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.