What happened
On 11 May 2007, an SD3-60 Variant 100, registration G-CLAS, was performing a commercial cargo flight near Stansted Airport. The aircraft was operating without a payload during a period of light rain. Shortly after climbing to approximately 1,500 ft, the flight crew noticed a burning odor within the cockpit.
As the smell intensified, the commander opened the cockpit door and observed a significant amount of what appeared to be smoke in the rear of the main cabin. Given the proximity to the landing runway, the commander opted not to don smoke hoods, instead instructing the first officer to declare an emergency and request an immediate return to the airport for an ILS approach to Runway 23.
Upon landing and clearing the runway, the commander verified that the smoke had dissipated. The aircraft taxied to a remote stand where airport fire services attended. Using thermal imaging equipment, the fire crew inspected the airframe but found no active heat sources.
The investigation
The investigation focused on the source of the burning smell and the visible smoke. Technical analysis revealed that a small wire within a lighting circuit, located behind panel 4P above the first officer's head, had been chafing.
It was determined that this chafed wire was the source of the burning smell. Regarding the visible smoke, investigators concluded that the substance seen in the cargo area was actually mist formed by the mixing of warm and cold airflows, which had been misidentified during the emergency as smoke.