The Boeing C-137 Stratoliner is a retired VIP transport aircraft derived from the Boeing 707 jet airliner used by the United States Air Force. Other nations also bought both new and used 707s for military service, primarily as VIP or tanker transports. In addition, the 707 served as the basis for several specialized versions, such as the E-3 Sentry AWACS aircraft. The designation C-18 covers several later variants based on the 707-320B/C series. The C-137 should not be confused with the similar Boeing C-135 Stratolifter; although they share a common ancestor the two aircraft have different fuselages, among other structural differences.
Development
US Air Force procurement of the Boeing 707 was very limited, amounting to three Model 707-153s designated VC-137A. When delivered in 1959 these had four 13,500 lb (6123 kg) dry thrust Pratt & Whitney J57 (JT3C6) turbojets; when subsequently re-engined with 18,000 lbf (80.1 kN) dry thrust TF33-P-5 (JT3D) turbofans they were redesignated VC-137B. Only one other variant served with the Air Force: this was the VC-137C Air Force One Presidential transport, the two examples of which were Model 707-320B Intercontinentals with specialized interior furnishings and advanced communications equipment. Two further C-137C aircraft lacking Presidential transport modifications were later added.
To supplement its VC-137s, the USAF converted several C-135 airframes to VC-135 VIP standard, and these were used for staff transport mainly within the United States.
Variants
C-18
The C-18 is the US military designation for the conversions of the 707-320B series.
C-18A Eight second-hand (former American Airlines) 707-323Cs bought as crew trainers for the EC-18Bs, four later converted to EC-18B, two converted to EC-18D, one to C-18B; one was not taken into service and was used for spares. C-18B One C-18A modified with instrumentation and equipment to support the Military Strategic and Tactical Relay System (MILSTAR). EC-18B Four C-18As modified alongside examples of the C-135 for Advanced Range Instrumentation Aircraft (ARIA) missions in support of the Apollo space program. The designation E-7 was originally applied to modified Boeing 707s before being replaced by the EC-18 designation. EC-18C Original designation for two prototype J-STAR aircraft, later redesignated E-8A. EC-18D Two C-18As modified as a Cruise Missile Mission Control Aircraft (CMMCA). TC-18E Two second-hand (former Trans World Airlines) 707-331 aircraft modified for E-3 pilot and crew training. TC-18F Two second-hand (former TAP Portugal) 707-382 aircraft modified for E-6 pilot training.
C-137 Stratoliner
The USAF purchased a number of 707s under the C-137 series of designations:
VC-137A Three 707-153s (58-6970, 58–6971, 58–6972) with a 22-passenger VIP interior and provision for use as an airborne command post, re-designated VC-137B. VC-137B The three VC-137As re-engined with four Pratt & Whitney JT3D-3 engines, operated by the 89th Military Airlift Wing, redesignated C-137B. C-137B The three VC-137Bs redesignated when downgraded from VIP role. VC-137C Two 707-353Bs (62-6000 and 72–7000) were purchased by the USAF (one in 1961 and one in 1972) for service as presidential transports with call signs SAM 26000 and SAM 27000; later redesignated C-137C. C-137C The two VC-137Cs were redesignated when downgraded from presidential use. SAM 26000 and SAM 27000 were retired in 1998 and 2001 respectively. Both are now in aviation museums. Two further C-137Cs were acquired by the USAF on 24 March 1988, one 707-396C (a seized aircraft formerly used for arms smuggling acquired in 1985) and one 707-382B bought second hand in 1987. Their assigned tail numbers were 85-6973 and 85–6974. Tail 85-6973 would later be converted into an E-8C JSTARS. EC-137D Two aircraft built as Early Warning and Control System prototypes. Later re-engined and re-designated E-3A. A further second-hand 707-355C aircraft was acquired and configured as an airborne special operations command post.