Incident Overview
An aircraft performed a precautionary landing after its crew received a fire warning indication for the number two engine. Initially, the fire warning light and bell were intermittent, but they later became steady. The crew secured the engine and discharged both fire bottles, but the indications did not cease.
Investigation Findings
Subsequent investigation revealed that no fire or overtemperature condition had occurred. The false warning was traced to a short circuit in the engine fire detection loop, affecting both elements. The short was caused by contaminants found on the connectors and pins of one end of the loop.
Contaminants and Testing
During initial inspection, contaminants were observed on the connectors. After handling and shipment to the operator's maintenance facility, the short disappeared and could not be replicated. The loop met all manufacturing specifications and functioned normally during subsequent testing. The contaminants appeared to be engine oil and hydraulic fluid.
Unresolved Questions
Investigators could not determine how or when the contaminants were introduced to the hermetically sealed connectors. No further analysis was conducted to establish the origin or timing of the contamination.