Seeking to succeed its successful, yet aging, line of Bo 105 light utility, twin-engine helicopters, the Eurocopter (now Airbus Helicopters) "EC135" was born from the Bo 108 prototype and has become a market success in its own right. The Bo 108, originally headed by Messerschmitt-Bolkow-Blohm (MBB) of West Germany, was drawn up as a technology demonstrator becoming something of an evolved form of the earlier Bo 105. A first-flight was recorded on February 15th, 1994 and service introduction followed in 1996 with over 1,000 being built since 1995. The militarized variant has become the "EC635" product detailed elsewhere on this site.
The Bo 108 began as a test program through and through. Relying on proven facets of the Bo 105, this gave engineers the freedom to incorporate smaller revisions to the overall design and led to the incorporation of a hingeless main rotor assembly, broadened use of composite construction, a more streamlined transmission arrangement, digital avionics fit in the cockpit and a bearingless conventional tail rotor unit. Power came from 2 x Allison 250-C20R-3 turboshaft engines seated over the passenger cabin. The front of the aircraft was heavily glazed as in the Bo 108 and a fixed landing skid acted as the undercarriage.
By early 1991, the Bo 108 program proved its worth and led to MBB pushing the design to a market product stage. It was also decided to rely on a different engine fit - be it the French Turbomeca "Arrius" series or a turboshaft offered by American-based Pratt & Whitney (PW206). A second prototype went airborne on June 5th, 1991 with a pair of Arrius engines in place.
The emergence of the Eurocopter brand label saw MBB and Aerospatiale both merged under it and, perhaps most important to the new helicopter program, this provided engineers unfettered access to the Aerospatiale-designed "Fenestron" shrouded tail rotor. In this arrangement, a multi-bladed unit sat within a fixed housing buried within the vertical tail fin promising reduced noise levels and optimal efficiency at the cost of complexity. With that, the Bo 108 was revised with the tail unit technology and given the new Eurocopter designation of "EC135". First flight of this product came on February 15th, 1994 and certification followed in June of 1996. Since both of the aforementioned engine fits proved successful in testing, both were offered to market customers in separate variants.
Like other helicopters of this weight class, the EC135 went on to find service careers in civilian, governmental and military circles around the world. Military operators included Australia, Brazil, Gabon, Germany, Ireland, Japan, Morocco, and Spain. Police and government security forces also took to the type in considerable numbers as showcased from its usage in the skies over Argentina and Australia to Turkey and the United States.
Several variants of the EC135 base design have been realized led by the initial EC135 P1 with its Pratt & Whitney engines of 621 horsepower each. The EC135 T1 fits the Arrius engine of 583 horsepower each and these have been followed by EC135 P2 and EC135 T2 with uprated engines from PW and Turbomeca respectively. Modern production versions carry the distinguishing "+" in their designation (i.e. "EC135 P2+") and the more powerful models are EC135 P3 and EC135 T3. A military training model is available as the TH-135 (based on the EC135 T2+) and the formal military model is the evolved EC635 / H135M, this offering originally developed against a Portuguese Army requirement.
The Eurocopter name is now a subsidiary of Airbus Group which uses Airbus Helicopters as its helicopter branch brand label. As such, the EC135 is now refered to as the H135.