Investigation Summary
The aircraft, V9977, was specifically chosen for H2S radar development because its design allowed for the installation of a large 8 foot perspex radome under the fuselage. On the day of the accident, the flight was intended to demonstrate the radar's improved performance to engineers from EMI who were preparing for mass production. Investigators determined that the fire originated in the number four (starboard outer) engine. A recent maintenance procedure, mandated by Rolls-Royce to address tappet valves working loose, had been performed on the engine just three hours before the flight. A lock nut on one of the 48 tappet valves was not sufficiently tightened, allowing the valve to unscrew and eventually fracture the valve stem. This failure allowed the fuel-air mixture to escape into the rocker cover area where it ignited. Because the propeller continued to windmill, the engine-driven fuel pumps continued to feed the fire. The crew was unable to reach the fuel shut-off valves, which were located behind the cockpit in a difficult position, and the fire extinguishers failed to suppress the flames, possibly because the bottles were empty or the fire had already bypassed the fireproof bulkhead.
Final Conclusions
The investigation led by AIB Chief Inspector Vernon Brown concluded that the primary cause was the improper tightening of a tappet valve nut by an RAF engine fitter. This maintenance oversight led to a catastrophic engine fire that the crew could not contain due to design flaws in the early Halifax Mk II, including the placement of fuel valves and extinguisher bottles on the wrong side of the fireproof bulkhead. The death of Alan Blumlein, a pioneer of stereophonic sound and television, was kept secret for three years to protect the security of the H2S radar project.
Video Analysis
No video analysis linked for this case file yet.
Crew Experience
First Pilot
P/O Douglas John Davies Berrington
Systems & Failure Modes
H2S Radar
A secret ground-mapping radar system using a cavity magnetron, mounted in a large ventral radome for navigation and targeting.
Rolls-Royce Merlin Engines
The aircraft was powered by four Merlin engines; the starboard outer engine suffered a mechanical failure of the tappet valve assembly.
Fuel Cut-off Controls
On early Halifax models, these controls were located far behind the cockpit, making them difficult for the crew to reach during an emergency.
Interesting Facts
- 01The crash is considered the deadliest accident in the history of British military test flying.
- 02Alan Blumlein, the inventor of stereo sound, was among the 11 fatalities.
- 03The H2S radar was the first airborne microwave ground-mapping radar system.
- 04The fire extinguishers failed to operate, with later suggestions that the bottles may have been delivered empty.
- 05The pilot had only 13 hours of experience on the Halifax type at the time of the accident.
Safety Actions & Advisories
H2S Priority Directive
Despite the loss of key personnel, Prime Minister Winston Churchill ordered that the H2S project maintain its highest national priority status.
Known Controversies & Unanswered Questions
Sabotage Allegations
Reference ↗Due to the extreme secrecy of the project and the lack of initial public information, the widow of Alan Blumlein and EMI research head Isaac Shoenberg initially suspected the aircraft had been sabotaged.