The Boeing KC-33A

The KC-33A was Boeing’s proposal to turn the 747 into an aerial refuelling tanker and heavy transport for the US Air Force, offered during the 1970s Advanced Cargo Transport Aircraft (ACTA) programme.

What it offered

On paper the case was strong. The 747 could carry more fuel than anything else available, and offload more of it further from base — which is precisely what a tanker is for. It could also carry outsize cargo on the main deck, giving the Air Force a single aircraft that could refuel and airlift.

The proposal put a refuelling boom under the rear fuselage, in the conventional American arrangement.

Why it lost

The competition was won by the McDonnell Douglas DC-10-30, which entered service as the KC-10 Extender.

The reasoning was mostly about size. The 747 offered more capability than the mission required, at a higher purchase and operating cost, and it needed longer runways and more ramp space — which restricted the airfields it could work from. The DC-10 carried enough fuel to do the job, cost less to buy and run, and fitted more places.

For a fleet purchase, “enough capability at lower cost, usable from more airfields” beat “more capability than needed, from fewer”.

Whether that was right

The judgement has been argued over ever since. Advocates of the KC-33A point out that a larger tanker offloading more fuel per sortie means fewer aircraft and fewer crews for the same effect, and that the Air Force spent the following decades wanting more tanker capacity than it had.

What is not in dispute is that the 747 never became a US tanker — though Iran operated a small number of 747 tankers, making it one of the few air forces to fly the configuration Boeing proposed.