Based on its L-39 Albatros, Aero Vodochody's L-159 is the latest and most capable version of the world's most prolific jet training aircraft. Since 1968, Czech manufacturer Aero has delivered over 3,000 L-39s and variants to air forces round the world. The subsonic L-159 was previously known as the Advanced Light Combat Aircraft (ALCA) and stems from a Czech air force requirement.
The L-159 is designed to perform several combat roles: air defence, close air support and tactical reconnaissance. Additionally, it will perform lead-in fighter and weapons trainer tasks.
The break-up of the Warsaw Pact and some members' desire to join NATO prompted the Czech air force to begin implementing a radical restructuring. Current Czech aircraft will be modified with NATO-compatible systems, while those of Soviet design will be replaced by new multirole aircraft. The nominal inventory of 100 combat aircraft will comprise a high technology/low technology mix of aircraft. About 24 Western supersonic multirole fighters (Lockheed Martin F-16/Boeing F/A-18/Saab Gripen-class) will be procured for the high end of the mix. The lower technology backbone will be made up of 72 Czech-built aircraft.
To reduce development costs and speed operational deployment, the air force required the L-159 to be based on the L-59 variant of the original L-39 line. Nearly identical in appearance to the L-39, the L-59 incorporated a hydraulic flight control system in the L-39's proven airframe. Turning a trainer airframe into a viable combat aircraft required the addition of avionics and weapon systems, as well as a more responsive and powerful engine. After partnering with Boeing in 1996, Aero assembled a world-class team of subcontractors: AlliedSignal, Fiar, GEC-Marconi and Honeywell. This combination of Eastern airframe and Western avionics expertise has developed what promises to be a capable light fighter and lead-in fighter trainer aircraft.
Advanced cockpit configuration
The most striking feature of the L-159 is its cockpit, similar to those of early model F-16s or F/A-18s. The combat survivability of these aircraft was enhanced by a GEC-Marconi Sky Guardian 200 radar warning receiver (RWR) and Vinten Vicon 78 Series 455 countermeasures dispensing system. Provision for two additional dispensers is available. Chaff or flares can be dispensed manually by a hands-on throttle and stick switch, or via semi or fully automatic modes. In the automated modes, the Sky Guardian's programmable threat library determines the sequence and type of countermeasures to be dispensed. One remarkable feature of the RWR is its ability to record emitter parameters during flight, useful when encountering a threat for the first time.
Rounding off the avionics is a NATO compatible communication, navigation and identification system. Two Rockwell Collins N-ARC-210 UHF/VHF radios allow several frequencies to be monitored at once, essential for the close air support role. Navigational accuracy is enhanced by a Honeywell H-764 G ring laser gyro inertial navigation system (INS) with an embedded global positioning system. An AlliedSignal APX-100 information friend or foe transponder should allow for safe passage through friendly air defence systems.
From a performance standpoint, the L-159's most impressive feature over its predecessors is its engine. Before the L-159, the most powerful engine installed in a L-39 series aircraft was the 4,045lb-thrust (18kN) AlliedSignal TFE 731-4. The L-159's AlliedSignal/ITEC F124-GA-100 is rated at 6,300lb at sea level and 15íC, a 50% increase. With half internal fuel, the single-seat L-159A has a sea level thrust to weight ratio of 57%. The dual spool non-afterburning turbofan engine is controlled by two full authority digital engine control units, allowing the pilot unrestricted throttle movement throughout the flight envelope. Maintenance and logistics staff will appreciate the engine's monitoring system and modular construction, which should help reduce life cycle costs.
Aero used four aircraft to complete its testing programme. A two-seat L-159B prototype was used for engine, performance and weapons separation tests, while a single-seat L-159A was used for radar and avionics integration. Aero was in the middle of certification testing for the air force when Flight International was invited to sample at first hand the L-159 at Aero's production facility north of Prague. A lull in the air force's certification process allowed time for me to fly a sortie in the two-seat prototype L-159B. First flown in August 1997, the aircraft had undergone the lion's share of the development programme. The prototype's cockpit and avionics, however, were from the previous generation L-59. For weight and balance purposes, a non-functional head-up display was fitted. Additionally, there was no radar or RWR installed. From aerodynamic and engine standpoints, however, it is representative of the aircraft that will be delivered to the air force.
I was accompanied on my flight by Aero's chief test pilot, Miroslav Schutzner. Notable features of the L-159 include the extended nose section, roughly 0.5m (1.5ft) longer than that of the L-59, which houses the Fiar Grifo radar (above). Six underwing hardpoints, capable of carrying a wide range of NATO standard stores, were clearly visible. The single centreline station can be fitted with an electronic countermeasures, reconnaissance or 20mm gun pod. The wingtip tanks were full, giving us a total of 1,280kg (2,820lb) of internal fuel (1,550kg for the single-seat L-159A). The robust trailing link landing gear allows for grass strip operations and rough landings by students. The inspection was straightforward and accomplished in several minutes. Integral boarding steps allowed easy access. I found the front cockpit's zero zeroVS-2 ejection seat to be comfortable, while its electrical height adjustment allowed me to attain easily the design sitting height. The +8/-4g aircraft had anti-g suit hook-ups, but neither Schutzner nor I wore one.
Before engine start, the single piece canopy was lowered. Field of view out of the front cockpit allowed me to check the rear six o'clock position. Prestart checks were completed in seconds and the battery and engine masters were switched on. While an external power cart was connected, the L-159 could have been started on battery power. The Safir auxiliary power unit (APU) was started by pushing a button on the left console. Within 20s, the green APU idle light illuminated on the right forward annunciator panel. I pushed the throttle out of cut-off into the idle position. Light off was immediate and the engine accelerated to 61% N2 in 25s. Peak exhaust gas temperature was 777°C, well below the limit of 871°C. Had the production INS been fitted, alignment would have taken about 4min. For planned scramble starts, an INS stored heading mode alignment can be accomplished in about 30s. After putting the generator on line, and checking the flaps and speedbrakes, we were ready to taxi.
At taxi speeds of less than 18kt (33km/h), +/-6° of nose wheel steering (NWS) was available. With it, I found I could track the taxiway centreline fairly easily. Holding a button on the stick increased the NWS range to +/-60°. The +/-60° mode was useful for making turns in excess of 45°, but was sensitive for straight-ahead taxiing. The toe wheel brakes have a unique feature. With increased NWS engaged, differential toe brakes were available. In +/-6 NWS mode, pushing either toe brake sent equal pressure to both wheels. The hydraulic wing flaps have three positions, up (0°), take-off (25°) and landing (44°). Flaps are positioned by pushing one of three selector buttons. I selected take-off flaps before taking the runway, and a light in the push button confirmed their positions.
Once lined up on runway 10, I advanced the throttle to maximum and released the toe brakes. The 5,600kg aircraft accelerated rapidly in the 24°C outside temperature. At 105kt, about 7kg of aft stick was required to attain a 10° nose high attitude. After an 800m ground run, and only 17°, the L-159 lifted off the runway. At 150kt, slight aft stick pressure was required to maintain pitch attitude when I selected flaps up. Had I forgotten to retract the flaps, they would have automatically retracted when the aircraft accelerated through 167kt. In the clean configuration (gear and flaps up), I levelled the L-159 at 2,000ft and accelerated to and maintained 340kt to prepare for our climb into the working area.
I pushed the throttle to maximum and started a climb to 20,000ft. During the climb, the aircraft easily trimmed to hold the desired airspeed. I levelled the L-159 out at 20,000ft and 270kt. We had averaged about 4,000ft/min (370m/s) rate of climb and burned only 120kg of fuel. While level, I set the throttle to 100¼ power lever angle, the cruise position for clean aircraft. The aircraft accelerated to and stabilised at 302kt and M0.64. Fuel flow was 1,020kg/h. Satisfied with the cruise performance, I selected maximum power and rolled into a steep turn. At 300kt, the 5,360kg L-159 was able to sustain a level 2.5g turn with no buffet. While roll control was excellent, there was a bit too much stick free play in the pitch axis.
Next I set up for an idle power clean configuration stall. Slowing the L-159 at 1kt/s, I found it responsive in all three control axes. Ailerons were particularly effective, no doubt due in part to the end plate effect of the wingtip fuel tanks. At 130kt I felt light buffet. At 120kt there was moderate airframe buffet. The wings remained level and I initiated a recovery by releasing back stick pressure. Recovery was positive and rapid. Once recovered, I climbed back to 20,000ft and set up for an 85% thrust landing configuration stall. Slowing as described above, light buffet occurred at 105kt. I recovered at the onset of moderate buffet at 100kt. Again, the recovery was positive and rapid. During both recoveries I rapidly advanced the throttle to maximum, with no adverse effects. While the landing configuration stall was benign, I would have preferred more warning before the onset of moderate buffet. I retracted the flaps and gear and climbed to 20,000ft.
Even though the L-159 was designed to be spin resistant, Schutzner allowed me to try a spin. While climbing at a 10° pitch attitude, I slowed the clean aircraft in idle power. At 130kt, I slowly put in full right rudder. When the aircraft rolled through 45° of right bank, I rapidly pulled the stick full aft and held it there. After this deliberate misapplication of controls, the L-159 rolled underneath and settled upright in a 30° nose low attitude. Yaw rate was about 120°/s. After about one complete 360° turn, the nose dropped to about 70° nose low and the yaw rate increased to 180°/s. Before completing another 360° of turn, I neutralised all controls. The aircraft recovered to steep nose low controlled flight in less than 90° of turn. I initiated a 3g wings level pull and levelled the aircraft at 14,000ft and 300kt. The L-159's benign stall and spin characteristics will allow pilots to manoeuvre with confidence in the lower speed region of the flight envelope.
Simulated flame-out approach
I descended to 15,000ft to determine sustained g capability at that altitude. With maximum power selected, the 5,175kg aircraft sustained a 3.7g (10°/s) turn at 310kt. Slightly better than the flight manual's predicted 3.5g. Continuing the descent, I opened the two lower fuselage-mounted speed brakes. They were effective at slowing the aircraft and had little effect on the pitch trim condition. Speed brakes extended and idle power, simulating a flamed out engine, we headed towards the abandoned Milovice AB. We arrived overhead at 4,000ft above the ground and 150kt best glide speed. Crossing the runway at a 90° angle, I continued descending in a left hand turn. Parallelling the runway and abeam the desired touchdown point, one-third of the way down the runway, the gear was lowered and flaps selected to land. I continued to turn towards the runway and set up for final approach. The relatively slow rate of descent and responsive flight controls greatly eased the energy management task required to complete a safe approach. At 200ft above the ground and in a position to land safely, I terminated the approach by advancing the power and retracting the gear and flaps.
Turning westward, I climbed to 1,000ft above the ground. Levelling, I accelerated to and stabilised at 400kt. Total fuel flow was about 1,680kg/h. While performing this manoeuvre, I found the aircraft to be overly sensitive in the pitch axis. Production L-159s will be autopilot equipped, with an integral stability augmentation system (SAS) for the pitch axis. The SAS will probably alleviate this trait. I next simulated a defensive break turn. Slamming the throttle to idle at 400kt, I rolled into a left hand 5g turn. Field of view to the rear was excellent as I rotated my tail away from the simulated threat. After 180° of turn in idle power, the resulting 310kt would allow a sustained maximum power 6g turn to be executed.
The winds were calm for our instrument landing system approach to runway 28. Lowering the gear at 180kt required about 3.5kg of aft stick to maintain level flight. Selecting flaps take-off at 160kt and land at 150kt required a slight increase in aft stick pressure to keep level flight attitude. Final approach was flown at 130kt for the 4,810kg aircraft. I found it easy to track the localiser and glidepath. Initially I flared slightly high, but was easily able to step the L-159 down to a soft touchdown at 105kt. Stick forces in the flare were light, requiring about one-quarter stick travel to attain the landing attitude of 10°. After touchdown, I selected maximum power and take-off flaps. The rudder and ailerons allowed me to easily track the runway centreline as we accelerated for the second part of our touch and go. Lift-off and climb to 900ft above the ground pattern altitude was uneventful. After completion of another touch and go to runway 28, we set up for the full stop landing. As with the previous two approaches, I found the L-159 to be very docile in the landing pattern. The rapid throttle response of the engine enabled timely and accurate airspeed corrections during all phases of flight in the landing pattern.
After touchdown on the full stop landing, I lowered the nose to the runway and selected up flaps. I easily tracked the runway centreline as the aircraft slowed. Nearing the turn-off point, I firmly applied the toe brakes to slow to a taxi speed. I accomplished the 90° turn off the runway using the increased +/-60 NWS. Engine shutdown and post flight procedures were simple and rapidly accomplished. Block to block time was 70min and we had burned 885kg of fuel during our 55min flight.
As the L-159 may be used as a trainer, I was eager to see how it landed from the back seat. While Schutzner took the front, I strapped into what must be one of the best back seats of any tandem trainer. Elevated above the front seat, the rear seat forward field of view was better than that of either the F-16 or Northrop T-38.
I accomplished two touch and goes and one full stop landing from the rear seat. During all three approaches, I was able to see the desired touchdown point through the canopy, just slightly above the top of the front ejection seat. Peripheral visual cues for the flare manoeuvre, while not as easily recognised as from the front seat, were more than adequate to demonstrate landings to novice pilots.
Responsive roll capabilities and predictable behaviour at high angles of attack should allow pilots to employ the L-159 effectively. The F124 engine showed itself to be highly responsive and allowed for carefree throttle movement throughout the flight envelope. While I was unable to sample the avionics and weapons systems installed in the production L-159, they are all proven off-the-shelf Western components. Five production aircraft will be delivered to the air force by year end and slots are available for export customers.
Source: Flight International