I believe that the rucksacks were used by the Glider Pilot Regiment mostly, if not exclusively, in the air landing brigades. It makes sense that the pilot and copilot could not wear their webbing at the controls, nor could they toss them aside behind the seats. I think they would have to stuff all their gear and weapons into the large rucksack, then extract them later. There would be no need for a small pack, since the partially emptied out rucksack would serve that purpose.
The jeep photo above has been credited to the 1st Battalion, Royal Ulster Rifles Regiment, serving with the 6th Air-landing brigade at Normandy. Authors David Orr and David Truesdale, authors of "The Rifles are There", the RUR's recent history, note that the two men in the photograph (Plate 14) wearing rucksacks are identifiable as the pilot and copilot, specifically because they were wearing rucksacks, as opposed to everyone else who is wearing standard webbing. It doesn't look like they had yet pulled their gear out when the picture was taken.
British glider pilots were all elite soldiers, all volunteers, taken from established infantry units, whose minimum rank after graduation as pilot was that of sergeant. Most volunteers didn't make the cut, so those that did were the cream of the crop.
Author Harlan Glenn writes that either they were all saddled with the awful Sten Mark III's, or that some carried some rifles and others stens, of which the latter were all mark III's.
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