1958 Tybee Island mid-air collision

The Tybee Island B-47 crash was an incident on February 5, 1958, in which the United States Air Force lost a 7600 lb Mark 15 nuclear bomb in the waters off Tybee Island near Savannah, Georgia, United States. During a practice exercise, the B-47 bomber carrying the bomb collided in midair with an F-86 fighter plane. To protect the aircrew from a possible detonation in the event of a crash, the bomb was jettisoned. Following several unsuccessful searches, the bomb was presumed lost somewhere in Wassaw Sound off the shores of Tybee Island.

Midair collision
The B-47 bomber was on a simulated combat mission from Homestead Air Force Base in Florida. It was carrying a single 7600 lb bomb. At about 2:00 AM, the B-47 collided with an F-86. The F-86 crashed after the pilot ejected from the plane, but the B-47, although damaged, remained airborne, albeit barely. The crew requested permission to jettison the bomb in order to reduce weight and prevent the bomb from exploding during an emergency landing. Permission was granted, and the bomb was jettisoned at 7200 ft while the bomber was traveling at about 200 knot. The crew did not see an explosion when the bomb struck the sea, and managed to land the B-47 safely at the nearest base, Hunter Air Force Base. The pilot, Colonel Howard Richardson, was awarded the Distinguished Flying Cross after this incident.

Bomb
Some sources describe the bomb as a functional nuclear weapon, but others describe it as disabled. If the bomb had a plutonium nuclear core installed, it was a fully functional weapon. If the bomb had a dummy core installed, it was incapable of producing a nuclear explosion but could still produce a conventional explosion. The 12-foot (4 m) long Mark 15 bomb weighs 7600 lb and bears the serial number 47782. It contains 400 lb of conventional high explosives and highly enriched uranium. The Air Force maintains that the bomb's nuclear capsule, used to initiate the nuclear reaction, was removed before its flight aboard B-47. As noted in the Atomic Energy Commission "Form AL-569 Temporary Custodian Receipt (for maneuvers)", signed by the aircraft commander, the bomb contained a simulated 150-pound cap made of lead. But according to 1966 Congressional testimony by then Assistant Secretary of Defense W.J. Howard, the Tybee Island bomb was a "complete weapon, a bomb with a nuclear capsule," and one of two weapons lost by that time that contained a plutonium trigger. Nevertheless, a study of the Strategic Air Command documents indicates that in February 1958, Alert Force test flights (with the older Mark 15 payloads) were not authorized to fly with nuclear capsules on board. Such approval was pending deployment of safer "sealed-pit nuclear capsule" weapons that did not begin deployment until June 1958.

Recovery efforts
Starting on February 6, 1958, the Air Force 2700th Explosive Ordnance Disposal Squadron and 100 Navy personnel equipped with hand held sonar and galvanic drag and cable sweeps mounted a search. On April 16, the military announced the search had been unsuccessful. Based on a hydrologic survey, the bomb was thought by the Department of Energy to lie buried under 5 to 15 feet (2 to 5 m) of silt at the bottom of Wassaw Sound.

In 2004, retired Air Force Colonel Derek Duke claimed to have narrowed the possible resting spot of the bomb to a small area approximately the size of a football field. He and his partner located the area by trawling in their boat with a Geiger counter in tow. Secondary radioactive particles four times naturally occurring levels were detected and mapped, and the site of radiation origination triangulated. Subsequent investigations found the source of the radiation was natural. In any case, a Geiger counter is not efficient in detecting alpha radiation from enriched uranium, because of the minimal ability of alpha particle to penetrate water (a few millimeter at most).

Ongoing concerns
The risk of corrosion of the bomb's alloy casing is lessened if it is completely covered in sand. But if part of the casing is exposed to seawater due to the shifting strata in which it is buried, rapid corrosion could occur, as demonstrated in simulation experiments. Eventually, the highly enriched uranium could leach out of the device and enter the aquifer surrounding the continental shelf in that area. Storms, hurricanes, and strong currents frequently shift the sand there.

To date, no undue levels of unnatural radioactive contamination have been detected in the regional Upper Floridan aquifer by the Georgia Department of Natural Resources (over and above the already high levels thought to be due to monazite, a locally occurring sand naturally high in radiation).

This event, as well as the 1956 B-47 disappearance, were the basis for NCIS Episode "Broken Arrow," which aired in 2010.