North-east Indian Railways in the Brahmaputra valley, with seven in Assam , four in the Bengal valley, and two near Calcutta, and terminated at six Chinaside airfields around Kunming. Through July 1944 the flight corridor for the India-China airlift was fifty miles wide with a highly restrictive vertical clearance. As bases expanded and the Low Hump route came into use, the corridor widened to and 25 charted routes, with a vertical clearance of 10,000–25,000 MSL in the south, permitting highly congested but controlled operations at all hours. Transport shortcomings A critical problem proved to be finding a cargo aircraft capable of carrying heavy payloads at the high altitudes required, and three types were tried before the opening of a low Hump route permitted the use of C-54s: C-47 and variants, C-46, and C-87/C-109. Initially, the India-China airlift was flown with the Douglas DC-3 and its military versions, the C-47, C-53 and C-39. However, the DC-3 design's fuselage was so high off the ground that loading from most truck beds was difficult, and its narrow door and weak floor could not sustain heavy cargo. Although C-47s had reinforced flooring and a wider door, they still required specialized loading equipment for much of the cargo needed in China and had a limited payload capacity. Also, performance specifications of the Douglas transports were not suited to high-altitude operation with heavy payloads, and could not normally reach an altitude sufficient to clear the mountainous terrain, forcing the planes to attempt a highly dangerous route through the maze-like Himalayan passes. The introduction in January 1943 of the Consolidated C-87 Liberator Express, a design modification of the B-24D heavy bomber, boosted tonnage figures. Its high-altitude capability enabled it to surmount the lower mountains (15,000–16000 feet MSL) without resorting to the passes, but the type had a high accident rate and was unsuited to the airfields then in use. Despite having four engines, the C-87 climbed poorly with heavy loads, as did its bomber counterparts, and often crashed on takeoff if an engine was lost. Its poor cockpit illumination was inadequate for bad weather flying and often failed during instrument takeoffs, its electrical and hydraulic systems frequently froze at high altitude and malfunctioned, and its flight deck heating system was prone to produce either stifling heat or none at all. Attributed by its pilots to its slim Davis wing, the C-87 also had a tendency to spin