The pilot of the Schweitzer 269CB helicopter reported that
during cruise on a repositioning ferry flight, the helicopter's
engine misfired and then stopped completely so he completed a power
off autorotation to a nearby dam wall. The pilot, the sole
occupant, received no injuries and the helicopter was not
damaged.
Troubleshooting by maintenance personnel isolated the problem to
the engine's carburettor. The carburettor was replaced and the
engine test run. No other anomalies were noted and the helicopter
was flown back to the maintenance base.
A technical disassembly of the Precision Airmotive carburettor,
model HA-6, part number 10-6030, serial number 75060303 was
initiated. The examination included fuel flow level checks. During
the checks, the throttle linkage was exercised through its full
range from idle to full power. Throttle operation was smooth and
unhindered. Several rapid accelerations were then carried out to
observe the operation of the accelerator pump. No fuel was observed
to flow from the accelerator pump discharge tube, positioned in the
carburettor venturi, during these tests. Further examination
revealed that the accelerator pump plunger mechanism was
disconnected at the upper circlip attachment point on the
plunger.
In addition, the accelerator pump/economiser linkages were found
to have severe wear on the cam lobe face corresponding to the full
throttle position and on the linkage plate that the camshaft lobe
acted upon. The linkage plate also had severe wear on the cam
lobe-mating surface and on the opposite non-cam lobe contact
surface. That indicated that the plate had been reversed at some
point during its operational life.
Carburettor background
At the time of the occurrence, the carburettor had accumulated
1,668.2 hours time in service (TIS). The carburettor had been
removed from the engine at 1,154.7 hours TIS for a discrepancy
reported as suspected low engine power. The repair documentation
noted an air-metering pin adjustment defect. Following
readjustment, the carburettor was bench tested and returned to
service.
At 794.4 hours TIS, the carburettor had been removed from the
engine, for a discrepancy reported as leaking fuel when in the full
lean/cut-off position. The repair documentation noted that a new
float needle and seat were fitted. The carburettor was bench tested
and returned to service.
