What happened
A multi-crew flight consisting of engineers and pilots from Bristol and KLM was conducting a test flight departing from Bristol-Filton Airport. Approximately seven minutes into the flight, the crew observed rising temperatures in the third engine. Following this observation, the engine was deactivated and subsequently restarted once temperatures had stabilized.
As the Bristol Britannia climbed to 10,000 feet, the temperature in the same engine increased again, resulting in an explosion. The subsequent fire proved impossible to extinguish. As a safety measure, the crew shut down the fourth engine and initiated an emergency return to Filton. During the approach phase, both left engines ceased operation but were successfully restarted by the crew. Due to the deteriorating situation, the captain executed a belly landing in the Severn estuary near Littleton-upon-Severn. All 13 occupants survived the incident with only minor injuries, though the aircraft sustained damage that rendered it beyond repair.
Findings
Investigations revealed that the explosion of the third engine was triggered by a failure within the reduction gear. Specifically, a pinion located at the front of the propeller shaft lost its teeth. Because the Proteus turboprop engine utilized a free turbine design, this mechanical failure left the propeller turbine without a load. This caused the turbine to accelerate rapidly until it disintegrated. The resulting fire was initiated when debris from the explosion punctured the engine oil tank, igniting the fuel. Additionally, the shutdown of engines one and two was attributed to an electrical short circuit caused by the ongoing fire.