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Final report

Summary

On the morning of 15 August 2003, the pilot hired a Victa Ltd Airtourer, registered VH-MVP, to practice basic aerobatics. Prior to flying to the training area, the pilot was conducting a number of touch and go circuits. Following the fourth takeoff and while climbing through about 200 ft, witnesses described hearing the aircraft engine surge then stop. Shortly after, witnesses saw the aircraft turn left. The turn steepened as witnesses lost sight of the aircraft behind trees and houses and a short time later they heard the aircraft impact the ground. The pilot was fatally injured in the accident.

The pilot was appropriately licensed and was reported by a family member to be fit and well and looking forward to the flight. The family member believed that the pilot was going to conduct circuits in another aircraft, not aerobatics in the Victa. However, the company flight details log sheet contained an entry for `aerobatics' and the pilot had discussed the aerobatic component of the flight with an instructor.

The Australian built, two seat, aerobatic rated, single-engine, low-wing aircraft was originally fitted with a 100 hp engine driving a fixed pitch propeller. However, the aircraft was later fitted with a 180 hp engine with a constant speed propeller.

Examination of the aircraft fuselage, flight controls, fuel system and engine including magnetos and spark plugs, by the Australian Transport Safety Bureau (ATSB) investigation team, provided no explanation for the sudden loss of power observed by witnesses. During the onsite phase, approximately 6.8L of fuel was removed from the aircraft fuel tank, which was still securely attached and not deformed. This included approximately 1L that drained from the tank as it was removed from the wreckage. The fuel was free of visible contaminants and was the correct colour. No other fuel was recovered from the site, nor was fuel staining or odour evident on the fuselage or the ground immediately below the wreckage. There was no evidence of excessive fuel consumption. Emergency workers and witnesses, who arrived at the accident site within minutes of the accident, could not recall a fuel smell. The ATSB did not receive any fuel related incident reports from other aircraft that had refuelled from the same source following the accident.

The fuel system consisted of a 130L (total capacity) rubber bladder fuel tank located in the fuselage. Fuel addition and manual contents checking was via an angled filler tube into the bladder tank through the side of the fuselage. Fuel was supplied to the engine via an engine-driven fuel pump and carburettor, with an electric boost pump as backup. Fuel quantity is checked via an electric fuel gauge and a flexible dipstick graduated in imperial gallons. The dipstick is made up of a number of five imperial gallon graduated segments, held taut by a chord under tension. Dipping of the fuel contents required a pilot to depress the button on the top of the dipstick to relax the tension on the segments and allow the flexible dipstick to travel down the angled fuel filler tube. After traversing the angled section of the filler tube, pressure on the button was required to be relaxed, thereby re-tensioning the segments prior to the end of the dipstick contacting the bottom of the tank. The procedure required the dipstick to be under tension prior to contacting the bottom of the tank and that the dipstick did not rest on a fold or ripple in the bladder. Failure to do this may result in an erroneous reading of the tank's content.

The approved aircraft flight manual stated that 1.3L of fuel was unusable. Although not stated, this is assumed to be for level flight. Calculations by the investigation indicated that about 6.2L of fuel would be unusable while the aircraft had a 5 degree nose up attitude in a climb. This would increase to about 12.4L at 10 degrees of nose up attitude.

Examination of the tank sender unit, a wire-wound wiper type, showed wiper shaft bearing surface wear. This would have allowed lateral movement of the wiper arm resulting in intermittent contact with the wire wound former and is likely to have caused the fuel gauge to display intermittent readings to the pilot. However, due to impact damage of the sender unit, it was impossible to determine the extent of the intermittent readings.

Three small pin size holes were located in the rubber bladder. A test indicated that 250 ml could have leaked from the tank from the time of the last refuelling to the time of the accident. The holes were not collocated and were not in the vicinity of the dipstick. It could not be determined if the holes were due to impact damage.

The tank had been filled on the day before the accident, prior to completing a 2.2 hour cross country flight. On the day of the accident, the pilot checked the tank contents and informed an instructor who assisted her to push the aircraft from the hangar that it held 15 imperial gallons (68L). However, presumably in error, the pilot entered 75L (16.5 imperial gallons) in the company flight details log sheet. The instructor did not see the pilot dip the tank or check the fuel gauge.

The investigation examined the aircraft engine manufacturer's fuel consumption tables and company flight details log sheet to establish the fuel consumption rate. Based on the record of total hours and fuel consumed, for the previous two weeks, the aircraft had consumed about 37.6L per hour for all modes of flying. Therefore the tank should have held about 47.3L after the previous day's cross country flight. If the tank contained 75L prior to the accident flight, as entered in the log by the pilot, the aircraft consumption rate would have been about 25L per hour. That fuel consumption rate was unlikely to be achieved during the cross country flight. The chief flying instructor commented that the company instructed students to plan using a fuel consumption rate of 40L per hour.

An instructor reported that the aircraft had completed three circuits prior to the accident, which would equate to roughly 35 minutes of taxi and flying time. Allowing for an average fuel burn of 37.6L per hour, there should have been about 25L remaining at the time of the accident. The ATSB fuel consumption calculations for some individual flights ranged from 35L to 50L for circuits, aerobatics and cross country flights. However, the log was incomplete, so it was not possible to derive a fuel consumption per flight for some of the previous flights. The reason for the discrepancy between what should have been in the tank and what was recovered at the accident site could not be determined.

The investigation examined a similar model aircraft and its fuel system. It was found that it is relatively easy to have the bottom (five imperial gallon) segment of the dipstick bend sideways, when the dipstick contacts the bottom of the tank. This will occur if the segments are not tensioned by releasing pressure on the tension button prior to the dipstick contacting the bottom of the tank. This results in the dipstick over-reading by about 4 to 5 imperial gallons (18.2L to 22.7L) and could have led the pilot to believe that there was adequate fuel for the flight.

Based on the examination of the recovered engine components, the witness reports of engine surging just prior to the engine failure, the lack of fuel odour at the accident site and the company fuel details log sheet, it is probable that there was insufficient fuel to complete the flight, either due to fuel exhaustion or starvation. It is possible that the pilot's operation of the dipstick provided an erroneous reading, which led her to believe that the aircraft's tank contained more fuel than it actually did. Additionally, the intermittent fuel gauge reading may have meant that she was not able to check the fuel quantity by a secondary means, other than the log entry for the previous day's flying.

In a take-off climb attitude of between 5 to 10 degrees, the fuel pickup point in the tank was probably unported, interrupting the fuel flow to the engine thus causing it to lose power. The flight path of the aircraft after the loss of engine power indicates that the pilot may have been attempting to turn the aircraft back to the runway at a low height and lost control with insufficient height to effect recovery.

 
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