The McDonnell Douglas DC-9 is an American five-abreast single-aisle aircraft designed by the Douglas Aircraft Company. It was initially produced by the developer company as the Douglas DC-9 until August 1967 and then by McDonnell Douglas.

After introducing its heavy DC-8 in 1959, Douglas approved the smaller, all-new DC-9 for shorter flights on April 8, 1963.

The DC-9-10 first flew on February 25, 1965, and gained its type certificate on November 23, to enter service with Delta Air Lines on December 8.

The aircraft has two rear-mounted Pratt & Whitney JT8D low-bypass turbofans under a T-tail for a cleaner wing aerodynamic, a two-person flight deck and built-in airstairs.

The Series 10 are 104 ft (32 m) long for typically 90 coach seats.

The Series 30, stretched by 15 ft (4.5 m) to seat 115 in economy, has a larger wing and more powerful engines for a higher maximum takeoff weight (MTOW); it first flew in August 1966 and entered service in February 1967.

The Series 20 has the Series 10 fuselage, more powerful engines and the -30 improved wings; it first flew in September 1968 and entered service in January 1969.

The Series 40 was further lengthened by 6 ft (2 m) for 125 passengers, and the final DC-9-50 series first flew in 1974, stretched again by 8 ft (2.5 m) for 135 passengers.

When deliveries ended in October 1982, 976 had been built.

Smaller variants competed with the BAC One-Eleven, Fokker F28 and Sud Aviation Caravelle, and larger ones with the original Boeing 737.

The original DC-9 was followed by the second generation in 1980, the MD-80 series, a lengthened DC-9-50 with a larger wing and a higher MTOW.

This was further developed into the third generation, the MD-90, in the early 1990s,, as the body was stretched again, fitted with V2500 high-bypass turbofans and an updated flight deck.

The shorter and final version, the MD-95, was renamed the Boeing 717 after McDonnell Douglas's merger with Boeing in 1997 and is powered by Rolls-Royce BR715 engines.

The DC-9 family was produced between 1965 and 2006 with a total delivery of 2441 units: 976 DC-9, 1191 MD-80, 116 MD-90 and 155 Boeing 717.

As of August 2022 a total of 250 aircraft remain in service, 31 DC -9 (freighter), 116 MD-80 (mainly freighter) and 103 Boeing 717 (passenger), while MD-90 was retired without freighter conversion.

Cielos del Sur S.A., operating as Austral Líneas Aéreas, more commonly known by its shortened name Austral, was a domestic airline of Argentina, the sister company of Aerolíneas Argentinas. It was the second-largest domestic scheduled airline in the country, after Aerolíneas Argentinas. As a subsidiary of Aerolíneas Argentinas, the company shares its headquarters with that airline, which is located in the Aeroparque Jorge Newbery of Buenos Aires, the main base of operations of the company.

Austral was currently fully integrated into Aerolíneas Argentinas; however, some differences arise, especially those relying upon the unions the staff of both companies are affiliated to, which lead to constant conflicts. In May 2020 it was announced that Austral would merge with Aerolíneas, saving an estimated 100 million US Dollars and eliminating Austral as a brand. Austral ended operations on 30 November 2020.As of October 2013, the airline operated an all-Embraer 190 fleet.