The McDonnell Douglas DC-9 (initially known as the Douglas DC-9) is a twin-engine, single-aisle jet airliner. It was first manufactured in 1965 with its maiden flight later that year. The DC-9 was designed for frequent, short flights. The final DC-9 was delivered in October 1982.
The DC-9-based airliners, MD-80, MD-90 and Boeing 717 later followed in production. With the final deliveries of the 717 in 2006, production of the DC-9/MD-80/90/717 aircraft family ceased after 41 years and over 2,400 units built.

Design and development
Origins
During the 1950s Douglas Aircraft studied a short-medium range airliner to complement their higher capacity, long range DC-8. (DC stands for Douglas Commercial.[1]) A medium-range four-engine Model 2067 was studied but it did not receive enough interest from airlines and it was abandoned. In 1960, Douglas signed a two-year contract with Sud Aviation for technical cooperation. Douglas would market and support the Sud Aviation Caravelle and produce a licensed version if airlines ordered large numbers. None were ordered and Douglas returned to its design studies after the cooperation deal expired.

In 1962 design studies were underway. The first version seated 63 passengers and had a gross weight of 69,000 lb (31,300 kg). This design was changed into what would be initial DC-9 variant. Douglas gave approval to produce the DC-9 on April 8, 1963. Unlike the competing but larger Boeing 727 trijet, which used as many 707 components as possible, the DC-9 was an all-new design. The DC-9 has two rear-mounted Pratt & Whitney JT8D turbofan engines, relatively small, efficient wings, and a T-tail. The DC-9's takeoff weight was limited to 80,000 lb (36,300 kg) for a two-person flight crew by Federal Aviation Agency regulations at the time. DC-9 aircraft have five seats across for economy seating. The airplane seats 80 to 135 passengers depending on version and seating arrangement.

The DC-9 was designed for short to medium routes, often to smaller airports with shorter runways and less ground infrastructure than the major airports being served by larger designs like the Boeing 707 and Douglas DC-8. Accessibility and short field characteristics were called for. The tail mounted engine design facilitated a clean wing without engine pods, which had numerous advantages. For example, flaps could be longer, unimpeded by pods on the leading edge and engine blast concerns on the trailing edge. This simplified design improved airflow at low speeds and enabled lower takeoff and approach speeds, thus lowering field length requirements and keeping wing structure light.

The second advantage of the tail-mounted engines was the reduction in foreign object damage from ingested debris from runways and aprons. Third, the absence of engines in underslung pods allowed a reduction in ground clearance, making the aircraft more accessible to baggage handlers and passengers. Turnarounds were simplified by built-in airstairs, including one in the tail, which shortened boarding and deplaning times. The problem of deep stalling, revealed by the loss of the BAC One-Eleven prototype in 1963, was overcome through various changes, including the introduction of vortilons, small surfaces beneath the wing's leading edge used to control airflow and increase low speed lift.

Into production
The first DC-9, a production model, flew on February 25, 1965. The second DC-9 flew a few weeks later, with a test fleet of five aircraft flying by July. This allowed the initial Series 10 to gain airworthiness certification on November 23, 1965, and to enter service with Delta Air Lines on December 8. The DC-9 was always intended to be available in multiple versions to suit customer requirements, The first stretched version, the Series 30, with a longer fuselage and extended wing tips, flew on August 1, 1966, entering service with Eastern Air Lines in 1967. The initial Series 10 would be followed by the improved -20, -30, and -40 variants. The final DC-9 series was the -50, which first flew in 1974.

In 1967, the US Air Force selected the DC-9 to meet their requirement for a medical evacuation (MedEvac) and passenger transport aircraft, designated as the C-9A Nightingale; deliveries began in 1968. The U.S. Navy also ordered five C-9Bs for passenger and cargo duties in April 1972. The C-9B aircraft have provided air logistics support for the U.S. Navy andMarine Corps; only C-9Bs are in service as of the summer having been replaced by the Boeing C-40A (737). A C-9B was also chosen by NASA for reduced gravity research.

Some C-9s were converted from former civilian DC-9s; three were also specially configured for VIP transport duties. Kuwait also operated a small C-9 fleet. The USAF retired all its C-9As by 2005, and its VC-9Cs were retired in 2011.

The DC-9 was a commercial success with 976 built when production ended in 1982. The DC-9 is one of the longest-lasting aircraft in operation. Its reliability and efficiency led to sales of its successors into the 21st century. The DC-9 family is one of the most successful jet airliners with a total of over 2,400 units produced; it ranks third behind the second-place Airbus A320 family with over 5,000 produced, and the first-place Boeing 737 with over 7,000 produced.

Studies aimed at further improving DC-9 fuel efficiency, by means of retrofitted wingtips of various types, were undertaken by McDonnell Douglas. However, these did not demonstrate significant benefits, especially with existing fleets shrinking. The wing design makes retrofitting difficult.

Legacy
The DC-9 was followed by the introduction of the MD-80 series in 1980. The MD-80 series was originally called DC-9-80 series. It was a lengthened DC-9-50 with a higher maximum takeoff weight (MTOW), a larger wing, new main landing gear, and higher fuel capacity. The MD-80 series features a number of variants of thePratt & Whitney JT8D turbofan engine having higher thrust ratings than those available on the DC-9.

The MD-80 series was further developed into the McDonnell Douglas MD-90 in the early 1990s. It has yet another fuselage stretch, a glass cockpit (first introduced on the MD-88) and completely new International Aero V2500 high-bypass turbofan engines. In comparison to the very successful MD-80, relatively few MD-90s were built.
The final variant was the MD-95, which was renamed the Boeing 717-200 after McDonnell Douglas's merger with Boeing in 1997 and before aircraft deliveries began. The fuselage length and wing are very similar to the DC-9-30, but much use was made of lighter, modern materials. Power is supplied by two BMW/Rolls-Royce BR715high bypass turbofan engines.

China's Comac ARJ21 is derived from the DC-9 family. The ARJ21 is built with manufacturing tooling from the MD-90 Trunkliner program.

Consequently it has the same fuselage cross section, nose profile and tail.

Variants
Series 10
The original DC-9 (later designated the Series 10) was the smallest DC-9 variant. The -10 was 104.4 ft (31.8 m) long and had a maximum weight of 82,000 lb (37,000 kg). The Series 10 was similar in size and configuration to the BAC One-Eleven and featured a T-tail and rear mounted engines. Power was provided by a pair of 12,500 lbf (56 kN) Pratt & Whitney JT8D-5 or 14,000 lbf (62 kN) JT8D-7 engines. A total of 137 were built. Delta Air Lines was the initial operator.

The Series 10 was produced in two main subvariants, the Series 14 and 15, although of the first four aircraft, three were built as Series 11s and one as Series 12. These were later converted to Series 14 standard. No Series 13 was produced. A passenger/cargo version of the aircraft with a 136 x 81 in side cargo door forward of the wing and a reinforced cabin floor, was certificated on March 1, 1967. Cargo versions included the Series 15MC (Minimum Change) with folding seats which can be carried at the rear of the aircraft, and the Series 15RC (Rapid Change) with seats removable on pallets. These differences disappeared over the years as new interiors have been installed.

The Series 10 was unique in the DC-9 family in not having leading edge slats. The Series 10 was designed to have short takeoff and landing distances without the use of leading edge high-lift devices. Therefore, the wing design of the Series 10 featured airfoils with extremely high maximum lift capability in order to obtain the low stalling speeds necessary for short field performance.

Series 10 features
The Series 10 has an overall length of 104.4 feet (31.82 m), a fuselage length of 92.1 feet (28.07 m), a passenger cabin length of 60 feet (18.29 m), and a wingspan of 89.4 feet (27.25 m).

The Series 10 was offered with the 14,000 lbf (62 kN) thrust JT8D-1 and JT8D-7.

All versions of the DC-9 are equipped with an AlliedSignal (Garrett) GTCP85 APU, located in the aft fuselage.

The Series 14 was originally certificated at an MTOW of 85,700 lb (38,900 kg) but subsequent options offered increases to 86,300 and 90,700 lb (41,100 kg). The aircraft's MLW in all cases is 81,700 lb (37,100 kg). The Series 14 has a fuel capacity of 3,693 US gallons (with the 907 US gal centre section fuel). The Series 15, certificated on January 21, 1966, is physically identical to the Series 14 but has the increased MTOW of 90,700 lb (41,100 kg). Typical range with 50 passengers and baggage is 950 nmi (1,760 km), increasing to 1,278 nmi (2,367 km) at long range cruise. Range with maximum payload is 600 nmi (1,100 km), increasing to 1,450 nmi (2,690 km) with full fuel.

The DC-9 Series 10, as with all later versions of the DC-9 is equipped with a two crew analog flightdeck.

The aircraft is fitted with a passenger door in the port forward fuselage, and a service door/emergency exit is installed opposite. An airstair installed below the front passenger door was available as an option as was an airstair in the tailcone. This also doubled as an emergency exit. Available with either two or four overwing exits, the DC-9-10 can seat up to a maximum certified exit limit of 109 passengers. Typical all economy layout is 90 passengers, and 72 passengers in a more typical mixed-class layout with 12 first and 60 economy-class passengers.

All versions of the DC-9 are equipped with a tricycle undercarriage, featuring a twin nose unit and twin main units.

Series 20
The Series 20 was designed to satisfy a Scandinavian Airlines request for improved short field performance by using the more powerful engines and improved wings of the -30 combined with the shorter fuselage used in the -10. Ten Series 20 aircraft were produced, all of them Model -21.

In 1969, a DC-9 Series 20 at Long Beach was fitted with an Elliott Flight Automation Head-up display by McDonnell Douglas and used for successful three month-long trials with pilots from various airlines, the Federal Aviation Administration, and theUS Air Force.

Series 20 features
The Series 20 has an overall length of 104.4 feet (31.82 m), a fuselage length of 92.1 feet (28.07 m), a passenger cabin length of 60 feet (18.29 m), and a wingspan of 93.3 feet (28.44 m).

The DC-9 Series 20 is powered by the 15,000 lbf (67 kN) thrust JT8D-11 engine.

The Series 20 was originally certificated at an MTOW of 94,500 lb (42,900 kg) but this was increased to 98,000 lb (44,000 kg), some 8 percent up on the higher weight Series 14s and 15s. The aircraft's MLW is 95,300 lb (43,200 kg) and MZFW is 84,000 lb (38,000 kg). Typical range with maximum payload is 1,000 nmi (1,900 km), increasing to 1,450 nmi (2,690 km) with maximum fuel. The Series 20, using the same wing as the Series 30, 40 and 50, has a slightly lower basic fuel capacity than the Series 10 (3,679 US gallons).

Series 20 milestonesFirst flight: September 18, 1968.
FAA certification: November 25, 1968.
First delivery: December 11, 1968 to SAS
Entry into service: January 27, 1969 with SAS.
Last delivery: May 1, 1969 to SAS.

Series 30
The Series 30 was produced to counter Boeing's 737 twinjet; 662 were built, about 60% of the total. The -30 entered service with Eastern Airlines in February 1967 with a 14 ft 9 in (4.50 m) fuselage stretch, wingspan increased by just over 3 ft (0.9 m) and full-span leading edge slats, improving takeoff and landing performance. Maximum takeoff weight was typically 110,000 lb (50,000 kg). Engines for Models -31, -32, -33 and -34 included the P&W JT8D-7 and JT8D-9 rated at 14,500 lbf (64 kN) of thrust, or JT8D-11 with 15,000 lbf (67 kN).

Unlike the -10, the Series 30 had leading edge devices to reduce the landing speeds at higher landing weights; full-span slats reduced approach speeds by 6 knots despite 5000 lbs greater weight. The slats were lighter than slotted Krueger flaps, since the structure associated with the slat is a more efficient torque box than the structure associated with the slotted Krueger. The wing had a six percent increase in chord, all ahead of the front spar, allowing the 15 percent chord slat to be incorporated.

Series 30 versions
The Series 30 was built in four main sub-variants.

DC-9-31: Produced in passenger version only. The first DC-9 Series 30 flew on August 1, 1966, and the first delivery was to Eastern Airlines on February 27, 1967 after certification on December 19, 1966. Basic MTOW of 98,000 lb (44,000 kg) and subsequently certificated at weights up to 108,000 lb (49,000 kg).

DC-9-32: Introduced in the first year (1967). Certificated March 1, 1967. Basic MTOW of 108,000 lb (49,000 kg) later increased to 110,000 lb (50,000 kg). A number of cargo versions of the Series 32 were also produced:

32LWF (Light Weight Freight) with modified cabin but no cargo door or reinforced floor, intended for package freighter use.
32CF (Convertible Freighter), with a reinforced floor but retaining passenger facilities
32AF (All Freight), a windowless all-cargo aircraft.

DC-9-33: Following the Series 31 and 32 came the Series 33 for passenger/cargo or all-cargo use. Certificated on April 15, 1968, the aircraft's MTOW was 114,000 lb (52,000 kg), MLW to 102,000 lb (46,000 kg) and MZFW to 95,500 lb (43,300 kg). JT8D-9 or -11 (15,000 lbf (67 kN) thrust) engines were used. Wing incidence was increased 1.25 degrees to reduce cruise drag.[19] Only 22 were built, as All Freight (AF), Convertible Freight (CF) and Rapid Change (RC) aircraft.

DC-9-34: The last variant was the Series 34, intended for longer range with an MTOW of 121,000 lb (55,000 kg), an MLW of 110,000 lb (50,000 kg) and an MZFW of 98,000 lb (44,000 kg). The DC-9-34CF (Convertible Freighter) was certificated April 20, 1976, while the passenger followed on November 3, 1976. The aircraft has the more powerful JT8D-9s with the -15 and -17 engines as an option. It had the wing incidence change introduced on the DC-9-33. Twelve were built, five as convertible freighters.

Series 30 features
The DC-9-30 was offered with a selection of variants of JT8D including the -1, -7, -9, -11, -15 and -17. The most common on the Series 31 is the JT8D-7 (14,000 lbf (62 kN) thrust), although it was also available with the -9 and -17 engines. On the Series 32 the JT8D-9 (14,500 lbf (64 kN) thrust) was standard, with the -11 also offered. The Series 33 was offered with the JT8D-9 or -11 (15,000 lbf (67 kN) thrust) engines and the heavyweight -34 with the JT8D-9, -15 (15,000 lbf (67 kN) thrust) or -17 (16,000 lbf (71 kN) thrust) engines.

Series 40
The DC-9-40 is a further lengthened version and entered service with Scandinavian Airlines System (SAS) in March 1968. With a 6 ft 6 in (2 m) longer fuselage, accommodation was up to 125 passengers. The -40 was fitted with Pratt & Whitney engines of between 14,500 and 16,000 lbf (64 and 71 kN). A total of 71 were produced.

Series 50
The Series 50 was the largest DC-9 to fly. It features an 8 ft 2 in (2.49 m) fuselage stretch and seats up to 139 passengers. It started revenue service in August 1975 with Eastern Airlines and included a number of detail improvements, a new cabin interior, and more powerful JT8D-15 or -17 engines in the 16,000 and 16,500 lbf (71 and 73 kN) class.

McDonnell Douglas delivered 96, all as Model -51. Some visual cues to distinguish this version from other DC-9 variants include side strakes or fins below the side cockpit windows and thrust reversers rotated about 22 degrees on the original configuration. However various maintenance replacements have seen the thrust reversers in the same position as the -30 and -40.

Military and government
C-9A Nightingale - 23 aeromedical evacuation aircraft for the United States Air Force received from 1968. The final active-duty flight of the C-9A was in September 2005. The last C-9A was flown to Andrews AFB Museum in September 2012.

C-9B Skytrain II - 24 convertible passenger/transport versions for the United States Navy and Marine Corps delivered from 1973 to 1976. An additional five C-9s were converted from passenger configured DC-9s.

VC-9C - 3 executive transport aircraft for the United States Air Force. The three aircraft were delivered to the US Air Force in late 1976.
C-9K - 2 DC-9-30C for the Kuwait Air Force.

Operators
A total of 90 DC-9 aircraft (all variants) were in commercial service as of August 2013, including Delta Air Lines (17), USA Jet Airlines (10), Everts Air Cargo (3), Aeronaves TSM (6), Aserca Airlines (4), LASER Airlines (3), Fly SAX (2), and other operators with fewer aircraft.

Delta Air Lines since acquiring Northwest Airlines, has operated a fleet of DC-9 aircraft, most of which are over 30 years old. With severe increases in fuel prices in the summer of 2008, Northwest Airlines began retiring its DC-9s, switching to Airbus A319s that are 27% more fuel efficient.

Because of the usage of the aging JT8D engines, as of late 2000s (decade) DC-9s are considered gas guzzlers when compared to other more recent airliner designs. Studies aimed at improving DC-9 fuel efficiency, by means of retrofitted wingtip extensions of various types, have not succeeded in demonstrating significant benefits.
With the existing DC-9 fleet shrinking, modifications do not appear to be likely to occur, especially since the wing design makes retrofitting difficult. Thus, DC-9s are likely to be further replaced in service by new Boeing 737, Airbus A320,Embraer E-Jets aircraft, or the new, emerging Bombardier CSeries airliner.However, it is probable that a modest number of DC-9s will continue to productively fly for many years to come. As the Northwest/Delta merger progressed, Delta pulled several stored DC-9s back into service.

One ex-SAS DC-9-21 is operated as a skydiving jump platform at Perris Valley Airport in Perris, California. With the steps on the ventral stairs removed, it is the only airline transport class jet certified to date by the FAA for skydiving operations as of 2008.