Incident Overview
During the climb phase of flight, a short circuit in the number 1 (left) engine sensing unit activated both aural and visual fire warnings for that engine. The flight crew followed the emergency checklist procedures, but the fire warning persisted. An emergency was declared, and the airplane returned to the departure airport for a landing with one engine inoperative.
Evacuation and Slide Deployment
After exiting the runway, the captain initiated an evacuation. The forward right (R1) manual slide did not fully deploy during the evacuation. Subsequent examination of the slide components and the door revealed signatures indicating that the flight attendant had manually inflated the slide before the door was opened enough to allow proper deployment.
Investigation Findings
The investigation determined that the root cause of the fire warning was a short in the engine sensing unit. No other causal factors were identified. The slide deployment issue was attributed to the timing of manual inflation relative to door opening.