Pilot report - Socata TB-21 GT

[Reprinted from Today's Pilot Magazine]


Fitting a turbo-charged engine can only make a good aircraft even better. Dave Unwin flies the turbocharged Trinidad GT.

The TB21 Trinidad GT Turbo is, as the name implies, a turbo-charged version of the popular TB20 Trinidad. Turbo-charging an aero-engine confers two significant advantages. Firstly, the reduction in performance caused by operations at high-density altitudes is greatly reduced and secondly, the much better performance at altitude makes it possible to flight plan more efficiently by utilising weather pressure patterns and high-altitude winds more effectively.

The cockpit is very well laid out. The instrument panel consists of three easily removable, padded boxes, the one in front of the pilot carrying all the primary flight instruments and a very comp rehensive annunciator panel. The one on the right contains the power gauges and some additional flight instruments, and the centre one houses a large vertical stack crammed with avionics and topped with vertical-reading gauges showing engine oil pressure and temperature, volts and fuel quantity. The avionics suite includes a Honeywell KMD 550 MFDS with colour GPS and a KFC 225 autopilot and flight director. Note the large stepped panel for the circuit breakers to the left of the pilot, the comprehensive annunciator panel above the flight instruments and the useful grab handle located under the panel. The large red knob under the instrument panel is for emergency undercarriage extension.

I was interested to see how many differences I could spot and whether or not Socata had implemented any further improvements (apart, of course, from fitting a turbocharged engine) to what was already a very well thought-out aeroplane. I particularly remember that the TB20's baggage bay door was perhaps not as large as it could have been. Therefore I was pleased to note that on the TB21 (and later model TB20s) the height of the door has been increased by eight inches.

The actual baggage capacity remains usefully high at 143lb. Other improvements were that the cabin is now five inches taller and boasts a new interior, and that the wingtips have been redesigned.

As I'd noted the last time that I'd flown a Socata-built aircraft, build quality is very good. Wherever possible the aeroplane's skin has been flush-riveted (which helps to reduce drag), while the windows are also flush-mounted. A carbon-fibre honeycomb sandwich is used to make the fuselage's upper panels and, interestingly, a CNC (computer numerically controlled) milling machine produces the main spar from a single piece of metal. This means that the wing's single spar has no joins at all, which makes it very strong wing, as any homogenous structure is inherently stronger than a modular structure. As on the TB20, the fin and rudder is set forward of the stabilator and a similar pair of ventral strakes are mounted under the fuselage. The main undercarriage legs are of the superior trailing link design. An electro-hydraulic pack is used to retract the undercarriage and, unusually, there are no mechanical up-locks. The wheels are retained in their wells solely by hydraulic pressure. Should the electro-hydraulic pack fail, the pilot merely releases the hydraulic pressure by actuating the emergency undercarriage lowering system. Springs then assist the undercarriage to free-fall into the extended position.

Entry to the spacious cockpit is first-class. Small steps (which retract when the undercarriage retracts) are mounted either side of the fuselage just aft of the trailing edge, making it easy to step up onto the wing and then down into the cockpit. As on the TB20, I was pleased to note that the non-slip wing root walkway is of a decent size. Too many aircraft have unreasonably narrow wing root walkways, making it only too easy for someone to walk where they shouldn't. Once in the cockpit my initial impression was that the cabin really does seem large for an aircraft in this class. The cabin is just over 50 inches wide and almost as tall, while the large windows give the cockpit a bright, airy feel. As with the TB20, the rear seat is of the bench type and is certified for three occupants. This bench seat is easily removed for carrying over-sized cargo up to 550lb. I thought the pilot's seat was extremely comfortable. A useful grab handle located under the panel makes it easy to pull yourself forward in the seat and the range over which it can be adjusted is generous. Inertia-reel seat belts are fitted as standard and the rudder pedals also adjust. If you cannot get comfortable in this cockpit, it's your own fault!

The panel is of an unusual but very practical design. It consists of three easily removable, padded boxes, with the one in front of the pilot carrying all the primary flight instruments and also a very comprehensive annunciator panel. The one on the right contains the power gauges and some additional flight instruments, and the centre one houses a large vertical stack crammed with avionics. As on the other aircraft in the TB range, the centre stack is topped with vertical-reading gauges that show engine oil pressure and temperature, volts and fuel quantity. As you'd expect on an aircraft in this class, the avionics suite is very comprehensive and for 2002 includes a Honeywell KMD 550 MFDS with colour GPS and a KFC 225 autopilot and flight director. And what makes the panel design practical? Well, both the left and right hand boxes 'tilt out', making for easy maintenance.

At the base of the avionics stack is a sizeable centre console that carries the throttle, prop and mixture levers, elevator trim wheel and adjacent pitch trim indicator, cigar lighter (which also functions as a 24-volt power point) and ashtray, as well as a knob for the rudder trim and markings to indicate rudder trim position. There is a rocker switch for the electric pitch trim on the pilot's yoke, which also incorporates buttons for the PTT and autopilot disconnect. Further back on the console is the rotary fuel tank selector, while immediately in front of the power control levers is a flat panel that carries a guarded pre-select switch for the flaps, a flap position indicator and the buttons for the electrics.

The system used for selecting the electrical services is worthy of comment, as Socata favours the use of thermal overload buttons. Consequently, each service has two push buttons a green one for 'on' and a red one for 'off', while the circuit breakers are located in a neat stepped panel next to the pilot's left knee.

One feature that I really approved of was that an electric fan supplements ram air for supplying fresh air to the cabin and de-mister, as on any car. Why don't more aeroplanes have this very useful device? Your guess is as good as mine! I liked the cockpit a lot. It is comfortable with good ergonomics and also has some very useful stowage areas for the Pilot's Operating Handbook, Airways manuals, flight guides, etc. In fact, my only real quibbles would be the lack of a DV panel and that the altimeter is of the old three-pointer design. As this aircraft is designed to cruise at 25,000 feet it really should have a drum-type altimeter with a single pointer.

The engine started readily and I was soon wending my way towards the active runway. The nosewheel steers through the rudder pedals, although differential braking can be used to reduce the turning circle.

With four people on board and full fuel tanks we were just about on the maximum take-off weight, and I was curious to see how the TB21 would perform on a hot day with nil wind. Having checked the mags, cycled the constant speed prop, adjusted the elevator and rudder trims to the 'take off' settings, turned the fuel pump 'on' and set the flaps to 10°, I rolled out onto the runway and smoothly opened the big Lycoming up to full power. As the TB21 is essentially the same airframe as that of the TB20 that I flew for an air test some time ago.

As the TB21 has a powerful rudder trimmer I needed less right rudder than when flying many other aircraft with substantially less horsepower and had absolutely no trouble tracking the centreline. Despite being close to gross weight and the day being hot with no appreciable wind acceleration was good and as 70kts came up I eased the yoke back and the TB21 rotated cleanly and lifted off. As the last of the runway disappeared beneath the nose, I dabbed the brakes and retracted the undercarriage before clicking the flap switch to 'up', trimming for 100kts and turning the fuel pump 'off'. I particularly liked the position of the flap switch, as it is possible to flick the switch forward without having to remove your hand from the throttle... which is nice!

Jacques had recommended leaving all three power controls fully forward, as he was keen for me to experience the difference that fitting a turbocharged engine has made. If he was looking to impress me he succeeded, for less than ten minutes later, we went surging past 10,000 feet with the needle of the VSI still showing in excess of 1,000ft/min. As we'd decided not to use the supplemental oxygen system, I levelled out at 12,500ft, trimmed forward and let the aircraft accelerate before easing the power levers back to '25 squared'. In no time at all, the ASI was indicating a true airspeed of almost 170kts, although fully exploring the TB21's capabilities would have required donning the oxygen masks and climbing to 25,000ft. At this altitude, the TB21 produces true airspeeds of around 190kts at only 75% power!

I was also pleasantly surprised at how quiet the cabin was. I'd felt that the TB20 I'd flown was a little bit noisy, but all aircraft in the TB range built since 2000t have a re-designed cabin and the ambient noise level certainly seemed lower, at least to me. Controlling the cabin environment was easy, with a good-sized air vent at both ends of the instrument panel and an automotive-type environmental control panel below the right hand instrument panel.

Visibility, as on the TB20, is excellent. The windscreen is wide, the windows large and the fact that the pilot sits just in front of the leading edge all combine to confer an outstanding outlook. Ride quality is also first class. With a wing loading of up to 24lb/sq ft, the TB21 has a substantially higher wing loading than most aircraft in this class. In fact, it flies like a much larger aircraft, and the ride is consequently agreeably firm. I think that this aircraft will prove to be very popular with passengers as well as pilots.

Investigation of its general handling qualities produced no real surprises. The control forces are reasonable, even at higher speeds, while I felt that the electric pitch trim was nicely geared. All three primary controls felt very crisp and positive, probably testimony to the fact that they are all driven by push-pull rods, including the rudder. This is most unusual, as the rudder on most GA types is cable operated. Harmony of control was also good, with light authoritative ailerons, a slightly heavier, effective elevator and a powerful rudder. Stick-free, the longitudinal stability was positive and the lateral stability almost neutral, while directional stability was fair. I briefly engaged the autopilot and found that it functioned very precisely, before disengaging it and trimming forward for a dive to the aircraft's 187kt Vne. As is to be expected, the primary controls firmed up as the airspeed increased and the cabin noise rose, although not to an unpleasant level.

Slow flight was equally easy. There is ample pre-stall buffet and also an audio stall-warning device. With the passengers on the back seat giving us an aft CG, I didn't let the stall fully develop but just let the aircraft nibble at the edge of the stall envelope. Nevertheless, the TB21 appeared to be a well-mannered beast, and I got the airspeed down to below 60kts with full flap and a smidgen of power.

As our speed was still slow, Jacques recommended that I try the undercarriage emergency extension system. This simply required pulling the circuit breaker for the electro-hydraulic power pack (to simulate its failure) before selecting the undercarriage lever to 'down' and then pulling the red knob situated below the left side air vent. This opens a valve in the hydraulics line, and with the hydraulic pressure removed gravity (assisted by springs) soon had the three wheels locked into place, an event confirmed by the three green, down-pointing arrows illuminating. Satisfied that the emergency extension system was functioning as advertised I pushed the red knob back in (to close the valve) re-set the circuit breaker and then selected the undercarriage up, before opening the throttle and setting course back to the airfield so that I could try a few circuits. Approaching the airfield at speed, I discovered that although the maximum speed for flight with the undercarriage extended is 139kts, the speed for operating the undercarriage is 10 knots slower at 129kts. I felt that this was less than satisfactory, as dropping the undercarriage early can be a great help in slowing down, and the TB21 is quite a slippery aircraft. The limiting speed for the first stage of flap is the same as the max undercarriage operating speed, so a little bit of forward planning is necessary when approaching an airfield if one is to arrive without an embarrassment of both airspeed and altitude.

With the undercarriage down I waited until I was nicely established on a long-ish final at 90kts before clicking the flap pre-selector to 'land', pushing the prop lever up to 'max rpm' and re-trimming for 75. Speed control all the way in was fine but alas, the touchdown was on the firm side. As we crossed the perimeter fence I began drawing the power off while initiating a nice smooth flare. However, this particular flight was at the end of a particularly long day, during which I'd already flown two other types of aircraft, and for whatever reason I ended up rounding out that little bit too high. Net result; the TB21 quit flying while still three feet up, and even the excellent trailing link undercarriage could not disguise the fact that this was not one of my better landings! Curiously, another circuit produced exactly the same result -- speeds and heights absolutely perfect all the way round, only for the whole affair to be spoilt by me rounding out just that little bit too high. To be fair, neither landing was especially bad, just not quite as neat an arrival as I usually aim to achieve.

So, in conclusion what did I make of this fine French tourer? Well, the cabin is easily one of the best of the light tourers and, now I think about it, the three-person rear bench seat may even be unique in this class. The handling is excellent, while the addition of a turbocharged engine and an oxygen system makes it a truly serious tourer, able to not only operate out of 'hot 'n high' airfields but also to make the most of the advantages of flying at high altitudes.

An admirable aircraft for any pilot who not only likes to tour, but also likes to arrive in style!

[END OF REPORT]