On 10 December 2005, at about 0953 ¹, a Piper Aircraft Corporation PA-28-161 Warrior, registered VH-UMB (UMB), with an instructor and student pilot on board, was being operated under the visual flight rules on circuit training at Coldstream Aerodrome, Vic. Another Piper Aircraft Corporation PA-28-161 Warrior, registered VH-BZA (BZA), also with an instructor and student pilot on board and being operated under the visual flight rules, was transiting from the local training area to Lilydale Aerodrome, when the two aircraft collided about 2 km north-east of Coldstream Aerodrome (Figure 1). Following the collision, the instructor pilots landed their aircraft safely and none of the pilots were injured.

Figure 1:     Relative position of VH-UMB and VH-BZA ²

The instructor on board UMB reported that he was instructing the student on his first session of circuit training. The circuit on which the collision occurred was the first of a series planned for the day. The instructor recalled making a broadcast when taxiing and prior to entering the runway. No recording was available of the Common Traffic Advisory Frequency (CTAF), nor was it required to be.

The instructor reported that as the aircraft proceeded on the downwind leg of the circuit, he was concentrating on assisting the student to maintain the correct spacing and orientation in the circuit and that the aircraft was 'a few hundred feet above circuit height'.

The circuit area at Coldstream Aerodrome did not conform to the typical aerodrome circuit pattern in shape or size. Due to the proximity of powerlines located a short distance to the south of the aerodrome, aircraft taking off from runway 17 were required to make an early left turn and the downwind leg was displaced further east than in a 'normal' circuit in order to maintain the aircraft clear of a noise sensitive area (Figure 1).

The student pilot of UMB climbed above the standard circuit height of 1,500 ft above mean sea level (AMSL) that was nominated in the Coldstream Aerodrome local instructions. The instructor of UMB was not certain of the maximum altitude reached. He reported that he did not see the other aircraft at any stage, nor did he recall hearing any inbound radio broadcast.

After the collision, the instructor on board UMB took over control of the aircraft from the student and after some difficulty the aircraft began to respond to his flight control inputs. He reported that he had initially considered an off-airport landing; however, after some degree of control was regained, he felt confident enough to proceed with a landing at Coldstream Aerodrome. The landing was conducted without extending flap. The right wing leading edge and the vertical stabiliser of UMB sustained substantial damage in the collision (Figure 2).

Figure 2: VH-UMB showing significant damage to right wing leading edge

The instructor on board BZA reported that they were returning to Lilydale Aerodrome after conducting a lesson in the local training area. The student was manipulating the flight controls at the time. The instructor reported that they overflew the Coldstream Aerodrome circuit area above 2,000 ft AMSL, which was specified as the overfly altitude in the local instructions for Coldstream Aerodrome.

Both the instructor and student on board BZA reported that, in accordance with flight school procedures, a radio broadcast of 'Lilydale traffic, Warrior BZA is approximately 5 NM east, 2,500 inbound Lilydale' was made. It was reported that this was the first opportunity to make an inbound broadcast as the training was being conducted within 10 NM from Lilydale (at about 5 NM). The instructor reported that he did not hear any broadcasts from potential conflicting traffic.

The two aerodromes were located approximately 3 NM apart, with Coldstream to the south-east of Lilydale (Figure 3). Both had a CTAF of 119.1 Mhz. A CTAF is a radio frequency on which pilots make positional broadcasts when operating in the vicinity of a non-towered aerodrome. The Lilydale Aerodrome was equipped with an Aerodrome Frequency Response Unit ³ (AFRU) or 'beep back'.

Figure 3: Location of Lilydale and Coldstream Aerodromes

Both aircraft were of the low-wing design. The student on board BZA reported seeing the other aircraft as the collision occurred. The instructor reported that he did not see the other aircraft until after the collision. The instructor took over control of the aircraft from the student and after verifying that it was responding normally to flight control inputs, manoeuvred to observe the other aircraft.

The instructor on board BZA reported that he heard a MAYDAY ⁴ broadcast from the flight crew of UMB. The instructor on board UMB reported that prior to the impact, he recalled 'hearing on the radio a couple of aircraft doing circuits at Lilydale', but did not recall hearing any inbound broadcast.

When it appeared likely that UMB was capable of completing an approach and landing at Coldstream Aerodrome, the instructor in BZA continued on to Lilydale Aerodrome and landed. BZA sustained minor damage to the left-wing tip, left aileron, engine cowl and nose gear.

Visibility at the time of the collision was not considered a factor in this occurrence with the student on board BZA stating that 'there was a bit of cloud around that day…but at 2,400 ft we could see quite clearly'.
Airservices Australia radar data indicated that as BZA overflew the Coldstream circuit area the aircraft was descending. The minimum recorded radar altitude was 2,000 ft AMSL, which was the displayed altitude at the time of the collision ⁵. Radar altitude data was not available for UMB until about one minute after the aircraft appeared to have collided, by which time the UMB was descending through 1,400 ft AMSL. The instructor on board UMB told the investigation that the aircraft transponder ⁶ was switched 'ON' for the duration of the flight, but that it had previous intermittent problems.

New procedures for operations at non-towered aerodromes were introduced as Stage 2c of the National Airspace System on 24 November 2005. Under the new procedures, defined volumes of airspace previously known as CTAF areas, were removed and replaced by new procedures. Under the new procedures, pilots of radio-equipped aircraft must continuously monitor and broadcast on the CTAF by no later than a distance of 10 NM from an aerodrome when operating in the vicinity of an aerodrome.

Although the carriage and use of radio was not mandatory at either Coldstream or Lilydale Aerodrome, the new procedures specified standard positional broadcasts including:

• by 10 NM when inbound to, or overflying an aerodrome
• just before turning onto the downwind leg of the circuit
• just before turning onto the base leg of the circuit
• just before turning onto final leg (with intentions).

1. The 24-hour clock is used in this report to describe the local time of day, Australian Eastern Daylight-savings Time A (EST), as particular events occurred. Australian Eastern Daylight-savings Time was Coordinated Universal Time (UTC) + 11 hours.
2. Positions of the aircraft in the figure represent the estimated flight paths and not a position relative to the other aircraft at any given time.
3. An automated recording which responds to broadcasts and indicates to the pilot that the radio is switched to the correct frequency and that the transmitter and receiver are working.
4. International radio broadcast for urgent assistance.
5. The accuracy of the radar displayed altitude information could not be verified.
6. Secondary Surveillance Radar (SSR) transmitter/receiver fitted to aircraft that transmits coded information when triggered by a correctly received signal.

On 10 December 2005, at about 0953, two Piper Aircraft Corp PA-28, Warrior, aircraft collided 2 km north-east of Coldstream Aerodrome.

The instructor and student on board one of the aircraft were conducting circuit training at Coldstream Aerodrome. The instructor reported that the aircraft had climbed above the nominated circuit height of 1,500 ft above mean sea level (AMSL) but was not certain of the maximum altitude their aircraft ultimately reached.

After the collision the instructor had initially considered an off airport landing; however, after some degree of control was regained, he felt confident enough to return to Coldstream Aerodrome. The right-wing leading edge and the vertical stabiliser of the aircraft sustained substantial damage in the collision.

The instructor and student onboard the other aircraft were returning to Lilydale Aerodrome after a period in the local training area. While transiting from the training area to Lilydale the aircraft overflew the Coldstream Aerodrome circuit area. After the collision, the instructor on board the aircraft took control of the aircraft and continued on to Lilydale Aerodrome located a short distance to the north-west. The aircraft sustained minor damage to the left-wing tip, left aileron, and engine cowl and nose gear.

The local instructions for aircraft operated at Coldstream Aerodrome indicated a circuit altitude of 1,500 ft AMSL and an overfly altitude of 2,000 ft AMSL. The instructor reported that they overflew the Coldstream Aerodrome circuit area above 2,000 ft AMSL, which was consistent with the unverified radar data.

Following the collision, the instructor pilots landed their aircraft safely and none of the pilots were injured.

The abnormally high collective pitch setting meant that the majority of the helicopter's forward cyclic control authority was required by the pilot to maintain level flight. The result was that, in response to the nose-up pitch, there was insufficient remaining forward cyclic control available for the pilot to recover the helicopter without also lowering the collective control. It appeared likely that the reported severe turbulence contributed to the helicopter's initial nose-up pitch.

At about 1315 hours Eastern Daylight-saving Time on 7 December 2005, a Kawasaki Heavy Industries BK 117 B-2 (BK 117 B-2) helicopter, registered VH-IME, was being operated on a medical flight at 7,000 ft above mean sea level, in moderate to severe turbulence and in visual meteorological conditions (VMC), when the helicopter sustained an uncommanded nose-up pitch of 40° to 45°. The pilot attempted to counter the nose-up pitch by applying full forward cyclic control, but without effect. The pilot then lowered the collective control, producing a nose-down pitching moment, before recovery to normal level flight could be achieved. The Mast Moment advisory light illuminate ¹ and the pilot continued the flight to the destination at reduced airspeed.

The pilot reported that, immediately prior to the nose-up pitch, the helicopter's indicated airspeed was about 120 kts, and the outside air temperature was 20° C. He indicated that the torque ² setting was about 68 % and within the helicopter's published limits at that time, and that he felt the position of the cyclic control may have been further forward than normal.

The helicopter's estimated all up weight (AUW) was 3,200 kg and its Velocity Never Exceed ³ was estimated to be 125.5 kts. The Eurocopter Deutschland GmbH ⁴ (ECD) Flight Safety Department advised that that speed should have only been possible in the BK 117 B-2 helicopter when in a descent.

The pilot reported that he flew through severe turbulence just prior to the incident, but that he felt no abnormal vibrations prior to the nose-up pitch. He also stated that, during an after-flight crew de-brief, one crew member reported that the helicopter rolled right after the nose-up pitch. The pilot could not recall that roll.

The incident was subsequently investigated by an ECD test pilot and engineer, in consultation with representatives of the helicopter's Japanese manufacturer. That investigation revealed that the published procedure for setting autorotation rotor RPM in the BK 117 B-2 maintenance manual did not include for aircraft operations at high gross weight, and that the application of that procedure in the incident helicopter had resulted in an inappropriately high collective pitch setting.

The effect of the helicopter's collective pitch stop is to limit the collective travel and, in turn, establish a maximum horizontal speed (VH ⁵) for the ambient conditions and AUW of the helicopter. The ECD Flight Safety Department advised that the helicopter's abnormally high collective pitch setting meant that the pilot would require increased forward displacement of the cyclic control in order to maintain level flight. That would result in a reduction in the longitudinal cyclic control authority ⁶ that was available to the pilot.

The ECD Flight Safety Department indicated that the severe turbulence reported by the pilot could have precipitated the helicopter's nose-up pitch in this incident, and that the reduced longitudinal cyclic control authority would have restricted the pilot's ability to recover the changing pitch. Retreating blade stall ⁷ was considered by the ECD Flight Safety Department as an unlikely contributory factor in this instance, as it was only considered possible when the helicopter was in a descent profile.

1. The illumination of the Mast Moment advisory light indicated that the rotor mast had exceeded its normal limits.
2. A measure of the power provided by the helicopter's engines to overcome the resistance of the rotor blades to rotation, and maintain constant main rotor RPM.
3. Never-exceed speed that was specified by the helicopter's manufacturer.
4. Co-designed with Kawasaki Heavy Industries. The incident helicopter was manufactured by Kawasaki Heavy Industries.
5. The maximum possible sustained airspeed in level flight at the helicopter's continuous engine power rating.
6. The forward and aft limits of travel of the cyclic control.
7. Stall of the retreating blades at high helicopter forward speeds. Occurs when the angle of the attack of the retreating blades becomes excessive, especially towards the tip of the retreating blades.

At about 1315 hours Eastern Daylight-saving Time on 7 December 2005, a Kawasaki Heavy Industries BK 117 B-2 (BK 117 B-2) helicopter, registered VH-IME, was being operated on a medical flight at 7,000 ft above mean sea level, in moderate to severe turbulence and in visual meteorological conditions (VMC), when the helicopter sustained an uncommanded nose-up pitch of 40° to 45°. The pilot attempted to counter the nose-up pitch by applying full forward cyclic control, but without effect. The pilot then lowered the collective control, producing a nose-down pitching moment, before recovery to normal level flight could be achieved. The Mast Moment advisory light illuminated, and the pilot continued the flight to the destination at reduced airspeed.

An investigation by the co-designers and manufacturer of the helicopter identified an incorrect collective pitch setting that reduced the longitudinal cyclic control authority available to the pilot. That reduced authority restricted the pilot's ability to recover the nose-up pitch.

A number of safety actions resulted from this investigation, including:

• advice to the operator from the helicopter's manufacturer to re-set the helicopter's collective pitch setting in accordance with the BK 117 C-1 model helicopter maintenance manual
• amendment of the BK 117 B-2 maintenance manual to include the relevant collective pitch setting procedure from the BK 117 C-1 manual
• the issue of Safety Recommendation R20050014, which recommended that the Civil Aviation Safety Authority (CASA) should alert Australian operators of the collective pitch setting discrepancy in BK 117 B-2 helicopters
• an interim alert was provided to Australian operators of the BK 117 B-2 helicopter by CASA to amend their operation of the BK 117 B-2 pending advice from the helicopter's manufacturer

The Boeing 747 (747) was cruising at FL320. The crew of the Boeing 767 (767) on a converging track, requested clearance to climb from FL300 to FL320. Yangon ATS cleared the 767 to climb. The 747 crew observed the 767 on TCAS at the same level as the 747 approximately three NM ahead. The Yangon ATS controller instructed the 747 crew to descend to FL310. The 747 was re-cleared to FL320 once the 767 had passed.

The occurrence was investigated by the State Administration of Ukraine for Aviation Safety Oversight. In accordance with para 5.23 of Annex 13 to the Convention on International Civil Aviation, ATSB appointed an Accredited Representative to assist the Ukrainian authority.

The absence of witnesses to the accident, and of an emergency radio broadcast from the pilot meant that there was no information available to the investigation about the pilot's situation immediately prior to the accident. However, the low-level manoeuvring carried out by the pilot overhead the sheltering sheep, and interaction via radio with the driver of the four-wheel drive vehicle was consistent with the pilot attempting to disturb the sheep from their position.

The steepness of the angle of bank and the nose-down pitch attitude at the aircraft's point of ground impact indicated that the aircraft was in a steep left turn at that time. Those indications and the minimal forward movement of the aircraft after ground contact were consistent with the aircraft having stalled and slipped out of the turn. The lack of aircraft rotation at impact indicated that there had been insufficient time for the stall to develop into a spin, consistent with it occurring at low level. It was likely that there was insufficient time for the pilot to recover before impacting the ground. Given the variety of opinion relating to the use of wing flap during mustering operations, the investigation was unable to determine the degree of influence that the lack of flap had on the development of the accident.

The pilot's probable focus on the sheltering sheep, together with the need to operate the UHF radio may have distracted the pilot from the primary task of flying the aircraft. In addition, any sensory illusion as a result of the pilot moving his head during the low-level manoeuvring, or inadvertent movement of the flight controls could have resulted in an unintentional increase in the aircraft's angle of bank. In either case, it was likely that the pilot was initially unaware that the aircraft was in such a steep turn, or that the airspeed was insufficient for the angle of bank. The stall warning probably sounded before the stall, but given warnings were often activated during aerial mustering, it may not have had a significant effect on the pilot's awareness of the impending stall.

A possible influence on the development of the stall was a decrease in available engine power. However, the damage to the propeller indicated that the engine was developing power at ground impact, there were no identified engine defects, there was adequate fuel on board and the pilot had flown for about 1.5 hours without any apparent performance degradation prior to the accident. That evidence indicated that the engine was capable of performing normally. Notwithstanding that evidence, the use of a mix of aviation gasoline and unleaded petrol, and the estimated dewpoint depression at about the time of the accident, meant that the investigation could not discount the possibility of the formation of carburettor icing. The result in that case was the possible loss of some engine power.

During the pilot's 18 years experience operating the Cessna 206, he would have become accustomed to the relatively large amount of engine power available during aerial mustering manoeuvres in that aircraft. The investigation considered whether the pilot might have unwittingly expected the same performance from the Cessna 150. However, the pilot had operated the Cessna 150 on aerial mustering operations for over 15 months, and it was concluded that the pilot would most probably have been aware of, and adjusted to that performance difference between the aircraft types during that time.

Although the pilot's flight reviews included the practice of stall recovery in turns, and the pilot had been mustering for 18 years, the lack of an aerial stock mustering permission meant that the pilot had not completed formalised training in all of the competencies inherent in the award of that permission. As a result, there was the potential that the pilot may have acquired and, over time, reinforced perhaps inappropriate responses to some of the risks inherent in the mustering environment. The completion by the pilot of the aeronautical experience requirements of Civil Aviation Order 29.10 would have provided some assurance that he had acquired the appropriate knowledge and skills necessary to manage the risks inherent in the low-level, low-speed, and high workload mustering environment.

Sequence of events¹

On 6 December 2005, the owner-pilot of a single-engine Cessna Aircraft Company 150G, registered VH-KPQ, was conducting aerial mustering operations on a family-owned station, 156 km north of Broken Hill, NSW. The pilot, who was the sole occupant, had departed a station airstrip at about 0710 Eastern Daylight-saving Time to coordinate the movement of sheep from an 18,000-acre paddock to shearing shed yards near the homestead. The paddock had medium to dense coverage of 2.5 m high scrub and was generally flat. Aerial mustering activity that day involved flying a north to south pattern progressively from the east to the west and directing ground-based mustering personnel to the location of the sheep. Those personnel were using two motorbikes and a four-wheel drive vehicle in the muster. All of the musterers were communicating with ultra-high frequency (UHF) radio.

At about 0835, the four-wheel drive vehicle was being used to move a few sheep along when the driver lost sight of them. The driver advised the pilot by UHF radio and shortly after the aircraft circled above the area a number of times at about 250 ft above ground level (AGL), but the pilot was reported to have not seen the sheep. The driver then sighted the sheep in thick scrub that the vehicle was unable to penetrate and advised the pilot of the situation. The pilot requested that the driver reverse and drive onto a nearby track. The driver asked the pilot to report when the sheep moved away from the scrub. There was no response. Shortly after the driver noticed smoke nearby and found that the aircraft had impacted the ground and there was an intense fire. The pilot was fatally injured.

The other musterers were in different areas of the paddock and did not observe the aircraft's descent and impact with the ground. No one heard an emergency radio broadcast from the pilot. One of the musterers observed the pilot's take-off from the station airstrip and heard the engine a number of times during the mustering before the accident and reported that it sounded normal.

Wreckage and site information

The aircraft wreckage was found approximately 400 m to the south-east from where the mustering vehicle was operating. The aircraft was upright with evidence of severe impact damage to the left wing, nose section and rear fuselage. There was evidence of intense fuel-fed fire damage to the cabin area and left wing. The main wreckage, approximately 8 m from the first ground impact mark, contained all the aircraft parts except for the nosewheel, which was found nearby.
The ground impact marks and damage to the left wing indicated that the initial impact with the ground was the outer leading edge of the left wing. In addition, the minimal damage to the shrubs surrounding the initial ground impact marks indicated that the aircraft impacted the ground with a steep left angle of bank between 70 and 80 degrees. The damage to the wing, and the nose impact position also indicated that the aircraft impacted in a nose down attitude with a high descent rate. There was no evidence of rotation.

Examination of the aircraft, including the flight control systems and engine, did not reveal any evidence of pre-impact defects. Damage to the propeller indicated that the engine was operating at ground impact. The wing flaps were found in the retracted position.

Pilot information

The pilot commenced flight training in 1987 and flew 18 hours in a Cessna 150 and 32 hours in a Cessna 172. The pilot purchased a Cessna 206 and completed his flight training in that aircraft. He was issued with a private pilot (Aeroplane) licence in 1988. There was no evidence of any low level or aerial stock mustering permission (the relevant regulations are outlined below).

Family members reported that the pilot used the Cessna 206 primarily for aerial mustering on pastoral properties that he owned in regional NSW. About 18 months prior to the accident, the pilot purchased the station north of Broken Hill and in August 2004, purchased the Cessna 150 primarily for mustering on that station.

The pilot's logbook was full and did not contain any entries after 1 October 2004. He had logged 36.5 hours flying the Cessna 150 over the station north of Broken Hill and a total of 2,041 hours. Although there were no flights logged by the pilot after 1 October 2004, family members reported that the pilot continued to fly the Cessna 206 and the Cessna 150 after that date.

The pilot's most recent flight review was competed on 24 April 2004 in his Cessna 206. The instructor who conducted the review reported that the pilot was competent and that the review had included steep turns and stall recovery. Steep turns were practiced at 3,000 ft AGL and between 45 and 60 degrees angle of bank. Stall recovery was practiced at 3,000 ft AGL and included recovery from a stall during a steep climbing turn with low power.

The pilot's logbook indicated that the pilot had completed stall and steep turn training in a Cessna 150 during his initial training in 1987/1988. There was no record of any stalls or steep turn training in a Cessna 150 since.

The Civil Aviation Safety Authority (CASA) issued the pilot with a Class 2 Medical Certificate on 30 June 2004 that was valid to 30 June 2006. That certificate contained a restriction requiring the pilot to wear distance vision correction and to have reading correction available during flight.

The pilot had been involved in mustering activities that included about two hours aerial mustering in the Cessna 150 per day during the previous two weeks. Two days before the accident the pilot did some aerial mustering in the Cessna 150 and then flew the Cessna 206 to a station in the Hay area. He was reported to have worked on the station until about 2030, before going to bed at 2300. The next morning the pilot awoke at 0600 and departed at 0830 for Wagga Wagga aerodrome, landing at 1000. At 1500, the pilot departed for the station north of Broken Hill via Hay and Broken Hill, arriving home at about 2000. The pilot worked in the sheep yards until returning to the homestead at 2130. Family members reported that the pilot was tired and had a sore shoulder for which he took a non-prescription painkiller. He retired to bed shortly after.

On the day of the accident, the pilot woke at about 0615 and at about 0630 flew the Cessna 206 from the airstrip near the homestead to another station airstrip where the Cessna 150 was hangered. It was reported that he was in good spirits and appeared well rested.

At the time of writing this report, the postmortem report was not available. There was no evidence of any physiological condition that may have contributed to the accident.

Aircraft information

The Cessna 150G was a two-seat, high wing aircraft equipped with a Continental O-200A engine rated at 100 HP (74.6 kW). The aircraft was fitted with a pneumatic aural stall warning system that activated 4 to 8 kts before the stall was reached. That was the only aural warning known to be fitted to the aircraft.

The pilot operated the Cessna 150 on a mixture of 10% aviation gasoline (Avgas) and 90% unleaded automotive petrol as authorised by a flight manual supplement. The flight manual supplement stated that when using automotive fuel, the onset of carburettor ice may occur earlier than with Avgas under the same atmospheric conditions. Two days before the accident, the aircraft fuel tanks were reportedly filled from clean drum stock in the hangar using a hand-pump that included an in-line filter. On the morning of the accident, the pilot was observed conducting his pre-flight checks, including of the aircraft's fuel system drains.

In addition to the aviation radios, a UHF transceiver was fitted to the aircraft for use during mustering operations. The audio output from that transceiver was wired into the aircraft's phones jack, and there was a hand microphone. The pilot was reported to use an aviation headset.

The pilot's Cessna 206 was a six-seat high wing aircraft equipped with a Continental IO-520F engine rated at 300 HP (223.7 kW).

Meteorological information

The applicable aviation area forecast was valid from 0800 and predicted isolated showers and thunderstorms with broken ² cloud at 10,000 ft. The wind at 2,000 ft was expected to be from the north-west at 20 kts. Turbulence was forecast to be moderate in the broken cloud and after 1200, light to moderate below 9,000 ft.

Automatic weather observations at 0830 from the nearest Bureau of Meteorology (BoM) site at Broken Hill recorded scattered cloud at 9,500 ft and a surface wind from the south-west at 10 kts. The temperature was 26 degrees, the dewpoint was 14 degrees and the barometric pressure was 1003 hPa. There was a report of a thunderstorm and 0.2 mm precipitation between 0730 and 0800. The BoM advised that the Broken Hill observations were representative of the weather conditions at the accident site.

One of the ground-based musterers reported that at the time of the accident, the wind was a light south-westerly and it was overcast and humid.

The 0830 Broken Hill temperature and dewpoint depression³ was plotted on a Carburettor icing - probability chart. That chart predicted moderate icing at cruise power, or serious icing at descent power in those conditions.

Current regulations

Civil Aviation Order 29.10 defines aerial stock mustering as 'the use of aircraft to locate, direct and concentrate livestock whilst flying below 500 feet above ground level'. The aeronautical experience requirements for a pilot to engage in mustering operations include that the pilot must complete 5 hours low flying training and an exam to confirm pilot proficiency, followed by 10 hours operational training. That training included:

• level, climbing and descending turns and recovery from the stall at up to 60 degrees angle of bank
• slow flying and the methods of losing height at low level
• steep climbing and descending turns away from and returning to a ground reference.

Aerial stock mustering

Aerial mustering of stock in aeroplanes such as the Cessna 150 usually involves low-level flight including steep turns at low airspeed to allow the pilot to monitor the location and movement of stock, and to guide ground-based personnel accordingly. It was reported that the pilot usually conducted aerial mustering at an estimated height of between 150 and 200 ft AGL, but if sheep found cover in dense scrub, the pilot would sometimes fly lower and apply power over the sheep to encourage them to move.

Turns are a significant risk during aerial mustering in aeroplanes due to the reduction in vertical lift component and significant increase in stall speed⁴ with bank angles over 60 degrees. To maintain height in a turn at a constant airspeed requires an increase in lift, which produces an increase in drag that necessitates an increase in engine power. Depending on the angle of bank and the conditions, maximum engine power may not be sufficient to prevent a descent.

The use of small angles of flap reduces the stall speed and lowers the nose angle for a particular airspeed. Although one low-level flying expert indicated that use of flap was advantageous during low-speed turns, opinion amongst pilots with experience in low-level operations regarding the use of flap during those operations varied.

One of the ground-based musterers reported that he heard a buzzer twice in the background of some of the pilot's UHF radio transmissions. That included while the aircraft was circling shortly before the accident. He also reported hearing the same buzzer during the pilot's previous mustering operations. An experienced aeroplane mustering pilot and instructor reported that it was common for the stall warning to activate in the steeply banked turns used during aerial mustering operations.

A number of potential sensory illusions can result in mustering pilots inadvertently applying excessive bank angles during turns. If a pilot's head is orientated into a turn (such as looking at the ground or stock) and is then quickly rotated away, the pilot's vestibular balance system can produce a sensation that the aircraft is underbanked. A visual illusion that the aircraft is skidding out of a turn can occur when turning from downwind to upwind while looking at the ground, also giving the sensation that the aircraft is underbanked.

When pilots are paying very close attention to stock by moving their head during a turn, they may tend to move the flight controls in sympathy with head-body movement. This can lead to inadvertent overbanking. Distractions during a turn, such as looking for stock or using a radio, can also increase the risk of inadvertently steepening the turn and/or allowing the airspeed to decay.

1. Only those investigation areas identified by the headings and sub headings were considered to be relevant to the circumstances of the occurrence.
2. Five to seven eighths of the sky obscured by cloud.
3. Dew point depression is calculated by subtracting the dewpoint from the ambient temperature.
4. Stall speed is the airspeed at which the stalling angle of attack (angle between effective wing chord line and relative airflow) occurs resulting in rapid decrease in lift

On 6 December 2005, the owner-pilot of a single-engine Cessna Aircraft Company 150G, registered VH-KPQ, was conducting aerial mustering operations on a family-owned station, 156 km north of Broken Hill, NSW.

At about 0835 Eastern Daylight-saving Time, the pilot was observed to circle some sheep at about 250 ft above ground level. Shortly after, ground mustering personnel noticed smoke nearby and found that the aircraft had impacted the ground and there was an intense fire. The pilot, who was the sole occupant of the aircraft, was fatally injured.

The aircraft wreckage was found approximately 400 m to the south-east from where the pilot was circling. The aircraft was upright with evidence of severe impact damage to the left wing, nose section and rear fuselage.

Examination of the aircraft, including the flight control systems and engine, did not reveal any evidence of pre-impact defects. Damage to the propeller indicated that the engine was operating at ground impact. The wing flaps were found in the retracted position.

The steepness of the angle of bank and the nose-down pitch attitude at the aircraft's point of ground impact indicated that the aircraft was in a steep left turn at that time. Those indications and the minimal forward movement of the aircraft after ground contact were consistent with the aircraft having stalled and slipped out of the turn. The lack of aircraft rotation at impact indicated that there had been insufficient time for the stall to develop into a spin, consistent with it occurring at low level.

The investigation concluded that the aircraft stalled at a height from which the pilot was unable to effect recovery.

On 17 November 2005, at 1327 Central Standard Time, a Boeing Company 717-200 (717) aircraft, registered VH-NXE, was inbound to Alice Springs, NT, on the 055 degree radial of the very high frequency omni-directional radio range (VOR) ground-based navigation aid, on a scheduled flight from Cairns, Qld. The 717 crew reported to the Alice Springs aerodrome controller that they passed almost overhead and within about 1,500 ft of a Cessna Aircraft Company C310R (C310) that was outbound from Alice Springs.

The C310, registered VH-JOI, was being operated on a pilot instrument rating renewal flight and, after becoming airborne off runway 12, was being tracked via a  GAFER ONE standard instrument departure (SID), on climb to operate in a training area located to the east of Alice Springs (Figure 1), not above 6,000 ft. A SID is a published instrument flight rules departure comprising obstacle clearance data to the minimum safe altitude and tracking data until the aircraft reaches a specified point on its air traffic control cleared route. The GAFER ONE SID provided altitude requirements and heading instructions that enabled the pilot to intercept the 055 degree radial from the VOR.

Figure 1:   Alice Springs visual terminal chart showing the 055 degree radial, the approximate track of the GAFER ONE standard instrument departure and the Eastern Training Area

The approved testing officer, who was also the pilot in command on board the C310, later reported that when the aircraft was at about 15 NM from Alice Springs, the pilot being tested turned the aircraft right, off the 055 degree radial of the VOR, and tracked towards the training area.

The Alice Springs airspace was not serviced by radar and as such the Alice Springs aerodrome controller was required to apply non-radar (procedural) control, in accordance with published procedures. The aerodrome controller was responsible for controlling aircraft operating within the Alice Springs control zone and control area up to 8,500 ft. Procedural control is achieved by the use of information from sources other than radar. The aerodrome controller intended to establish a lateral separation standard between the C310 and the inbound 717 once the C310 was established in the training area.

The aerodrome controller initially applied a vertical separation standard of 1,000 ft between the C310 and the 717. At 1326, the pilot of the C310 reported, to the aerodrome controller, that the C310 was established in the training area to the east of Alice Springs. That training area was procedurally separated, using a lateral separation standard, from the 055 degree radial from the Alice Springs VOR. Once the pilot of the C310 reported established in that training area, the C310 was laterally separated from the 717. The aerodrome controller then cleared the crew of the 717 to descend below the altitude of the C310.

About a minute later, the crew of the 717 reported that they had the C310 in sight about 1,500 ft below their aircraft. The pilot in command of the C310 later reported that he heard that broadcast made by the crew of the 717, but despite conducting a search for the 717, did not see the aircraft.

On 17 November 2005, a Boeing 717-200 (717) aircraft departed Cairns, Qld, on a scheduled flight to Alice Springs, N.T. It reportedly passed within 1 NM horizontally and 1,500 ft vertically of a Cessna Aircraft Company C310R (C310) aircraft that was outbound from Alice Springs, NT, on an instrument flight rules (IFR) training flight.

The information provided to the Alice Springs aerodrome controller indicated that a lateral separation standard existed between the aircraft. On that basis, the aerodrome controller cleared the crew of the 717 to descend below the level of the C310. However, the position of the Cessna C310 relative to the 717 at the time the aerodrome controller cleared the 717 to descend through the level of the C310 meant that no procedural separation standard existed between the two aircraft. The aerodrome controller had no indication that the information from either source may have been erroneous.

Alice Springs airspace is not serviced by radar, and there was no recorded information on board either aircraft that could be used to determine the exact proximity between the two aircraft at their closest point. It was not possible for the investigation to determine why the lateral separation standard applied by the aerodrome controller was compromised.

Pilots should be mindful of the importance of the accuracy of the information they provide to air traffic control, in maintaining the integrity of published separation standards.