What happened
During the boarding process, the flight crew received notification that smoke was emanating from a passenger lavatory. In response to the report, the crew instructed flight attendants to perform an evacuation of the DC9. All passengers and crew members successfully exited the aircraft without incident; there were no injuries.
The investigation
An inspection of the aircraft following the event identified that wiring inside the water heater unit had become discolored and melted near a terminal. Investigators noted evidence of chafing on the wire in the area adjacent to the terminal screw.
Findings
A joint investigation by the Federal Aviation Administration and Midwest Express personnel determined that the water heater installation was missing a grounding strap. This absence prevented the circuit breaker for the water heater from tripping during the electrical fault. Following the incident, Midwest Express Airlines conducted inspections of its DC9 fleet to ensure all required ground straps were properly installed.