Boeing 727 - Design

Design

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.

Early 727s had nose gear brakes fitted to reduce braking distance on landing, but these were soon removed from service as they provided little reduction in braking distances, while adding weight and increasing maintenance needs.

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.