Sequence of events
On 22 June 2003, a Cessna Aircraft Company 172M, registered VH-TUR, drifted to the right shortly after take off from runway 35 at Wedderburn airfield in NSW and impacted the ground to the north-east of the airfield. The aircraft was destroyed and the four occupants were fatally injured.
The pilot held a valid Private Pilot Licence (aeroplane) and current class 2 medical certificate. There was no evidence that any physiological or psychological factors had affected the pilot's performance.
A witness at the airfield videoed the aircraft as it took off. Examination of the video revealed that the aircraft became airborne after a take-off roll of about 500 m, with 10 degrees of wing flap extended. As it climbed, the aircraft drifted to the right, and entered a right-wing-low sideslip with a nose-up attitude. Witnesses at the airfield observed the aircraft between gaps in the trees to the north-east of the airfield banked to the right in a steep descent and then heard the sound of an impact.
Runway 35 was 1,000 m long and sloped down slightly to the north. The first half of the runway was bitumen and the second half was a mix of hard clay/gravel. Trees about 10 m high surrounded the runway, and sheltered it during crosswind conditions. At the time of the accident, the wind was gusting from the south-west.
The investigation found that the aircraft had been descending steeply in a right turn when it impacted the ground in a westerly direction. Data recovered from a global positioning system found in the wreckage supported other evidence, which indicated that the aircraft entered a spin during a right turn after take-off.
Examination of the wreckage revealed no evidence of pre-existing mechanical defects that may have contributed to the accident. The wing flaps were in the 10 degrees extended position at the time of the accident. The aircraft stall warning system was recovered from the wreckage, tested and found serviceable. The aircraft had sufficient fuel onboard for the planned flight.
An assessment of the aircraft weight indicated that it was approximately 30 kg above maximum allowable take-off weight, and the centre of gravity was calculated to have been towards the aft limit of its normal centre of gravity range. That extra weight would have increased the aircraft stall speed by 1.4% (less than 1 kt), and reduced its climb performance slightly.
Wedderburn was an uncontrolled airfield. It was normal procedure at uncontrolled airfields to maintain runway heading after take-off until the aircraft had reached a height of 500 ft above the airfield, and then to turn left.
As the aircraft climbed out of the shielding effect of the trees beside the runway, the crosswind from the left would have increased markedly. If the aircraft was continuing to climb as it turned to the right, it would have then been in an increasing tailwind. An increasing tailwind will lead to a momentary reduction in aircraft indicated airspeed. The wind was also strong and gusting, and if there had been a wind gust at the same time, it would have caused a greater momentary reduction in aircraft indicated airspeed.
When an aircraft turns away from the wind, at low level, the groundspeed increases. In such circumstances, the view of the ground accelerating below an aircraft may give an illusion of an increased airspeed. For any given nose attitude, an aircraft will fly slower if wing flaps are extended. With 10 degrees of wing flap extended, the aircraft was therefore flying at a lower airspeed in the climb than if the wing flaps had been retracted, if the same attitude was maintained. The aircraft's climb performance would also have been reduced by using 10 degrees of wing flap, compared with using no wing flap. The aircraft take-off performance data in the aircraft operating handbook indicated take-off performance with wing flap retracted, and provided information that 10 degrees of wing flap should be used for take off from a soft surface.
The amount of aerodynamic lift produced by a wing in flight can be changed by a pilot in a number of ways. If the speed of the air flowing over the wing is increased, aerodynamic lift normally increases. The shape of the wing can be changed to an extent by moving the control surfaces, which adjusts the amount of aerodynamic lift. The angle at which the airflow impinges on the wing can be adjusted, which will also change the aerodynamic lift. This angle is known as the angle of attack. In normal flight, if the angle of attack is increased, the aerodynamic lift is also increased, up to a certain angle of attack known as the stalling angle. In contrast, if the angle of attack is increased beyond that stalling angle, the amount of aerodynamic lift decreases. An aircraft flown at a greater angle of attack than the stall angle is commonly described as being aerodynamically 'stalled'.
In steady flight there is a relationship between speed and the angle of attack of the wing. The stall speed is the speed at which the angle of attack coincides with the stall angle for a given configuration.
One effect of flap extension is to increase the relative pitch angle (incidence) between the wing and the fuselage. As a result, in steady flight, an aircraft with flaps extended will fly at a lower speed than one at the same attitude with flaps retracted. If the aircraft was flown at the normal flapless climb attitude, but with 10 degrees of flap extended, it would fly at a lower airspeed. The additional drag produced by the flap extension would also have reduced the climb performance. The aircraft would also have stalled at a lower nose attitude when 10 degrees of wing flap was extended, compared with when the wing flap was retracted.
An aircraft's stall speed increases by a factor of the square root of the secant of the angle of bank, all other things being equal. The aircraft was in a gentle turn at the time the stall occurred. The wing is generally less effective at producing aerodynamic lift when an aircraft is flown out of balance. There is also an increased likelihood of one wing stalling before the other leading to a roll input at the onset of the stall. The accident aircraft had been flown out of balance shortly before the onset of the stall, but it was not known if it was out of balance at the onset of the stall.
The aircraft was observed flying slowly during its climb after takeoff. If the aircraft's airspeed became sufficiently slow in a steady climb, the aircraft would stall. The circumstances were consistent with the aircraft entering a stall and a spin at a height from which it was considered impossible to recover. Some or all of the following factors could have contributed to the aircraft entering a stall:
- The aircraft exceeded the maximum allowable take off weight, which would reduce its climb performance
- The aircraft was climbing into an increasing tailwind, which would create a momentary reduction in airspeed
- The wind was gusting, which could have created a further momentary reduction in airspeed
- The takeoff was downwind, which would have led to a higher groundspeed that would give an illusion of higher airspeed. The pilot may have compensated for this illusion by raising the aircraft's nose
- The aircraft was turned away from the wind at low level, which could have led to an illusion of increasing airspeed. The pilot may have compensated for this illusion by raising the aircraft's nose
- The aircraft was flown out of balance for parts of the flight, which would have reduced its performance
- The use of 10 degrees of wing flap would have reduced its climb performance, and meant that the aircraft would have been flying slower for any given nose attitude.
|Date:||22 June 2003||Investigation status:||Completed|
|Time:||1443 hours EST|
|State:||New South Wales||Occurrence type:||Collision with terrain|
|Release date:||13 January 2005||Occurrence category:||Accident|
|Report status:||Final||Highest injury level:||Fatal|
|Aircraft manufacturer||Cessna Aircraft Company|
|Type of operation||Private|
|Damage to aircraft||Destroyed|
|Departure point||Wedderburn, NSW|
|Departure time||1442 hours EST|
|Role||Class of licence||Hours on type||Hours total|