Blog on World of Civil Airplanes : Boeing 727

The Boeing 727 is a mid-size narrow-body three-engine jet aircraft built by Boeing Commercial Airplanes. It can carry 149 to 189 passengers and later models can fly up to 2,400 to 2,700 nautical miles (4,400 to 5,000 km) nonstop. Intended for short and medium-length flights, the 727 can use fairly short runways at smaller airports. It has three Pratt & Whitney JT8D engines below the T-tail, one on each side of the fuselage with a center engine that connects through an S-duct to an inlet at the base of the fin. The 727 is Boeing's only trijet aircraft, as well as the only one without a conventional tail.

The 727 followed the 707 quad-jet airliner with which it shares its upper fuselage cross-section and cockpit design. The 727-100 first flew in February 1963 and entered service with Eastern Air Lines in February 1964; the stretched 727-200 flew in July 1967 and entered service with Northeast Airlines that December. The 727 became a mainstay of airlines' domestic route networks and was also used on short- and medium-range international routes. Passenger, freighter, and convertible versions of the 727 were built.

The 727 was heavily produced into the 1970s; the last 727 was completed in 1984. In July 2011, 23 727-100s and 227 727-200s were in airline service.Airport noise regulations have led to 727s being equipped with hush kits

DEVELOPMENT

The Boeing 727 design was a compromise between United Airlines, American Airlines, and Eastern Air Lines requirements for a jet airliner to serve smaller cities with shorter runways and fewer passengers.United Airlines wanted a four-engined aircraft for its flights to high-altitude airports, especially its hub at Stapleton International Airport at Denver, Colorado.American, which was operating the four-engined Boeing 707 and Boeing 720, wanted a twin-engined aircraft for efficiency. Eastern wanted a third engine for its overwater flights to the Caribbean, since at that time twin-engined commercial flights were limited by regulations to routes with 60-minute maximum flying time to an airport (see ETOPS/LROPS). Eventually the airlines agreed on a trijet.

Production of the 727In 1959 Lord Douglas, chairman of British European Airways (BEA), suggested that Boeing and de Havilland Aircraft Company (later Hawker Siddeley) work together on their trijet designs, the 727 and D.H.121 Trident, respectively.The two designs had a similar layout, the 727 being slightly larger. At that time Boeing intended to use three Allison AR963 turbofan engines, license-built versions of the Rolls-Royce RB163 Spey used by the Trident. Boeing and de Havilland each sent engineers to the other company's locations to evaluate each other's designs, but Boeing eventually decided against the joint venture. De Havilland had wanted Boeing to license-build the D.H.121, while Boeing felt that the aircraft needed to be designed for the American market, with six-abreast seating and the ability to use runways as short as 4,500 ft.

In 1960 Pratt & Whitney was looking for a customer for its new JT8D turbofan design study, based on its J52 (JT8A) turbojet,while United and Eastern were interested in a Pratt & Whitney alternative to the RB163 Spey. Once Pratt & Whitney agreed to go ahead with development of the JT8D, Eddie Rickenbacker, Chairman of the Board of Eastern, told Boeing that the airline preferred the JT8D for its 727s. Boeing had not offered the JT8D as it was about 1,000 lbs heavier than the RB163, though slightly more powerful; the RB163 was also further along in development than the JT8D. Boeing reluctantly agreed to offer the JT8D as an option on the 727 and it later became the sole powerplant.

With high-lift deviceson its wing the 727 could use shorter runways than most earlier jets (e.g. the 4800-ft runway at Key West).

Later 727 models were stretched to carry more passengers and replaced earlier jet airliners such as the Boeing 707 and Douglas DC-8 as well as aging prop airliners such as the DC-4, DC-6, DC-7 and the Lockheed Constellations on domestic routes.For over a decade more 727s were built per year than any other jet airliner; in 1984 production ended with 1,832 built and 1,831 delivered, the highest total for any jet airliner until the 737 surpassed it in the early 1990s.

DESIGN

Boeing 727-200 of Air Algérie in September 1981

Lufthansa Boeing 727 at Paris Orly Airport in 1981

The airliner's middle engine (engine 2) at the very rear of the fuselage gets air from an inlet ahead of the vertical fin through an S-shaped duct.This S-duct proved to be troublesome in that flow distortion in the duct induced a surge in the centerline engine on the take-off of the first flight of the 727-100. This was fixed by the addition of several large vortex generators in the inside of the first bend of the duct.

The 727 was designed for smaller airports, so independence from ground facilities was an important requirement. This led to one of the 727's most distinctive features: the built-in airstair that opens from the rear underbelly of the fuselage, which initially could be opened in flight. D. B. Cooper, a hijacker, parachuted from the back of a 727 as it was flying over the Pacific Northwest. Boeing subsequently modified the design with the Cooper vane so that the airstair could not be lowered in flight. Another innovation was the auxiliary power unit (APU), which allowed electrical and air-conditioning systems to run independent of a ground-based power supply, without having to start one of the main engines. An unusual design feature is that the APU is mounted in a hole in the keel beam web, in the main landing gear bay. The 727 is equipped with a retractable tail skid that is designed to protect the aircraft in the event of an over-rotation on takeoff. The 727's fuselage has an outer diameter of 148 inches (3.8 m). This allows six-abreast seating (three per side) and a single aisle when 18 inches (46 cm) wide coach-class seats are installed. An unusual feature of the fuselage is the 10 inch difference between the lower lobe forward and aft of the wing as the higher fuselage height of the center-section was simply retained towards the rear.

Lloyd Aéreo Boliviano 727-200 at Jorge Wilsterman Airport. The rear air stairs are visible at the 727's tail.

The 727 proved to be such a reliable and versatile airliner that it came to form the core of many start-up airlines' fleets. The 727 was successful with airlines worldwide partly because it could use smaller runways while still flying medium-range routes. This allowed airlines to carry passengers from cities with large populations but smaller airports to worldwide tourist destinations. One of the features that gave the 727 its ability to land on shorter runways was its unique wing design. With no wing-mounted engines, leading-edge devices (Krueger, or hinged, flaps on the inner wing and extendable leading edge slats out to the wingtip) and trailing-edge lift enhancement equipment (triple-slotted,aft-moving flaps) could be used on the entire wing. Together these high-lift devices produced a maximum wing lift coefficient of 3.0 (based on the flap-retracted wing area). The 727 was stable at very low speeds compared to other early jets, but domestic carriers learned after review of various accidents that the 40-degree flaps setting could result in a higher-than-desired sink rate or a stall on final approach. These carriers' Pilots' Operation Handbooks disallowed using more than 30 degrees of flaps on the 727, even going so far as installing plates on the flap slot to prevent selection of more than 30 degrees of flaps.

With high-lift deviceson its wing the 727 could use shorter runways than most earlier jets (e.g. the 4800-ft runway at Key West).

DESIGN

Boeing 727-200 of Air Algérie in September 1981

Lufthansa Boeing 727 at Paris Orly Airport in 1981

Lloyd Aéreo Boliviano 727-200 at Jorge Wilsterman Airport. The rear air stairs are visible at the 727's tail.

NOISE

The 727 is one of the noisiest commercial jetliners, categorized as Stage 2 by the U.S. Noise Control Act of 1972, which mandated the gradual introduction of quieter Stage 3 aircraft. The 727's JT8D jet engines use older low-bypass turbofan technology, whereas Stage 3 aircraft utilize the more efficient and quieter high-bypass turbofan design. When the Stage 3 requirement was being proposed, Boeing engineers analyzed the possibility of incorporating quieter engines on the 727. They determined that the JT8D-200 engine could be used on the two side-mounted pylons, but the structural changes to fit the larger-diameter engine (49.2 inches (125 cm) fan diameter in the JT8D-200 compared to 39.9 inches (101 cm) in the JT8D-7) into the fuselage at the number two engine location were prohibitive.