Boeing E-3 Sentry

The Boeing E-3 Sentry is an airborne early warning and control (AEW&C) developed by Boeing as the prime contractor. Derived from the Boeing 707, it provides all-weather surveillance, command, control and communications, and is used by the United States Air Force (USAF), NATO, Royal Air Force (RAF), French Air Force and Royal Saudi Air Force. The E-3 is distinguished by the distinctive rotating radar dome above the fuselage. Production ended in 1992 after 68 aircraft were built.

In the mid-1960s, the USAF was seeking an aircraft to replace its piston-engined EC-121, which had seen 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's 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. NATO, as a single identity, also had eighteen aircraft manufactured, basing them in Germany. The E-3 was also sold to the United Kingdom (seven) and France (four) and Saudi Arabia (five, plus eight E-3 derived tanker aircraft). In 1991, by which time the last aircraft had been delivered, E-3s participated in Operation Desert Storm, playing a crucial role of directing Coalition aircraft against the enemy. Throughout the aircraft's service life, numerous upgrades were performed to enhance its capabilities. In 1996, Westinghouse Electric was acquired by Northrop before being renamed Northrop Grumman Electronic Systems, which currently supports the E-3's radar.

Background
In 1963, the USAF asked for proposals for an AEW&C aircraft 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 which allowed airborne radars to "look down" and detect low-flying aircraft, even over land, which was previously impractical due to 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 and the 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's radar antenna was going to be used whichever company won the radar competition, since Westinghouse had pioneered in the design of high-power RF phase-shifters.



Boeing initially proposed a purpose-built aircraft, but tests indicated that 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, or carrying 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 endurance requirement led to retaining the normal 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 during March–July that year. Favorable test results saw the selection of Westinghouse's radar for the production aircraft. Hughes's radar was initially thought to be a certain winner, simply because much of its design was also going into the new F-15 Eagle's radar program. 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 fighter. 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 full-scale development of the AWACS system. To allow further development of the aircraft's systems, orders were placed for three pre-production aircraft, the first of which performed its maiden flight in February 1975. To save costs, the endurance requirements were relaxed allowing the new aircraft to retain the four JT3D (US Military designation TF33) engines. IBM and Hazeltine were selected to develop the mission computer and display system. The IBM computer receiving the designation 4PI, and the software is written in JOVIAL. A Semi-Automatic Ground Environment (SAGE) or 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, Europe and Korea over Link-11 (TADIL-A).

Modifications to the Boeing 707 for the E-3 Sentry included a rotating radar dome, single-point ground refueling, air refueling, and a bail-out chute. The original design called for two bail-out chutes (one forward, and one aft) but the aft bail-out chute was deleted as a way to cut mounting costs. Engineering, test and evaluation began on the first E-3 Sentry in October 1975. During 1977–1992, 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—but the E-10 program was canceled by the Department of Defense. The USAF is now performing a series of incremental improvements, mainly to avionics, to bring the E-3 up to current standards of performance. Boeing is flight testing its Block 40/45 E-3s. This modified E-3 contains upgrades of the mission crew and air battle management sections, as well as significantly upgraded electronic equipment.

Another program that the Air Force is considering is the "Avionics Modernization Program" (AMP). AMP would equip the E-3s with glass cockpits. The Air Force also wants modified E-3s with jet engines that are more reliable than the original ones, and also with at least 19% higher fuel efficiencies. New turbofan engines would give these E-3s longer ranges, longer time-on-station, and a shorter critical runway length. If the modification is carried out, the E-3s could take off with full fuel loads using runways only 10000 ft long, and also at higher ambient temperatures and lower barometric pressures, such as from bases in mountainous areas. Now that the E-8 Joint STARS are being fitted with the new Pratt & Whitney JT8D-219 turbofans, which are stated as having one-half the cost of the competing engine, the CFM56, the Air Force is again studying the possibility of replacing the E-3's original turbofan engines with more-efficient ones.

Overview
The E-3 Sentry's airframe is a modified Boeing 707-320B Advanced model. USAF and NATO E-3s have an unrefueled range of some 4000 mi or eight 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 about 5000 mi. 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.

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; whilst 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 fighter-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 dome is 30 feet (9.1 m) in diameter, six feet (1.8 m) thick at the center, and is held 11 feet (3.4 m) above the fuselage by two struts. It is tilted down 6° at the front to reduce its air drag during take-offs, and while flying endurance speed (which is corrected electronically by both the radar and SSR antenna phase shifters). The dome uses both bleed-air and cooling doors to remove the heat generated by electronic and mechanical equipment. The hydraulically rotated antenna system permits the Westinghouse Corporation's AN/APY-1 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. Consoles display computer-processed data in graphic and tabular format on video screens. Console operators perform surveillance, identification, weapons control, battle management and communications functions. The radar and computer subsystems on the E-3 can gather and present broad and detailed battlefield information. This includes position and tracking information on enemy aircraft and ships, and location and status of friendly aircraft and naval vessels. The information can be sent to major command and control centers in rear areas or aboard ships. In times of crisis, data can be forwarded to the National Command Authority in the U.S. via RC-135 or naval aircraft carrier task forces.

Electrical generators mounted on each of the E-3's four engines provide one megawatt of electrical power that is required by the E-3's radars and other 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 "beyond the horizon" 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 to provide a look down to detect, identify and track enemy and friendly low-flying aircraft while eliminating ground clutter returns.

Upgrades
Starting in 1987, USAF E-3s were upgraded under the "Block 30/35 Modification Program" to enhance the E-3's capabilities. On 30 October 2001, final airframe to be upgraded under this program was rolled out. Several major enhancements were made, firstly the installation of electronic support measures (ESM) and an electronic surveillance capability, for both active and passive means of detection. The Joint Tactical Information Distribution System (JTIDS) provides rapid and secure communication for transmitting information, including target positions and identification data, to other friendly platforms. Global Positioning System (GPS) capability was also added. Onboard computers were also overhauled to accommodate JTIDS, Link-16, the new ESM systems and to provide for future enhancements.

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 dramatically improve 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 digital computers that utilised a High-level programming language instead of assembly language. Significant improvement came from replacing the old 8-bit FFT with 24-bit FFTs, and the 12-bit A/D (Sign + 12-bits) with a 15-bit A/D (Sign + 15-bits). 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 were completed in December 2000. Along with the RSIP upgrade was installation of the Global Positioning System/Inertial Navigation Systems which dramatically improve positioning accuracy. In 2002, Boeing was awarded a contract to add RSIP to the small French AWACS squadron. Installation was completed in 2006.

Operational history
In March 1977 the 552nd Airborne Warning and Control Wing (now the 552d Air Control Wing) at Tinker AFB, Oklahoma received the first E-3 aircraft. The 34th and last USAF Sentry was delivered in June 1984. In March 1996, the USAF activated the 513th Air Control Group (513 ACG), an ACC-gained Air Force Reserve Command (AFRC) AWACS unit under the Reserve Associate Program. Collocated with the 552 ACW at Tinker AFB, the 513 ACG which performs similar duties on active duty E-3 aircraft shared with the 552 ACW.



The USAF have 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).

In 1977 Iran placed an order for ten E-3's, however this order was cancelled following the 1979 revolution.

NATO acquired 18 E-3As and support equipment for a NATO air defense force. Since all aircraft must be registered with a certain country, the decision was made to register the 18 NATO Sentries with Luxembourg, a NATO member that previously did not have any air force. The first NATO E-3 was delivered in January 1982. 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. Presently 17 NATO E-3As are in the inventory, since one E-3 was lost in a crash.

The United Kingdom and France are not part of the NATO E-3A Component, instead procuring E-3 aircraft through a joint project. The UK and France operate their E-3 aircraft independently of each other and of NATO. The UK operates six aircraft (with a seventh now retired) and France operates four aircraft, all fitted with the newer CFM56-2 engines. The British requirement came about following the cancellation of the British Aerospace Nimrod AEW3 project to replace the Avro Shackleton AEW2 during the 1980s. The UK E-3 order was placed in February 1987, with deliveries starting in 1990. The other operator of the type is Saudi Arabia which operates five aircraft, all fitted with CFM56-2 engines.

E-3 Sentry aircraft were among the first to deploy during Operation Desert Shield, where they immediately established as an around-the-clock radar screen to defend against 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 history. 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 and RAF E-3s participated in the international military operation in Libya.

Variants

 * EC-137D
 * Two prototype AWACS aircraft with JT3D engines, one fitted with a Westinghouse Electric radar and the other 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 five for Saudi Arabia with CFM56 engines.


 * KE-3A
 * These are not AWACS aircraft but CFM56 powered tankers based on the E-3 design. Eight were sold to Saudi Arabia.


 * E-3B
 * E-3As with improvements, 24 conversions.
 * E-3C
 * 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 Royal Air Force to E-3C standard with CFM56 engines and British modifications designated Sentry AEW.1, seven built.


 * E-3F
 * Production aircraft for the French Air Force to E-3C standard with CFM56 engines and French modifications, four built.


 * E-3G
 * USAF Block 40/45 modification.

Operators



 * NATO
 * Based in Geilenkirchen, Germany, 18 E-3 AWACS were purchased – one lost in Greece. All of these aircraft are officially registered as aircraft of Luxembourg, a NATO member with no other air force. Responsible for monitoring airspace for NATO operations around the world.


 * Squadron 1
 * Squadron 2
 * Squadron 3
 * Training wing


 * 🇫🇷 France: The French Air Force purchased four E-3F aircraft similar to the British E-3D aircraft.
 * All planes are assigned to the Escadron de Détection et de Commandement Aéroporté (ECDA, Air detection and command squadron) and are based at Avord Air Base.


 * 🇸🇦 Saudi Arabia: The Royal Saudi Air Force purchased five E-3A aircraft and eight KE-3A tanker aircraft in 1983.
 * No. 18 Squadron


 * 🇬🇧 United Kingdom: The Royal Air Force purchased seven E-3Ds by October 1987 but six were operational as of 2012. The aircraft are designated Sentry AEW.1.
 * No. 8 Squadron
 * No. 23 Squadron – (disbanded in 2009)
 * No. 54 Squadron


 * United States of America: The United States Air Force currently has 32 E-3s.
 * 552d Air Control Wing has 27 E-3s stationed at Tinker AFB, Oklahoma
 * 552d Operations Group
 * 960th Airborne Air Control Squadron
 * 963d Airborne Air Control Squadron
 * 964th Airborne Air Control Squadron
 * 965th Airborne Air Control Squadron
 * 966th Airborne Air Control Squadron
 * 513th Air Control Group (Air Force Reserve Command)
 * 970th Airborne Air Control Squadron


 * 3d Wing, Elmendorf AFB, Alaska
 * 3d Operations Group
 * 962d Airborne Air Control Squadron
 * 18th Wing, Kadena AB, Japan
 * 18th Operations Group
 * 961st Airborne Air Control Squadron

Incidents and accidents


The E-3 has been involved in three hull-loss accidents.


 * On 22 September 1995, a U.S. Air Force E-3 Sentry (callsign Yukla 27, serial number 77-0354), crashed shortly after take off from Elmendorf AFB, Alaska. The plane lost power to both port side engines after these engines ingested several Canada Geese during takeoff. The aircraft went down about two miles (3 km) northeast of the runway, killing all 24 crew members on board.


 * On 14 July 1996, a NATO E-3A, LX-N90457, c/n 22852, ex-79-0457, overran the runway and dipped into the sea on takeoff from Preveza AB, Preveza, Greece. The fuselage broke into two, destroying the aircraft, but there were no casualties among the 16 crew members on board. It allegedly suffered a birdstrike during take off, but no evidence of a birdstrike was found.


 * On 28 August 2009, a USAF E-3C, 83-0008, was severely damaged while landing at Nellis Air Force Base, after experiencing a nose-wheel failure. The failure resulted in a fire that caused a reported US$100 million in damage. The accident was determined to be pilot error.