The Boeing YC-14; Taking on the King

October 24, 2023

The Vietnam War played a huge part in forming the doctrine of United States air power. Issues in air-to-air combat led to the establishment of dedicated schools teaching Air Combat Maneuver tactics, as well as hugely influencing the design of the next generation of United States Air Force fighters, most notably the McDonnell Douglas F-15 and the General Dynamics F-16. Ground attack experience led to the rapid development of precision weapons and in the creation of Fairchild A-10 “Warthog”.

But less well remembered is the impact that Vietnam had on the USAF’s thinking on transport aircraft, especially tactical ones.

Because despite all the reams and reams of dramatic footage of MiGs being shot down and napalm explosions, in fact arguably the most important contribution that the USAF made to the entire war effort was in running logistics.

Let’s be blunt, it was the ability of the USAF transport fleet, running their seemingly steady and boring flight routes all across South Vietnam, supplying both US and allied forces with all of the minutiae that makes war possible, that allowed the conflict to be fought.

If you want a classic example of the importance of this, we can compare two of the most famous battles to occur during the long conflict; Dien Bien Phu in 1954 and Khe Sanh in 1968. While both were obviously extremely complex, multifaceted operations that relied on a whole host of factors to determine the respective outcomes, I would say that the most important difference is that the Frech defeat at Dien Bien Phu was due to their inability to supply their troops by air, while the US victory was principally possible because they were able to.

While this importance is easy to overlook, the USAF were certainly thinking about the value of air transport and the aircraft that they had available to perform the job. And though the respective development program in fighters and attack aircraft are well known and often discussed, less well remembered is that USAF Transport Command did decide due to their Vietnam experience that though their existing transports, principally the Lockheed C-130 Hercules was pretty damn good, they wanted something even better.

After all, why should just the fast boys get all the pretty new toys?

In 1968 the USAF began a study to examine requirements for a future medium tactical transport that would be capable of replacing the C-130., which was followed in 1972 with a requirement for a new aircraft, the Advanced Medium STOL Transport (AMST).

This was to be broadly in the same class as the Hercules, but with much greater capabilities, capable of using shorter landing strips with greater payloads at higher speeds, creating a far more efficient transport.

Especially of interest was improved Slow Take Off and Landing (STOL) abilities. The requirement hoped for an aircraft that could clear a fifty-foot obstruction in a takeoff distance of 2,000 feet (610m) with a payload of 14 short tons (12,700kgs). For comparison, the then is service C-130 required 3,500 feet (c.1,070m) of strip to clear a similar obstruction with only 10 short tons (c.9,000kg).

The new aircraft should also be capable of carrying bigger vehicles than possible with the C-130, in particular the Army’s M109 155mm and M110 203mm self-propelled howitzers. Additionally, with much higher speed potentially offered by jet propulsion, the USAF calculated that a new aircraft would be more than twice as efficient in ton-miles as the C-130.

Obviously, the prospect of replacing the USAF’s entire Hercules fleet, let alone additional export sales, made the prospect hugely lucrative potentially and basically all of the United States major aircraft producers issued proposals. In November 1972 this was slimmed down to just two options, those of Boeing and McDonnell Douglas, and orders were issued for two prototypes of each companies offering.

I’ll cover McDonnell Douglas’ YC-15 in my next article, and today will look at the Boeing YC-14.

Boeing opted for a rather innovative design that actually placed twin turbofans above the wing rather than below it as was conventionally recognized, as well as tucking the engines in tight to the fuselage, giving the YC-14 a very distinctive look.

The concept had been previously examined by NASA, and Boeing was able to utilize their research into the idea and how correct flap construction would enable the jet thrust to stick to the upper wing surfaces and be directed downwards to provide huge upward thrust, essential to provide the amounts of lift the aircraft would require.

Additionally, Boeing made a smart decision with the engine choice, selecting the General Electric CF6-50D, which each produced 51,000lb of thrust. This was then a brand-new design that Boeing engineers were studying for use on other aircraft that they built and would ultimately go on to be used by some operators of Boeing’s 747 airliner as well as being adopted by the USAF as the F103.

On the YC-14 it was combined with huge thrust reversers that allowed the aircraft to stop in an amazingly short distance.

The amount of development work testing the new wing-and-engine concept meant that the YC-14 took longer to get ready than the rival YC-15, and it wasn’t until August 1976 that the first prototype was able to conduct the type’s first flight, a year after the McDonnell Douglas aircraft.

But from the first the YC-14 proved an excellent flyer. With just an eight hundred foot (244m) take off roll the YC-14 demonstrated a capability to lift off and climb at 6,000 feet per minute (c.1830m/min), more than three times that of the in-service C-130 variants. The shortest landing recorded during the flight tests was 387 feet (118m), which was achieved due to a 21-knot head wind, but is still a truly remarkable figure for such a big aircraft. Even in low wind conditions average landing was still only eight hundred feet (244m).

The aircraft also had an impressive ability at reversing because of the deflectors directing air flow over the wings. Minimum landing speed was recorded as 68 mph (109 km/h) while a top speed of 504 mph (811 km/h) at 38,000 feet (11,600 m) was recorded.

Payload was planned to be a maximum of 31,400kgs (69,000lbs) while maximum load for STOL operations was 12,300kgs (27,000lb), or alternatively a load of 150 paratroopers.

However, with flight testing almost completed the Air Force suddenly announced they wanted to see if the new designs could carry a somewhat heavier load…an M60A2 battle tank. At 49,500Kgs (c.109,000lbs) that was a pretty big leap in requirement. But the YC-14 managed it, proving able to load the tank in less than two minutes, though it did not fly with the massive load.

If this wasn’t all impressive enough, Boeing also added a new innovation that, as far as I know, had never been fitted to transport aircraft up until that time;  digital flight information readout.

We might be used to the concept of the “glass cockpit” today, but in the YC-14 Boeing were really demonstrating the future.

Of course, for such an innovative aircraft the YC-14 did have some problems. Flight tests showed that drag was greater than expected on the YC-14, but Boeing tweaked the aircraft by adding vortex generators on the wings which helped resolve the problem.

But remarkably that was the only real issue and in 1977 the aircraft completed trials with flying colours. The YC-14’s then conducted a tour of several other countries as Boeing began to market the type to NATO air forces, causing a great deal of interest.

Things would have appeared to have rather rosy for Boeing’s interesting new transport. Unfortunately for the YC-14, and indeed the competing YC-15, the USAF’s priorities had changed in the years since the requirement had been issued.

Vietnam was over, and with it the perceived need to replace the C-130. That aircraft still had plenty of development potential, hence people literally ordering new build versions right up to the current day.

In fact, the USAF was now more concerned with its strategic transport mission rather than tactical. The Vietnam War had fueled a huge market for contract air transport services that had massively supplemented the USAF’s own capabilities. But the end of US involvement in Southeast Asia – and the massive oil crisis post the 1973 Yom Kippur War – meant that these private companies had seen their contracts end and as a result the divestment of much of their air transport fleets.

Additionally, though the C-5 Galaxy was providing the USAF with a substantial heavy lift capability, most of the strategic lift capacity was provided by the Lockheed C-141 Starlifter. These aircraft, though a fine design, had seen much heavier usage than anticipated and as a result would need to undergo substantial life extension programs.

Because of all this the USAF decided that what they wanted was something between the AMST design and the C-5 Galaxy, capable of operating in rough field STOL conditions but over much greater distances with much greater loads.

But that story needs to be told in relation to the McDonnell Douglas YC-15, which as said will follow shortly.

The long and the short of it was that the YC-14, though handsomely exceeding the requirements it was expected to meet, was now redundant when the USAF cancelled the AMST program in December 1979, despite the huge potential demonstrated by the YC-14. Indeed, it is arguably one of the best aircraft design’s built that never got to serve as, had it fulfilled the great promise it seemed to be showing, it would have been a truly superb transport.

But that was not to be and the two YC-14’s were retired, though both still exist; one is on display at the Pima Air Museum in Arizona while the other is currently stored at the USAF’s Davis Monthan “boneyard”.

There is one other detail that I should mention though. A year after the YC-14 flew the Soviet Antonov bureau flew this, the An-72.

Given the similarities of the design with its over wing engine design, this has understandably led to allegations that the An-72, which received the NATO designation of “Coaler”, was a product of Soviet espionage.

That is a possibility, though the Coaler is a far lighter aircraft, with roughly a third the maximum capability of the YC-14. Instead, I suspect that the An-72 was rather inspired by the initial Boeing design proposals, which were hardly secret, and the Antonov bureau thought that the over wing scheme a pretty good idea. Seeing as the An-72 and its various offshoots are still in service today and the type has a pretty solid reputation, it seems they were correct.

So perhaps the Boeing YC-14 did leave something of a legacy after all.

Sources/Related:

YC-14 STATUS REPORT

The AMST Program’s Lasting Legacy

https://en.wikipedia.org/wiki/Advanced_Medium_STOL_Transport

https://en.wikipedia.org/wiki/Boeing_YC-14

https://www.autoevolution.com/news/boeing-yc-14-proof-old-school-boeing-succeeded-even-when-they-failed-210153.html

https://www.youtube.com/watch?v=hOATpg4VP1U&t

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