Investigation Summary
The flight was the Kano to Tripoli leg of a long haul service originating in Lagos and destined for London. At the time of departure, moderate rain was falling, but conditions appeared acceptable for takeoff. The aircraft, named 'Argo', departed Runway 25 at 17:21 GMT. After a normal initial climb to 240 feet, the crew retracted the flaps, at which point the airspeed was approximately 125 to 130 knots. Almost immediately, the aircraft entered a localized thunderstorm cell that was not visible to the pilot or predicted by the meteorological office. The encounter resulted in a sudden drop in airspeed to 103 knots, dangerously close to the stall speed of 97 knots. The pilot applied full power and attempted to lower the nose to regain speed, but the aircraft entered a high rate of descent. At an altitude of only 15 to 20 feet above ground level, the pilot observed a tree directly in the flight path and attempted to bank right to avoid it; however, during this maneuver, the left wing struck a different 35 foot tall tree, rupturing fuel lines and causing the aircraft to crash and catch fire.
Final Conclusions
The Board of Inquiry concluded that the accident was caused by a loss of height and airspeed resulting from an encounter with an unpredictable thunderstorm cell. This cell produced a sudden reversal of wind direction, heavy rain, and downdrafts. The investigation cleared the pilot in command of any blame, noting that the cell was neither visible nor predictable with the technology available at the time. The inquiry recommended that ICAO investigate the specific hazards of operating in close proximity to thunderstorms.
Photographic Evidence (1)
Video Analysis
No video analysis linked for this case file yet.
Systems & Failure Modes
Rolls-Royce Merlin 622 Engines
Liquid-cooled V12 piston engines; the pilot applied full power to these units in an attempt to arrest the descent, but the downdraft and wind shear exceeded the aircraft's performance capabilities.
Flaps
Retraction of the flaps at 240 feet coincided with the encounter with the thunderstorm cell, which exacerbated the loss of lift during the sudden airspeed drop.
Master RPM Lever
The master RPM lever was unserviceable at the time of the accident. This prevented the co-pilot from increasing the engine speed to the maximum rated 3,000 RPM during the emergency descent.
Interesting Facts
- 01The aircraft was powered by four Rolls-Royce Merlin 622 engines.
- 02The stall speed of the Canadair C-4 Argonaut in the accident configuration was 97 knots.
- 03The crash site was located approximately 1.5 miles (2.4 km) from the end of the runway.
- 04The thunderstorm cell encountered by the crew was not visible from the cockpit before takeoff.
- 05The accident resulted in 32 fatalities (29 passengers and 3 crew members) out of 45 people on board (38 passengers and 7 crew).
- 06Due to a localized pressure rise of 2-3 millibars within the storm cell, the aircraft's altimeters under-read, meaning the actual altitude was 50 to 100 feet higher than indicated.
Safety Actions & Advisories
ICAO Thunderstorm Hazard Study
The Board of Inquiry recommended that the International Civil Aviation Organization (ICAO) urgently investigate the special hazards to aircraft inherent in taking off or landing in close proximity to thunderstorms.