The ATSB has released its final report into a fatal accident involving a Cessna 172S that impacted with terrain in Queensland on October 18, 2010.
History of the flight
At about 1030 Eastern Standard Time on October 18, 2010, a Cessna 172S registered VH-VSK was operating at low level near Durham Downs Homestead, Queensland with pilot and one passenger on board. The pilot was assisting a ground party locate two horses.
Earlier that morning, the pilot flew the passenger from Durham Downs Homestead to Woomanooka, about 40km to the southeast to repair a disabled truck. During the return flight, and within a few kilometres of the homestead, the aircraft was heard by a ground party, on motor cycles, that was herding horses. The pilot was asked via radio by a member of the ground party to see if he could locate two horses that had separated from the main group.
Members of the ground party observed the C172 flying low above treetops and turning steeply. Shortly after, the pilot broadcast that he had located the two horses and suggested that the ground party ‘get moving with the horses’. The ground party turned their focus back to the main group of horses. A few minutes later, a member of the ground party observed two horses emerge from under some trees. That person could no longer see the aircraft and, feeling some concern, attempted to contact the pilot via radio. There was no reply.
The ground party immediately initiated a search and subsequently located the aircraft wreckage near a dry creek bed in the area where it was last seen. When they arrived at the accident scene, they detected a strong smell of aviation gasoline and observed fuel leaking from the aircraft’s wing fuel tanks. The pilot and passenger were fatally injured and the aircraft seriously damaged.
Members of the ground party stated that the weather conditions at the time of the occurrence were fine, with a light wind. They did not notice anything abnormal in terms of the aircraft’s operation or the pilot’s radio broadcasts.
It was reported that there had been an argument between the pilot and a person at Woomanooka before the aircraft departed on the occurrence flight. That person recalled that he spoke sternly to the pilot about ‘saying things behind his back’. Afterwards, he and the pilot and others had morning tea together. The pilot did not appear withdrawn or otherwise to have reacted adversely to the episode in the period before the aircraft departed for Durham Downs.
Pilot information
The pilot held a CPL-A, a mustering endorsement and a Grade 2 Flight Instructor rating. His total flying experience was about 1500 hours. His flight crew record indicated that he completed low-flying training and was endorsed to conduct aerial mustering operations on November 5, 2008. The pilot flew the C172 regularly and had been employed as a pilot at Durham Downs for about two years. On June 19, 2010 the pilot was appointed senior pilot for the company which owned the aircraft and three other Cessna 172s. That position included responsibility for all of the company’s air operations.
The pilot held a valid Class 1 Medical Certificate, which included the restriction that reading correction was to be available whilst exercising the privileges of the licence.
Witnesses reported that on October 11 and 12, 2010 the pilot had experienced flu-like symptoms, including a fever and a very sore throat. He was examined by a paramedic at Bellara, Qld on October 13 and prescribed antibiotics.
On October 14 the pilot drove a vehicle from Durham Downs to Woomanooka. That journey normally took three to four hours but flooding in the area made some roads impassable and the journey was reported to have taken longer than 12 hours and the pilot did not arrive at the destination until late that night. The pilot’s fever was reported to have gone by October 16, but he appeared ‘pretty tired’ that day.
Aircraft information
The aircraft (serial number 172S 8648) was manufactured in the US in 2000. It was one of four Cessna 172s owned and operated by a pastoral company on properties across Australia. The company reported that the average flight time for each aircraft in 2010/2011 was about 367 hours.
At the time of the occurrence, the aircraft had current certificates of registration and airworthiness and a valid maintenance release. The last scheduled 100-hourly maintenance check was carried out on July 23, 2010. At the time of the occurrence, the engine had exceeded the engine manufacturer’s overhaul time of 2000 flight hours by 107 hours. There were no entries in the maintenance release regarding any aircraft defects.
In June 2011, the ATSB received a report that, a few days prior to the occurrence, the pilot had told another pilot that the aircraft’s stall warning system was inoperative. Neither the aircraft owner/operator nor the aircraft maintenance organisation received any information regarding the serviceability of the stall warning system prior to October 18.
Wreckage examination
The aircraft was seriously damaged by the ground impact forces. There was no evidence of fire. Deformation damage to the wing leading edges indicated that the aircraft’s attitude at impact was about 55 degrees nose-down. The right wing sustained more damage than the left, indicating that the right wing impacted the ground before the left wing. The initial impact position was about 4m from the final location of the wreckage.
There was no evidence of any pre-existing fault in any of the flight control systems. The wing flaps were in the retracted position at impact. There was no indication that any part of the aircraft sustained a birdstrike or that the aircraft struck a tree or other obstacle prior to the impact with the ground.
The engine crankshaft had fractured immediately behind the propeller. Examination of the crankshaft fracture surfaces revealed evidence of an overload fracture due to severe side loads on one of the propeller blades. Damage to the propeller blades was consistent with the engine delivering power to the propeller at the time of impact. Witness marks on the face of the engine tachometer and instrument glass showed that the tachometer was indicating between 2100 and 2300 RPM at impact.
ANALYSIS
The extreme nose-down attitude of the aircraft at impact was well outside the normal envelope of operation for the aircraft type. It was consistent with loss of control following aerodynamic stall at an altitude where a ground collision was unavoidable.
The facts relating to the occurrence allowed a number of issues to be eliminated as having directly contributed to the development of the occurrence. The benign weather conditions at the time were unlikely to have caused any control difficulties for the pilot. The wreckage examination did not indicate any fault with the aircraft’s engine, its fuel system, or the flight controls that may have led to a loss of control. Further, there was no evidence that the pilot was under any pressure to undertake the flight.
Stall warning system
The evidence relating to the stall warning system, however, was not clear-cut and deserved some discussion. The investigation considered that the pilot was unlikely to have told the other pilot that the system was inoperative if that had not been the case. In terms of importance and relevance, the pilot would have been aware of the requirement for a serviceable stall warning system, and it was the type of information that one pilot would share with another who flew the same aircraft. At the same time, there was no evidence that the system underwent any repair in the period before the occurrence flight, or that the matter had been reported to company management or CASA. There were, therefore, three possibilities:
- the stall warning system was returned to a serviceable status before the flight, possibly as a result of the pilot removing a blockage in the plumbing;
- the pilot operated the aircraft with the stall warning system inoperative; or
- the stall warning system became inoperative during the occurrence flight.
From the aircraft’s maintenance release, and information provided by the other pilot who flew the aircraft, the stall warning system must have become inoperative after the other pilot flew the aircraft on October 11. The accident pilot could have become aware that the system was inoperative during a routine pre-flight check of the aircraft, or during the flight on October 13.
The overwhelming majority, if not all, of the pilot’s flying experience would have been in aircraft with a serviceable stall warning system and it would be entirely reasonable to expect him to have been aware of the importance of the system to safe flight. Against that background, the investigation considered it less, rather than more likely, that the pilot flew the aircraft with the stall warning system inoperative. Nevertheless, flight with an inoperative stall warning system could not be discounted entirely.
Stall avoidance during flight with an inoperative stall warning system depended on pilot attention to, and interpretation of, airspeed, aircraft attitude and flight control ‘feel’. In situations where a pilot’s attention was directed outside the aircraft, the risk of a late and/or inappropriate response to airspeed and attitude changes increased, raising the possibility of an inadvertent aerodynamic stall.
Flight activity when the loss of control occurred
None of the witnesses continued to watch the aircraft after the pilot advised that he had found the horses and ‘a few minutes’ elapsed before they noticed that the aircraft was missing. In other words, no information was available regarding the aircraft’s flightpath during those ‘few minutes’; neither was there any information as to when, during that time frame, the loss of control occurred.
The position of the wing flaps and ad hoc nature of the task
Until the pilot received the request to assist the ground party, the flight entailed a transit from Woomanooka to Durham Downs. The pilot was unlikely to have expected to become involved in ‘mustering’ activities during the flight. As a result, he may not been mentally prepared for the task and may have forgotten to select 10° of flaps before commencing a low-level manoeuvring flight.
Against the background of the pilot’s mustering experience and the aircraft configuration information he had included in the company procedures manual during its draft, it was considered unlikely that the pilot would have deliberately conducted low-level manoeuvring flight in a clean (flaps retracted) configuration. That contrasted with the retracted position of the flaps at impact. A possible explanation for the as-found position of the flaps was that the pilot, having located the horses and completing the low-level manoeuvring phase of the flight, retracted the flaps to return to the aircraft’s clean configuration before continuing on to Durham Downs Homestead.
In any case, with flaps retracted, the aircraft would have stalled at a higher speed than the pilot may have been anticipating for the flight environment at the time. The unexpected nature of such an event, particularly at low level, could have placed extreme pressure on the pilot’s capacity to respond appropriately to the situation. The possibility that the pilot retracted the flaps after he lost control of the aircraft but before ground impact was considered unlikely.
Pilot action to avoid a collision
The pilot may have detected an obstacle in the aircraft’s flightpath, such as a large bird or a tree and taken action to avoid a collision. In such a situation, and irrespective of the flaps position, it was possible for one or both wings to have aerodynamically stalled and placed the aircraft in an extreme attitude from which the pilot was unable to recover before ground impact. However, the absence of any direct evidence of the aircraft’s flightpath at the time of the occurrence prevented a conclusion that collision avoidance was a factor in the accident.
Residual effects of the pilot’s recent illness
Any residual effect from the pilot’s recent throat infection, such as reduced alertness or distraction due to pain or physical discomfort, had the potential to affect the pilot’s ability to safely manoeuvre the aircraft. However, no expert medical assessment of his condition had been undertaken since he was prescribed antibiotics on October 13. Consequently, no positive conclusion could be drawn regarding the pilot’s recent illness as a factor in the accident.
Interference with the controls
In the context of the occurrence, the possibility that the passenger interfered with the controls was considered. Any unexpected or unintended movement of the flight or engine controls while the pilot was manoeuvring the aircraft had the potential to cause an in-flight upset and subsequent loss of control. However, considering the passenger’s previous experience as a passenger in the aircraft, the likelihood that he interfered with the controls was considered low.
FINDINGS
From the evidence available, the following findings are made. They should not be read as apportioning blame or liability to any particular organisation or individual.
Contributing safety factors
- For reason(s) which could not be determined, the pilot lost control of the aircraft, probably following an aerodynamic stall.
Other key findings
- The engine was operating at the time of impact.
- The operating status of the aircraft’s stall warning system could not be determined.
Safety action
The ATSB has been advised of the following proactive safety action in response to this incident by the aircraft operator. Following the accident, the operator reviewed:
- The use of fixed-wing aircraft for mustering;
- The type of aircraft used for mustering;
- Pilot training and ongoing monitoring; and
- Aircraft Operations Policies and Procedures.
It was determined that there was a role for fixed-wing aircraft in mustering and it was possible to safely conduct mustering operations. A review of available aircraft types suggested that the Cessna 172 was the most suitable VH registered aircraft for mustering operations. There were a number of non type certified or sport kit aircraft that had good flying performance, but these were rejected because of concerns regarding the reliability of construction, maintenance standards and the ability to recruit pilots.
The operator’s standard of pilot training was deemed satisfactory, but a four-monthly flight check with an instructor was introduced to ensure pilots were maintaining proper standards.
(ATSB)
