From The Cockpit

By mid 1988, worldwide Tobago sales exceeded 450, whilst 240 orders for TB20s had been taken. Socata have been particularly aggressive in penetrating the US market and not scared off by the product liability issues contained therein. To this end, the Trinidad has been displayed at the annual NBAA Airshows. Most recently Socata is investigating a licence production agreement for Trinidad production in China.

Technically Speaking

In designing the TB series, Socata dared to be different in appearance, both outside and in. The Tampico, Tobago and Trinidad are all of a similar airframe configuration – low cantilevered constant chord wing, all moving fuselage mounted tailplane (stabilator), conventional fin and rudder, and tractor engine. Attention to aerodynamic detail is prevalent: Aerospatiale introduced a customised wing section for a wing of the comparatively low area – 128sq ft, compared to the American ‘standard single engined aircraft wing area’ of 175sq ft. A lower wing area increases the wing loading, which is normally far less than optimum for light singles, in order to keep the stalling speed in the landing configuration below the 61 knot maximum speed airworthiness requirement. Higher wing loading results in greater aerodynamic efficiency in the cruise by making the maximum LID (lift to drag ratio) speed closer to the normal cruise speed appropriate to 65-75% power.

Other attention to drag reduction detail was paid in the form of flush riveting, raked forward fuselage lines and ventral strakes on the rear fuselage underside. The frontal area of the fuselage has been kept down whilst accommodating an excellent cockpit width – by ‘minimising’ cockpit height. Although satisfactory for an average height pilot such as myself, the upper height bracket do find some difficulty in sitting comfortably without being restricted by the folded back sun visor in the roof. It is difficult to gain additional height in the cockpit, for the seats are located over and on the main spar carry through structure.

The squat rear fuselage tapers to an empennage consisting of fin/rudder located ahead of the tailplane mounted off the rear fuselage frame. The all moving tailplane has a full span trim tab geared to provide an antibalance action upon stabiliser deflection – very effective as it turns out in increasing longitudinal control forces with speed displacements. The Tobago tailplane attachment bearings have been subject to premature wear from slipstream-induced vibration and have been strengthened by redesign.

The compact wing carries fuel tanks (integral type), plain ailerons and slotted flaps. Presently the mainspar is safe-lifed at 9,000 hours. The wide flaps deflect to 40° on the Tampico and Trinidad and 30° on the Tobago. The reduction was possibly necessary for static lateral stability considerations; the wing dihedral on the Tobago is the lowest I know on a GA low wing. It is about doubled in comparison on the Trinidad – to about 9°, with which the larger flap deflection is reinstated. Eyeballing the plain hinged smallish ailerons the temptation is there not to give them half a chance of doing the job of rolling the aircraft adequately. But this they do.

The TBs have their wet wing fuel tanks in the leading edge of the wings – one either side. On the Tobago, usable capacity is 102 litres a side, increasing to 163 with much larger tanks on the Trinidad. For these tanks, unfortunately the greater dihedral prevents visual inspection of the tank contents for quantities below about 120 litres.

The lower rear fuselage has a pair of ventral strakes just aft of the wing trailing edge; the reason for their inclusion has not been clearly disclosed by the manufacturer but it is reasonably surmised that they are separation control devices for the purpose of drag or large eddy turbulence reduction.

The TBs’ systems are simple, both on the Tobago and Trinidad. The pitot-static system has an underwing pitot tube on the left wing and static vents located mid height either side of the fuselage aft of the baggage compartment bulkhead. Curiously the Approved Flight Manuals for the types schedule negligible position error correction for the pitot-static system over the whole of the normal flight speed range – Socata certainly got the positioning of the static vents correct.

Similarly the aircrafts’ 12v electrical systems are simple.

Powered by a 40amp hr battery located in the engine compartment and standard engine driven alternator, the system energises, apart from instruments and avionics, the flaps and on the TB20, the hydraulic pump used for LG operation. The Trinidad’s LG is itself simple and rugged in design. The main landing gear is of trailing link type suspension, probably the best design for cushioning vertical loads and reacting to drag loads on landing; in spite of this the gear is reasonably stiff on landing. With the gear retracted neither the whole of the NLG nor the mainwheels are faired over. With an evident concern for drag reduction in the TB series, such a concession can only be in favour of system simplicity and reliability. In its favour the gear has a super easy freefall emergency extension system.

The Tobago’s fixed gear is of standard oil/gas oleo construction, with loads taken directly by the wing structure and by the fuselage longerons via a welded steel NLG mount. The wing rib structure used for NLG attachment has been subject to fatigue around the mounting holes for the strut fixing bolts and is the subject of an AD for their strengthening. The NLG has a spring centering device which assists in increasing rudder control forces somewhat.

Entry to the TB cockpit is via left and right clamshell opening doors from up on the wingroots. (Watch the flap skin, NO STEP and of an easily dented gauge thickness. ) No need for a Beechcraft bash or a Cessna crash with these doors. They have a superb pull-in latching system for the two sill hooks per door, so that you simply lower the door gently and rotate the over-centre locking lever. Not so good are the doors’ piano-type hinges which are subject to fatigue cracking at the end segments (be wary on the ground of wind buffeting with the doors up).

In the Driving Seat

It is inside that Socata has gone to considerable pains to give the pilot and front seat passenger something different. The layout of the panels and surrounds has been described as ‘sportscarish’ and that’s a reasonable description; it is dominated by the large central pedestal and console which tapers aft but extends full length to the rear seat. Instead of a gearstick, the console has quadranted engine controls, elevator (and on the TB20, rudder) trim wheels and electric switches (which themselves are a departure from the standard US rocker switch breakers; instead consisting of separate ON and OFF pushbuttons for each function). I do not think that cockpit spaciousness does suffer significantly by the presence of the pedestal.

The CB panel is located on the left sidewall of the cockpit. Up front the instrument panels are entirely different from any other on GA aircraft that I know. They are located above a structural crossmember, used as a subpanel for ignition, park brake, fuel selector etc – and are grouped with rectangular padded trim borders into separated subpanels: flight/nav (left), avionics (central) and engine instruments (right). From an instructional point of view the flat top of the coaming provides an excellent reference for attitude and roll angle setting on the horizon.

If there has to be an ergonomic criticism of the TB10 cockpit it has to be arguably a serious one: on the subpanel below the pilot’s control column are located the parking brake control (a rotating knob with OFF being in the 12 o’clock position) and to its immediate right the fuel selector (also a rotating knob between L-OFF-R, OFF being likewise straight up). I have witnessed on more than one occasion student pilots mistakenly turning the fuel selector OFF instead of the park brake. That is picked up if on the ground and the brake is still on, but if the brake hadn’t been set, the situation could result in an engine failure at the worst time. Further still a colleague has reported an occasion wherein the same thing was done during the prelanding brake action and setting checks. Consideration should be given to separating the two controls or guarding the fuel selector. For rear seat passengers the seat pitch is a bit low, detracting from legroom and making conditions somewhat cramped. For the pilot, maps and charts storage is well catered for.

Flying the TBs

Ground visibility in the TB10 is excellent; more so with some loss of compression in the nose oleo, a typical problem associated with oil leakage around the bottom seal. Nosewheel steering (NWS) forces are firm, but the response is positive. The TB20 Trinidad has a lengthened noseleg to cater for the aircraft’s much larger diameter propeller. This detracts from ground vision ahead, which is then merely adequate. Pedal forces for NWS in the Trinidad are firmer and I consider close to being excessively heavy.

The ground attitudes of the TB10 and 20 figure also during the normal takeoff manoeuvre. The Tobago requires a substantial back elevator force and travel in order to become airborne, because of its nose-low ground attitude; thus rotation needs to be commenced 5-8 knots before desired liftoff speed. The TB20 also requires a definite rotation, although not to the same magnitude as the Tobago. The Trinidad’s acceleration on takeoff is brisk; once airborne the aircraft can easily slip through its normal climb speed of 95 knots IAS if the attitude is not checked.

The TBs all have excellent visibility airborne; normal cruise straight and level attitude flying initially needs familiarisation due to the low nose providing little horizon reference details. What the Tobago lacks in its mediocre climb performance (about 5/600fpm at normal climb power in typical ISA+ 10°C temps), the Trinidad more than makes up for. In the cruise the tanks allow for a total endurance of over the usual five hours at 65-70% power and TASs of 115/150 knots for the TB10/20 respectively are typical flight plan figures, for fuel consumptions of 38 and 53 litres/hour.

Control harmony and feel in the Tobago and Trinidad are adequate if not exhilarating. Control and stability in the pitching plane is the best of the three axis components. Lateral stability on the TB9/10 with flaps lowered and 75% power is the lowest I have encountered in a production GA aircraft; in fact I have roughly assessed it as being negative, ie statically unstable and wonder how it made the grade against ‘the (certification) book’. Concerning lateral control – the ailerons have significant breakout forces and consequently a dead response band between these applied forces, an awkward feel. The ailerons heavy up considerably with airspeed and in ‘early’ TB20s are very heavy at cruise speed. Later Trinidads have nicer feel ailerons, less heavy, presumably through an increase in the degree of aerodynamic balancing of the ailerons. Rudder control forces are somewhat light on the Tobago, but nevertheless satisfactory if adjusted correctly with the fixed tab.

Finally, in the cruise on a still day, watch for elevator pecking- random small amplitude and sharp edged movement of the tailplane which is seen and felt by control column response. Present on both the TB10 and TB20, it is generally more noticeable on the Trinidad. Aerodynamic in nature the buffeting is indicative of vortex bursting, source unknown, in the vicinity of the tailplane.

The TBS are most amenable to circuit training. Vision is good, flap (particularly full flap on the Trinidad) is effective, flight path perception cues are impressive and the aircraft must be fully controlled through the flare and landing. The Tobago’s crosswind control capability is particularly pleasing-little into wind aileron is required to hold the sideslip after the nose is straightened and the aircraft is quite responsive to rudder. As for the takeoff, a lot of back pressure is needed to hold off fully before touchdown.

Trim change with flap selection (at approach power) is mild; however, in an overshoot the nose up trim change with power and full flap lowered is large. Later models of TB20 have preselect flap (Up-T/O, 10°- Land, 40°), a simple and far preferable control than variable position flap if, like me, you think that for GA aircraft per se, two flap positions are all that are required. The 40° of (Full flap on the Tobago is 30°) flap gives the Trinidad a very steep glide approach path angle with gear down at 85 knots; Tobago glide speed with flap is 80 knots. The flaps up best glide speeds are somewhat high at 86 and 91 knots IAS for the TB10 and 20 respectively.

With a standard empty weight of 650kg, the Tobago has enough load capacity for full fuel, 4 on board and about 40kg of baggage to make up to the MTOW of 1,150kg. Full fuel and 4 on board, the Trinidad pretty well takes care of all the useful load capacity, with the aircraft at its MTOW of 1,335kg.

The TB Market

Socata has been aggressive with TB 10/20 sales in the early/ mid 1980s; prices have been most competitive against other light aircraft (when produced) and raises the question of Aerospatiale subsidisation. Socata has gone into the US market with resolve, casting aside the product liability spectre (seeing the lack of a product liability tail as being the decisive factor).

TB10s and 20s have sold well in Australia; new sales, like all others, largely dried up in the mid 1980s and essentially remain in the same state. With far less than a decade in service the aircraft has still been finding its feet working the gremlins out of the airframe via a spattering of ADs. Used Trinidads are on the local used market at about A$90-100,000 and up to A$130,000 or more and Tobagos about A$50-60,000.