McDonnell Douglas DC-9

The McDonnell Douglas DC-9 is an American five-abreast, single-aisle aircraft designed by the Douglas Aircraft Company. It was initially produced as the Douglas DC-9 prior to August 1967, after which point the company had merged with McDonnell Aircraft to become McDonnell Douglas. Following the introduction of its first jetliner, the high-capacity DC-8, in 1959, Douglas was interested in producing an aircraft suited to smaller routes. As early as 1958, design studies were conducted; approval for the DC-9, a smaller all-new jetliner, came 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 DC-9 is powered by two rear-mounted Pratt & Whitney JT8D low-bypass turbofan engines under a T-tail for a cleaner wing aerodynamic. It has a two-person flight deck and built-in airstairs to better suit smaller airports. The aircraft was capable of taking off from 5,000 ft runways, connecting small cities and towns in the jet stream of air travel where jet service was previously impossible. The Series 10 aircraft 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 Series 30's 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; it 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-9s, 1191 MD-80s, 116 MD-90s, and 155 Boeing 717s. As of August 2022, 250 aircraft remain in service: 31 DC-9s (freighter), 116 MD-80s (mainly freighter), and 103 Boeing 717s (passenger), while the MD-90 was retired without freighter conversion.
During the late 1950s, Douglas Aircraft studied a short- to medium-range airliner to complement their then-sole jetliner, the high-capacity, long-range DC-8 (DC stands for Douglas Commercial). The Model 2067, a four-engined aircraft sized for medium-range routes was studied in depth, but work on it was abandoned after the proposal did not receive enough interest from airlines. In 1960, Douglas signed a two-year contract with the French aeronautics company Sud Aviation for technical cooperation; under the terms of this contract, Douglas would market and support the Sud Aviation Caravelle and produce a licensed version if sufficient orders were forthcoming from airlines. However, none were ever ordered from the company, leading to Douglas returning to its design studies after the co-operation deal expired.
In 1962, design studies were underway into what would become the DC-9, known as Model 2086. The first envisioned version seated 63 passengers and had a gross weight of 69,000 lb (31,300 kg). This design was changed into what would be the initial DC-9 variant. During February 1963, detailed design work commenced. On April 8, 1963, Douglas announced that it would proceed with the DC-9. Shortly thereafter, Delta Air Lines placed the initial order for the DC-9, ordering 15 aircraft along with options for another 15. By January 1965, Douglas had garnered orders for 58 DC-9 as well as options for a further 44.
Unlike the competing but larger Boeing 727 trijet, which used as many 707 components as possible, the DC-9 was developed as an all-new design. Throughout its development, Douglas had placed considerable emphasis on making the airliner as economic as possible, as well as to facilitate its future growth. The adoption of the Pratt & Whitney JT8D low-bypass turbofan engine, which had already been developed for the Boeing 727, enabled Douglas to benefit from the preexisting investment. Pratt & Whitney had long collaborated with Douglas on various projects, thus their engine was a natural choice for the company. In order to reduce the considerable financial burden of its development, Douglas implemented one of the first shared-risk production arrangements for the DC-9, arranging for de Havilland Canada to produce the wing at its own financial cost in return for promises on prospective future production orders.
The pace of development on the program was rapid. 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. Several key refinements to the aircraft were made during flight testing, such as the replacement of the original leading-edge slat design to achieve lower drag. The flight test program proceeded at a rapid pace; the initial Series 10 received airworthiness certification from the Federal Aviation Administration on November 23, 1965, permitting it to enter service with Delta Air Lines on December 8.
Through the DC-9, Douglas had beaten rival company Boeing and their 737 to enter the short-haul jet market, a key factor that contributed to the DC-9 becoming the best selling airliner in the world for a time. By May 1976, the company had delivered 726 aircraft of the DC-9 family, which was more than double the number of its nearest competitor. However, following decades of intense competition between the two airliners, the DC-9 would eventually be overtaken as the world's best selling airliner by Boeing's 737.
From the onset of its development, the DC-9 had been intended to be available in multiple versions to suit varying 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 was followed by the improved -20, -30, and -40 variants. The final DC-9 series was the -50, which first flew in 1974.
The DC-9 series, the first generation of the DC-9 family, would become a long term commercial success for the manufacturer. However, early production of the type had come at a higher unit cost than had been anticipated, leading to DC-9s being sold at a loss. The unfavorable early economics of the type negatively impacted Douglas, pushing it into fiscal hardship. However, the high customer demand for the DC-9 made the company attractive for either an acquisition or a merger; Douglas would merge with the American aerospace company McDonnell Aircraft to form McDonnell Douglas in 1967.
The DC-9 family is one of the longest-lasting aircraft in production and operation. It was produced on the final assembly line in Long Beach, California, beginning in 1965, and later was on a common line with the second generation of the DC-9 family, the MD-80, with which it shares its line number sequence. Following the delivery of 976 DC-9s and 108 MD-80s, McDonnell Douglas stopped series production of the DC-9 in December 1982. The last member of the DC-9 family, the Boeing 717, was produced until 2006. The DC-9 family was produced in total 2441 units: 976 DC-9s (first generation), 1191 MD-80s (second generation), 116 MD-90s, and 155 Boeing 717s (third generation). This compared to 2,970 Airbus A320s and 5,270 Boeing 737s delivered as of 2006.
Studies aimed at further improving DC-9 fuel efficiency, by means of retrofitted wingtips of various types, were undertaken by McDonnell Douglas, but these did not demonstrate significant benefits, especially with existing fleets shrinking. The wing design makes retrofitting difficult. Between 1973 and 1975, McDonnell Douglas studied the possibility of replacing engines on the DC-9 with the JT8D-109 turbofan, a quieter and more efficient variant of the JT8D. This progressed to the flight-test stage, and tests achieved noise reduction between 8 and 9 decibels depending on the phase of flight. No further aircraft were modified, and the test aircraft was re-equipped with standard JT8D-9s prior to delivery to its airline customer.
Two further developments of the original or first generation DC-9 series used the new designation with McDonnell Douglas initials (MD- prefix) followed by the year of development. The first derivative or second generation was the MD-80 series and the second derivative or third generation was the MD-90 series. Together, they formed the DC-9 family of 12 aircraft members (variants), and if the DC-9- designation were retained, the family members would be: First generation (Series 10, Series 20, Series 30, Series 40, and Series 50), second generation (Series 81, Series 82, Series 83, Series 87, and Series 88), and third generation (Series 90 and Series 95). The Series 10 (DC-9-10) was the smallest family member and the Series 90 (MD-90) was the largest.
The original DC-9 series was followed in 1980 by the introduction of the second generation of the DC-9 family, the MD-80 series. This was originally called the DC-9-80 (short Series 80 and later stylized Super 80). 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 the JT8D turbofan engine that had higher thrust ratings than those available on the original DC-9 series. The MD-80 series includes the MD-81, MD-82, MD-83, MD-88, and shortest variant, the MD-87.
The MD-80 series was further developed into the third generation, the MD-90 series, in the early 1990s. It has yet another fuselage stretch, an electronic flight instrument system (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 shorter and final variant, the MD-95, was renamed the Boeing 717 after McDonnell Douglas's merger with Boeing in 1997 and before aircraft deliveries began. The fuselage length and wing are very similar to those of the DC-9-30, but much use was made of lighter, modern materials. Power is supplied by two BMW/Rolls-Royce BR715 high-bypass turbofan engines.
China's Comac ARJ21 (C909) is derived from the DC-9 family. The ARJ21/C909 is built with manufacturing tooling from the MD-90 Trunkliner program. As a consequence, it has the same fuselage cross-section, nose profile, and tail.
The DC-9 was designed for short to medium-haul routes, often to smaller airports with shorter runways and less ground infrastructure than the major airports being served by larger airliners like the Boeing 707 and Douglas DC-8, where accessibility and short-field characteristics were needed. The DC-9's takeoff weight was limited to 80,000 lb (36,300 kg) for a two-person flight crew by the then-Federal Aviation Agency regulations at the time. The commercial passenger aircraft have five abreast layout for economy seating that can accommodate 80 to 135 passengers, depending on version and seating arrangement. Turnarounds were simplified by built-in airstairs, including one in the tail, which shortened boarding and deplaning times. The DC-9 was originally designed to perform a maximum of 40,000 landings.
The DC-9 has two rear-mounted JT8D turbofan engines, relatively small, efficient wings, and a T-tail. The tail-mounted engine design facilitated a clean wing without engine pods, which had numerous advantages. First, the 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, but with this position, the engines could ingest ice streaming off the wing roots. The third was the absence of engines in underslung pods, which permitted a reduction in fuselage ground clearance, making the airliner more accessible to baggage handlers and passengers. The cockpit of the DC-9 was largely analogue, with flight controls mainly consisting of various levers, wheels, and knobs.
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 wings' leading edges used to control airflow and increase low-speed lift. The need for such features is a result of the rear-mounted engines.
The DC-9 series, the first generation of the DC-9 family, includes five members or variants and 10 subvariants, which are the production versions (types). Their designations use the Series (DC-9-) prefix followed by a two-digit numbering with the same first digit and the second digit being a zero for variant names and a nonzero for version/type designations. The first variant, Series 10 (DC-9-10), has four versions (Series 11, Series 12, Series 14 and Series 15); the second variant, Series 20, has one version (Series 21); the third variant, Series 30, has four versions (Series 31, Series 32, Series 33 and Series 34); the fourth variant, Series 40, has one version (Series 41); and the fifth or final variant, Series 50, has one version (Series 51).
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) 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 combi version of the aircraft, with a 136-by-81-inch (3.5 by 2.1 m) side cargo door forward of the wing and a reinforced cabin floor, was certificated on March 1, 1967. Combi versions included the Series 15MC (minimum change) with folding seats that can be carried in 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 were 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 to obtain the low stalling speeds necessary for short-field performance.
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 10, as with all later versions of the DC-9, is equipped with a two-crew analog flightdeck.
The Series 14 was originally certificated with 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 an increased MTOW of 90,700 lb (41,100 kg). Typical range with 50 passengers and baggage is 950 nautical miles [nmi] (1,760 km; 1,090 mi), increasing to 1,278 nmi (2,367 km; 1,471 mi) at long-range cruise. Range with maximum payload is 600 nmi (1,100 km; 690 mi), increasing to 1,450 nmi (2,690 km; 1,670 mi) with full fuel.
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.
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 as the Model -21. The -21 had slats and stairs at the rear of plane.
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 the US Air Force.
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), eight percent more than 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; 1,200 mi), increasing to 1,450 nmi (2,690 km; 1,670 mi) 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).
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 Series 10, the Series 30 had leading-edge devices to reduce the landing speeds at higher landing weights; full-span slats reduced approach speeds by six knots despite 5,000 lb 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.
The Series 30 was built in four main sub-variants.
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.
The DC-9-40 is a further lengthened version. With a 6 ft 6 in (2 m) longer fuselage, accommodation was up to 125 passengers. The Series 40 was fitted with Pratt & Whitney engines with thrust of 14,500 to 16,000 lbf (64 to 71 kN). A total of 71 were produced. The variant first entered service with Scandinavian Airlines System (SAS) in March 1968. Its unit cost was US$5.2 million (1972) (equivalent to US$28.73 million in 2023) .
The Series 50 was the largest version of the DC-9 to enter airline service. It features an 8 ft 2 in (2.49 m) fuselage stretch and seats up to 139 passengers. It entered revenue service in August 1975 with Eastern Airlines and included several 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 the Model -51. Some visual cues to distinguish this version from other DC-9 variants include side strakes or fins below the side cockpit windows, spray deflectors on the nose gear, and thrust reversers angled inward 17 degrees compared to the original configuration. The thrust reverser modification was developed by Air Canada for its earlier aircraft, and adopted by McDonnell Douglas as a standard feature on the series 50. It was also applied to many earlier DC-9s during regular maintenance.
As of May 2024, a total of 30 DC-9 series aircraft remain in service, of which 20 are operated by Aeronaves TSM and two passenger aircraft in service with African Express Airways, and the rest in cargo service.
With the existing DC-9 fleet shrinking, modifications do not appear to be likely to occur, especially since the wing design makes retrofitting difficult. DC-9s are therefore likely to be further replaced in service by newer airliners such as Boeing 737, Airbus A320, Embraer E-Jets, and the Airbus A220. However one former Scandinavian Airlines 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 2006. This is the last and only -21 series still airworthy, and after being out of service for over a decade, it returned to the sky on May 7th, 2024
During the mid 1990s, Northwest Airlines was the largest operator of the type in the world, flying 180 DC-9s. After its acquisition of Northwest Airlines, Delta Air Lines operated a sizable fleet of DC-9s, most of which were over 30 years old at the time. 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. As the Northwest/Delta merger progressed, Delta returned several stored DC-9s to service. Delta Air Lines made its last DC-9 commercial flight from Minneapolis/St. Paul to Atlanta on January 6, 2014, with the flight number DL2014.
As of June 2022, the DC-9 family aircraft has been involved in 276 major aviation accidents and incidents, including 156 hull-losses, with 3,697 fatalities combined (all generations of family members)= (1st gen., DC-9 series): 107 hull-losses & 2,250 fatalities + (2nd gen., MD-80 series): 46 hull-losses & 1,446 fatalities + (3rd gen., MD-90 series including Boeing 717): 3 hull-losses & 1 fatality.
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