Civil Aircraft
Airline cabins are frequently classified as narrow-body if there is a single aisle with seats on either side, or wide-body if there are two aisles with a block of seats between them in addition to the seats on the side. The number of seats abreast is affected by the aircraft width. On very small aircraft such as the Beechcraft 1900 there are only individual seats on each side of the aisle (1+1 seating). The widest narrow body aircraft such as the Airbus A320 and Boeing 737 have six abreast seating in a 3+3 layout. Asymmetrical layouts also exist, the Embraer Regional Jets have 1+2 seating while the Douglas DC-9 aircraft typically feature 2+3 seating.
In the arrangement of the cabin of a "narrow-body" transport such as the 707 or 727 the cabin is divided into a small first-class compartment with four-abreast seating and a large tourist-class cabin with six-abreast seating. A single aisle runs the entire length of the cabin with three seats located on either side. For an aircraft of large passenger capacity, the fuselage of the narrow-body type tends to become very long, which, in turn, may dictate a long, heavy landing gear in order to permit the desired rotation angle on takeoff without scraping the rear end of the fuselage on the runway. The long aisle also causes lengthy delays in passenger loading and difficulty for the cabin attendants in serving meals and refreshments.
A narrow six-abreast single-aisle configuration usually has slightly less wetted area, and thus less drag, than a six-abreast twin-aisle arrangement designed for the same number of passengers. Apparently, passengers are willing to accept the single-aisle layout for short flights but prefer the more spacious wide-body design for flight times greater than several hours.
The use of the term "wide body" in describing these aircraft is derived from the interior arrangement of the passenger cabin. A typical interior cabin arrangement of a wide-body transport has first-class cabin consisting of a small four-abreast compartment in the forward part of the fuselage and a large nine-abreast tourist cabin. The tourist cabin is divided by two longitudinal aisles that run the length of the cabin. In this arrangement shown, three seats are located on either side of the aircraft next to the windows, and three seats are disposed about the centerline of the cabin with an aisle on either side. On wide body-aircraft the center block of seats between the aisles can have as many as 5 seats on planes like the layout on some McDonnell Douglas DC-10 and Boeing 777 aircraft, although Boeing recommends the 3+3+3 over the 2+5+2 layout.
Very wide planes such as the Boeing 747 or the Airbus A380 have ten seats abreast, typically in a 3+4+3 layout, although this layout is also sometimes used as a high density layout on aircraft normally seating nine abreast, such as the 777 or DC-10. Current high-density versions of the Boeing 747, for example, may seat as many as 550 passengers in a 10-abreast arrangement.
For large-capacity aircraft, the double-aisle arrangement offers easy passenger loading and simplifies the serving problem for the cabin attendents. The design may also offer the passenger somewhat wider seats and a feeling of greater spaciousness. The landing-gear problem is alleviated by the relatively short fuselage offered by the wide-body design for a given passenger capacity.
Section 25.815 species the minimum aisle width dimensions for transport category airplanes based on the passenger capacity. For airplanes with 20 or more passenger seats, a minimum 15-inch width at heights 25 inches or less above the main aisle floor and a minimum 20-inch width at heights greater than 25 inches above the floor must be maintained. Aisle width is measured at any point along the aisle, normal to the centerline of the aisle. The main aisle envisioned by the regulations would run in a straight line from one end of the passenger cabin to the other and would satisfy these width criteria.
Long standing FAA policy has permitted slight deviation from a straight line where there is a transition from one cabin section to another, or where there are interior features which dictate that the aisle move laterally. For example, from tourist class to first class there may be a change from five-abreast seating to four-abreast seating which moves the aisle centerline laterally. This has been accepted provided the required widths are maintained at all heights normal to the path that an individual would take. This type of offset normally occurs at one or two points in a main cabin aisle. In addition, there is no offset permitted in the aisle vertically; that is, the required 15-inch dimension must lie completely below the projected 20-inch dimension at all points along the aisle.
