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Factual Information

Summary

The owner/builder of the the Vans RV-6A aircraft was conducting a flight from Townsville to Toowoomba. The aircraft departed Townsville at 0846 local time. The pilot subsequently contacted Oakey Approach at 1324, and the aircraft was identified on radar at 3500 feet. The pilot was instructed to maintain that altitude. When the aircraft was about 26 nautical miles from Toowoomba, the pilot transmitted a mayday distress message stating that the aircraft's engine had failed. A short time later he reported that a propeller blade had failed. No further transmissions were heard from the aircraft. About 30 minutes later, a searching helicopter located the wreckage in a flat clear area amongst hilly, tree covered terrain.

The area apparently chosen by the pilot for an emergency landing was about 300 m long and relatively flat. There were trees under the likely approach path of the aircraft and rising ground at the far end. The surrounding hills were above the level of the landing area.

Examination at the accident site revealed that the aircraft struck the ground while banked about 90 degrees left, and descending at an angle of about 34 degrees. A 200 mm section of one propeller blade tip had broken off and could not be located at the accident site. Witness marks indicated that the propeller was rotating under power when the blades contacted the ground. Aside from the broken tip, the propeller blades were in good condition. No other faults were found that might have prevented the aircraft from operating normally.

During the impact sequence, the section of the fuselage forward of the pilot's seat was deflected upward relative to the rear fuselage. The cockpit sides had buckled outwards. That resulted in the pilot striking the instrument panel, even though his shoulder harness remained fastened.

Aircraft information

The pilot purchased the aircraft in 1996 and he first flew it in 1998. At the time of the accident, the aircraft had accumulated 383 hours time in service.

The pilot fitted a new engine and propeller to the aircraft during construction. The engine was modified to improve and balance the airflow through the valves of each cylinder to enhance engine performance. In an apparent further attempt to improve engine performance, the pilot replaced one magneto with an electronic ignition system that was capable of varying the ignition timing in response to changes in engine RPM and manifold pressure. That variation contrasted with the fixed timing ignition provided by the other "standard" magneto fitted to the engine.

The aircraft's wing was a constant chord, low aspect ratio wing. A characteristic of low aspect ratio wings is high induced drag at low speed. Unless the pilot controls the speed carefully, the effect on aircraft performance can be a rapid speed loss and a high rate of descent. If such a situation arose during an approach to land, the only means of regaining speed and arresting the rate of descent would be to increase engine power. If little or no engine power was available, the outcome could range from a heavy landing to a loss of control of the aircraft.

Examination of the failed propeller blade

A fatigue crack had initiated near the leading edge of the blade 216 mm from the blade tip. Crack growth had occurred as a result of alternating thrust loads, and had propagated along the thrust face (rear surface) of the blade. The characteristics of the crack indicated that it had grown under constant amplitude loading. There was no evidence of flight by flight striations. The propeller material was of the correct type and no damage or other reason for the crack to initiate was found.

All propellers are subject to alternating thrust loads during normal operation. Propellers are designed so that those loads will not exceed the design value, thus preventing the development of fatigue cracks during operation. The firing of each cylinder in a reciprocating engine produces torsional vibrations. That means that the crankshaft momentarily speeds up at each firing stroke, and then slows down again prior to the next firing stroke. The vibration leads to alternating thrust loads in the propeller.

Examination of the engine connecting rod big-end bearings revealed distress on the bearing surfaces. That distress was indicative of firing loads exceeding the designed capacity of the bearing lubrication. For optimum operation of spark ignition engines, the peak pressure developed by the combustion of the fuel air mixture should occur approximately 15 degrees after the crank has located past top centre.

Ignition timing was a critical factor, influencing engine power, fuel economy, and the operating condition of the engine. Timing depended on the rate of propagation of the flame front through the fuel-air mixture. Increased or advanced ignition timing resulted in increased combustion chamber pressures. Magneto timing was fixed and was optimised for the operating range of the engine. If the response of the electronic ignition system to reductions in manifold pressure created by part throttle opening was to advance the timing of ignition, that could increase cylinder head pressures and increase the magnitude of torsional vibration.

Pilot information

The pilot, aged 64, held a New Zealand Private Pilot Licence and a current Class 2 Medical Certificate. The medical certificate was subject to three restrictions; the pilot was required to wear spectacles, he was granted an exemption regarding his hearing standard, and he was subject to a restriction regarding a drug he was taking for tinnitus (a ringing or similar sensation in the ears, due to disease of the auditory nerve).

The post-mortem examination of the pilot revealed that he had previously suffered at least one myocardial infarction and had coronary artery disease. Specialist examination of the pilot's electrocardiogram traces over a number of years did not reveal any pre-existing signs of a heart problem. Specialist medical opinion was that the possibility of the pilot suffering a heart attack induced by high stress levels after the propeller failed could not be excluded.

According to the pilot's logbook, he had a total flying experience of 1,179.6 hours at the time of the accident, of which 1,109.9 hours were in command. He had 383.2 hours on the accident aircraft, all of which were in command. On the day before the accident, the pilot had flown the aircraft from Auckland, New Zealand to Townsville, Queensland. That flight took 13 hours.

 
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