Incident Summary
During cruise at 37,000 feet, approximately 95 nautical miles south of Salt Lake City, a flight attendant reported a fire in the left aft lavatory. Another flight attendant was actively suppressing the flames with a Halon extinguisher. The captain declared an emergency, requested priority handling, and subsequently landed without further incident.
Examination
Post-flight inspection of the aircraft revealed damage to the lavatory toilet water level sensor. The sensor was charred and melted, with the top left corner of its circuit board consumed. Several wires attached to the circuit board at the melted area were also melted and consumed, including a 115-volt AC input wire and a 28-volt DC ground wire. The left side of the circuit relay, adjacent to the consumed portion of the board, exhibited melting and had melted solder on its exterior. Additionally, the left hexagon-shaped mounting screw for the relay was melted.
Findings
The manufacturer determined that the melting point of the screw was between 2,550 and 2,650 degrees Fahrenheit. The investigation team concluded that for such temperatures to occur, one of the wires in the area of the left screw must have arced.