The aircraft was being used for endorsement training of a
foreign tourist. It had been operating in the training area east of
Jandakot for approximately one hour and was engaged in general
handling manoeuvres and aerobatics. Having completed these
manoeuvres, the instructor decided to conduct practice forced
landing training. During the first approach, from an altitude of
2,500 ft, the instructor reported that the throttle had been opened
twice and the engine had responded normally.
The instructor reported that at 200 to 300 ft, with 30 degrees
of flap set, he instructed the student to commence a go-around. He
reported that the student raised the nose attitude and opened the
throttle but the engine did not respond. The student was flying the
approach at 65 knots. The stall speed in the finals configuration
was approximately 42 knots. The recommended climb speed was 60
knots.
The instructor reported that he checked that the throttle was
fully open. He also noticed the airspeed reducing. He took over and
attempted to lower the nose attitude to prevent the aircraft
stalling but the aircraft did not seem to accelerate. He called to
the student to check that the fuel was on.
The instructor reported that he had turned right, then left to
avoid a tree and line up on a paddock. He did not assess the rate
of descent as being excessive until he tried to flare the aircraft
for landing. He did not recall the engine power recovering before
impact.
Wreckage evidence indicated that the engine was delivering high
power at impact. The wreckage and ground marks also indicated that
the aircraft had struck the ground in a flat attitude at relatively
low ground-speed. There was sufficient fuel and the fuel selector
had been selected to the left tank.
The engine was a Gypsy Major 10, Mk 2, correctly modified for
use in the Chipmunk aircraft. During the subsequent engineering
examination, the engine was removed and successfully run on a test
stand. An examination of the fuel components including the
carburettor, fuel pump and filters indicated that there were no
pre-existing faults. The engine's original Fairey metal propeller
had been replaced by an approved Hoffman wooden propeller.
Operators reported that the wooden propeller appeared to have less
"flywheel" effect and although the engine could accelerate faster,
it would also stop more readily.
Previously reported problems associated with Gypsy Major engine
response during go-arounds was pinpointed to worn needle valve
seats causing carburettor flooding. A modification was introduced
in 1957 to address the fault. The modification was incorporated in
the accident engine and the subsequent examination revealed that
the valve seat was not worn.
The Gypsy Major engine does not incorporate an accelerator pump
and rapid throttle openings have been known to induce a delay due
to the fuel/air mixture becoming, momentarily, lean. Whilst this
appears to be a known characteristic of the engine, there is no
warning in the aircraft's handling notes relating to the
consequences of opening the throttle too rapidly.
Evidence indicated that the engine was delivering significant
power at impact. The post-accident engine examination and ensuing
test run revealed that it was unlikely that a mechanical fault
caused the engine to lack power during the go-around. If, however,
the throttle was opened too rapidly during the go-around, the
engine may have momentarily lost power through a lean cut. The
wooden propeller's low inertia associated with the aircraft's low
airspeed may have caused the engine to almost stop, further
delaying the power response.
By the time the instructor took over, the airspeed had probably
reduced rapidly towards the aircraft's stall speed as a result of
the drag genrated by the full flap and the pitch attitude. Whilst
the manoeuvres conducted by the instructor late in the approach
indicated that the aircraft had not stalled, there was probably
insufficient airspeed to permit an effective flare before landing.
The aircraft may have stalled during the flare, causing it to
impact the ground in a near flat attitude at low speed.