Military


Boeing 767

Production of the venerable 707 airframe ended in May 1991. Following studies of the most suitable follow-on aircraft for the AWACS mission, Boeing announced in December 1991 that it would offer a modified 767 commercial jetliner as the platform for the system. By 2003 the 767 had a new multi-decade role as an aerial tanker.

The 767-200 provides several advantages over the 707. Because of its wide-body configuration, the 767 offers 50 percent more floor space and nearly twice the volume of the 707. The 767 can carry a heavier payload, has a greater range and flies higher than the 707. The two-person flight crew and high-reliability twin engines also provide economic advantages. More than 590 commercial 767 aircraft are in service with 54 of the world's airlines, and there is a wide base of suppliers, spare parts and support equipment. The 767 is produced at a rate of about four each month.

Equipped for extended-range twin-engine operations (ETOPS), the 767 has enabled commercial airlines to realize economic benefits and retire less-efficient three- and four-engine airplanes from long-range routes. The 767 has become the dominant aircraft on North Atlantic routes, making more flights daily between Europe and North America than any other commercial aircraft model. Of all commercial transatlantic flights, more than one-third are flown with the 767. The 767 has safely flown over 600,000 extended-range flights and adds 10,000 more each month.

Normally, the FAA first certifies a twin-engine plane for flights of not more than one hour from an airport, then two hours, and finally, after a couple year's service, a full three hours so the plane could fly anywhere in the world. The 767, powered by Pratt & Whitney JT9D-7R4D/E turbofan engines, became the first Boeing twin to win 120-minute approval in May, 1985, but not until after it had flown for two years.

Boeing's conservative approach was illustrated in the 1970s and 1980s when it decided not to include in its 767 more advanced systems such as fly-by-wire, fly-by-light, flat panel video displays, and advanced propulsion systems (Holtby, 1986). Even though the technology existed, Boeing did not believe it was mature enough for the 767. Boeing also used what Gansler defines as a design-to-cost constraint. After Boeing defines a program it evaluates cost before going into production. Its cost evaluations include trade offs of performance, technology, and manufacturing investments.

With a first flight date in September 1981, the Boeing 767-200 entered airline service in the late summer of 1982.

The Boeing 767-200 is a 290-passenger, double-aisle, wide-body airliner designed to replace the aging Boeing 707 and McDonnell Douglas DC-8 transports now used on domestic and foreign medium range route segments. Average stage lengths over which the aircraft will be operated are estimated by Boeing to lie between 850 and 1150 miles. Maximum range is, of course, much greater and includes a nonstop United States coast-to-coast capability; the twin-engine 767-200 is not intended for long over-ocean flights.

The configuration of the Boeing 767-200 is conventional with the wing located in the low position at the bottom of the fuselage and with one of the two engines pylon mounted beneath each wing. Location of the engines under the wing, rather than to the rear of the fuselage, allows the horizontal tall to be mounted in the low position. A low tall position is helpful in minimizing pitching-moment nonlinearities that are often characteristic of sweptback wings at angles of attack in the vicinity of the stall. The main landing gear consists of two struts, each with a four-wheel bogie, that retract inward into the wing root.

Although of conventional configuration, the detailed aerodynamic design of the 767-200 is highly refined, as might be expected by the nearly 25 000 hours of wind-tunnel time required in the development of the aircraft. To place this wind-tunnel effort in perspective, 14 000 and 4000 wind-tunnel hours were expended in developing the Boeing 747 and 727, respectively.

The Boeing 767-200 has been widely advertised as being much more fuel efficient than earlier generations of jet transports. Although the careful aerodynamic design just mentioned contributes to the efficiency of the aircraft, the high-bypass-ratio turbofan engines employed on the 767-200 are primarily responsible for its high fuel efficiency. At present, the 767-200 is offered with two versions of both the Pratt & Whitney JT9D and the General Electric CF6 turbofan engines. Both of these engines are in the 48 000- to 50 000-pound thrust range and have bypass ratios between 4.5 and 5.0 and compressor pressure ratios between 25 and 30. Specific fuel consumption of these engines, expressed in pounds of fuel per pound of thrust per hour, is between 20 and 25 percent lower than that of the Pratt & Whitney JT3D engine that powers both the McDonnell Douglas DC-8 and the Boeing 707.

Comparison of certain characteristics of the 767-200 with those of the older Boeing 707-320B is of interest. The wings of the two aircraft are nearly the same size, with only small differences in area and span. Sweepback angle and aspect ratio of the new 767-200 are 31.5 and 8.0, respectively, as compared with 35 and 7.1 for the 707-320B. These differences in wing geometry would be expected to increase aerodynamic efficiency by a small amount. Incorporated in the wing of the 767-200 is a new Boeing-developed supercritical-type airfoil section. The basic technology of the supercritical airfoil section was pioneered by Richard T. Whitcomb of the NASA Langley Research Center. Use of such sections allows increased wing thickness ratio without corresponding reductions in the Mach number at which large adverse compressibility effects begin to occur. Reduced wing structural weight, increased aspect ratio and reduced wing sweepback angle-or some combination of the three-are accordingly possible. Incorporation of this new type of airfoil section on the Boeing 767-200 contributes to high overall efficiency.

High-lift devices on the wing consist of full-span leading-edge slats and a combination of both single- and double-slotted flaps on the trailing edge, with the double-slotted flaps placed on the inboard part of the wing. Inboard and outboard ailerons in combination with spoilers are used for lateral control. When deployed symmetrically, the spoilers help decelerate the aircraft on the landing rollout and aid in rapid in-flight descents. An elevator and adjustable stabilizer are used for longitudinal control, and a conventional rudder is provided for control about the yaw axis. All controls are of the fully powered, irreversible type.

New techniques for navigation and flight control are used on the 767-200. These techniques herald an entirely new relationship between the aircraft and the flight crew. An automatic flight control system coupled with a computer allows storage of an entire flight plan and gives automatic guidance and control of the aircraft from takeoff to landing. Included in the system are not only navigation functions but vertical flight-path control to minimize fuel consumption. To a large extent, the traditional electromechanical instrumentation has also been replaced by more simple cathode-ray-tube displays that provide different types of information at the command of the crew. A detailed description of this new equipment and its use are beyond the scope of the present discussion. Let it be noted, however, that aircraft such as the 767-200 may herald the end to most hands-on flying of transport aircraft and introduce an age in which the pilot is increasingly a button-pushing systems manager. Automatic flight management techniques such as those employed in the new Boeing transports will certainly result in more efficient fuel utilization in future airline operations.

All versions of the 767-200 can accommodate a maximum of 290 passengers seated in a 7-abreast double-aisle configuration. The aircraft is now offered in six variants, with gross weights falling in the range from 302 000 to 337 000 pounds. Listed in table VII are the characteristics of the 337 000-pound version of the aircraft, which is powered by two Pratt & Whitney JT9D-7R4E turbofans of 50 000 pounds thrust each. This particular variant of the aircraft, available in 1984, has nearly the same gross weight as the Boeing 707-320B but carries about 100 more passengers over a much shorter range. Maximum cruising speed of the 767-200 is about 40 miles per hour slower than that of the 707-320B, and takeoff and landing field lengths of the new aircraft are significantly shorter than those of the 707. These differences in speed and field length reflect the differing requirements of a long-range aircraft designed for international operations and one designed for medium-range domestic use. Aerodynamic efficiency of the 767-200 can be judged by the maximum lift-drag ratio, estimated to be about 18. The larger ratio of wetted area to wing area of the 767-200, as compared with that of the 707-320B, results in a value of (L/D) max somewhat lower than that of the older aircraft. The much larger passenger capacity and more efficient engines, however, make the new aircraft more efficient in terms of cost-per-seat-mile.




NEWSLETTER
Join the GlobalSecurity.org mailing list