What happened
On 18 November 2000, a Boeing 767-300, registration N657UA, was performing a scheduled passenger flight from London Heathrow Airport. Shortly after departing the airfield, cabin crew members noticed smoke coming from the right-hand business class toilet area.
In response to the sighting, the crew used a Halon fire extinguisher to spray the toilet waste disposal chute. The flight crew was notified of the potential fire, prompting the commander to declare an emergency with Air Traffic Control. The aircraft was instructed to return to Heathrow, with a request for emergency services to be on standby for its arrival.
Upon landing safely back at the airport, the smoke dissipated. Personnel from the Airport Fire Service boarded the aircraft to conduct a thorough inspection of the lavatory, but they were unable to find any visible evidence of an active fire.
The investigation
Investigators looked into the source of the smoke and the mechanical state of the aircraft's systems. The maintenance organization responsible for the aircraft's upkeep conducted a follow-up inspection of the onboard equipment.
Findings
It was determined that the smoke was caused by a short-circuit within the toilet's water heater, which led to the unit overheating. Following the discovery, the faulty water heater was replaced to prevent a recurrence.