U.S. Army Air Force Glider Aerial Retrieval System in the Second World War.
Military gliders have been used by the militaries of various countries for carrying glider infantry troops and heavy equipment to a combat zone, mainly during the Second World War. These engineless aircraft were towed into the air and most of the way to their target by military transport planes, e.g., the Douglas C-47 Skytrain or Dakota, or bombers such as the Short Stirling, which were relegated to secondary activities.
The gliders, once released at some distance from the actual target, were silent in their descent and difficult for the enemy to identify as they attempted to land on any convenient open terrain close to target, while sustaining as little damage to the cargo and crew as possible. Most Landing Zones were far from ideal, however, and the one-way nature of the missions meant that the gliders were treated as semi-expendable, leading to construction from common and inexpensive materials such as wood.
During the Second World War, only the Waco Aircraft Company was able to deliver the experimental glider prototypes that satisfied the requirements of the U.S. Army Materiel Command, with the Waco nine-seat CG-3 and fifteen-seat CG-4. The shock of the Japanese attack on Pearl Harbour on 7 December 1941 prompted the United States to set the number of glider pilots needed at 1,000 to fly 500 eight-seat gliders and 500 fifteen-seat gliders. The number of pilots required was increased to 6,000 by June 1942,  but by this time, the U.S. Army Air Force (USAAF) glider program had already decided that the big troop and cargo carrying gliders were simply too expensive to be abandoned after just one combat mission. The fields where gliders landed were usually too small and too uneven for a plane to land and retrieve the gliders, so another means of recovery had to be found.
The USAAF turned to All American Aviation (AAA) of Wilmington, Delaware. This company had developed an aerial retrieval system to pick up postal pouches from the ground by an aircraft in flight, and had also demonstrated that aerial retrieval of gliders was feasible. Various tests and redesigns took place, and at the end of 1942 contracts were signed between USAAF and AAA to provide pickup equipment initially for use with training gliders and finally for gliders weighing from 8,000 to 16,000 pounds.
A new Model 80 heavy duty pick up system was subsequently designed incorporating improved braking systems that made use of a 'time delay' function where the first several drum rotations had a reduced braking force to allow the drum to accelerate without applying high cable loads, and then the final brake force was gradually applied until the pre-set line tension was established. A 182 ft. nylon tow rope, 15/16 in. in diameter was initially used. One end of the tow rope was connected to the nose of the glider and the other end to a closed loop of nylon rope, 80 ft. in circumference, which was draped over the top of two 12 ft. stanchions spaced 20 ft. apart and positioned ahead of and slight to the right of the glider.
Du Pont nylon ropes greatly enhanced glider "snatch pickups", because the woven nylon rope was elastic and stretched 25 - 30 percent of its length and absorbed much of the shock as the glider became airborne. The rope would then return to its original length. The glider would usually become airborne in no more than 200 feet.
Douglas C-47s were the primary aircraft used to test the Model 80C pickup system which was bolted to the floor on the left side of the cabin about 6 ft. from the front bulkhead. The plane approached the pick-up station at a 45 degree angle about 20 ft. above the terrain at 130 mph to 145 mph, depending on the gross weight of the glider. At the moment the tow rope was snagged by the aircraft's trailing hook, the winch drum began to feed out cable rapidly, then more slowly as the brake took effect. Some of the initial shock was absorbed by the unwinding cable and by the elasticity of the nylon rope. In a matter of about seven seconds the glider would accelerate from 0 to 120 mph and be airborne in as little as 60 ft. At the same time the extra weight of the glider slowed the plane to about 105 mph. The cable drum slowly braked to a full stop and then reversed. The glider, now 500 ft. or more behind the plane, was slowly winched-in to the tow rope hook. Various tests continued and once the aerial retrieval system became operational, glider pilot students were required to participate in at least one snatch pickup as first pilot and one as co-pilot.
Glider snatch pick was employed by the USAAF in France, Holland, Germany and Burma during the Second World War. Records show 13 gliders from Normandy, 256 from Holland and 148 from Germany being successfully recovered from downed locations. However, there were also many problems with landings in heavily treed fields, or damage done on landing that was beyond repair, or by German shelling. On 22 March 1945, an historic medical evacuation mission took place when two CG-4As landed in a clearing near the Remagen bridgehead to evacuate 25 severely injured American and German casualties. Each glider was fitted with six stretchers suspended by nylon straps on each side of the cargo area. Once loaded the gliders were successfully snatched from their landing site by a C-47 transport and flown to a military hospital in France.
Once the retrieval of a single glider was perfected, experiments began to retrieve two gliders with a single tow plane. Two gliders were properly lined up with two separate tow ropes and two separate ground stations for holding each pickup loop. The C-47 tow picked up the first glider and then circled back and snatched the second glider. The difficulty in the double retrieval was the transfer of the first rope from the winch pickup cable to the normal tow point on the tail of the tow plane. This was necessary so that the winch cable and pickup hook could be reset on the boom in order to pick up the second glider. With practice this procedure eventually could be done in less than 15 minutes.
 Taken from an article by Major Leon Boyce Spencer, USAF, (17 September, 1924 – 28 December, 2016)
Second World War Glider Pilot Historian.
 Devlin, Gerald M. (1985). Silent Wings. W. H. Allen & Co.
1 A U.S. Army Air Force Waco CG-4A-WO glider (s/n 42-79211) in 1943. (Public Domain)
2 Aerial view of a C-47 Dakota as it tows off a CG-4A Waco glider from a British airfield en route for Holland, on 17 September 1944. (Public Domain)
3 Laurinburg-Maxton Army Air Base CG-4 Glider taking off after snatch pickup in 1942. (USGOVPD)