On January 21, 1968, a B-52G Stratofortress with the callsign "HOBO 28" from the 380th Strategic Bomb Wing at Plattsburgh Air Force Base, New York was assigned the "Hard Head" mission over Thule and nearby Baffin Bay. The bomber crew consisted of five regular crew members including Captain John Haug, the aircraft commander. Also aboard was a substitute navigator (Captain Curtis R. Criss), and a mandatory third pilot—Major Alfred D'Mario.
Before take-off, D'Mario placed three cloth-covered foam cushions on top of a heating vent under the instructor navigator's seat in the aft section of the lower deck. Shortly after take-off, another cushion was placed under the seat. The flight was uneventful until the scheduled mid-air refueling from a KC-135 Stratotanker, which had to be conducted manually because of an error with the B-52G's autopilot. Approximately one hour after refueling, while the aircraft was circling above its designated area, Captain Haug directed co-pilot Svitenko to take his rest period. His seat was taken by D'Mario. The crew was uncomfortable because of the cold, despite the heater's rheostat being turned up—D'Mario opened an engine bleed valve to increase the cabin's temperature by drawing additional hot air into the heater from the engine manifold. As a result of a heater malfunction, the temperature drop between the engine manifold and the cabin's heating ducts was negligible; during the next half hour, the cabin's temperature became uncomfortably hot, and the stowed cushions ignited. A member of the crew reported smelling burning rubber, and a search was mounted for the fire. The navigator searched the lower compartment twice before discovering the fire behind a metal box. He attempted to fight it with two fire extinguishers, but could not put out the blaze.
Thule Air Base in the foreground with North Star Bay, which was covered in sea ice at the time of the accident, in the background.
At 15:22 EST, approximately six hours into the flight and 90 miles (140 km) south of Thule Air Base, Haug declared an emergency. He advised Thule air traffic control that he had a fire on board and requested permission to perform an emergency landing at the air base. Within five minutes, the aircraft's fire extinguishers were depleted, electrical power was lost and smoke filled the cockpit to the point that the pilots could not read their instruments. As the situation worsened, the captain realised he would not be able to land the aircraft and advised the crew to prepare to abandon it instead. They awaited word from D'Mario that they were over land—when he confirmed that the aircraft was directly over the lights of Thule Air Base, the four crewmen ejected, followed shortly thereafter by Haug and D'Mario. The co-pilot, Svitenko, did not have an ejection seat and sustained fatal head injuries when he attempted to bail out through one of the lower hatches.
The pilotless aircraft initially continued north, then turned left through 180° and crashed onto sea ice in North Star Bay—about 7.5 miles (12.1 km) west of Thule Air Base—at 15:39 EST. The conventional high explosive (HE) components of four 1.1 megaton B28FI model hydrogen bombs detonated on impact, spreading radioactive material over a large area in a similar manner to a dirty bomb. A nuclear explosion was not triggered. The extreme heat generated by the burning of 225,000 pounds of aviation fuel during the five to six hours after the crash melted the ice sheet, causing wreckage and munitions to sink to the ocean floor.
The gunner (center), SSgt Calvin Snapp, is rescued after ejecting onto the ice
Haug and D'Mario parachuted onto the grounds of the air base and made contact with the base commander within ten minutes of each other. They informed him that at least six crew ejected successfully and the aircraft was carrying four nuclear weapons. Off-duty staff were mustered to conduct search and rescue operations for the remaining crew members. Three of the survivors landed within 1.5 miles (2.4 km) of the base and were rescued within two hours. Captain Criss, who was first to eject, landed 6 miles (9.7 km) from the base—he remained lost on an ice floe for 21 hours and suffered hypothermia in the −23 °F (−30.6 °C) temperatures, but he survived by wrapping himself in his parachute.
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