Cessna 150G, VH-KPQ

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

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.

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