Incident Description
On 7 September 2021, a Cessna 441, registered VH-JFU, was operating a passenger charter flight from Sawfish Camp to Darwin, Northern Territory. During cruise, the pilot observed abnormal indications from the right engine, including torque fluctuations, high oil pressure, and high oil temperature. The pilot diverted to Tindal. Maintenance checks revealed that the air/oil cooler return line and air/oil separator vent line had been incorrectly transposed during a recent engine change. The lines were rectified, and the aircraft was released to service.
On 28 September 2021, during another flight, the pilot observed abnormal indications from the right engine, specifically torque and oil pressure fluctuations. After landing, oil was found throughout and under the right engine cowling. The reduction gearbox scavenge pump was found to be unserviceable.
ATSB Findings
The Australian Transport Safety Bureau (ATSB) found that the oil lines could be easily transposed because they were flexible, long enough to reach both adjacent ports, identical in size and thread, and similar in appearance. This created a risk of transposition without hindrance. It was not possible to determine individual or environmental factors associated with the error. The requirements for an independent inspection did not include checking the oil lines.
The incorrect oil flow resulting from the transposed lines damaged the air/oil separator, which increased the reduction gearbox scavenge pump pressure. This compromised the structural integrity of the pump housing, leading to its failure and abnormal engine indications on the later flight.
The engine manufacturer had issued a service information letter (SIL) in 1990 advising that such transpositions had occurred previously. A limited review of previous occurrences did not identify any that led to a complete loss of power or an accident.
Actions Taken
Following the occurrence, the operator Chartair commenced a fleet-wide program to add markings to engine oil tanks, conducted toolbox talks with engineering staff about distractions during maintenance, and began documenting each stage of engine changes. Honeywell, the engine manufacturer, reissued the SIL with additional information and guidance, and indicated it would revise inspection/repair manuals with instructions to re-mark engine oil tanks.
Safety Message
The ATSB reminds maintenance engineers to check relevant documentation rather than relying on experience and memory, and to remain familiar with manufacturer service information letters. Since maintenance documents may not always provide advice on non-routine technical situations, operators and maintainers should seek technical advice from the manufacturer to ensure non-routine problems are fully rectified before releasing an aircraft to service.