Mod­erni­sa­tion

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.

Aviation Classics - - CONTENTS -

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

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