High-bypass turbofans were introduced into the airliner market around 1970, when the first widebody jetliners were released (the 747 was first, followed in rapid succession by the DC-10, L-1011, and A300); however, narrowbodies continued to rely on low-bypass turbofans (primarily the Pratt & Whitney JT8D) for considerably longer. To the best of my knowledge, the first narrowbody powered by high-bypass engines was the DC-8-70 (a re-engining of the DC-8-60), which entered service in 1982; the first narrowbody built with high-bypass turbofans, the 757, did not enter service until 1983. As for the other narrowbodies of the time:

  • 707, 720, 1-11, F28: No high-bypass version ever became available (although high-bypass versions of the 707 and 1-11 were proposed, they never entered production).

  • 727: The only 727s that ever flew with high-bypass engines were four 727-100s reengined and renamed 727-100QFs in 1992.

  • 737: The first high-bypass version, the 737-300, was only introduced in 1984.

  • DC-9: Stubbornly held onto the JT8D into the mid-1990s, only switching to high-bypass engines with the DC-9-90.

What kept the narrowbodies from immediately, or almost immediately, taking advantage of the greater power and lesser fuel consumption and noise of high-bypass turbofans?

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    $\begingroup$ switching engines, in general, isn't easy. finding a proper engine with proper thrust and weight isn't easy either. it simply takes time for CFM56 to emerge and engineered to be a usable replacement for JT8D. $\endgroup$ – user3528438 Dec 19 '18 at 4:50

There was no suitable high-bypass-ratio turbofan engine available.

What were the narrow bodies flying in the 1960s and 70s? Here is a list:

  • Sud Aviation Caravelle
  • Boeing 707
  • Douglas DC-8
  • Hawker-Siddeley Trident
  • BAC One-Eleven
  • Boeing 727
  • Tupolev 134
  • Tupolev 154
  • Douglas DC-9
  • Vickers VC-10
  • Ilyushin Il-62
  • Boeing 737

The thrust needed for those aircraft, about 10 tons per engine, was not covered by the first generation of high-bypass-ratio engines. Remember, their development was started with the Heavy Logistics System (CX-HLS) program which eventually resulted in the General Electric TF-39 and the C-5 Galaxy. The maximum thrust of the TF-39 (43,300 lbf or 193 kN) is twice that of the Pratt & Whitney JT8D (21,000 lbf or 93.4 kN) which powered the Boeing 707, 727 and 737, the DC-8 and -9 and the Mercure. Another result of the CX program was the Pratt & Whitney JT9D which was in the same thrust class as the TF-39 and its civilian spin-off CF6.

Other early engines with higher bypass ratio like the General Electric CF700 were much too small with 4,200 lbf or 18.68 kN static thrust and did not offer enough of a jump in bypass ratio to become interesting for airliners.

It was not before the CFM56 became available in the 1980s that a suitable high-bypass-ratio engine in the 10-ton class existed. One reason for the long development time of the CFM56 was the delay in an export license for the engine core technology which was derived from the military F101 design. And when the CFM56 entered the market, it was first used to re-engine old Boeing KC-135 (the engines for the French KC-135s were the first big order), 707 and DC-8 aircraft which benefited much more from the new engine than the smaller mid- and short-range airliners.

The competition to the CFM56 (IAE V2500 and Pratt & Whitney 6000 series) both entered the market much later. There simply was no suitable engine available any sooner!

  • $\begingroup$ The CFM56 didn't take that long to trickle down to the smaller narrowbodies; the French government chose it to reengine its KC-135s in 1978, United Airlines decided to upgrade its DC-8s with the new engine in 1979, and Boeing released the design for the 737-300 in mid-1980, just six months after the engine won the USAF KC-135 contract. $\endgroup$ – Sean Dec 21 '18 at 3:58
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    $\begingroup$ @Sean: Yes, but it was still more than one decade later than the first widebodies with high bypass turbofans. The question asks why it took so long and this is the answer: There was nothing available before. $\endgroup$ – Peter Kämpf Dec 21 '18 at 22:37
  • $\begingroup$ @PeterKämpf In general it's easier to make a smaller engine than a big one, so if there was demand for a small high-bypass engine it would come first. For example, JT9D had fan case distortion issues which is directly related to such a large fan. Another example today is geared turbofans: it took 45 years to advance from bizjet engines to 30k thrust, and they haven't shown up anywhere greater. $\endgroup$ – user71659 Dec 22 '18 at 21:02
  • $\begingroup$ @user71659 You are right about engine size. The big difference, however, was that the US military funded only 20 ton class turbofans and left the 10 ton class to the civilian sector. And the Snecma-GE cooperation had to use a military-funded hot section (from the F101) to build their CFM56. Civilian funds were simply not available earlier, even though the smaller engine would pose less risk. $\endgroup$ – Peter Kämpf Dec 22 '18 at 22:08

Early high-bypass engines were expensive and unreliable.

The effort in developing new technology and designs, plus the cost of building a larger fan add cost. Then all of the new technology is inherently less reliable than existing proven designs.

The JT9D was notoriously unreliable, causing major problems in the development of the 747. Development problems and spiraling costs from the RB211 bankrupted Rolls-Royce, had huge failure rates, nearly causing an accident (Eastern Airlines Flight 855), and ultimately killing the L1011. CF6 fan disk failures caused National Airlines 27, sucking a passenger out the window, and the fatal United Sioux City crash.

Even what we think of as the most reliable engine today, the CFM56, when new had a fan failure leading to a fatal crash (Kegsworth), combined with water ingestion issues that nearly lead to a crash (TACA 110).

When you have such compromised technology, you narrow it in the place that makes sense. High bypass engines brought high efficiency, reducing fuel burn. They also had more thrust, allowing heavier planes, carrying more fuel. Both advantages combine to enable long-haul direct flights which passengers like. These are flown with widebodies, so naturally these engines were used in widebodies.

Short-haul narrowbodies carry less fuel per flight, and can refuel frequently, so there wasn't an advantage here. Only as the compromises with high-bypass engines were solved, then did the technology trickle down.

You see a similar thing today with carbon fiber composite fuselages. They're expensive to build, expensive to fix, and there is uncertainty over long-term costs. But the weight savings opens up ultra-long-haul flights, hence the A350 and 787.

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    $\begingroup$ Note, too, that the high bypass engines all debuted on >2 engined aircraft, so if an engine failure occurred, you continued on 2/3 or 3/4 of engine power, not 1/2. Until the tech matured and reliability increased, that extra 1/6 to 1/4 of designed power made a difference. $\endgroup$ – FreeMan Dec 19 '18 at 19:42
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    $\begingroup$ @FreeMan: Given that the (twin-engined) A300 first flew in 1972, that would imply that high-bypass technology matured fairly rapidly. $\endgroup$ – Sean Dec 19 '18 at 21:37
  • $\begingroup$ @Sean I was going to note the A300 was twin engined, but if you look at its history, it really didn't sell well until about 10 years after everybody else, placing it more in the 767 era. (Famously Airbus was stuck with a bunch of white tails, unordered planes that were built) Then Airbus turned around and put out a 4 engined plane, the A340, at the point the market went to twins. $\endgroup$ – user71659 Dec 19 '18 at 21:41
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    $\begingroup$ @user71659: The A300 didn't sell poorly because of unreliability; it sold poorly because ETOPS wasn't around yet. $\endgroup$ – Sean Dec 19 '18 at 21:44
  • $\begingroup$ @Sean That would only affect transatlantic flights. The A300 still was perfectly fine for the high-density short- to medium-haul it was designed for (the air bus missions). It ended up selling well doing just that around southeast Asia and Europe. $\endgroup$ – user71659 Dec 19 '18 at 22:07

One reason would be that high bypass engines tend to be larger, especially having a larger diameter.

This requires often quite significant redesign of the aircraft to fit them when the aircraft wasn't designed with them in mind from the outset.

The 737 for example required a longer undercarriage to allow the larger diameter engines to fit under the wing. Aircraft with tail mounted engines require almost certainly strengthening of the rear fuselage, especially the mounting pylons and their support structures, which adds weight, changes the center of gravity, etc. etc..

It probably just wasn't worth the cost until noise regulations made aircraft with turbojets harder to sell and operate.

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    $\begingroup$ The 737 didn't get a longer undercarriage until the MAX; the Classic and NG 737s have the same-length landing gear as the 737-100/200 (otherwise they wouldn't need their hamster-pouch engine intakes). $\endgroup$ – Sean Dec 19 '18 at 21:42

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