Incident
During passenger boarding, the flight crew was notified of smoke emanating from the passenger lavatory. In response, the crew directed the flight attendants to evacuate the aircraft. All passengers and crew members exited the aircraft normally without reported injuries.
Inspection Findings
Following the incident, an inspection of the aircraft revealed that wiring inside the water heater was discolored and melted near a terminal within the unit. The wire showed signs of chafing adjacent to the terminal screw. Additionally, the water heater installation was found to be missing a grounding strap.
Determination
A Federal Aviation Administration (FAA) inspector working with Midwest Express personnel determined that the absence of the grounding strap prevented the water heater's circuit breaker from tripping during the electrical fault.
Aftermath
Subsequent to the incident, Midwest Express Airlines inspected their DC9 aircraft to ensure the presence of the grounding strap on all similar installations.