Boeing 720

Boeing 720
Trans Polar Boeing 720 arriving at Stockholm-Arlanda Airport in June 1970
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
Retired	September 29, 2010[1]
Status	Retired
Primary users	United Airlines Western Airlines Eastern Airlines Northwest Airlines
Produced	1958–1967
Number built	154[2]
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.

Two primary versions of the aircraft were built. The original 720 with Pratt & Whitney JT3C turbojet engines entered service in 1960, while the improved 720B with Pratt & Whitney JT3D turbofans entered service in 1961. Some 720s were later converted to 720B specification.

Although only 154 were built, the Boeing 720 and 720B was 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. Some 720s are on display.

Development

Shorter range 707

Boeing announced its plans to develop a new version of the 707 in July 1957.^[2] It was developed from the 707-120 to provide for short to medium range flights from shorter runways. The model was originally designated 707-020 before being changed to 720 at the input of United Airlines.^[3] Compared to the 707-120, it has four fewer frames in front of the wing and one fewer aft: a total length reduction of 8 feet 4 inches (2.54 m).^[4]

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, lowering take-off and landing speeds—thus shortening runway length requirements—and a thickened inboard leading edge section, with a slightly greater sweep. This modification increased the top speed over the -120.^[4] It had four Pratt & Whitney JT3C-7 turbojet engines producing 12,500 lbf (55.6 kN) each.^[5]

At one point in the development phase, it was known as the 717-020,^[3] 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.^[6]

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.^[7] The first 720 took its maiden flight on November 23, 1959.^[4] 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.^[4] 65 of the original version were built.^[8]

Further developments

The 720B version of the 720 had JT3D turbofan engines,^[4] producing 17,000 lbf (75.6 kN) each.^[5] The JT3D engines had lower fuel consumption and higher thrust.^[9] 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 Airlines in March 1961.^[5] 89 720Bs were built,^[8] in addition to conversions of American's ten existing 720s.^[10]

As a modification of an existing model the 720 had minimal research and development costs, which allowed it to be successful despite few sales.^[11] The company built 154 Boeing 720s and 720Bs from 1959 to 1967.^[12] The 720's wing modification was later added on the -120B and on -120s retrofitted to the B standard.^[13]

Design

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,^[3] and had a lighter structure through use of lighter forged metal parts and thinner fuselage skins and structures.^[14]

Fuselage

The rearmost of the 707's over-wing emergency exits was deleted on each side which reduced passenger capacity, while two over-wing exits were an option for higher-density configurations.^[15]

Wings

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.^[16][17] 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 Mach 0.02.^[18]

Engines

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.^[9]

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 and associated humps are visible just above the main engine inlets on the two inner engine pods of all 720s and 720Bs; the lack of the turbocompressor inlet on the outer starboard pod (number 4 engine) helps spotters distinguish 720/720Bs from most 707s, which had three turbocompressors.

Other equipment

The Boeing 720 lacked an APU, and was reliant instead on ground power and pneumatic air to power the aircraft's systems, provide air conditioning, and start the engines while on the ground. The normal practice for Boeing 720 aircraft was to start the number 3 (inner starboard) engine first, disconnect ground power and air. After start of the first engine the use of bleed air from that engine could be used to provide necessary air pressure to start the other engines one by one. However, on ground, with ground starting crew already at hand, all four engines were usually started with ground crew help as this was more reliable and faster.

Operational history

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 of the 720 was by United Airlines on July 5, 1960^[8] on the Los Angeles-Denver-Chicago route. American Airlines followed by putting the 720 in commercial operation on July 31 that same year. The 720 was supplanted by the Boeing 727 in the mid-1960s in its medium-range, high-performance market.^[3] The late 1960s, 720 and 720B aircraft were used by the US military to shuttle troops to the Far-East war efforts. The interior of these planes were stripped of class partitions. Some of these flights originated at Travis AFB California and flew non-stop to Japan. At least, one of the landing sites was Yokota AB, Japan before the troops traveled to their final destinations.

After disposal of 720s by the major airlines, many were acquired by second rank operators in South America and elsewhere.

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. The performance of fire-retardant fuel was also tested.^[19]

The first 720 (N7201U) was later renamed "The Starship" and become a private charter jet used mainly by touring rock bands. Its main user was Led Zeppelin in the 1970s. The seating capacity was reduced and a bar with a built-in electric organ were added, along with beds, a shower, a lounge area, a TV and video cassette player.^[20]

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.^[21] Honeywell replaced their aircraft with a Boeing 757.

Pratt & Whitney Canada operated the last flying 720 until 2010. Its final operational flight occurred on September 29, 2010. Pratt & Whitney Canada replaced the testbed with a Boeing 747SP.^[1] In May 2012, the former PWC 720 was flown to CFB Trenton, ON to be put on display at the National Air Force Museum of Canada.^[22]

Variants

720

First production variant with four Pratt & Whitney JT3C turbojet engines.

Several high-density seat configurations delivered to Eastern Airlines included 4 over-wing escape hatches and brake cooling fans to effect quick turns on short-haul sectors.. These aircraft, designated "720-025", were certificated to carry up to 170 passengers, provided that certain safety requirements were met.^[23]

720B

Improved variant with four Pratt & Whitney JT3D turbofan engines. American Airlines converted its 720s to 720B standard.^[9]

Operators

The following operators flew Boeing 720/720Bs:

 Belize

• Belize Air

 Colombia

• AeroCóndor 2 x 720B, used from 1972 to 1980
• Avianca 4 x 720B, two delivered in 1961, one in 1965 and one in 1969^[24]
• SAM Colombia 3 x 720B used from 1977 to 1980

 Ethiopia

• Ethiopian Airlines 3 x 720Bs, two delivered in 1962 and one in 1965^[25]

 Germany

• Lufthansa 8 x 720B delivered 1961–1962^[26]

 Greece

• Olympic Airways 7 x 720B delivered 1972–1973^[26]

 Ireland

• Aer Lingus 3 x 720 delivered 1960–1961^[27]

 Israel

• El Al 2 x 720B delivered in 1962^[28]

 Kenya

• Kenya Airways 720B

 Lebanon

• Middle East Airlines 720B

 Malta

• Air Malta 5 x 720B used from 1974 to 1988

 Pakistan

• Pakistan International Airlines 4 x 720B delivered 1961–1962 and one in 1965^[29]

 Rhodesia

• Air Rhodesia 3 x 720 acquired 1973^[30]

 United States

• Alaska Airlines operated both the 720 and the 720B but did not take delivery of either type new.^[31][32]
• American Airlines 10 x 720 delivered in 1960 and 15 x 720Bs delivered in 1961, the ten 720s were converted to turbofan powered B models in 1961^[33]
• Braniff International Airways 5 x 720 delivered 1961–1963, an additional aircraft was sold to the Federal Aviation Agency before delivery^[34]
• Continental Airlines 8 x 720B delivered 1962–1966^[35]
• Eastern Airlines 15 x 720 delivered 1961–1962^[36]
• Federal Aviation Agency one 720 delivered in 1960^[37] or 1961^[38]
• Northwest Airlines 13 x 720B delivered 1961–1964^[39]
• Pacific Northern Airlines 2 x 720 delivered in 1962^[29]
• Pan American World Airways 9 x 720B delivered in 1961^[40]
• United Airlines 29 x 720 delivered 1960–1962^[41] (several also leased by United to Capital Airlines in 1960)
• Western Airlines 27 x 720Bs delivered 1961–1967^[42]

Accidents and incidents

The Boeing 720 has had 23 hull-loss accidents during its career; it was also involved in a number of incidents including nine hijack incidents and one aircraft destroyed by a bomb in mid-air in 1976. Only 12 of the hull-loss accidents included fatalities which totaled 175 deaths in addition to the 81 deaths on the aircraft destroyed in mid-air by a bomb.^[43]

The worst accidents were:^[43]

• On February 12, 1963, Northwest Airlines Flight 705, a Boeing 720-051B (reg. N724US), suffered an in-flight break-up over the Florida Everglades approximately 12 minutes after leaving Miami, bound for Chicago. All 35 passengers and eight crew died. The cause of the crash was determined to be an unrecoverable loss of control due to severe turbulence.^[44]
• On May 20, 1965, Pakistan International Airlines Flight PK 705, 720-040B (reg AP-AMH), crashed short of the runway at Cairo International Airport, killing 121 of the 127 people on board.^[45]
• On January 1, 1976, Middle East Airlines Flight 438, a Boeing 720-023B (reg. OD-AFT), was destroyed en route from Beirut to Dubai by a bomb in the forward cargo hold. All 66 passengers and 15 crew were killed.^[46]

Aircraft on display

• 720-051B 18351 a former Republic of China Air Force VIP aircraft on display at Kangshan, Taiwan.^[47]
• 720-030B HK-749 on display at Museo de los Ninos (Children's Museum of Bogotá), Bogotá, Colombia in Avianca Colombia livery.^[48] It was the first jet airplane delivered to a Colombian airline, Avianca.^[49]
• 720-047B AP-AXL on Display at PIA Planetarium, Lahore (Pakistan) standing in a park for public.^[50]

Specifications

	720	707-120B	707-320B
Cockpit crew	Four (Pilot, Co-Pilot, Flight Engineer, Navigator)
Passengers	149 170 (720-025, certification[23])	110 (2 class) 189[51] (1 class)	147 (2 class) 189 (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 / 621 mph (1000 km/h)		525 kn/604 mph (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)

Sources: Boeing^{[3][52][53][54]}

See also

Related development

• Boeing 367-80
• Boeing 707
• Boeing C-135 Stratolifter
• Boeing KC-135 Stratotanker

Aircraft of comparable role, configuration and era

• Convair 880
• Tupolev Tu-114

Related lists

• List of jet airliners
• List of airliners
• List of Boeing customers

References

Notes

Bibliography

External links

Wikimedia Commons has media related to Boeing 720.

• Boeing 720 family on Boeing.com

Boeing 7x7 aircraft production timeline, 1955–present
1950s					1960s										1970s										1980s										1990s										2000s										2010s
5	6	7	8	9	0	1	2	3	4	5	6	7	8	9	0	1	2	3	4	5	6	7	8	9	0	1	2	3	4	5	6	7	8	9	0	1	2	3	4	5	6	7	8	9	0	1	2	3	4	5	6	7	8	9	0	1	2	3	4	5	6	7	8	9
			Boeing 707
			Boeing 720
																																											Boeing 717 (MD-95)
								Boeing 727
													Boeing 737 Original																Boeing 737 Classic														Boeing 737 NG																			Boeing 737 MAX
															Boeing 747 (Boeing 747SP)																			Boeing 747-400																						747-8
																											Boeing 757
																											Boeing 767
																																						Boeing 777																									Boeing 777X
																																																						Boeing 787
= Narrow-body																														= Wide-body