Boeing E-3 Sentry
The Boeing E-3 Sentry is an American airborne early warning and control (AEW&C) aircraft developed by Boeing. E-3s are commonly known as AWACS (Airborne Warning and Control System).  Derived from the Boeing 707 airliner, it provides all-weather surveillance, command, control, and communications, and is used by the United States Air Force, NATO, French Air and Space Force, and Royal Saudi Air Force. The E-3 is distinguished by the distinctive rotating radar dome (rotodome) above the fuselage. Production ended in 1992 after 68 aircraft had been built.

In the mid-1960s, the U.S. Air Force (USAF) was seeking an aircraft to replace its piston-engined Lockheed EC-121 Warning Star, which had been in service for over a decade. After issuing preliminary development contracts to three companies, the USAF picked Boeing to construct two airframes to test Westinghouse Electric and Hughes's competing radars. Both radars used pulse-Doppler technology, with Westinghouse's design emerging as the contract winner. Testing on the first production E-3 began in October 1975.

The first USAF E-3 was delivered in March 1977, and during the next seven years, a total of 34 aircraft were manufactured. E-3s were also purchased by NATO (18), the United Kingdom (7), France (4) and Saudi Arabia (5).

In 1991, when the last aircraft had been delivered, E-3s participated in the Persian Gulf War, playing a crucial role of directing coalition aircraft against Iraqi forces. The aircraft's capabilities have been maintained and enhanced through numerous upgrades. In 1996, Westinghouse Electric's Defense & Electronic Systems division was acquired by Northrop Corporation, before being renamed Northrop Grumman Mission Systems, which currently supports the E-3's radar.

Development

Background
In 1963, the USAF asked for proposals for an Airborne Warning and Control System (AWACS) to replace its EC-121 Warning Stars, which had served in the airborne early warning role for over a decade. The new aircraft would take advantage of improvements in radar technology and computer-aided radar data analysis and data reduction. These developments allowed airborne radars to "look down", i.e. to detect the movement of low-flying aircraft, and discriminate, even over land, target aircraft's movements; previously this had been impossible due to the inability to discriminate an aircraft's track from ground clutter. Contracts were issued to Boeing, Douglas, and Lockheed, the latter being eliminated in July 1966. In 1967, a parallel program was put into place to develop the radar, with Westinghouse Electric Corporation and Hughes Aircraft being asked to compete in producing the radar system. In 1968, it was referred to as Overland Radar Technology (ORT) during development tests on the modified EC-121Q. The Westinghouse radar antenna was going to be used by whichever company won the radar competition since Westinghouse had pioneered the design of high-power radio frequency (RF) phase-shifters, which are used to both focus the RF into a pencil beam and scan electronically for altitude determination.

Boeing initially proposed a purpose-built aircraft, but tests indicated it would not outperform the already-operational 707, so the latter was chosen instead. To increase endurance, this design was to be powered by eight General Electric TF34s. It would carry its radar in a rotating dome mounted at the top of a forward-swept tail, above the fuselage. Boeing was selected ahead of McDonnell Douglas's DC-8-based proposal in July 1970. Initial orders were placed for two aircraft, designated EC-137D, as test beds to evaluate the two competing radars. As the test beds did not need the same 14-hour endurance demanded of the production aircraft, the EC-137s retained the Pratt & Whitney JT3D commercial engines, and a later reduction in the endurance requirement led to retention of the JT3D engines in production.

The first EC-137 made its maiden flight on 9 February 1972, with the fly-off between the two radars taking place from March to July of that year. Favorable test results led to the selection of Westinghouse's radar for the production aircraft. Hughes' radar was initially thought to be a certain winner due to its related development of the APG-63 radar for the new F-15 Eagle. The Westinghouse radar used a pipelined fast Fourier transform (FFT) to digitally resolve 128 Doppler frequencies, while Hughes's radars used analog filters based on the design for the F-15. Westinghouse's engineering team won this competition by using a programmable 18-bit computer whose software could be modified before each mission. This computer was the AN/AYK-8 design from the B-57G program, and designated AYK-8-EP1 for its much expanded memory. This radar also multiplexed a beyond-the-horizon (BTH) pulse mode that could complement the pulse-Doppler radar mode. This proved to be beneficial especially when the BTH mode is used to detect ships at sea when the radar beam is directed below the horizon.

Full-scale development
Approval was given on 26 January 1973 for the full-scale development of the AWACS system. To allow further development of the aircraft's systems, orders were placed for three preproduction aircraft, the first of which performed its maiden flight in February 1975. IBM and Hazeltine were selected to develop the mission computer and display system. The IBM computer was designated 4PI, and the software was written in JOVIAL. A Semi-Automatic Ground Environment (SAGE) or back-up interceptor control (BUIC) operator would immediately be at home with the track displays and tabular displays, but differences in symbology would create compatibility problems in tactical ground radar systems in Iceland, mainland Europe, and South Korea over Link-11 (TADIL-A). In 1977, Iran placed an order for ten E-3s, however this order was cancelled following the Iranian Revolution.

Engineering, test and evaluation began on the first E-3 Sentry in October 1975. Between 1977 and 1992, a total of 68 E-3s were built.

Future status
Because the Boeing 707 is no longer in production, the E-3 mission package has been fitted into the Boeing E-767 for the Japan Air Self Defense Forces. The E-10 MC2A was intended to replace USAF E-3s—along with the RC-135 and the E-8 Joint STARS, but the program was canceled by the Department of Defense.

NATO intends to extend the operational status of its AWACS until 2035 when it is due to be replaced by the Alliance Future Surveillance and Control (AFSC) program. The Royal Air Force (RAF) chose to limit investment in its E-3D fleet in the early 2000s, diverting Sentry upgrade funds to a replacement program. On 22 March 2019, the UK Defence Secretary announced a $1.98 billion contract to purchase five E-7 Wedgetails.

Design

Overview

The E-3 Sentry's airframe is a modified Boeing 707-320B Advanced model. Modifications include a rotating radar dome (rotodome), uprated hydraulics from 241 to 345 bar (3500–5000 PSI) to drive the rotodome, single-point ground refueling, air refueling, and a bail-out tunnel or chute. A second bail-out chute was deleted to cut mounting costs.

USAF and NATO E-3s have an unrefueled range of  or 8 hours of flying. The newer E-3 versions bought by France, Saudi Arabia, and the UK are equipped with newer CFM56-2 turbofan engines, and these can fly for about 11 hours or more than . The Sentry's range and on-station time can be increased through air-to-air refueling and the crews can work in shifts by the use of an on-board crew rest and meals area. The aircraft are equipped with one toilet in the rear, and a urinal behind the cockpit. Saudi E-3s were delivered with an additional toilet in the rear.

When deployed, the E-3 monitors an assigned area of the battlefield and provides information for commanders of air operations to gain and maintain control of the battle; while as an air defense asset, E-3s can detect, identify, and track airborne enemy forces far from the boundaries of the U.S. or NATO countries and can direct interceptor aircraft to these targets. In support of air-to-ground operations, the E-3 can provide direct information needed for interdiction, reconnaissance, airlift, and close-air support for friendly ground forces.

Avionics
The unpressurized rotodome is  in diameter,  thick at the center, and is held  above the fuselage by 2 struts. It is tilted down at the front to reduce its aerodynamic drag, which lessens its detrimental effect on take-offs and endurance. This tilt is corrected electronically by both the radar and secondary surveillance radar antenna phase shifters. The rotodome uses bleed air, outside cooling doors, and fluorocarbon-based cold plate cooling to maintain the electronic and mechanical equipment temperatures. The hydraulically rotated antenna system permits the  and AN/APY-2 passive electronically scanned array radar system to provide surveillance from the Earth's surface up into the stratosphere, over land or water.

Other major subsystems in the E-3 Sentry are navigation, communications, and computers. 14 consoles display computer-processed data in graphic and tabular format on screens. Its operators perform surveillance, identification, weapons control, battle management and communications functions. Data may be forwarded in real-time to any major command and control center in rear areas or aboard ships. In times of crisis, data may also be forwarded to the National Command Authority in the U.S. via RC-135 or aircraft carrier task forces.

Electrical generators mounted in each of the E-3's four engines provide 1 megawatt of electrical power required by the aircraft's radars and electronics. Its pulse-Doppler radar has a range of more than 250 mi (400 km) for low-flying targets at its operating altitude, and the pulse (BTH) radar has a range of approximately 400 mi (650 km) for aircraft flying at medium to high altitudes. The radar, combined with a secondary surveillance radar (SSR) and electronic support measures (ESM), provides a look down capability, to detect, identify, and track low-flying aircraft, while eliminating ground clutter returns.

Upgrades

Between 1987 and 2001, USAF E-3s were upgraded under the "Block 30/35 Modification Program". Enhancements included:
The installation of ESM and an electronic surveillance capability, for both active and passive means of detection. 
Installation of the Joint Tactical Information Distribution System (JTIDS), which provides rapid and secure communication for transmitting information, including target positions and identification data, to other friendly platforms.
Global Positioning System (GPS) capability was added. 
Onboard computers were overhauled to accommodate JTIDS, Link-16, the new ESM systems and to allow for future enhancements.

RSIP
The Radar System Improvement Program (RSIP) was a joint US/NATO development program. RSIP enhances the operational capability of the E-3 radars' electronic countermeasures, and improves the system's reliability, maintainability, and availability. Essentially, this program replaced the older transistor-transistor logic (TTL) and emitter-coupled logic (MECL) electronic components, long-since out of production, with off-the-shelf computers that utilised a High-level programming language instead of assembly language. Significant improvement came from adding pulse compression to the pulse-Doppler mode. These hardware and software modifications improve the E-3 radars' performance, providing enhanced detection with an emphasis towards low radar cross-section (RCS) targets.

The RAF had also joined the USAF in adding RSIP to upgrade the E-3's radars. The retrofitting of the E-3 squadrons was completed in December 2000. Along with the RSIP upgrade was installation of the Global Positioning System/Inertial Navigation Systems which improved positioning accuracy. In 2002, Boeing was awarded a contract to add RSIP to the small French AWACS squadron. Installation was completed in 2006. Saudi Arabia began RSIP upgrades in 2013; the first aircraft being upgraded by Boeing in Seattle with the four  remaining aircraft upgraded in Riyadh between 2014 and 2016.

NATO Mid Term Program
Between 2000 and 2008 NATO upgraded its E-3s to Mid Term Program (MTP) standard. This involved technical upgrades and a total multi-sensor-systems integration

DRAGON
In 2009, the USAF, in cooperation with NATO, entered into a major flight deck avionics modernization program in order to maintain compliance with worldwide airspace mandates. The program, called DRAGON (for DMS Replacement of Avionics for Global Operation and Navigation), was awarded to Boeing and Rockwell Collins in 2010. Drawing on their Flight2 flight management system (FMS), almost all the avionics were replaced with more modern digital equipment from Rockwell Collins. Main upgrades include a Digital Audio Distribution System, Mode-5/ADS-B transponder, Inmarsat and VDL datalinks, and a terrain awareness and warning system (TAWS). The centerpiece flight deck hardware consists of five 6x8 color graphics displays and two color CDUs. DRAGON laid the foundation for subsequent upgrades including GPS M-Code, Iridium ATC, and Autopilot.  USAF DRAGON Production began in 2018.

USAF Block 40/45
In 2014 the USAF began upgrading block 30/35 E-3B/Cs into block 40/45 E-3Gs. This upgrade replaces the main flight computer with a Red Hat Linux-based system, as well as replacing the DOS 2.0-like operating system with a Windows 95-like system on the operator workstations. In 2016, a three-week long cybersecurity vulnerability test revealed that the 40/45 block and its supporting ground equipment were vulnerable to cyber threats, and were thus deemed "not survivable." This caused a delay of approximately two years. Twenty-four E-3s are projected to complete this upgrade to 40/45 by the end of fiscal year 2020, while seven aircraft will be retired to save upgrade costs and harvest out-of-production components.

NATO Final Lifetime Extension Program
NATO intends to extend the operational status of its AWACS until 2035 by significantly upgrading fourteen aircraft in the Final Lifetime Extension Program (FLEP) between 2019 and 2026. Upgrades include the expansion of data capacity, expansion of bandwidth for satellite communications, new encryption equipment, new HAVE QUICK radios, upgraded mission computing software and new operator consoles. The supporting ground systems (mission training center and mission planning and evaluation system) will also be upgraded to the latest standard. NATO Airborne Early Warning & Control Program Management Agency (NAPMA) is the preparing and executing authority for the FLEP. FLEP will be combined with the standard planned higher echelon technical maintenance.

Operational history

In March 1977 the 552nd Airborne Warning and Control Wing received the first E-3 aircraft at Tinker AFB, Oklahoma. The 34th and last USAF Sentry was delivered in June 1984. The USAF has a total of thirty-one E-3s in active service. Twenty-seven are stationed at Tinker AFB and belong to the Air Combat Command (ACC). Four are assigned to the Pacific Air Forces (PACAF) and stationed at Kadena AB, Okinawa and Elmendorf AFB, Alaska. One aircraft (TS-3) was assigned to Boeing for testing and development (retired/scrapped June 2012).

E-3 Sentry aircraft were among the first to deploy during Operation Desert Shield, where they established a radar screen to monitor Iraqi forces. During Operation Desert Storm, E-3s flew 379 missions and logged 5,052 hours of on-station time. The data collection capability of the E-3 radar and computer subsystems allowed an entire air war to be recorded for the first time. In addition to providing senior leadership with time-critical information on the actions of enemy forces, E-3 controllers assisted in 38 of the 41 air-to-air kills recorded during the conflict.

NATO, UK, French and USAF AWACS played an important role in the air campaign against Serbia and Montenegro in the former republic of FR Yugoslavia. From March to June 1999 the aircraft were deployed in the NATO bombing of Yugoslavia (operation Allied Force) directing allied strike and air defence aircraft to and from their targets. Over 1,000 aircraft operating from bases in Germany and Italy took part in the air campaign which was intended to destroy Yugoslav air defenses and high-value targets such as the bridges across the Danube river, factories, power stations, telecommunications facilities, and military installations.

On 18 November 2015, an E-3G was deployed to the Middle East to begin flying combat missions in support of Operation Inherent Resolve against ISIL, marking the first combat deployment of the upgraded Block 40/45 aircraft.

France and United Kingdom
In February 1987 the UK and France ordered E-3 aircraft in a joint project which saw deliveries start in 1991. The British requirement arose due to the cancellation of the BAE Nimrod AEW3 project.  While France operates its E-3F aircraft independently of NATO, UK E-3Ds formed the E-3D Component of the NATO Airborne Early Warning and Control Force (NAEWCF), receiving much of their tasking directly from NATO. However, RAF E-3Ds remain UK manned and capable of independent, national tasking. This has been done on numerous occasions, notably when E-3Ds were committed to operations over Afghanistan in 2001 and Iraq in 2003.

The UK fleet has slowly been reduced from 7 since 2011. In 2009, the UK effectively limited the service life of the E-3D fleet by de-funding the Project Eagle upgrade which would have seen it upgraded in line with the USAF Block 40/45 standard. AirForces Monthly reported that by December 2020, just 2 aircraft were available for operations at any one time. The Strategic Defence and Security Review 2015 had announced the intention to retain the E-3D fleet until 2035, however in March 2019, the Ministry of Defence announced that the E-3Ds would be replaced by five E-7 Wedgetails from 2023. The £1.51 billion contract was awarded to Boeing without a competitive procurement process, a decision criticised by both competitors of Boeing and the UK's Defence Select Committee. The 2021 Integrated Defence Review confirmed a reduced order of three aircraft. On 27 January 2015, the RAF deployed an E-3D Sentry to Cyprus in support of U.S.-led coalition airstrikes against Islamic State militants in Iraq and Syria. The last intended operational flight by an RAF E3 Sentry was supposed to be in July 2021 with the Sentry retired from service, however RAF ZH101 and ZH106 have both flown patrols over Poland / Eastern Europe during Russia's incursions into Ukraine during late February / March 2022.

France operates four aircraft, all fitted with the newer CFM56-2 engines.

NATO
NATO acquired 18 E-3As and support equipment, with the first aircraft delivered in January 1982. The aircraft are registered in Luxembourg. The eighteen E-3s were operated by Number 1, 2 and 3 Squadrons of NATO's E-3 Component, based at NATO Air Base Geilenkirchen. 

NATO E-3s participated in Operation Eagle Assist after the September 11 attacks on the World Trade Center towers and the Pentagon. NATO and RAF E-3s participated in the military intervention in Libya.

Presently, 16 NATO E-3As are in the inventory, since one E-3 was lost in a crash and one was retired from service in 2015. The latter was due for its six-year cycle Depot Level Maintenance (DLM) inspection which would have been very costly. The "449 Retirement Project" resulted in reclamation of critical parts with a value of upwards of $40 million which will be used to support the 16 active aircraft. Some of the parts to be removed are no longer on the market or have become very expensive.

Variants

EC-137D
2 prototype AWACS aircraft with JT3D engines, 1 fitted with a Westinghouse Electric radar and 1 with a Hughes Aircraft Company radar. Both converted to E-3A standard with TF33 engines.
E-3A
Production aircraft with TF33 engines and AN/APY-1 radar, 24 built for USAF (later converted to E-3B standard), total of 34 ordered but the last 9 completed as E-3C. One additional aircraft retained by Boeing for testing, 18 built for NATO with TF33 engines and 5 for Saudi Arabia with CFM56 engines.
KE-3A
These are not AWACS aircraft but CFM56 powered tankers based on the E-3 design. 8 were sold to Saudi Arabia.
E-3B
USAF Block 30 modification. E-3As with improvements, 24 conversions.
E-3C
USAF Block 35 modification. Production aircraft with AN/APY-2 radar, additional electronic consoles and system improvements, ten built.
JE-3C
One E-3A aircraft used by Boeing for trials later redesignated E-3C.
E-3D
Production aircraft for the RAF to E-3C standard with CFM56 engines and British modifications designated Sentry AEW.1, 7 built.  Modifications included the addition of a refuelling probe next to the existing boom AAR recipticle, CFM-56 engines, wingtip ESM pods, an enhanced Maritime Surveillance Capability (MSC) offering Maritime Scan-Scan Processing (MSSP), JTIDS and Havequick 2 radios.
E-3F
Production aircraft for the French Air and Space Force to E-3C standard with CFM56 engines and French modifications, 4 built.
E-3G
USAF Block 40/45 modification. Includes hardware and software upgrades to improve communications, computer processing power, threat tracking, and others, and automates some previously manual functions. Initial operating capability (IOC) declared in July 2015.

Operators

 The Chilean Air Force purchased three second-hand E-3D Sentry aicraft from the Royal Air Force.

 The French Air and Space Force purchased four E-3F aircraft.
 Escadron de détection et de contrôle aéroportés 36 Berry (36th Airborne Detection and Control Squadron "Berry") based at Avord Air Base.

 18 E-3 AWACS were purchased – 1 was written off in Greece, 3 were retired from service. Mainly responsible for monitoring European NATO airspace, they have also been deployed outside the area in support of NATO commitments. The 20 multinational crews are provided by 15 of the 28 NATO member states.
 NATO Airborne Early Warning and Control Force – E-3A Component. Based at Geilenkirchen (Germany), with forward operating bases at Konya (Turkey), Preveza/Aktion (Greece) and Trapani/Birgi (Italy) and a forward operating location at Ørland (Norway). 
 Aircrew Training Squadron
 Flying Squadron 1
 Flying Squadron 2
 Flying Squadron 3 – disbanded in 2015

 The Royal Saudi Air Force purchased five E-3A aircraft in 1983. In 2004, modifications began to convert KE-3A tankers into RE-3 electronic intelligence gathering aircraft. 
 RSAF No. 6 Wing (Prince Sultan Air Base – Al Kharj)
السرب الثامن عشر (al-Sarab al-Ththamin Eshr – No. 18 Squadron) 
No. 19 Squadron – RE-3A/B (as well as Beechcraft 350ER-ISR)
No. 23 Squadron – KE-3A

 The Royal Air Force purchased seven E-3Ds by October 1987, designated Sentry AEW.1 in British service. As of December 2020, only three remained in service after one was withdrawn from service in 2009 to be used as spares, two were withdrawn in March 2019 and a further one withdrawn in January 2020. The fleet had been given an out of service date (OSD) of December 2022. They form the E-3D Component of the NATO Airborne Early Warning and Control Force. However, that date was accelerated pursuant to the 2021 defence review and the aircraft made its final flight in U.K. service in August 2021.
 RAF Waddington, Lincolnshire, England
No. 8 Squadron (1991–2021)
No. 23 Squadron (1996–2009)
No. 54 Squadron (Operational Conversion Unit 2005–?)
No. 56 Squadron (Operational Evaluation Unit 2008–?)

 The United States Air Force has 31 operational E-3s as of December 2019
Tactical Air Command 1976–1992
Air Combat Command 1992–present
 552d Air Control Wing – Tinker Air Force Base, Oklahoma
960th Airborne Air Control Squadron 2001–present (NAS Keflavik, Iceland 1979–1992)
963d Airborne Air Control Squadron 1976–present
964th Airborne Air Control Squadron 1977–present
965th Airborne Air Control Squadron 1978–1979, 1984–present
966th Airborne Air Control Squadron 1976–present
 380th Air Expeditionary Wing – Al Dhafra Air Base, United Arab Emirates
968th Expeditionary Airborne Air Control Squadron 2013–present (Thumrait Air Base, Oman 2002–2003) 
Air Force Reserve Command
 513th Air Control Group (Associate) – Tinker AFB, Oklahoma 
970th Airborne Air Control Squadron 1996–present (Personnel only, aircraft loaned by co-located 552nd ACW as needed)
 413th Flight Test Group – Robins AFB, Georgia
 10th Flight Test Squadron – Tinker AFB, Oklahoma 1994–present
Pacific Air Forces
 3d Wing – Elmendorf AFB, Alaska
962d Airborne Air Control Squadron 1986–present
 18th Wing – Kadena AB, Japan
961st Airborne Air Control Squadron 1979–present

Incidents and accidents

E-3s have been involved in three hull-loss accidents, and one radar antenna was destroyed during RSIP development (see photo under Avionics).
 On 22 September 1995, a U.S. Air Force E-3 Sentry (callsign Yukla 27, serial number 77-0354), crashed shortly after takeoff from Elmendorf AFB, Alaska. The plane lost power to both left side engines after ingesting several Canada geese during takeoff. The aircraft went down about  northeast of the runway, killing all 24 crew members on board.
 On 14 July 1996, a NATO E-3 Sentry (tail number LX-N90457) overran the runway and crashed into a sea wall at Préveza-Aktion Airport in Greece when the pilot attempted to abort takeoff after mistakenly believing that the aircraft had suffered a bird strike. The aircraft overran the runway and struck a sea wall, where it came to a halt. There were no injuries and the aircraft was written off. Investigators could find no evidence that a bird strike and ingestion had occurred. 

 On 28 August 2009, a U.S. Air Force E-3C Sentry (serial number 83-0008) participating in a Red Flag exercise at Nellis Air Force Base, Nevada experienced a nose gear collapse on landing, resulting in a fire and damaging the aircraft beyond repair. All 32 crew members evacuated safely.

Specifications (USAF/NATO)

See also

References
Notes

Citations

Bibliography

External links

 Royal Air Force E-3 Sentry information
 NATO AWACS-Spotter Geilenkirchen website
 Airborne Early Warning Association website

Aircraft first flown in 1972
AWACS aircraft
E-03 Sentry
Boeing 707
E-03 Sentry
Quadjets
1970s United States military reconnaissance aircraft