Airtest – TBM 700

The TBM 700 is an indigenous design; it has adopted the Malibu’s strongpoint – cabin dimensions – and draws upon Mooney 301 flight test experience and Caravan I practice with regard to wing sizing and configuration – such as wing and flap areas and spoiler roll augmentation – whilst maintaining that familiar TB empennage and rear fuselage look.

The transatlantic GA French Connection was initiated when M Alex Couvelaire bought out Mooney of Kerrville, Texas. At the time Mooney had, under the direction of LoPresti, completed a proof of concept flight test program on the Mooney 301, a pressurised piston single six seater at the very cutting edge of advanced configuration technology applicable to GA aircraft. Separately to this program, ‘turboprop single’ preliminary marketing and design analysis studies conducted by Aerospatiale’s Socata and Mooney were concluded in September 1985, to be followed shortly thereafter by a draft TBM International agreement between the two companies. A thorough design process was initiated, resulting in a cabin mock-up at the Paris Airshow of 1987. At Paris two years later the prototype was flying, after having amassed about 200 flight test hours, and TBM was taking orders. TBM had been formed on the 3rd May 1989 with a capital split of 70:30 between Socata and Mooney International to manage the overall program, including marketing. Development, certification and industrialisation are entirely subcontracted to Socata, whilst manufacturing is shared between the two partners in accordance with their participation. The ‘industrialisation’ of the program was launched at Paris ’89 and production initiated.

Since the prototype first flew on July 14, 1988 at the hands of Chief Test Pilot Bernard Dorance, a number of changes have been incorporated. These include an engine upgrade to the twin exhaust stacked PT6A-64 engine, 45cm internal extension of the rear baggage area, incorporation of a ‘clamshell’ airstair door, 5cm high wing fences at midspan, an increase in fin and dorsal areas (achieved on the fin by drawing forward the leading edge to increase the chord length ahead of the spar) and the addition of rear fuselage ventral strakes.

The development and certification pace of the project has been driven by TBM and bears witness to the financial advantages of ownership by a giant, in the form of the Aerospatiale group. It also means early customer acquisition of this exciting aircraft. French and US certification, using a total of three prototypes all told, is expected early 1990 – a meagre 18 months from first flight. Socata’s facility at Tarbes-Lourdes aerodrome, which has been the centre of all TBM 700 development, also undertakes a substantial amount of heavy metal bashing for parent Aerospatiale, including transall fuselage section fabrication and Dassault Falcon airframe component manufacture, in addition to TB9/10 and 20 production. The former probably accounts for the bread and butter of the facility, and begs the question of cross subsidisation of the lightplane line (a commercially advantageous position). One may also question how the indirect costs of the factory inspection and overhead systems required for large aircraft component manufacture, are suppressed for the lightplane segment of the business. On the plus side however, the most advanced CIM (computer integrated manufacturing) facilities are in place to be applied to the lightplane lines, thereby reducing direct production costs. In any case there is no doubt about TBM’s pride in its facilities, nor their modernness.

As mentioned, in the TBM 700 configuration there are evocations of the lineage from the TB series. There is also some shifting towards large aircraft design philosophy. The TB’s primary structures are safe lifed – wingspan throwaway at 9,000 hours. As should be expected on a pressurised cabin airframe, the structure of the ‘700 is fail safe, with low stressed elements, multiple load paths, small panel sizes and tear strips about more critical fatigue elements. Weight conscious stressing is clearly a success, for the aircraft has a standard empty weight to MTOW ratio of 0. 58, which is very good; fatigue in service remains to be seen.

Material selection is conventional aluminium alloy, with the exception of Nomex honeycomb for the empennage surfaces, flaps and all control surfaces. The 8. 2 aspect ratio wing contains a moderate area of 193 square feet, giving, at 32. 3Ib/sq ft, the highest wing loading as possible for a 61 knot stall speed (to improve design point cruise aerodynamic efficiency) as the maximum allowable under the FAR23 certification basis. To keep the stalling speed down, the wing trailing edge has a Fowler flap extending 71% of span; subsequently, with little span available, aileron roll control has been augmented by spo. ilers. Both the wings and horizontal stabiliser have 6. 5° of dihedral. Wing deicing is to be proficed by low profile pneumatic boots on the leading edges, or alternatively a TKS system, depending on the results of icing trials. Wing section is similar to that on the TB series, in part the RA1643 by courtesy of the Aerospatiale computer. The inclusion of wing fences presumably has been to overcome wing drop at the stall, which can be quite smart on the TB10 when stalling with a slight amount of sideslip,

The cabin has a width of 48 inches and a length of 13. 4 feet. Standard entry is rear left side, an airstair of 46 inches height and width 25. 8 inches. An overwing emergency exit is located leftside. With a hefty cabin pressure differential of 6. 2psi the TBM 700 is easy on sinuses and ears – holding sea level pressure up to FL144 and maintaining 8,830ft cabin altitude at the maximum certification altitude of 30,000ft.

The -700 prototype has been re-engined in order to achieve the desired 300 knot cruise speed at altitude, a PT6A-64 for a -40. The Dash 64 at 1,495shp has a greater thermodynamic rating and thus has an expanded flat rated envelope at 700shp, in fact to FL260 in ISA. It is also likely that application of twin exhaust stacks reduce engine installation losses. The engine change incurred a weight penalty of only 30lb. Involved with it were several installation changes apart from the exhaust system, namely repositioning the oil cooler and ECS intakes, and introducing larger servicing doors in the cowling.

The choice of the empennage configuration is interesting. TBM have stuck with the TB series configuration – a low tailplane set aft of the swept fin. It is handsome and is probably in the best position for spin recovery. Although a Normal Category aircraft, the ‘700mustdemonstrate recovery from a one turn spin a further turn using both normal recovery technique (rudder then stick) and reversed technique (stick then rudder) – a low rear tail is insensitive to the difference (Prototype No. 2 has completed the spin certification test program, conducted at the French national flight test centre near Marseilles).

A T tail is also generally insensitive to the difference and for a corporate aircraft minimisation of cabin vibration is an important design objective, one which is aided by keeping the tailplane out of the prop slipstream. As none of the prototypes so far have had a full interior, assessing cabin noise and vibration objectively remains to be done. Another detractor of the low/rear mounted tailplane, almost invariably in my experience, is that it is easy to over-cook the aerodynamic balancing of the rudder, making it ‘soft’ and reducing rudder free static directional (weathercock) stability and rudder free yaw damping (leading to low damped Dutch Roll, with on turboprops, a low roll to yaw amplitude ratio, ie a ‘snaking’ motion). The empennage changes detailed previously suggest that this has also been the case on the TBM 700. The ‘700 also has a yaw damper incorporated, and an aileron rudder interconnect.

For the pilot, the production aircraft instrument panel is well thought out. With the full two pilot panel TBM have opted for the corporate/airliner standard of multiple (3) ADls and no rate of turn indicator. The panel has a surprising lack of switches, which are grouped overhead on an eyebrow panel. The centre pedestal has a thoughtful arrangement of engine controls, mechanical elevator trim wheel, trim indicators, and flap selector/indicator. The fuel selector is unfortunately placed at the back of the pedestal: being out of direct scan it should be highly coloured. It is surprising to see separate L and R tank selections as well as continuous transfer (R to L engine); being a turbine, requiring continuous fuel feed, I would expect to see an on/off cock only, with joint feed from the wing tanks to a collector tank. Fuel is tanked in two integral wing tanks; total capacity is about 298USG and the tanks have been coated to protect against resinae, a welcome inclusion in the warm Australian environment, where within prospers this micro organism which breeds in aviation turbine fuel and ends up clogging an aircraft’s fuel system.

Concerning turboprops generally, it is very fashionable to ascribe excellent reliabilities to contemporary engines. Simple power losses do occur, not through blade separation or disc disintegration but through leakage, separation, cracking or blockage (most likely) of the air pressure sensing lines from compressor pick off points to the fuel control unit (FCU), the pressures being used by the FCU to regulate fuel pressure to the combustor fuel nozzles; as I recall for the PT6A-34AG for example the Py line can be troublesome, on the Allison 250, the PO line can. If any of the irregularities occur they will lead to power restriction or uncommanded idle power only (roll back). As recently as late 1989 P&WC reached a $US6 million settlement over the 1985 crash of a Beech King Air on approach to Millville, NJ. As part of the settlement P&WC have revised a 1981 service bulletin to cover the “installation of an air filter in the pneumatic line leading to the fuel control on . . . PT6 engines” to prevent sudden roll back to idle. Therefore Cessna, on the Caravan I, designed a mechanical power back up lever that bypasses the FCU governing function and supplies fuel pressure directly to the engine (albeit cautious operation is a must to prevent overfuelling and consequent over-temping), to be used in emergency upon failure of the FCU. It is pleasing to see a similar emergency power lever in the TBM 700.

The ‘700’s electrical system is powered by a 28 volt DC/250A engine driven starter/generator, backed up by an emergency 28v/70A alternator. The battery is a 25Ahr/24v NiCad mounted on the lower left forward side of the firewall. The generator(s) power a distribution system consisting of three parallel busbars. Flaps are electrically actuated. The LG is hydraulic, powered by a 1,700psi electrically driven hydraulic pump; gear extension speed is high at 180KIAS, but when retracted the undercarriage is completely enclosed by gear doors. Emergency gear extension is achieved by manual operation of the hydraulic system (which releases the landing gear for extension) and mechanical downlocks.

Pressurisation and air conditioning systems utilise engine compressor bleed air, the pressure differential being controlled by dual outflow valves in the aft pressure bulkhead. Bleed air also drives an air cycle machine to provide conditioned air in the cabin. In addition to the standard airframe de-ice system elements, the windshield is heated like that of the Malibu/Mirage, a treat in light GA aircraft.

For trimming function, the prototype has control wheel-mounted electrical pitch and yaw trim activation switches (opposite to the mil standard of pitch/roll on the stick, but on a high powered single, yaw retrim is required far more often); for production it is planned that all three trimmers are on the wheel. TBM are sales posturing emphatically for the States where the main market is. With no competition in its breed though, the world is its oyster.