The U.S. Navy is concerned about the growing use of anti-aircraft missiles by submarines. To deal with that problem, it is equipping some Mk-54 torpedoes, that are normally dropped into the water at a low altitude, by P-3 patrol aircraft, with an add on glide kit. This system, called "LongShot", consists of wings, control flaps, a flight control computer, battery and GPS for navigation. The kit would allow a Mk54 torpedo to be released at 20,000 feet, which is outside the range of submarine launched anti-aircraft missiles, and glide down to about 300 feet altitude, where the glide kit would be jettisoned, and the Mk-54 would enter the water and seek out the sub. Normally, the P-3 has to descend to under a thousand feet to launch the torpedo. This takes time, and puts stress on the aircraft. All existing P-3s are quite old, and it will be a few years before a replacement is ready, so reducing stress on the current ones is a major issue. Indeed, it may be the main issue for introducing the glide kit. The P-3 stress problem is rather larger than the number of subs out there equipped with anti-aircraft missile systems. These systems have been around for years, and many are basically shoulder fired type missiles adapted for launch from a water-proof container that is released by a submerged sub.
There are other reasons for LongShot. Many subs have sensors that are sensitive enough to detect low flying helicopters (the main target for the subs anti-aircraft missiles) and aircraft. The P-3 is also more effective if it can stay at high altitude all the time. Moreover, the glide kit is easy to build, since it can use items already used for smart bombs (JDAM) and earlier glide kits.
A P-3 usually carries eight torpedoes. The Mk54 is a new model, introduced three years ago. It is a 12.75 inch weapon, weighing about 700 pounds and with a warhead containing a hundred pounds of explosives. It's guidance system has been designed to work well in shallow coastal waters.