The plan for the flight was for the tug aircraft to air-launch
the glider and then to fly to Tipton to retrieve another glider.
The glider pilot indicated to the tug pilot that he wanted to be
towed to a thermal to the north west of the strip. Flying
conditions in the area were reported to have been moderately
turbulent.
Radio communications for the initial tow-rope hook-up were normal,
with the tug pilot responding normally to the glider pilot's
instructions. After becoming airborne, the tug pilot made a right
turn through about 40°, onto a heading of 340°, and continued the
climb. When the aircraft were about 1,200 ft AGL they encountered a
strong thermal. After both aircraft had stabilised in the thermal,
the glider pilot noted that the variometer was indicating lift in
excess of 10 kt. He then disconnected the tow and transmitted 'rope
gone' over the radio while commencing a right turn. He observed the
tug aircraft begin a gentle turn to the left.
The glider pilot completed a turn through 360° and decided to fly
another orbit, but at a greater angle of bank to better utilise the
thermal. When heading approximately south-west at about 1600 ft,
the glider suddenly yawed violently to the right and the control
column was torn from the pilot's hand. At the same time, the glider
adopted a steep nose down altitude. The pilot was able to regain
control of the glider at a height of about 11 00 ft above ground
level and saw the tug aircraft below spiralling anti-clockwise
towards the ground.
The pilot was able to control the glider by the use of left rudder
and almost full right aileron. A right circuit was flown and the
glider landed on the departure strip.
A ground witness observed the release of the tow but did not
observe the mid-air collision. He and another ground witness
observed the tug aircraft spiral and dive vertically into the
ground. The tug aircraft impacted the ground at an indicated
airspeed of 116 kts and was destroyed.
Damage to the glider was restricted to its left wing. The
inboard half of the left aileron had been broken off in a downward
motion. The trailing edge of the left wing was split for most of
its length, and a small dent was present on the trailing edge about
500 mm from the wing root.
Examination of the impact marks on both aircraft indicated that
the left wing of the tug had collided with the trailing edge of the
left wing of the glider when it was banked right at about 40°. The
strut attachment area on the front spar of the tug's wing had
impacted the glider about 300 mm inboard of the aileron while the
tug was moving outboard relative to the glider. The leading edge of
the tug wing had then broken off part of the glider aileron and
become snagged on the wing at the aileron cutout. This tore the
wing fabric, destroying the aerofoil shape of the outboard section
of the left wing. The top of the tug's left wing was then dragged
inboard and rearwards across the lower surface of the glider's left
wing. The top left corner of the cockpit roof of the tug probably
made light contact with the trailing edge of the left wing of the
glider at a point about 500 mm from the wing root. This indicated
that the angle of bank of the tug was about 23° greater than that
of the glider at the time of collision.
Analysis of the possible flight paths from glider release to the
impact point indicated that the tug pilot might not have heard the
glider pilot call 'rope gone' and could have been still trying to
maintain the glider in the thermal, pending tow release by the
glider pilot. It could not be determined why the tug remained in
the thermal. It is possible that the tug pilot was aware that the
glider pilot had released and was using the strong thermal updrafts
in the manner of a glider. Other tug pilots report that it is
difficult to determine if the glider is still under tow in
turbulent conditions. Thus it is possible that the tug pilot was
unaware that the glider pilot had released.
The glider pilot reported that there was a significant amount of
radio traffic on the local airstrip frequency at the time he
released the tow. This was caused by the number of gliders in the
air in both the local and Kingaroy areas. Gliding organisations use
one radio frequency for operations at the airstrip and another for
area operations. However, it was reported that the airstrip
frequency was used almost exclusively while the area frequency
received little use. In this instance, the frequency for airstrip
operations was also the Kingaroy common traffic advisory frequency
(CTAF).
Significant factors
The following factors were considered relevant to the development
of the accident:
- The tug remained in the thermal for reasons that could not be
determined. - The tug pilot did not see the glider in time to avoid a
collision. - The tug pilot was unable to regain control of his aircraft due
to the damaged wing.
Safety Action
Recommendation
The use of radios and control of glider operations have been the
subject of investigation in three recent, fatal, mid-air collisions
between gliders and fixed wing aircraft:
- Tocumwal NSW 2 November 1990 VH-CAG/GXO
- Tocumwal NSW 8 February 1992 VH-UTKlGOR, and
- This accident, Jondaryan Old 1 November 1992 VH-SCT/WGR.
The Bureau of Air Safety Investigation therefore recommends
that:
- The Civil Aviation Authority, in conjunction with the Gliding
Federation of Australia, develop and implement national standard
operating procedures for radio use in glider operations.