KC-135 Stratotanker

KC-135 Stratotanker

Boeing built 732 KC-135 Stratotankers for the US Air Force between 1957 and 1965. The US Air Force still has about 550 KC-135 Stratotankers in service (active duty, 253; Air National Guard, 222; Air Force Reserve, 70) and has made substantial investment in a series of upgrade programs, including reskinning of the lower wing surfaces, the installation of new CFM56 engines and new avionics systems.

411 of the aircraft in service are the upgraded 135R models and 134 the older 135E. The KC-135 is also in service with the air forces of France (11 aircraft), Turkey (seven) and Singapore (four).

The US Air Force received testing equipment from Cubic Defence Systems as part of the contract signed in February 2010. The testing equipment, which includes small toolbox-sized test sets, will be used to examine and troubleshoot the auxillary power unit (APU) fitted in the tail section of KC-135 refuelling aircraft. The APU produces the electricity that fires up the KC-135’s engines.

KC-135 aerial refuelling method

The primary air fuel transfer method is through the tanker’s flying boom, controlled by an operator stationed at the rear of the fuselage. USAF aircraft have primarily used this boom and receptacle refuelling technique.

“The KC-135 Stratotanker is the US Air Force’s air-to-air refuelling tanker aircraft.”

A shuttlecock drogue can be trailed behind the boom and used to refuel aircraft equipped with refuelling probes. Aircraft fitted with the boom drogue cannot refuel boom and receptacle aircraft.

Stratotanker hose and drogue air refuelling

About 45 US Air Force KC-135R Stratotankers are fitted with mk32B wingtip hose and drogue air refuelling pods, which are supplied by Flight Refuelling Ltd.

These are capable of refuelling Navy and Nato aircraft, which use a probe and drogue system instead of a boom and receptacle. The receiving aircraft approaches the tanker and its probe makes contact with a hose reeled out and trailing from the tanker.

The additional system allows the KC-135R to refuel both probe / drogue and boom / receptacle aircraft on a single mission and to refuel two probe / drogue aircraft simultaneously.

The installation of wingtip refuelling pods involves a major modification and refit to the entire aircraft, including modifications to the wing and fuselage fuel tanks, additional fuel control systems and the installation of indicators and circuit breakers on the flight deck.

Inside the refuelling pods, a collapsible funnel-shaped drogue is attached to a hose, which is reeled out to trail behind the wing of the aircraft. The hose is fitted with a constant tension spring to give stability to the hose while it is extended.

A joint semi annual aerial refuelling training exercise was performed by the US Air Force’s KC-135 Stratotanker in December 2009 with 30 Royal Danish Air Force’s F-16 fighting falcons to strengthen operational readiness and mutual trust between the Nato allies.


Passengers and up to 37,650kg cargo can be carried on the cargo deck above the refuelling systems.

KC-135 Stratotanker modernisation programme

The USAF selected Rockwell Collins to carry out the modernisation of the avionics system under the KC-135 Pacer CRAG (compass, radar and global positioning system) programme.

“The primary air fuel transfer method is through the tanker’s flying boom.”

The upgraded avionics include cockpit enhancements with the Collins FMS-800 integrated flight management system, Collins FDS-255 liquid crystal flat-panel multifunction flight display, and the Collins WXR-700X forward-looking predictive windshear weather radar.

The flight management system is integrated with a traffic alert and collision avoidance system (TCAS) and an enhanced ground proximity warning system (EGPWS).

Subsequent to the Pacer CRAG programme, the USAF selected Rockwell Collins to update the KC-135 flight deck in support of the global air traffic management (GATM) initiative. The GATM upgrade program will focus primarily on upgrading the aircraft’s communication and navigation systems to free operation in civil airspace. 50 of the 550 USAF aircraft received the upgrade under the Phase I low-rate initial production contract and the first production aircraft was delivered in August 2003.

The second LRIP contract, signed in July 2003, provided for a further 25 aircraft to be upgraded. The phase II full rate production contract for 30 more systems was signed in April 2004. As of March 2008, 229 upgraded aircraft had been delivered.

GATM additions to the communications system include an Aero-I SAT-2000 satellite communications system and two FANs (future air navigation) capable CMU-900 Communication Management Units for data link applications. Also, the addition of two Collins integrated processing centres provides an integrated, modularised platform for hosting several partitioned GATM-related software applications.

GATM additions to the navigation system include two Collins GNLU-955M multi-mode receivers, which contain individual modules for GPS, MLS and FM-Immune VOR/ILS (VHF omni-directional radio range steering guidance linked with an instrument landing system) capabilities. Additionally, one existing and one newly added embedded global positioning system / inertial navigation system (EGI) unit will possess 12-channel, all-in-view GPS receivers.

Rockwell Collins was chosen by the US Air Force to provide engineering, manufacturing and development (EMD) of the KC-135 as part of the block 45 cockpit upgrade programme.

The contract will overhaul the aircraft’s flight deck with advanced autopilot, flight director, radar altimeter and electronic engine instrument display to enhance navigtional capabilities. The unused equipment will be replaced with digital avionics to increase safety, reliability and efficiency.

Two prototype aircraft will be modified during the EMD phase which will establish the production baseline for 415 additional aircraft to receive the Block 45 upgrade.

Mission variants

“Some KC-135 aircraft have been configured for other missions such as reconnaissance.”

Some KC-135 aircraft have been configured for other missions such as reconnaissance and as a flying command post.

The US Air Combat Command operates 17 RC-135V/W rivet joint reconnaissance aircraft and the OC-135B open skies observation aircraft and the US Strategic Command operates the EC-135 flying command post.

The US Strategic Command has one EC-135C on alert status at all times, ready to take command if ground control is compromised.


The KC-135R aircraft has been updated with more efficient CFM International CFM-56 engines, which burn up to 25% less fuel and provide greater thrust than previously fitted engines. The efficiency of the engines enable the aircraft to offload 50% more fuel on a medium-radius, 1,725-mile refuelling mission. 410 of the USAF fleet of 546 KC-135s have been re-engined. The remainder have Pratt & Whitney TF33-102 engines.


The KC-135 can climb at the rate of 1,490m a minute. The maximum and cruise speed of the aircraft are 933km/h and 852km/h respectively. The range is 2,419km. The ferry range and service ceiling of are 17,766km and 15,200m.

The aircraft weighs around 44,663kg and the maximum take-off weight is 146,300kg.


  • KC-135A: Original production version powered by four Pratt & Whitney J57s, 732 built. Given the Boeing model numbers 717-100A, 717-146 and 717-148.
  • NKC-135A: Test-configured KC-135A.
  • KC-135B: Airborne command post version equipped with turbofan engines, 17 built. Provided with in-flight refueling capability and redesignated EC-135C. Given the model number 717-166.
  • KC-135D: All four RC-135As (Pacer Swan) were modified to partial KC-135A configuration in 1979. The four aircraft (serial numbers 63-8058, 63-8059, 63-8060 and 63-8061) were given a unique designation KC-135D as they differed from the KC-135A in that they were built with a flight engineer’s position on the flight deck. The flight engineer’s position was removed when the aircraft were modified to KC-135 standards but they retained their electrically powered wing flap secondary (emergency) drive mechanism and second air conditioning pack which had been used to cool the RC-135As on-board photo-mapping systems. Later re-engined with Pratt & Whitney TF33 engines and a cockpit update to KC-135E standards in 1990 and were retired to the 309th AMARG at Davis-Monthan AFB, AZ in 2007.
  • KC-135E: Air National Guard and Air Force Reserve KC-135As re-engined with Pratt & Whitney TF33-PW-102 engines from retired 707 airliners (161 modified). All E model aircraft were retired to the 309th AMARG at Davis-Monthan AFB by September 2009 and replaced with R models.
  • NKC-135E: Test-configured KC-135E. 55-3132 NKC-135E “Big Crow I” & 63-8050 NKC-135B “Big Crow II” used as airborne targets for the Boeing YAL-1 Airborne Laser carrier.
  • KC-135Q: KC-135As modified to carry JP-7 fuel necessary for the SR-71 Blackbird, 56 modified, survivors to KC-135T.
  • KC-135R (1960s): 4 JC/KC-135As converted to Rivet Stand (Later Rivet Quick) configuration for reconnaissance and evaluation of above ground nuclear test (55-3121, 59–1465, 59–1514, 58–0126; 58-0126 replaced 59-1465 after it crashed in 1967). These aircraft were powered by Pratt & Whitney J57 engines and were based at Offutt AFBNebraska.
  • KC-135R: KC-135As and some KC-135Es re-engined with CFM56 engines, at least 361 converted.
  • KC-135R(RT): Receiver-capable KC-135R Stratotanker; eight modified with either a Boeing or LTV receiver system and a secure voice SATCOM radio. Three of the aircraft (60-0356, -0357, and -0362) were converted to tankers from RC-135Ds, from which they retained their added equipment.
  • KC-135T: KC-135Q re-engined with CFM56 engines, 54 modified.
  • C-135F: A new-built variant for France as dual-role tanker/cargo and troop carrier aircraft. 12 were built for the French Air Force with the addition of a drogue adapter on the refueling boom. Given Boeing model numbers 717-164 and 717-165.
  • C-135FR: 11 surviving C-135Fs upgraded with CFM International F108 turbofans between 1985 and 1988. Later modified with MPRS wing pods.
  • KC-135Y: An airborne command post modified in 1984 to support CINCCENT. Aircraft 55-3125 was the only EC-135Y. Unlike its sister EC-135N, it was a true tanker that could also receive in-flight refueling. Pratt & Whitney TF33-PW-102. Retired to 309th AMARG at Davis-Monthan AFB, AZ.

Specifications (KC-135R)

Crew 3 (pilot, co-pilot and boom operator ; some KC-135 missions require the addition of a navigator
Capacity up to 80 passengers / 83,000 lb (38,000 kg)
Length 136 ft 3 in (41.53 m)
Wingspan 130 ft 10 in (39.88 m)
Heigh 41 ft 8 in (12.70 m)
Wing area 2,433 sq ft (226.0 m2)
Empty weight 98,392 lb (44,630 kg)
Gross weight 297,000 lb (134,717 kg)
Max take off weight 322,500 lb (146,284 kg)
Power plant (Dry thrust) 4 × CFM International F108-CF-100 turbofan engines, 21,600 lbf (96.2 kN) thrust each
Power plant (Thrust with afterburner)   
Maximum speed (Sea level)  
Maximum speed (High altitude) Mach 0.9 (580 mph, 933 km/h)
Combat radius 1,303.5 nmi (1,500.0 mi, 2,414.1 km) with 150,000 lb (68,039 kg) of transferable fuel
Ferry range 9,572 nmi (11,015 mi, 17,727 km)
Service ceiling 50,000 ft (15,000 m)
Rate of climb 4,900 ft/min (25 m/s)
Wing loading  
Design load factor  


  • Chilean Air Force – Operates 3 KC-135Es. It received its first KC-135E in February 2010.
  • French Air and Space Force – Operates 11 C-135FRs and 3 KC-135Rs, which are as of 2018-2023 being replaced by 15 Airbus A330 MRTTs, designated Phénix in French service.
  • Turkish Air Force – Operates 7 KC-135Rs.
  • United States Air Force – Operates 398 KC-135s (156 Active duty, 70 Air Force Reserve, and 172 Air National Guard) as of May 2017
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