KAI’s T-50 is designed to bridge the gap between current trainers and fourth-generation fighters. Our test pilot was the first non-South Korean to fly it
Many air forces around the world are just ordering or receiving the latest “fourth-generation” fighters, including the Boeing F/A-18E/F, Dassault Rafale, Eurofighter Typhoon, Lockheed Martin F-16E/F and Saab Gripen, These aircraft carry such an array of advanced systems and weapons that the pilot has become a mission commander, requiring skills that far exceed conventional piloting.
These aircraft also all feature digital fly-by-wire (FBW) flight control systems providing carefree handling, high thrust-to-weight ratio, high sustained g and high angle-of-attack capabilities, supersonic cruise, high altitude ceiling, high attack speed, and have advanced digital avionics, radar, sensors and weapons.
Given this level of capability, air forces need highly trained pilots to get the most from the potential a fourth-generation fighter offers. But can available advanced jet trainers produce a pilot that can cope with this new technology and performance mix? Trainers in use widespread today such as the Aermacchi MB339, BAE Systems Hawk and Dassault/Dornier Alpha Jet have analogue instruments and conventional controls and are showing their age.
Bridging the widening gap between the ‘old’ advanced trainer and the ‘new’ fighters means the continuing need for an extensive and expensive operational conversion unit (OCU) phase using a large number of new fourth-generation fighters themselves. Given the lack of early exposure of the student pilot to advanced systems and aircraft characteristics, pilot wastage at the OCU remains high and further adds to training cost and inefficiency.
Matching capability
The answer would appear to be a matching “fourth generation” advanced trainer and that is the concept behind the Korea Aerospace Industries (KAI) T-50 Golden Eagle, which enters service this month with the South Korean air force. The T-50 is the first trainer designed from the outset to meet the needs of air forces re-quipping with fourth-generation fighters and with performance, avionics, systems and weapons capabilities to match.
But an advanced trainer must still retain handling qualities that are docile enough to allow the progressive teaching of students, many of whom may just have graduated from basic training, and provide a rear-seat instructor environment that makes the training truly effective. KAI offers both the T-50 advanced trainer and the enhanced T-50 lead-in fighter trainer (LIFT), which includes an APG-67 multi-mode radar, internal 20mm gun, five pylons and weapons capability.
How well the T-50 could meet modern training demands was the subject of Flight International’s evaluation at KAI’s facilities at Gwanju airbase (simulator and ground training) and Sachon airbase (production and flight test). I was the first foreign test pilot to be invited to fly the T-50 after it received its Korean military airworthiness certification in January. The aircraft first flew in 2002 and development involved a total of 1,411 test flights to certification.
KAI is the consolidation of all Korean aerospace companies and is privately held by industrial giants Doosan (formerly Daewoo), Hyundai and Samsung. As well as the T-50, KAI also produces the KT-1 turboprop trainer at Sachon. South Korea co-produced over 140 KF-16s, and Lockheed Martin, a major partner on the T-50 programme, contributing its experience in fighter design and manufacture, is helping to market the T-50.
KAI hopes to sell up to 140 T-50s to the South Korean air force, but sees a potential export market for over 600 aircraft with Greece, Israel, Singapore and the United Arab Emirates being the best near-term candidates. The Eurotraining requirement is also starting to look promising as the programme is further delayed. The T-50 was first displayed internationally at Dubai last November.
Clean sheet
Concept studies started in the early 1990s to identify the requirements for a “clean sheet” advanced trainer and KAI appears to have timed completion of T-50 development to perfection. The major competitor is the BAE Systems Hawk 128, which has similar advanced avionics but an airframe design that dates from the 1970s. Even with the latest aerodynamic and engine upgrades, the Hawk 128 has much lower quoted performance levels than the T-50 and lacks radar and FBW. The Aermacchi M346 is still under development and may not come to market for another two to three years.
The KAI training centre and simulator building to support the T-50’s introduction into South Korean service is sited at Gwanju airbase 200km north of Sachon. The new building has computer-based training rooms for up to 80 students and a large auditorium for computer-assisted instruction. For ground crew there are several large workshop/training rooms featuring actual T-50 hardware such as the wing, engine and landing gear.
For the pilots there are two operational flight trainers (OFT) and two more advanced full mission trainers (FMT) with 300° by 135° dome visuals. The FMTs can be linked together for battle pair or 1 vs 1 training. With about 45min in each simulator type I was able quickly to get familiar with the cockpit and flight characteristics. The OFT and FMT were both excellent in terms of visual fidelity, control feel and control response. The overall impression was of a company committed to supporting its product at a level equal to anything on offer by a Western aircraft manufacturer.
The T-50 tested was aircraft 001, the first prototype to fly. This aircraft has additional flight-test flight instrumentation, including a large panel to change flight-control system parameters in place of the left-hand multi-function display (MFD), so is not truly representative of the production cockpit. The aircraft had no radar fitted, the wing was clean and no external fuel tanks were carried. Internal fuel was 2,180kg (4,800lb).
Basic operating empty weight for the production T-50 is 6,620kg, but the additional test instrumentation took our maximum all-up weight to 8,870kg, with a centre of gravity position of 35.5% mean aerodynamic chord. Limitations on the prototype include a maximum speed of M1.3 (M1.5 for the production T-50) and ceiling of 42,000ft (12,800m – 55,000ft for the production version). In all other aerodynamic and control respects the prototype was identical to a production aircraft.
The T-50 is powered by a General Electric F404-102 afterburning turbofan producing 11,900lb (53kN) of military power and 17,700lb thrust with reheat. Quoted thrust/weight ratio with full internal fuel is 0.92. Dual-channel full-authority digital engine control is fitted.
Safety pilot for my flight was Lt Col Choong Hwan Lee, a test pilot with the 52nd Test and Evaluation Group and a member of the joint KAI/South Korean air force T-50 test team based at Sachon. I occupied the front seat and would fly the entire mission. After take-off from Sachon’s runway 06L, we were to be chased by an air force KF-16 and our training area over the sea and coast, about 50km south west of the base, featured strong turbulence at lower levels.
With the outside air temperature at 0°C we boarded from a platform inside the test shed, but normal “on the line” access is via an aircraft-specific boarding ladder placed and removed by the ground crew. Stepping into the cockpit was simple and strapping into the Martin Baker Mk16 zero-zero ejection seat straightforward. Although it lacked the negative-g strap common on many other Martin Baker seats it seemed to make no difference during the flight.
I found the air force flying ensemble particularly bulky at the sides of my chest and visually intrusive when locating cockpit switches on the lower side console shelves at or behind my body’s “3-9” line. The canopy is closed using a large, electrically operated central jack. In an emergency the canopy is blown off by rockets as part of the automatic ejection-seat sequence, or can it be jettisoned manually by the crew using a separate handle or by ground personnel externally using a 3.7m pull-out cord
Cockpit comfort
The seat is raked back 17° and setting the design eyepoint for the HUD and forward field-of-view (FoV) using an electric height adjustment, and then adjusting the rudder pedals, was delightfully easy. I found the T-50 to have one of the most comfortable cockpits I have flown in. After positioning the arm support for the flight-control sidestick on the right-hand side of the cockpit, I never readjusted a thing. Total FOV was superb and I could see at least 320° in azimuth. The canopy arch is there to give increased birdstrike protection to the curved windscreen at low altitude, but I did not find the structure in any way obtrusive.
As in the sidestick-equipped F-16, the unobstructed space in the centre of the cockpit struck me as incredible. I believe KAI could productively use this middle space for a third MFD. The MFDs could then be programmed to replicate the type of display pages, including radar, weapons and moving map seen in the latest fighters. KAI is considering this third MFD option, and it is planned for the F-50 variant.
The T-50’s hands-on-throttle-and-stick (HOTAS) controllers incorporate 15 switches. The throttle principally controls the radar and the stick the weapons. Although I had not flown a fighter with sidestick control for some time, I found the stick natural to use. For F-16-operator Korea a sidestick makes sense, but what about for customers flying fighters with centre sticks, like the Eurofighter Typhoon or Saab Gripen?
Irrespective of the centre versus sidestick argument, I think the most important aspect is not the position, but to have a HOTAS design that is as similar as possible between trainer and fighter. This will aid standardisation as the pilot becomes practised in using these two inceptors as the principal interface with the aircraft. KAI believes this commonality can be offered as a hardware option on export versions.
Complementing the head-down MFDs is the up front controller (UFC) placed just below the HUD, controlling multiple functions for communication, navigation and identification, including inserting waypoints and setting bingo fuel. The UFC buttons were large, easy to use with a gloved hand, logical and the screen display easy to read.
The wide-angle HUD has comprehensive F-16-type flight symbology with a pitch ladder and sharply defined digits and lines. I am no great fan of full velocity-vectored HUDs or typical US fighter HUD symbology because I feel there are too many rolling digits. When manoeuvring hard you never know where to look for them. HUD formats are always a personal pilot preference, but KAI again believes it would be a simple option to alter the symbology to match the customer’s fighter HUD format.
Initial cockpit checks took no more than a couple of minutes. Canopy closure took about 7s. The engine was fired up by first starting the auxiliary power unit (APU) and about 20s later, with a green engine start light illuminated and core speed (N2) showing at least 15%, moving the throttle to idle. The FADEC did the rest. The engine stabilised after about 15s at 63% N2, an exhaust gas temperature around 450°C and fuel flow of 700lb/h.
Up and running
The after-start checks consisted principally of bringing the inertial-navigation/global-positioning system (INS/GPS) to alignment, which took about 4min and in the meantime checking the flight control system and central alerting and warning system (CAWS). My preference would be for the capability of having the aircraft APU running to allow the INS/GPS to be aligned before pilot arrival so that waypoint or mission data could be updated through the UFC before engine start.
Five minutes after starting we were ready to taxi. A small increase of about 7% N2 started us rolling and taxy speed could then be controlled with the throttle almost at idle. The engine has a ground idle RPM setting below 80kt (145km/h) with weight on wheels and an open engine nozzle, which makes it simple to maintain taxi speed with just a quick dab of the main brakes about every 45s. The parking brake automatically disengages as power increases, a nice design feature. Nosewheel steering is powerful, with a small rudder pedal input generating an almost instantaneous turn. Initially my response was quite jerky, but once I corrected my technique it allowed for accurate centreline tracking.
With our KF-16 chase in position next to us on the runway we were ready for take-off. An earlier simulator assessment showed the aircraft could safely take off with fully mis-set trims in all axes (as this does not generate a configuration warning), but the take-off checklist should ensure all trims are zero. Because of an extended hold at the threshold for another formation, our fuel at take-off was 1,995kg. After a quick check of the engine instruments at 80% N2, brakes were released and military power selected.
Acceleration was brisk with the rotatation speed of 124kt indicated reached after about 600m of ground roll and 10s. Rotation to 12° nose-up pitch was achieved accurately, with a small pitch overshoot, using about 10mm back stick deflection equating to about 4kg stick force. Gear was raised immediately (although the T-50 has a generous 300kt operating limit) and no pitch change was felt as the gear travelled and the automatic trailing-edge flaperons were rescheduled to come up.
Runway heading was maintained until 2,000ft and then a 180° climbing turn commenced onto a HUD heading of 240°, with 350kt captured and maintained easily and accurately at a pitch attitude of over 20° during the turn. This was converted to M0.8 at just over 20,000ft, which was passed at 2min 30sec, and the 40,000ft level off was achieved just 5min 10sec after brake release. We had used about 270kg of fuel. It was truly eye-watering performance from a trainer, and all using just military power.
Zero freeplay
In the climb I was able to evaluate the sidestick control mechanical characteristics in the longitudinal and lateral axes. Breakout was about 1kg in both axes, a little higher than my personal preference of about 0.5kg. Control freeplay was zero, centering when controls were released from deflection was absolute and there were no noticeable control system oscillations. Unlike the F-16’s sidestick controller, the T-50’s stick moves about 20mm in all directions from the neutral point. Stick force at full deflection was about 7kg fully forward, 11kg fully aft and 6kg in either lateral direction. All control force/displacement gradients were linear. I would prefer a little less force laterally at full deflection, but control harmony seemed fine. Unlike the F-16, there was no further control possible beyond the stick deflection limits even if more force was applied.
Rapid aileron rolls through 360° in the climb at 350kt showed a roll rate in excess of 200°/s. Four-point rolls showed the bank could be stopped accurately at the 90° point. Roll-rate onset was rapid. Using clouds as simulated targets, vigorous manoeuvring to point the nose for guns or infrared missiles looked responsive and accurate.
Having stabilised briefly at 40,000ft and M0.8 for the chase, I lowered the nose to about 10° nose-down pitch, selected afterburner for 3-4sec and then back to military power, and watched the aircraft as we went rapidly transonic then supersonic. Apart from the Mach digits moving on the HUD, there were no other indications in the cockpit of passing M1 such as transonic pitch changes, pitot-static effects or increased noise. I stabilised at M1.1 and then slammed the throttle to idle and used a 3g turn to slow down so that we were properly subsonic before 30,000ft (Korean rules).
The aircraft had showed itself to be truly supersonic in performance, almost without effort and to have such good handling characteristics that a student pilot would find accelerating and decelerating through the transonic/supersonic region to be almost a non-event.
I levelled at 25,000ft and slowed to 150kt. There were no noticeable pitch changes on airbrake selection in or out at any speed. Maintaining speed with power, I pitched 60° nose-up, closed the throttle and released the controls. The aircraft basically flew itself out by pitching gently nose down to reduce the angle of attack (AoA). HUD indicated airspeed fell to zero during the manoeuvre, but this had no effect. Slow-speed flight at 25° AoA (as set by the FBW control laws) was easily achievable and resulted in a stable airspeed of around 120kt.
In the simulator I had previously evaluated a 250kt, 90° nose-up pitch release, airbrake out and with full pro-spin controls applied and held at the zenith of the zero-speed zoom. Even at this extreme, the aircraft remained completely docile and essentially just gently flew itself out by ‘flopping’ to about 60° nose-down. It was another validation of the FBW control-law design, which is student friendly while replicating how a fourth-generation fighter will behave.
I descended to 15,000ft for a quick look at some stability and manoeuvre boundary test points. My designated test speed was 450kt. The flight control system is fully digital, triple channel, with three independent and redundant electrical power sources.
AoA limiting
The basis for the control laws at low AoA (less than 15°) is pitch attitude hold. At low airspeeds, above 15° AoA, the FCS blends pitch demand and AoA limiting. Maximum positive load factor is +8.5G and the FCS will always prevent this being exceeded so the aircraft cannot be over-stressed.
With its full-authority fly-by-wire control system, the aircraft exhibited no change in stick displacement or out-of-trim forces away from neutral with increasing or decreasing speed, and this is exactly right for a fighter trainer, I believe. There was never any need for me to trim longitudinally throughout the entire sortie. There were no pitch up/pitch down changes due power effects with throttle “slams” from idle to full afterburner or back again. There was no phugoid. The aircraft responded instantly once the stick was moved out of “breakout”.
A wind-up turn in military power maintaining 450kt at approximately 15° nose-down showed what I had already seen in the simulator, that the stick force and displacement per g are linear with no discontinuities. At +4g the stick force to hold the g was about 5kg. The small nose-down angle needed to hold airspeed as g increased showed the induced drag created by the wing was low and the aircraft possessed good specific excess power. Additionally there was no noticeable effect on stick force or displacement per g gradients with variations in speed, altitude or power throughout the sortie.
Increasing g in the turn to check the manoeuvre boundary at 450kt and military power showed buffet onset at around +4.5g, but at about +5.7g I encountered heavy limiting wing rock that prevented me from pulling any further positive g. I repeated the wind-up in afterburner. I again encountered a lift boundary limit due to a heavy wing rock, but this time at +6.7g.
Rock puzzle
It was a puzzling phenomenon given that KAI’s performance tables show a “corner speed placement” at about 320kt for that altitude, so +8.5g at 450kt in a wind-up turn should be perfectly attainable. I asked Lt Col Lee to try. He pulled much more rapidly and achieved and sustained about +8.3g as we seemed to “pass through” the worst of wing-rock region. But manoeuvre boundaries are the only item during my assessment that I would re-visit on a second evaluation flight and back-up with detailed analysis of the telemetry data.
Loaded, full-deflection aileron rolls at +4g showed a slower roll rate, but otherwise were completely carefree. Inverted, level -1g flight could be held with a push force of 3-4 kg. Maximum negative load factor is -3g, but I was not brave enough to push much past -1.5g.
Laterally and directionally, the FCS took care of everything and automatically minimised sideslip at all times with the rudder centred. Spiral stability was neutral. No Dutch roll modes were present. The aircraft rolled with rudder in the direction of the applied pedal. I never needed to trim either laterally or directionally throughout the sortie.
Slowing to 180kt, gear up, showed I could generate about 8° of sideslip at full rudder deflection set against 15° of opposite bank in a steady-heading sideslip. This indicated that the T-50 could easily cope with a “kick off drift” manoeuvre in the landing flare from a crabbed approach at the aircraft’s 25kt crosswind limit. But KAI presently teaches that the aircraft be landed from a crosswind approach without de-crabbing.
As the chase positioned into my front port quarter about 300m ahead at 300kt, I accelerated to overtake then slowed to come into close formation. The F-16-style split speedbrakes were effective and caused little airframe vibration. They can be set at any angle up to a maximum of 57° after a 2s “blip” on the throttle-mounted speedbrake button, but the head-down indicator only showed open/transit/closed so an exact intermediate angle could not be set. Position keeping in close was easy and stable, but I found myself over-controlling in pitch by a fraction. I think the cause may be that the breakout force was just a little higher than I was used to.
After a couple of bank angle changes in close formation without problem, I cleared the chase and descended to low level. Korean rules prevented us flying below 1,500ft, but we easily accelerated to 550kt level in military power and the ride was comfortable despite a lot of low-level turbulence. The simulator showed that, in afterburner, the clean aircraft is capable of speeds close to 650kt (M0.98) at low level. Given the absence of a radar and constraints due having only internal fuel, there was not much weaponry simulation I could evaluate, so we climbed to 6,000ft and headed back to base for some visual circuits and roller landings.
I had practised simulated flame-out procedures in the simulator. High key (approach altitude over the runway) is entered at around 6,500ft and 200kt; low key at around 3,500ft. Final turn is made at 170kt and, when landing is assured, alternate gear and alternate flap lowered for a nominal flare and touchdown at around 130kt and about 2,000ft in from threshold. The simulated flame-out was easy to fly and differed little from the Hawk in terms of aspect or pattern size.
Few checks
Approach checks required were just “landing lights on” and the break (closed pattern) was entered level at 1,500ft and 300kt. Downwind checks were just “gear down”. This was my sort of aircraft. There was a slight pitch-up change of trim as the gear lowered because this also triggered the first lowering of the flaperons as flaps. Full flaperon extension was then a function of speed.
The gear lowering also automatically brought up the landing symbology on the HUD. This included an AoA “bracket” that rose in the display as speed decreased to come alongside the HUD flight-path vector with 9° AOA at the top of the bracket and 13° at the bottom. The turn on to final was flown initially at 170kt, slowing to 9° AoA, then 11° AoA (mid bracket) on short finals with the flight-path vector also placed over the intended touchdown point. AoA was controlled with short ‘bursts’ of power and, although the bracket was quite lively in the crosswind and gusts, it was easy to hold the desired AoA accurately.
The aircraft was flown virtually to the ground, with a small check flare to achieve 13° AoA as the aircraft touched down with a little power still on. Two roller landings and one full stop landing were made. Although the aircraft has no brake parachute or thrust reverser, aerodynamic and wheel braking below 100kt seemed effective. I shut down with 455kg of fuel remaining after a flight time of 1h 10min.
My overriding impression of the T-50 was trainer that was easy to fly, with carefree aircraft and engine handling, and combined stunning performance with modern avionics. It also has the digital capacity to enable future upgrades that will allow the tariner to mimic closely the fourth-generation fighters it will support throughout its life. KAI has produced the T-50 at the right time and with the right specification to capitalise on a growing need for more advanced trainer.
All the elements of a complete T-50 export “package” appear to be in place. The price must be competitive, but once KAI has achieved its first T-50 export order, others look likely to follow worldwide.
PETER COLLINS / SACHON
Source: Flight International
