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.
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.
The displaced circlip from the upper end on the accelerator pump plunger, discovered during the technical disassembly of the carburettor, would have resulted in insufficient fuel being available to the engine during rapid high power requirements.
The engine logbook documentation did not reveal any maintenance performed to the carburettor since it's last repair and reinstallation. It was unlikely that the circlip had become displaced during the repairs of the carburettor, as bench testing would have revealed the problem.
If the maintenance action of the reversing of the linkage plate had been carried out without removing the top plate of the carburettor, it would have been difficult to verify the continued connectivity of the accelerator linkage and the proper functioning of the accelerator pump mechanism within the carburettor.
The investigation could not conclusively establish the reason for the disconnection of the accelerator pump plunger mechanism.
|Date:||02 November 2001||Investigation status:||Completed|
|Time:||0930 hours EST|
|Location:||9 km N Mareeba, Aero.|
|State:||Queensland||Occurrence type:||Forced/precautionary landing|
|Release date:||04 February 2002|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Schweizer Aircraft Corp|
|Type of operation||Aerial Work|
|Damage to aircraft||Nil|
|Departure point||Mareeba, QLD|