The end of the Cold War saw a whole new market open for the L-39 and its derivatives, the next generation of which would be an entirely different breed of cat. The agile, simple trainer was about to become a very capable and well equipped combat aircraft.
From the beginning of the 1990s it was realised that, in order to attract customers to the L-39 from the new range of countries now open to Aero, the aircraft would have to be extensively ‘westernised’. The Russian style instruments would have to be replaced with the common standard of equipment found in aircraft throughout the western world and a suitable replacement engine for the Ukrainian manufactured AI25TL would need to be found. The need for the engine was not because there was anything significantly wrong with the AI-25, it had performed reliably since the L-39 first flew, but because it was realised that western customers would prefer a more local supply chain for spares and maintenance. In terms of size and power output, the best choice available at the time was the Garrett TFE 731, a geared turbofan from 1972 which had been fitted to an international range of business jets and three jet trainers, including Spain’s CASA C-101 Aviojet. The two engines were remarkably similar in physical size, both the AI-25TL and TFE 731 having a diameter of 39in (1m), the Garrett engine being shorter and 37lb (17 kg) lighter. The TFE 731-4 was chosen, which produced 4080lb (1850kg) of thrust, about 300lb (136kg) more than the AI-25, but it was in the fuel consumption that the biggest difference occurred. The TFE 731 burned 15% less fuel than the earlier engine, bestowing a commensurate increase in range and endurance.
In 1991, a test installation of this engine was being developed, the aircraft being designated the L-139. A new engine mount had to be devised and many of the engine driven systems had to be modified. The Saphir 5 auxiliary power unit, used for engine starting and providing ground power at austere operating sites, was no longer required because the engine gearbox was already fitted with a starter generator. Likewise, the emergency ram air turbine was no longer needed as the engine systems included an emergency generator, both of these deletions saving a great deal of weight. Flight Visions, the company that had cooperated with Aero on the integrated weapons delivery and navigation system (WDNS) for the L-59, along with Bendix King, developed a similar WDNS for the L-139. This presented its data to the pilots on a Head Up Display (HUD) and Electronic Flight Instrument System (EFIS) display in the front cockpit and a HUD repeater screen in the rear. The HUD data could also be captured during flight to assist in debriefings. Two other major system changes were made. An on-board oxygen generating system (OBOGS) that took bleed air from the engine replaced the standard oxygen bottles and a fatigue and stress monitoring system was installed to determine the actual service life of the airframe. Like the L-39MS, the ejection seats were the Czech designed and manufactured VS-2 zero-zero units, but the canopy had remained the two-piece design of the earlier aircraft rather than the single piece design of that aircraft, because the basic front fuselage structure was identical to the L-39. On May 8, 1993, Ladislav Snýdr and Stanislav Vohanka took the prototype L-139 into the air for the first time. After a short test flight programme, the L-139 made its public debut at the Paris Air Show later that year. A number of American test pilots, including Steve Barter from General Dynamic and Allan Frazer from Mcdonnell Douglas, gave the L-139 favourable reviews, so it was decided to enter the type into the Joint Primary Aircraft Training System (JPATS) competition in the US. The competition was intended to provide a new basic trainer for the US Air Force and US Navy, a considerable contract as it included over 500 aircraft and their associated training and ground support equipment. One of the stipulations of the competition was that the aircraft be a proven, off the shelf design, provided in part through partnership with an American company. To this end, Aero teamed up with General Dynamics to enter the L-139 as it more than fulfilled all of the performance requirements. Unfortunately, General Dynamics sold its aircraft division to Lockheed in March 1993, and Lockheed already had a partner for the JPATS competition, Alenia Aermacchi with the MB-339 T-bird II, so the Aero L-139 JPATS bid came to naught. Surprisingly to many observers, the winner of the competition emerged on June 25, 1995, as the Beechcraft Raytheon T-6 Texan II, a modified version of the Pilatus PC-9 Mk II, a turboprop aircraft.
ABOVE: The prototype of the L-139 in the colour scheme it appeared in at the Paris Air Show in 1993. Pavel Kucˇera RIGHT: The Garrett TFE 731 turbofan being fitted to the L-139 prototype. Pavel Kucˇera