Investigation Summary
The flight was part of Operation Chrome Dome, a Cold War mission involving airborne nuclear-armed bombers. During the mission, a spare pilot stowed three cloth-covered foam cushions on top of a heating vent in the lower deck, and a fourth cushion was added shortly after take-off. When the cabin became uncomfortably cold, the pilot opened an engine bleed valve to increase the heat flow. Due to a heater malfunction, the air entering the cabin was significantly hotter than intended, causing the stowed cushions to ignite. The fire quickly became uncontrollable, depleting the onboard fire extinguishers and filling the cockpit with dense smoke that obscured the flight instruments. As electrical power failed, the commander ordered an emergency ejection over Thule Air Base. The pilotless aircraft flew for several minutes before impacting the ice at a 20 degree angle. The impact triggered the high explosives in all four nuclear weapons, creating a 'dirty bomb' effect that contaminated approximately 250,000 square feet of ice with plutonium and americium.
Final Conclusions
The accident was caused by the improper storage of combustible materials (foam cushions) over a heating vent, combined with a malfunction in the heating system that allowed excessively hot bleed air into the cabin. The loss of the aircraft and the subsequent nuclear contamination led to the immediate termination of the SAC Chrome Dome airborne alert program. The cleanup operation, known as Operation Crested Ice, involved removing massive amounts of contaminated ice and debris. The investigation also led to the development of insensitive high explosives (IHE) for use in nuclear weapons to prevent similar dispersals in future accidents.
Video Analysis
Broken Arrow Over Greenland: Thule, 1968 — A detailed look at the cabin fire and subsequent crash of a nuclear-armed B-52G near Thule Air Base, including the massive cleanup effort known as Operation Crested Ice.
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Systems & Failure Modes
Heating and Ventilation
A system using engine bleed air to warm the cabin; a malfunction prevented the air from cooling sufficiently before entering the ducts.
Ejection Seats
Six of the seven crew members were equipped with upward or downward firing ejection seats; the seventh had to bail out manually.
B28FI Thermonuclear Weapons
Nuclear gravity bombs containing conventional high explosives designed to trigger the nuclear reaction; these explosives detonated on impact.
Interesting Facts
- 01The aircraft was carrying four 1.1 megaton B28FI thermonuclear bombs.
- 02The fire was caused by foam cushions placed over a heating vent under the instructor navigator's seat.
- 03One of the nuclear weapon's secondary stages was never officially accounted for after the recovery operation.
- 04The crash site was located approximately 7.5 miles west of Thule Air Base.
- 05Temperatures at the time of the rescue and cleanup reached as low as -40 degrees Fahrenheit.
Safety Actions & Advisories
Termination of Operation Chrome Dome
The Strategic Air Command immediately discontinued airborne alert missions with nuclear-armed bombers due to the safety and political risks.
Development of Insensitive High Explosives
Research was accelerated into explosives like TATB that are less likely to detonate in a fire or impact, reducing the risk of radioactive dispersal.
Known Controversies & Unanswered Questions
Missing Nuclear Component
Reference ↗Despite an intensive cleanup, reports persisted that the secondary stage of one nuclear weapon was not recovered and may have melted through the ice into the seabed.
Danish Nuclear Policy
Reference ↗The accident caused a political scandal in Denmark when it was revealed that the government had tacitly allowed nuclear weapons in Greenland despite a public 'nuclear-free' policy.