Astir. The pilot of the Astir CS
was reported to have been well rested prior to the day's flying
operations. Earlier that day he had completed a short flight in the
same aircraft, returning due to the lack of thermal currents. A
damaged pair of sunglasses was found in the wreckage of the Astir
CS but it could not be determined if they were being worn at the
time of the collision.
The pilot of the Pawnee held a Commercial pilot licence and
valid Class 1 medical certificate. She had obtained her basic
glider-towing permit on 27 November 1998 and had conducted aero
towing at Waikerie since, accumulating some 117 hours in aero
towing operations. As a prerequisite for aero towing, prospective
tug pilots were required to have undertaken gliding training to
solo standard. She had completed this training, which included spin
recovery techniques. The instructor who carried out that spin
training reported she was competent and had displayed sound
recovery techniques. She had previously qualified and been
certified for solo aerobatic manoeuvres, including spin recoveries,
in powered aircraft. On the day of the accident she had completed 8
towing flights before the accident flight.
The pilot of the Twin Astir was appropriately qualified for
passenger-carrying flights in gliders. Additionally, he held a
Private Pilot's Licence and a valid Class 2 Medical Certificate. He
was also qualified to conduct aero towing and had flown an aero tow
launch earlier that day.
The investigation did not find any pre-existing medical
condition that could have impaired the performance of the three
pilots involved in the occurrence.
Radio requirements
The gliding organisation relied extensively on the patronage of
overseas pilots. Many of those pilots were from non-English
speaking backgrounds. Their English language skills were assessed
as part of the requirement for the issue of a Radiotelephone
Operators Authorisation. This was a requirement for the use of
glider radios on the CTAF. Issue of an authorisation was dependent
on a satisfactory oral examination on radio use and knowledge of
procedures. This exam was conducted by a GFA Level 1 instructor who
held either a radio licence or authorisation.
The CTAF area at Waikerie operated on a frequency of 126.7 MHz
and extended from the surface to 3,000 ft above the aerodrome, and
to a radius of 5 NM. Within the CTAF area pilots had to use the
designated frequency, however outside the CTAF boundary glider
pilots could use one of three allocated gliding frequencies. The
GFA Manual of Standard Procedures (MOSP) required pilots of
radio-equipped gliders to broadcast their intentions before
entering the CTAF area or when about to take-off. It also stated
that "Radio-equipped gliders must also respond to calls when
appropriate when operating within a CTAF area". Other radio calls
could be made at the pilot's discretion. The gliding organisation
had a local rule that required an additional broadcast to be made
on downwind.
Each of the aircraft involved in this occurrence was equipped
with a serviceable VHF radio, however impact damage to the radio in
the Astir CS made it impossible to determine the frequency selected
at the time of the accident. The pilot of the Twin Astir reported
that the pilot of the Pawnee had broadcast she was taxiing for aero
towing operations from runway 26 before the launch. He did not
recall hearing any response from other traffic.
Procedures
The local topography around Waikerie produced favourable soaring
conditions to the south of the field. This characteristic was known
to both tow pilots and glider pilots and was often exploited by
them for gaining altitude. However, this area encroached on the
designated left traffic pattern from runway 26 and at times
resulted in aero towing aircraft and gliders sharing the same
airspace. There was no segregation of gliding and aero towing
activity. Aeronautical Information Publication procedures permitted
gliders to use thermal lift on the "live" side of the circuit. This
was provided that glider pilots monitored the CTAF and maintained
adequate separation from other traffic in the circuit area. The
local gliding operator discouraged the practice below 1,000 ft
above ground level. The GFA Rules of the Air required pilots of
gliders to maintain separation from other gliders and from towing
combinations by at least 200 feet horizontally and vertically.
Vision
Although the see-and-avoid technique was the primary means of
collision avoidance for aircraft operating under visual flight
rules (VFR) there were limitations to its effectiveness, even for
relatively slow aircraft such as gliders and light aircraft. The
slim frontal profile of gliders made them more difficult to see
than most powered aircraft. In 1991 the then Bureau of Air Safety
Investigation published a research report "Limitations of the
See-and-Avoid Principle" which documented the physiological and
psychological limitations associated with seeing other aircraft.
The report recommended that pilots should be aware of those
limitations and not rely entirely upon vision to avoid collisions.
In recommending the use of alerted see-and-avoid the report
concluded that visual acquisition for collision avoidance should be
the last defence in achieving separation.
Chapter 4 of the GFA publication Basic Gliding Knowledge (Fourth
Edition) titled "The Development of Effective Lookout" described in
detail the visual detection of other aircraft and the limitations
of human vision in the airborne environment. It also stressed the
need for pilots to ensure there was no other traffic in the
airspace before manoeuvring and to avoid allowing one's attention
to dwell in the cockpit. It did not mention the use of radio to
assist in the detection of other traffic. The pilot of the Astir CS
should have been familiar with scanning techniques and traffic
lookout taught during his initial training. His subsequent training
at Waikerie emphasised the need to continuously maintain a lookout.
The Astir CS was fitted with a single-piece canopy that provided
the pilot with an unobstructed forward and upward view. Visibility
from the cockpit of the Astir CS was better than that from other
aircraft whose canopies incorporated a windshield bow.
Survival
The pilot of the Astir CS had been wearing an emergency
"slim-pack" type parachute. The gliding organisation required
pilots of all single-seat gliders to wear a parachute and to be
briefed on the emergency escape procedure. The investigation was
unable to determine if the pilot was capable of performing the
escape functions to parachute to safety following the collision
with the Pawnee. The pilot of the Pawnee was unable to regain
control of the damaged aircraft following the collision.
Information obtained during the investigation suggested that damage
to the Pawnee wing structure would probably have made the aircraft
uncontrollable. The pilot of the Pawnee was not equipped with a
parachute nor was she required to wear one.
