It didn’t take long for these huge single tires to prove their impracticality. Both of these aircraft were so heavy when fully loaded that the single tires exerted enough pressure to crush through the pavement of almost any runway in existence. In fact, only three airfields in the entire country had specially reinforced concrete runways capable of accommodating these aerial giants. This serious limitation prompted designers to retrofit both the XC-99 and B-36 with smaller four-wheeled main landing gear bogies. These spread the weight over a wider area and greatly expanded the number of airfields these aircraft could safely use.
The flight crew of the XC-99 typically consisted of the pilot, copilot, navigator, radio operator and two flight engineers. In addition two scanners were also routinely included, whose job it was to advise the pilot about engine performance, flaps and landing gear operation. None of these functions were visible from the flight deck.
For long flights, a relief crew also went along. These off-duty crewmen traveled in relative luxury, having at their disposal bunks and a galley equipped with hot plates, an electric oven, icebox, dining table, chairs and food storage compartments. In addition, there was hot and cold running water, an electrical incinerating toilet and even a private compartment for the aircraft commander.
The XC-99 boasted a maximum fuel capacity of 21,116 gallons, giving it an unmatched range of 8,100 miles with a limited load of 10,000 pounds. With a full 100,000-pound load, it could still fly an impressive 1,720 miles at just over 300 mph.
Amazingly, the XC-99’s huge control surfaces – almost as large in area as the entire wing of a B-24 bomber – were operated without any power boost. Instead, the controls were linked to smaller trim tabs that aerodynamically moved the larger control surfaces. Although understandably somewhat sluggish, the aircraft reportedly handled well in spite of its size and its unconventional control mechanism.
The XC-99 established its place in aviation history as it broke almost two dozen international records for cargo hauling and distance. It amazed the world for almost a decade by airlifting massive loads thousands of miles nonstop. Everywhere it landed, the aerial giant was the object of attention, and it was a regular feature both in the press and on the air.
As a consequence of its far-reaching travels and imposing appearance, the much-celebrated XC-99 became a sort of ambassador of the air for the Air Force. It was a source of great pride for Americans, and equally important, a highly visible symbol of U.S. military dominance during a period of high international tension.
The civilian transport version of the XC-99 never made it off the drawing board. The massive double-deck fuselage of the so-called Model 37 was designed to carry 204 passengers plus more than seven tons of cargo. It was anticipated that customers would pay well to travel luxuriously between New York and London in only nine hours. Pan American World Airways was so impressed that it ordered 15 of these “Super Clippers” in February 1945. They later cancelled this order, but kept an option open on three aircraft.
When the intended engine for the Model 37 – a 5,000 hp gas turbine –failed to become available, the airline was forced to cancel the order altogether. The operating costs of the existing 3,500 hp radials were too great to allow the proposed airliner to be flown profitably. Besides, it was becoming apparent that post-war airline demands would not support the use of such a large capacity aircraft. This ended any prospect for a Convair “Super Clipper.”
Almost inexplicably, the Air Force also declined to order additional copies of the XC-99, in spite of its many aeronautical successes. It could carry more cargo farther and cheaper – at 16 cents per ton-mile – than any other military transport in existence at the time. Because of this, the Air Force operated its single XC-99 longer and more extensively than any other original prototype – 7,400 flight hours and almost eight years. Why then after years of outstanding service did no orders for additional aircraft materialize?
Probably the biggest enemy of the XC-99 was timing. The era of piston-engine, propeller-driven aircraft was nearly at its end by the time the Air Force accepted the XC-99 in 1949. Some argued that even the B-36 was obsolete from the outset and should have been scrapped in favor of the newer and faster generation of jet bombers, such as the B-47. Thus, a large investment in the relatively slow piston-engine transport was just not in the best interests of the Air Force.