What happened
On 30 August 2019, a Saab 340B, registration ES-NSD, was performing a scheduled passenger flight from Carlisle Airport to London Southend. While climbing through approximately 2,500 ft, the commander noticed that the propeller RPM on the left engine was low and attempted to adjust the climb power. However, the condition lever for the left engine failed to respond to inputs.
Following established procedures, the flight crew initiated an engine shutdown of the left engine and declared an emergency. The aircraft returned to Carlisle Airport and landed safely twelve minutes after departure. There were no injuries to the three crew members or the 13 passengers on board, and no damage was reported to the aircraft.
The investigation
An inspection of the left engine revealed significant damage to the condition control cable, specifically the section running between the hydromechanical unit (HMUT) and the pitch control unit (PCU). The outer stainless steel casing and vinyl covering showed large holes consistent with chafing against the accessory gearbox.
The investigation examined the aircraft's maintenance history and the implementation of previous service bulletins. While the aircraft was fitted with plastic tubing intended for chafing protection, the manufacturer determined that this protection was not positioned correctly to shield the cable from the gearbox. Furthermore, while a 1988 service bulletin (SB 340-76-027) recommended such protection, there was no formal record of this bulletin being embodied on ES-NSD.
Findings
- The primary cause of the control failure was the degradation of the condition control cable due to chafing against the engine's accessory gearbox.
- Although chafing protection had been installed on the cable, it was placed in an incorrect location and failed to protect the vulnerable area.
- There was a discrepancy between the manufacturer's build-up manual and the aircraft maintenance manual regarding the specific requirements for bending control cables during installation, which can impact cable longevity.
Safety action
- The operator has introduced periodic inspections of engine control cables into its maintenance programme.
- The aircraft manufacturer has committed to updating the aircraft maintenance manual to include detailed guidance on proper cable installation and bending procedures.