What happened
During the climb phase following departure, flight attendants detected the smell of burning. A passenger subsequently observed a "glow" inside a right-side air vent near their feet and reported that the cabin floor felt warm. In response, Halon was sprayed into the vent, which caused the glow to disappear.
The pilot declared an emergency and returned to the departure airport. Once the aircraft stopped on the runway, an emergency evacuation was conducted based on reports from emergency personnel.
The investigation
Investigators identified thermal damage and soot on the right-side underside floor structure, a nearby fiberglass bin wall panel, and a potable water bottle. A 5-foot by 5-foot section of insulation—composed of fiberglass with metallized mylar on both sides—had been burned.
Located under this insulation was the right-side alternate static port heater. The investigation revealed that the insulation on one of the wires leading to the heater had deteriorated and melted at the point where the wire was bent around the thermostat case. Investigators found evidence of metal transfer between the solid-core wire and the adjacent heater case, along with soot covering both the wire insulation and the heater case. The heater had been manufactured with the specific wire bend that caused the issue.