The Beechcraft Baron aircraft was being operated on a freight charter flight from Groote Eylandt to Darwin.
The pilot said that as the aircraft descended through 5,000 ft the left engine fuel flow gauge reading decreased to zero and the left tachometer indication reduced to about 1,700 RPM. The cylinder head temperature and oil temperature readings also reduced. The pilot advanced the engine control levers to full power and tried to fly the aircraft at the best single engine rate of climb speed. The aircraft continued to lose altitude and the pilot realised there was not enough height remaining to reach an aerodrome. As the left engine tachometer was still indicating about 1,700 RPM, the pilot, believing the engine was still developing power, did not feather the left propeller.
The pilot landed on a two-lane highway. The aircraft was substantially damaged during the landing roll when it slid off the roadway and entered a ditch. The pilot, who was the sole occupant, was not injured.
The pilot had about 1450 hours aeronautical experience and 105 hours on the aircraft type.
Examination of the left engine found the fuel mixture control cable had failed near the fuel control unit control lever. This allowed the lever to move downward under its own weight into the fuel cut-off position. No other pre-existing damage or fault was found that may have contributed to the failure of the engine.
Both engines were only slightly damaged in the forced landing. They were fitted to an engine test cell where they operated in accordance with the manufacturer's standards.
The aircraft's maintenance records showed that the fuel mixture control cable was fitted to the aircraft about 230 hours previously. Specialist metallurgical examination of the cable, which was constructed of one central strand surrounded by six smaller diameter strands, showed it had broken because of fatigue cracking. The fatigue cracking showed significant alternating stresses had been applied to the cable during aircraft operation. The examination also found sliding contact wear close to the break, caused by abnormal alternating loads applied to the cable at the point where it was swaged to the fitting. Possible misalignment of the cable during installation may have led to the failure.
The pilot did not realise that the left engine had failed and the unfeathered propeller was driving the engine, because the RPM indication was higher than he had expected. He therefore was not aware that the unfeathered propeller was causing excessive drag preventing the aircraft from maintaining height.