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.
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.
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.
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.
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.
Aero towing operations from runway 26 resulted in a flight path through an area where there was at times a concentration of aircraft. On this occasion, the presence of at least two gliders either thermalling or about to join the circuit pattern for a landing, created an area of potential conflict with the towing combination. Although each aircraft was radio-equipped, the required traffic broadcasts did not provide their pilots with an awareness of each other's presence. Effective use of radio communication could have alerted them to a possible conflict. Instead of relying solely on visual acquisition of the other aircraft for collision avoidance, each pilot would have had an awareness of the proximity of the other aircraft and used that information to mutually resolve the conflict and aid visual separation.
The towing combination should have been readily visible to the pilot of the Astir CS at a distance far enough to allow adequate time for him to take avoiding action. However, the Astir CS was not seen to deviate from its flight path until just before colliding with the towline, suggesting that its pilot had not seen the towing combination until too late to avoid a collision. On more than one occasion during his training the pilot of the Astir CS had been advised of the need to keep a continuous lookout by his instructor. Despite his training the pilot of the Astir CS did not see the towing combination.
The pilot of the Pawnee was responsible for traffic lookout and avoidance during the towing phase of the flight. It was possible that she had seen the other glider in the area but not the Astir CS. It had approached the towing combination from behind and would not have been in the Pawnee pilot's normal traffic scan. The unaddressed warning broadcast by the pilot of the Twin Astir may have conveyed a sense of danger to the pilot of the Pawnee. However, in the few seconds available to her it was unlikely that she would have been able to understand the significance of the warning and take appropriate avoiding action.
The pilot of the Twin Astir found himself in a situation for which there was no emergency procedure. His expectation that the pilot of the Astir CS would give way to the towing combination was not unrealistic. Confronted with an unexpected and rapidly developing dangerous situation, and without time to formulate any course of action, he reacted instinctively and broadcast an unaddressed warning. There was no obvious response from either pilot to this warning. The Twin Astir pilot, in releasing the towline when a collision between the Astir CS and the towline was unavoidable, saved his glider from also coming into collision with the other aircraft.
Aero towing operations from Waikerie were conducted through areas where gliding activity resulted in traffic conflicts that relied solely on visual acquisition for separation.
Inadequate use of the CTAF by pilots for traffic alerting.
The collision warning broadcast by the pilot of the Twin Astir was not effective.
Traffic lookout by the pilot of the Astir CS was not effective.
Investigations into five fatal collisions between gliders and aero towing aircraft from 1986 cited the presence of unalerted traffic in a see-and-avoid environment as a contributing factor. The investigation into another two fatal mid air collisions involving gliders cited the lack of alerted traffic information as a contributing factor. In nearly all of these occurrences the aircraft involved were radio-equipped and the pilots of the gliders either did not broadcast their intentions or did not respond to the other pilots' transmissions. Following the accident investigation of Occurrence 199700049 the Bureau issued recommendation R19970092 to the Gliding Federation of Australia on 8 August 1997 that related in part to traffic alerting procedures and high-visibility marking of gliders. The response received from the GFA dated 8 September 1997 stated in part,
"The GFA recognizes that the rate of mid-air collisions involving gliders is unacceptably high in the circuit areas of aerodromes and will implement measures to improve discipline in flying the pattern and making better use of the radio".
The Bureau classified the response as CLOSED - PARTIALLY ACCEPTED.
Information in the 1991 research report "Limitations of the See-and-Avoid Principle" published by the Bureau of Air Safety Investigation and other research into the difficulties associated with visual acquisition of aircraft in an airborne environment, suggested that reliance on the see-and-avoid principle to provide reliable safe separation from other aircraft was inadequate without additional traffic alerting information. Repeated emphasis to pilots to be more vigilant and maintain an effective lookout has not reduced the incidence of mid air collisions in gliding activity. Implementing additional defences into a system that recognises and takes into consideration these limitations, such as more effective use of radio for traffic information, conspicuity and traffic segregation can improve the level of safety.
With the conclusion of the investigation into this occurrence the following recommendations are made:
The Australian Transport Safety Bureau recommends that the Gliding Federation of Australia in conjunction with its member clubs incorporate the use of radio for effective traffic alerting into standard operating practices as a matter of priority.
The Australian Transport Safety Bureau recommends that Civil Aviation Safety Authority review the assessment process for the issue of a radiotelephone operator certificate of proficiency or equivalent, as specified by Civil Aviation Regulations subregulation 83A(2) and subregulation 83E(1)(a) and establish competency standards for those applicants for whom English is a second language, especially in respect of a candidate's ability to effectively communicate and comprehend traffic information.
The Australian Transport Safety Bureau recommends that the Gliding Federation of Australia adopt measures to make all aircraft engaged in gliding activities more conspicuous.
The Australian Transport Safety Bureau recommends that the Gliding Federation of Australia consider developing procedures that permit segregation of aero towing and gliding activity
|Date:||02 March 1999||Investigation status:||Completed|
|Time:||1457 hours CSuT|
|Location:||3 km S Waikerie, Aero.|
|State:||South Australia||Occurrence type:||Airborne collision|
|Release date:||27 September 2000||Occurrence class:||Airspace|
|Report status:||Final||Occurrence category:||Accident|
|Highest injury level:||Fatal|
Aircraft 1 details
|Aircraft manufacturer||Piper Aircraft Corp|
|Type of operation||Private|
|Damage to aircraft||Destroyed|
|Departure point||Waikerie, SA|
|Departure time||1454 hours CSuT|
|Role||Class of licence||Hours on type||Hours total|
Aircraft 2 details
|Aircraft manufacturer||Grob - Burkhart Flugzeugbau|
|Type of operation||Gliding|
|Damage to aircraft||Destroyed|
|Departure point||Waikerie, SA|
|Departure time||0140 hours CSuT|
|Role||Class of licence||Hours on type||Hours total|