A twin-engine, T-tail, regional to medium-range jet airliner produced from 1965 until 2006, first by Douglas, followed by McDonnell Douglas, and then, finally, by Boeing.

The McDonnell Douglas DC-9 (originally the Douglas DC-9) is a T-tailed regional twinjet, produced, in several different variants, for 41 years, from 1965 until 2006. The DC-9 was produced by from 1965 until 1967, by from 1967 until 1997, and by from 1997 until 2006.

The DC-9 airframe was produced in eight variants during its history:

  • The DC-9-10 series was the first DC-9 version, entering service with Delta Air Lines in December 1965; it was also the smallest DC-9, and the only one without leading-edge . The DC-9-10 through -50 used the Pratt & Whitney JT8D engines (in the -10's case, the JT8D-1 and JT8D-7). There were two main variants, the DC-9-14 and DC-9-15, although three very early aircraft were built as DC-9-11s, along with a single DC-9-12. A total of 137 DC-9-10s were built from 1965 until 1968.

  • The DC-9-20 series, despite its numbering, actually entered service after the DC-9-30; it was designed specifically for excellent short-field performance, combining the fuselage of the DC-9-10 with the wings and engines of the DC-9-30. Only ten DC-9-20s were produced, in 1968 and 1969. The DC-9-20 series had only one variant: the DC-9-21.

  • The DC-9-30 series was Douglas's answer to the , and featured both a stretched fuselage, larger wings, and more powerful engines; the DC-9-30's wings had leading-edge slats, unlike the DC-9-10's, but like those on all future DC-9s. The DC-9-30 came in four variants; from smallest to largest, these were the DC-9-31, DC-9-32, DC-9-33, and DC-9-34. 662 DC-9-30s were built and sold from 1967 through 1982.

  • The DC-9-40 series was essentially a stretched DC-9-30 with more powerful engines; 71 were built from 1968 through 1979, all of the DC-9-41 variant.

  • The DC-9-50 series was a further stretch, slightly longer and with slightly more powerful engines than the DC-9-40, along with a number of other minor aerodynamic improvements. There was only one DC-9-50 variant, the DC-9-51; 96 were produced, from 1975 through 1981.

  • The MD-80 (tag: ) series (initially known as DC-9-80; also known as Super 80) was the most-produced DC-9 ever; it was yet a further stretch, featuring a considerably longer fuselage (except for the -87) compared to even the DC-9-50 (made possible by its more powerful JT8D-200 engines), and seeing a total of 1,191 produced from 1980 through 1999 in five variants (the MD-81, -82, -83, -87, and -88).

  • The MD-90 was the largest DC-9 ever built, and also the first to use high-bypass engines (the IAE V2500) instead of the relatively inefficient, low-bypass JT8D. 116 were built from 1993 through 2000.

  • The Boeing 717 (MD-95 before the merger) was a smaller version, built to replace the DC-9-30, and using Rolls-Royce BR715 engines. 156 were built from 1998 through 2006.

Aside from a handful of old DC-9-30s, those that remain in service are from the MD-80/-90 and 717 series. High fuel consumption and rising maintenance costs make the older DC-9 versions generally uneconomical to operate.

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