Posted on September 13 2021
The E-4B National Airborne Operations Center (NAOC) is a conversion of the Boeing 747-200 developed for the U.S. Air Force to exercise command and control of U.S. strategic nuclear forces. It is powered by four General Electric CF6-50E2 high-bypass turbofan engines supplying 52,500 lbf. (233.52 kN) of thrust.
Early Development of the U.S. National Military Command System
The mission performed by the E-4B today dates to the early 1960s when U.S. Strategic Air Command (SAC) began hardening its ground command posts to improve their survivability in the event of a Soviet missile attack. In July 1960, it stationed a Boeing 707-derived KC-135A with special communications gear on ground alert as a communications relay between Washington and SAC forces. In February 1961, this became a continuous airborne alert, with a SAC general officer and a battle staff always in the air.
Around the same time, a ground alert KC-135A for Andrews AFB, Maryland was proposed as a National Emergency Airborne Command Post (NEACP) for the President. The first NEACP was operational in February 1962. Meanwhile, the Joint Chiefs of Staff (JCS) and Office of the Secretary of Defense worked to unify the military’s global communications system and to link it directly to the JCS via a new National Military Command Center (NMCC) in the Pentagon. The NMCC would supersede the older Joint War Room and combined with NEACP and its seaborne equivalent NECPA would link the President, the JCS, and the Secretary of Defense with global U.S. military forces. An interim NMCC was largely operational by the end of 1965, with construction of a dedicated facility underway.
The NEACP KC-135 was only deployed on an interim basis and was replaced with specially modified aircraft starting in July 1962. Three aircraft were modified to ensure one could always remain on ground alert at Andrews. All C-135s used as command posts bore the KC-135 designation until they were redesignated as EC-135s in 1965. Throughout the 1960s they received upgrades to their communications equipment, new TF-33-9 turbofan engines, improved flight controls and an aerial refueling port.
Meanwhile, Air Force EC-135s continued to fulfill the “Looking Glass” mission providing an alternative airborne facility capable of exercising full control over U.S. strategic nuclear forces. Looking Glass is designed to assure operational control of these forces through a general officer, while NEACP supports the civilian national command authority (the President and Secretary of Defense).
EC-135 Obsolescence and Replacement
In the early 1970s, the Air Force sought a replacement for the aging EC-135s that had filled the NEACP role since its inception. The Advanced Airborne Command Post (AABNCP) program was authorized in the fiscal 1973 budget. Under the program, the Air Force would acquire seven 747-200s from Boeing, six with fiscal 1973 funds and the seventh with fiscal 1974 funds. The first three would be equipped with the same systems used aboard the outgoing EC-135 aircraft as an interim capability.
The fourth aircraft was intended as a testbed for a new NEACP mission system, and for electromagnetic pulse (EMP) hardening. Because NEACP aircraft were intended primarily to exercise command and control of strategic nuclear forces (NC3), the Air Force was interested in reducing their vulnerability to EMP phenomena generated by nearby nuclear explosions.
The final three aircraft were intended as the permanent NEACP capability. These would feature a super-high-frequency satellite communications terminal; fleet satellite terminals; a high-powered low-frequency and very low-frequency transmitter; automated data processing; and ground data links to the NMCC, SAC (now STRATCOM), the North American Air Defense Command (NORAD), and the Defense Support Program (DSP) ground systems. DSP is an Air Force early warning satellite constellation designed to detect missile launches. After these aircraft entered service, the first three would be withdrawn and brought to the same standard. These first three aircraft were dubbed E-4As, and the final four were dubbed E-4Bs.
Compared to the EC-135, the E-4B was expected to have 75% more floor space, 25% more endurance and 53% more payload capacity.
The requirement for automatic data processing was quickly dropped as an imprecise requirement. It also quickly became apparent that more significant than expected modifications would have to be made to the 747-200’s hydraulic, electric and air conditioning systems to support the NEACP mission system.
The Air Force awarded a $59 million ($358.32 million in 2020 USD) contract to Boeing for the first two E-4As on Feb. 28, 1973. This contract included an option for a third aircraft, which was exercised. The first E-4A was accepted at Andrews AFB on Dec. 23, 1974. A second followed on May 16, 1975.
In August 1975 Boeing delivered the E-4B testbed aircraft, which was plumbed for aerial refueling. Finally, the third and final E-4A was delivered on Sep. 15, 1975 after completing its modifications at E-Systems, Inc.
In July 1976, Boeing rolled out the E-4B testbed. It was then modified by E-Systems, which finished work on Sep. 11, 1977. By July 1978, the Air Force plan for E-4B acquisition stipulated the purchase of four in fiscal 1980 and another in fiscal 1981. Final operational capability (FOC) would be declared in October 1983. This timetable was revised again in October, with one E-4B to be delivered every fiscal year from 1980 to 1984 and the FOC declaration to occur in November 1986. By February 1978, the program had been truncated to four aircraft, with the E-4B testbed and the three E-4As eventually to be upgraded to the same operational E-4B standard. Because of this truncation, the E-4 could not simultaneously fulfil the NEACP and Looking Glass missions. When the EC-135 fleet was retired in the 1990s, the U.S. Navy took over the Looking Glass role with its E-6 Mercury fleet.
The E-4B testbed was turned over to Strategic Air Command on Jan. 7, 1980. On June 26, a fixed price incentive contract valued at $417.89 million ($1.31 billion in 2020 USD) was awarded to Boeing to retrofit the three E-4As to the E-4B standard.
In the 1990s the NEACP mission was formally expanded and the nomenclature changed to refer to the E-4B as the National Airborne Operations Center (NAOC). The redesignation coincided with the addition of a secondary mission for the fleet: supporting the Federal Emergency Management Agency.
E-4B Obsolescence and Replacement
Since the mid-2000s the Air Force has investigated replacing the E-4 fleet. The youngest E-4B (75-0125) has now been in the Air Force inventory for over forty years and despite decades of incremental modernization still contains plenty of obsolete equipment. The latest initiative to retire the aircraft is the Survivable Airborne Operations Center (SAOC) program. This effort is rooted in the 2018 Nuclear Posture Review, which called for a broad modernization of the NC3 enterprise. It also directly referenced the need to “upgrade and modernize” the NAOC, ABNCP and TACAMO aircraft.
SAOC is still in its early stages. In December 2020, the Presidential Airlift Directorate of the Air Force Life Cycle Management Center announced that it would solicit proposals for a SAOC derived from a commercial aircraft. The Presidential Airlift Directorate is broadly responsible for the SAOC program. The Directorate has also disclosed that it wants a “very large” platform for the SAOC, pointing to a widebody aircraft comparable in size to the E-4B. It had originally wanted to issue a request for proposals around this time, but was delayed due to the designation of SAOC as an ACAT 1D program, a designation for major weapons systems procurements requiring oversight from the Undersecretary of Defense for Acquisition and Sustainment.
Prior to SAOC, the Department of Defense’s Cost Assessment and Program Evaluation (CAPE) office conducted a study of replacement requirements and options for the E-4B, the Boeing C-32A and the E-6B Mercury. This so-called NEAT study was initiated in 2019 and completed in 2020. The name referred to the NAOC, the C-32A (“Executive Airlift”), the ABNCP, and TACAMO. Following the completion of the study, the Navy has begun its own effort to replace the E-6B, most likely with a derivative of the Lockheed Martin C-130J Super Hercules.
The Air Force intends to issue early contracts for SAOC sometime in 2021. Plausible aircraft capable of fulfilling the role are limited to the Boeing 747-8 and Boeing 777 if the SAOC mission does require a comparable size aircraft to the E-4B. The 777 remains in production, with the new 777X series expected to enter production in 2022. 747-8 production will be terminated in 2022 or 2023 when the final outstanding orders are fulfilled. It nevertheless remains possible that a SAOC aircraft is based on a 747-8, as the Air Force could acquire surplus aircraft from a civil operator or broker as it has done for the VC-25B procurement effort. Systems integrators for the program could include L3Harris, Lockheed Martin, Northrop Grumman, or Boeing, though none are yet known to have firmly indicated interest.
$76.417 million of Fiscal 2021 Air Force RDT&E funds were appropriated for E-4B recapitalization, which principally includes the SAOC program. A milestone B decision authorizing engineering and manufacturing development is scheduled for the second quarter of 2023. The budget also included $3.468 million in Fiscal 2021 Air Force RDT&E funds for E-4B upgrades
This section describes the original E-4B configuration, though some information reflects upgrades to its mission systems conducted since the aircraft entered service.
All E-4s are converted Boeing 747-200B passenger aircraft with three decks. The 747-200B is a wide-body airliner with four engines mounted underneath a 37.5-deg. swept wing and a fuselage mounted tail. Its main landing gear is comprised of four legs with four-wheels on each and a dual-wheel nose gear.
The main deck contains a briefing area (17 seats), conference room (9), senior executive suite and a battle staff area (35). Altogether up to 114 people can be seated in the aircraft. For crew rest, there are 14 bunks aboard the E-4B. Behind the flight deck, the upper deck houses a crew rest area for maintenance personnel with seating and six bunks. Another crew rest area aft of the communications center contains seating and the final eight bunks. A room behind the briefing room was originally used to house 35mm projection equipment, but the system has been phased out in favor of LCD displays.
The aircraft is designed to be hardened against the effects of electromagnetic pulse (EMP) phenomena generated by nuclear detonations. The most obvious sign of this is the wire mesh visible on the aircraft’s windows.
The E-4B can fly for up to 12 hours unrefueled and 72 hours with in-flight-refueling. Fully refueling the aircraft through its aerial refueling port often requires two full fuel offloads from KC-135R Stratotankers.
The E-4B carries four General Electric CF6-50-E2 high-bypass turbofan engines, which the Air Force dubs the F103-PW-100. Each engine supplies 52,500 lbf. (233.5 kN) of thrust, for a total of 210,000 lbf. (934.1 kN). Unlike the civil aircraft upon which the E-4 is based, the E-4B features two generators per engine, for a total of eight (the 747-200B only carries four). The engines have cascade-type thrust reversers.
All of the communications systems aboard the E-4B originally featured point-to-point wiring with a bus interface. The aircraft carries an interphone system for communications between stations aboard the aircraft.
Terrestrial Radio Communications
The lower lobe cargo area is converted to house a VLF trailing wire assembly and operator console. To facilitate VLF communications, a five-mile-long wire antenna is extruded from the rear of the aircraft. The wire is tipped with a metal drogue for aerodynamic stability. The VLF system can transmit with 20 kW of output power. It enables direct one-way communication with submerged ballistic missile submarines and with strategic bombers, providing a more survivable alternative to VLF ground sites. This VLF capability permits airborne communications with nuclear strike platforms in the event TACAMO aircraft are destroyed but an E-4B survives.
The E-4B carries five HF transceivers and four receive-only HF radios. This equipment is linked to KY-75 Parkhill analog secure-voice sets and USC-43/KYV Advanced Narrowband Digital Voice Terminals. Data modems include the TE-204, CV-786, and KL-43.
The E-4B also has a complex telephone communications system that has been expanded over time. This system includes unsecure and secure lines that are electrically and physically isolated from each other. Owing to the age of the aircraft, this system originally featured analog switching. Calls are placed or received over the Defense Switched Network (DSN) via the HF-band High Frequency Global Communications System (HF-GCS). Before the introduction of DSN in the 1990s the aircraft could connect to the legacy AUTOVON system.
The E-4B can also use the HF Mystic Star network for voice and data communications between E-4Bs, VC-25s VH-3Ds, VH-60s and ground stations. Mystic Star also has a UHF satellite layer for global coverage.
Critically, the E-4B interfaces with the Minimum Essential Emergency Communications Network (MEECN). MEECN provides a secure jam-resistant communications link between the three legs of the U.S. nuclear triad; the NMCC in Washington; and TACAMO, ABNCP and NAOC aircraft. The airborne component of the system aboard the E-4B uses the aircraft’s VLF/LF transmission capability to communicate with Minuteman intercontinental ballistic missile (ICBM) launch control centers and a Miniature Receive Terminal that can receive emergency action messages (EAMs) from ground stations. EAMs can be transmitted by the NMCC; the Alternate NMCC at Raven Rock, PA; or by the NAOC itself. EAMs are issued by the national command authority to order the use of nuclear force. When received by STRATCOM, an EAM is retransmitted globally via HF-GCS.
The E-4B was originally designed with an AN/ASC-24 SHF satellite communications (SATCOM) system to integrate it into the Defense Satellite Communications System (DSCS) II network. The AN/ASC-24 consists of a computer-augmented antenna pointing sub-system, an RCA 12 kW SHF parabolic dish antenna, a Collins 11 kW SHF terminal and a Sylvania Electronic Systems wideband anti-jam digital data modem. The SHF is housed in a radome mounted ventrally on the forward fuselage in a distinctive “hump.” The AN/ASC-24 is an upgraded variant of the developmental AN/ASC-18 tested aboard a C-135 in the early 1970s.
As the U.S. has introduced new SATCOM constellations the E-4B has received upgrades to make use of their capabilities. The EHF-band MILSTAR constellation of six satellites launched between 1994 and 2003 and required the integration of a new terminal aboard the aircraft. When the Advanced EHF (AEHF) constellation entered service, the E-4B required another new terminal to replace the MILSTAR equipment. AEHF uses a higher data rate waveform than MILSTAR but is backwards compatible with MILSTAR’s low and medium data rate modes. AEHF functionality for the aircraft includes Presidential National Voice Conferencing (PNVC) capability, which replaces the MILSTAR-based Survivable Emergency Conferencing Network.
The E-4A was the original configuration of the E-4, incorporating equipment from the EC-135.
The E-4B was the upgraded and final production standard for the E-4 fleet. After testing and evaluation of the only new-build E-4B was completed, the three E-4As were converted to E-4Bs.
In the mid-1990s, the E-4B received a major series of upgrades to its underlying computing and networking systems. All four aircraft were routed with fiber optics in place of the old point-to-point copper wiring. They also received an asynchronous transfer mode data bus. At the same time the E-4Bs were outfitted with UHF-band SATCOM equipment for the first time, which enabled the use of commercial Iridium SATCOM.
In June 2011, a contract was awarded to Boeing to modify the E-4B fleet with new communications navigation surveillance/air traffic management (CNS/ATM) equipment. These upgrades were completed the next year. Boeing provided a 737-based commercial off-the-shelf solution.
Modification Block 1
A series of block upgrades were contemplated beginning in 2001 that would modernize various systems aboard E-4Bs. The upgrade program was renamed to Modification Block 1 after a brief period in which it was called Block 5 and was authorized in 2005 under a $2 billion ($2.67 billion in 2020 USD) contract.
The block upgrade included an audio infrastructure update (AIU), Global Air Traffic Management II equipment and the Senior Leaders Communication System. The AIU replaced obsolescent analog audio equipment with digital counterparts. The upgrade also included an enhanced local area network.
Low Frequency Transmit System Replacement
The Low Frequency Transmit System (LFTS) replacement program was established to replace the E-4’s now over 40-year-old VLF/LF system. The new LFTS has been under development by Boeing since October 2014, when a $9.8 million ($10.8 million in 2020 USD) contract was awarded to the company. The first aircraft received its new LFTS alongside upgrades to its AEHF and PNVC capability in early 2021. The new system reduces the weight of the aircraft by about a ton.
In fiscal 2020 replacement of USC-42 UHF radios with multi-user objective system (MUOS) compatible radios was authorized. MUOS is a satellite constellation providing data and full duplex voice communications. Integration of the new radios will improve interoperability with other aircraft and ground stations using the MUOS waveform.
Production and Delivery History
The first E-4A was delivered to the Air Force on Dec. 23, 1974. It was followed by the second aircraft on May 16, 1975. The E-4B testbed was delivered in August 1975 but was not fully fitted-out and delivered to SAC until Jan. 7, 1980. Thereafter, the three E-4As were converted to the E-4B standard, and all four have remained in service ever since. They are based at Offutt AFB, Nebraska.
An E-4B typically accompanies Air Force One (the Presidential transport, presently the 747-8-derived VC-25A) on overseas flights. They are also used to transport the Secretary of Defense because of their sophisticated communications equipment, which permits the Secretary to conduct classified business in transit and ensures that they remain constantly connected to the U.S. military’s global command and control system. Finally, when not accompanying the President or the Secretary of Defense, one E-4B maintains a constant ground alert at Andrews AFB, Maryland.
The E-4B is the most expensive aircraft in the American inventory to sustain. In Fiscal 2018 the operations and sustainment cost per individual E-4B stood at $85.34 million (then-year dollars).
Unfortunately, the fleet has also suffered from low mission capable and availability rates due to the small size of the force and the sophistication of the NAOC mission system. According to the Government Accountability Office, the E-4B fleet met its availability goals in none of the fiscal years from 2011 to 2019 and met its mission capability goals in three of the nine years. With an average of 18,557 flight hours on the four airframes and civilian 747-200s entering retirement, the fleet will only be harder to maintain in the future.
In May 2010 an E-4B was involved in a tail incident at Offutt AFB. The incident represented millions of dollars in damage but incurred no fatalities or injuries, and the aircraft was returned to service after repairs.
In June 2017, two E-4Bs were damaged during a tornado at Offutt AFB. Both aircraft were in shelters with their tails exposed; winds from the tornadoes acted upon the tails of the aircraft and pushed them around on the ground. 11 weeks of repairs were necessary to return the aircraft to service.
A historic flood in March 2019 caused significant damage to Offutt AFB and forced a brief relocation of the E-4B fleet. While the aircraft themselves were not damaged, 137 buildings were damaged, 60 so severely that they were slated for demolition. This included the E-4B alert facility, though the hangar for the aircraft is thought to be salvageable.
For eighteen months beginning in March 2021 the E-4Bs are operating out of Lincoln Airport, Nebraska while the runway at Offutt AFB is replaced. The original runway dated to 1941 and required intensive maintenance to maintain safety of flight.