Incident Overview
On 6 April 2005, a Shorts SD3-60-100 (registration EI-SMB) was conducting a nighttime Royal Mail cargo flight from Edinburgh to Kinloss. The weather was good with a surface wind from 180° at 10-15 kt, and Runway 12 was in use. The aircraft carried two crew members and no passengers.
Discovery of Vapour
A few minutes after takeoff, while passing FL55, the commander detected an unusual smell. A visual inspection of the cargo area through the cockpit door revealed a white vapour emanating from under the cargo net and filling the cargo compartment. The commander assumed the vapour was smoke, but the central warning panel gave no smoke or fire warnings.
Return to Edinburgh
The crew informed Air Traffic Control of a fire on board and requested an immediate return to Edinburgh. While being radar vectored for an SRA approach to Runway 12, a further visual inspection showed that the cargo section was now filled with dense white smoke. The crew completed the Cargo Fire checklist and made oxygen masks and goggles available. Once stabilised on finals for a visual approach, another inspection revealed that the smoke had cleared.
Post-Landing Actions
The aircraft landed without further incident and was brought to a halt on the runway and shut down. The Airport Fire Service (AFS) attended immediately, and the crew evacuated the aircraft. The AFS used a thermal imaging camera to inspect the aircraft, finding no signs of heat or fire. All packages in the cargo hold appeared normal, intact, and undamaged. The AFS could not identify the contents of the cargo because no shipper's declaration was available from the commander or the Royal Mail; however, such a declaration was not required as there were no dangerous goods on board.
Subsequent Investigation
After the aircraft was taxied to a stand, the cargo was offloaded for further investigation with Royal Mail personnel present. Nothing untoward was found on the aircraft that could have caused the smoke in the cabin. The cargo was transported by road to Inverness and held separately at a sorting office. Royal Mail personnel conducted a visual external search of all packages before release for distribution and delivery. No evidence of leakage or damage to packaging was found.
Conclusion
It has not been possible to identify the source of the smoke or vapour that led the crew to believe there was a fire on board.