| Boeing 720 | |
|---|---|
| A Middle East Airlines Boeing 720 in April 1982 | |
| Role | Narrow-body jet airliner |
| National origin | United States |
| Manufacturer | Boeing Airplane Company |
| First flight | November 23, 1959 |
| Introduction | July 5, 1960 with United Airlines |
| Status | Out of production, in very limited use |
| Primary users | United Airlines Western Airlines Eastern Airlines Northwest Airlines |
| Produced | 1958–1967 |
| Number built | 154[1] |
| Developed from | Boeing 707 |
The Boeing 720 is a four-engine narrow-body short- to medium-range passenger jet airliner. Developed by Boeing in the late 1950s from the Boeing 707, the 720 has a shorter fuselage and less range. The 720 first flew in November 1959 and the model entered service with launch customer United Airlines in July 1960.
Though only 154 were built, the Boeing 720/720B was still profitable due to the low research and development costs of it being a slightly modified version of the 707-120. It was later replaced by the Boeing 727.
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Boeing announced its plans to develop a new version of the 707 in July 1957.[1] It was developed from the 707-120 to provide for airline operation for short to medium range flights from shorter runways. The model was originally designated 707-020 before being changed to 720 because of all the design changes involved. Compared to the 707-120, it has four fewer frames in front of the wing, and one less aft, for a total of 8 feet 4 inches (2.54 m) shorter.[2]
The new model was designed to a lower maximum takeoff weight with a modified wing and a lightened airframe. The wing modifications included Krueger flaps outboard of the outboard engines to lower takeoff and landing speeds and thus shorten field length, and a thickened inboard section at the leading edge, which had a slightly greater sweep. This modification increased the top speed over the -120.[2] It had four Pratt & Whitney JT3C-7 turbojet engines producing 12,500 lbf (55.6 kN) each.[3]
At one point in the promotion stage to airlines, it was known as the 717, although this was the Boeing model designation of the KC-135 and remained unused for a commercial airliner until it was applied to the MD-95 following Boeing's merger with McDonnell Douglas in 1997.[4]
Because the aircraft systems were similar to the Boeing 707 no prototype Boeing 720 was built; any different systems were tested on the Boeing 367-80.[5] The first 720 took its maiden flight on the November 23, 1959.[2] The Type Certificate for the 720 was issued on June 30, 1960. It was first put into service by United Airlines on July 5, 1960.[2] 65 of the original version were built.[6]
The 720B was the version of the 720 with JT3D turbofan engines,[2] producing 17,000 lbf (75.6 kN) each.[3] As a turbofan variant of the JT3C it had lower fuel consumption and higher thrust. The maximum takeoff weight for the 720B was increased to 234,000 lb (106,000 kg). The 720B first took to the skies on October 6, 1960 and received certification and entered service with American in March 1961.[3] 89 720Bs were built,[6] in addition to conversions of American's ten existing 720s.[7]
As a modification of an existing model the 720 had minimal research and development costs, which allowed it to be profitable despite few sales. Boeing built 154 720s and 720Bs from 1959 to 1967.[8] The 720's wing modification was later added on the -120B and on -120s retrofitted to the B standard.[9]
The Boeing 720 is a four-engined low-wing cantilever monoplane. Although it was similar to the Boeing 707, compared with the 707-120 it was 9 ft (2.73 m) shorter in length and was built using a lighter gauge of aluminum to reduce weight.
Compared to the 707-120, the 720 had one 20 inch frame section removed ahead of the wing and four removed aft for a total reduction of 100 inches. The rearmost of the 707's over-wing emergency exits was deleted on each side which, under FAA exit regulations, limited passenger capacity to 149. However, two over-wing exits were an option, which allowed a capacity increase to 165 passengers.
The 720 uses an improved wing based on the 707 wing. The wingspan remained the same as the 707-120. For the 720, the wing was changed between the fuselage and inner engines by adding a wing root glove.[10][11] This glove reduced the drag of the wing by decambering the root, which reduced the "middle effect", thereby increasing the effective local wing sweep. It is reported that the wing root glove increased the drag divergence Mach number of the wing by .02 Mach.[12]
Though initially fitted with turbojet engines, the dominant engine for the Boeing 720 was the Pratt & Whitney JT3D, a turbofan variant of the JT3C with lower fuel consumption and higher thrust. JT3D-engined 720s had a "B" suffix; some of American's 720Bs were conversions of JT3C-powered 720s.
Like the 707, the 720/720B used engine-driven turbocompressors to supply high-pressure air for pressurization. The engines could not supply sufficient bleed air for this purpose without a serious loss of thrust. The small air inlets are visible just above the main engine inlets on the inner engine pods of all 720s and 720Bs; the lack of the turbocompressor inlets on the outer pods helps spotters distinguish 720/720Bs from most 707s.
The first aircraft was a production aircraft for United Airlines which flew on November 23, 1959. The Type Certificate for the 720 was issued on June 30, 1960. The first service was by United Airlines on July 5, 1960[6] on the Los Angeles-Denver-Chicago route, American Airlines followed by putting the 720 in commercial operation on July 31, 1960. The 720 remained in service until airlines replaced it with the Boeing 727 after the mid-1960s.
In 1984 a Boeing 720 flown by remote control was intentionally crashed at Edwards AFB as a part of the FAA and NASA Controlled Impact Demonstration program. The test provided peak accelerations during a crash.[13]
Honeywell operated the last Boeing 720 in operation in the United States, flying out of Sky Harbor airport in Phoenix. The aircraft had been modified with an extra engine nacelle mounted on the right side of the fuselage to allow testing of a turbine engine at altitude, operating on special certification allowing it to be used for experimental use. This 720B was scrapped on June 21 and 22, 2008.[14] Honeywell replaced their aircraft with a Boeing 757.
Pratt & Whitney Canada flew the last 720 until 2010. The final operational flight took place on September 29, 2010 with a decision still to come regarding whether it will be scrapped or placed in a museum. Pratt & Whitney Canada replaced their testbed with a Boeing 747SP.[15]
The following operators took delivery of new Boeing 720/720Bs:
| 720 | 707-120B | 707-320B | |
|---|---|---|---|
| Cockpit crew | Three | ||
| Passengers | 167 | 110 (2 class) 179 (1 class) |
147 (2 class) 202 (1 class) |
| Length | 136 ft 2 in (41.25 m) | 145 ft 1 in (44.07 m) | 152 ft 11 in (46.61 m) |
| Wingspan | 130 ft 10 in (39.90 m) | 145 ft 9 in (44.42 m) | |
| Tail height | 41 ft 7 in (12.65 m) | 42 ft 5 in (12.93 m) | |
| Maximum Takeoff Weight (MTOW) | 222,000 lb (100,800 kg) | 257,000 lb (116,570 kg) | 333,600 lb (151,320 kg) |
| Empty weight | 103,145 lb (46,785 kg) | 122,533 lb (55,580 kg) | 146,400 lb (66,406 kg) |
| Runway needed at MTOW | 8,300 ft (2,515 m) | 11,000 ft (3,330 m) | 10,840 ft (3,280 m) |
| Fuel Capacity | 16,060 US gal (60,900 l) | 17,330 US gal (65,590 l) | 23,820 US gal (90,160 l) |
| Landing run | 5,750 ft (1,740 m) | 6,200 ft (1,875 m) | 5,950 ft (1,813 m) |
| Operating range (Max Payload) | 3,680 nmi (6,820 km) | 3,735 nmi (6,920 km) | |
| Range at MTOW (max fuel) | 3800 nmi (7,040 km) | 4,700 nmi (8,704 km) | 5,750 nmi (10,650 km) |
| Cruising speed | 540 kn (1000 km/h) | 525 kn (972 km/h) | |
| Fuselage width | 12 ft 4 in (3.76 m) | ||
| Powerplants (4 x) | Pratt & Whitney JT3C-7: 12,000 lbf (53.3 kN) |
Pratt & Whitney JT3D-1: 17,000 lbf (75.6 kN) |
PW JT3D-3: 18,000 lbf (80 kN) PW JT3D-7: 19,000 lbf (84.4 kN) |
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