When the SA227 Metro aircraft was turning onto the downwind leg of the circuit, the crew noticed that the left-engine fire warning annunciator light was illuminated. The phase-one emergency procedures were immediately initiated. Those procedures included immediate recall and checklist prompted actions. Accordingly, the engine was shut down. Shortly after, the fire warning light extinguished. The pilot in command (PIC) then decided not to activate the fire bottle. A single-engine approach and landing was conducted.
This was the third left-engine fire indication the aircraft had experienced since September 1999, the previous two having occurred over a two-week period. The PIC was involved in all three events. During the first and second events, the fire warning indications had remained illuminated following engine shutdown, and the PIC had discharged the corresponding fire extinguisher into the engine fire zone on both those occasions. However, the PIC chose not to discharge the fire extinguisher during the most recent event as the fire-warning indications had extinguished following the engine shutdown. The company's chief pilot supported that decision.
Following the first event, maintenance investigation carried out by the operator revealed that the insulation on a wire in the fire-warning system wiring harness had chafed through while in contact with a nearby bracket. That damage was rectified. After the second event, the fire-detection system was checked for operation. No fault that could have contributed to the activation of the fire-warning system was found. Notwithstanding, it was considered that the lower-turbine fire detector was possibly too close to the engine, and may have resulted in a spurious fire indication. The detector was re-positioned and the aircraft returned to service.
Investigation of the most recent event showed that one of the fire detectors was activating at an incorrect temperature. The aircraft was returned to service following replacement of that detector with one capable of activating within the correct temperature range. At the time of writing this report, similar problems had not re-occurred.
The phase-one emergency checks are described in detail in the operator's aircraft flight operations manual and in the aircraft flight manual. The immediate recall actions require that the engine be shut down with the "stop and feather" control, and that the fuel and hydraulic oil be isolated, before the fire extinguishing system is discharged. The procedure is to then be continued from a written checklist. Items identified as recall actions are intended as a non-discretionary response to a fire warning.