1952. The US Army became the largest user of the Otter, and other military users included Australia, Canada, and India. Some Otters have been converted to turboprop power by Cox Air Services using a Pratt & Whitney Canada PT6A turboprop, yielding a lower empty weight and a higher maximum speed, and a number of other aftermarket PT6A conversions have been offered. The Otter found a significant niche as a STOL bush aircraft and many remain in service. DHC-4 Caribou The DHC-4 Caribou was a rugged STOL design like the Beaver and Otter, but it had two engines and was conceived primarily as a military transport, designed in response to a US Army requirement for a tactical airlifter to supply the battlefront with troops and supplies and evacuate casualties on the return journey. The DHC-4 first flew on 30 July 1958. The US Army ordered five for evaluation and went on to become the largest Caribou operator, taking delivery of 159, initially as the AC-1 and later as the CV-2. These aircraft were transferred to the USAF in 1967 and redesignated as the C-7, seeing extensive service during the Vietnam War, where some were captured by North Vietnamese forces and operated until the late 1970s. Other notable military operators included Canada, Australia, Malaysia, India, and Spain. The type's ruggedness and STOL capabilities also appealed to some commercial users and US certification was awarded on 23 December 1960. Other Caribou entered commercial service after being retired by the military, and some civil Caribou have been fitted with turboprop engines. DHC-5 Buffalo Known originally as the Caribou II, the DHC-5 Buffalo was basically an enlarged DHC-4 with turboprop engines and a T-tail, developed to meet a US Army requirement for a STOL tactical airlifter with greater load-carrying ability than the DHC-4. The Buffalo made its maiden flight on 9 April 1964, but the US Army soon transferred heavy fixed-wing aircraft operations to the USAF, which had little interest in the Buffalo; only four evaluation aircraft were delivered to the US military, two of which were later transferred to NASA for research. The improved DHC-5A transport was acquired by Canadian Armed Forces (as the CC-115), the Brazilian Air Force and the Peruvian Air Force, and the production line was then shut down, but it was reopened after the enhanced DHC-5D was introduced in response to interest by other military users. In the early 1980s,