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Flight Test: Grob G120TP – The twenty first century trainer

PUBLISHED: 13:00 27 June 2016

Flight Test: Grob G120TP  Photo: Christoph Schubert/Grob

Flight Test: Grob G120TP Photo: Christoph Schubert/Grob

FLIGHT TEST

A high performance aircraft selected for military training in the UK, the G120TP would also make a fabulous sportplane

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

As we slew sideways through the sky I roll upright and grimace, “You know, I think those Immelmans still need a lot more work, Tom”. “Well,” he replies diplomatically, “I definitely do see some improvement!”

As Grob’s Tom Reinert and I approach the Grob G120TP, gleaming in the late afternoon summer sun, I can see that aerodynamically it is very clean−the only anomaly being a relatively large ventral strake under the fin. It’s a very attractive aircraft that exudes power and purpose, while the big five-blade propeller gives a clue that a lot of horses are hiding beneath the ‘shark-nose’ cowling.

Based on the successful piston-engined G120A, the G120TP is a much more potent machine, courtesy of its powerful RollsRoyce M250-B17F turboprop. This very compact turboshaft engine has an excellent power-to-weight ratio (it weighs only 96kg yet can produce up to 456 shaft horsepower) and spins a five-blade MT constant-speed/ reversible propeller. Access to the power unit is excellent, as a large section of the top cowling hinges open on both sides. Fuel is contained in a pair of integral wing tanks with a total capacity of 290kg.

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

The demonstrator we are flying today is trialling dual Berringer hydraulic brakes on the mainwheels of the retractable tricycle undercarriage (Cleveland brakes are standard). The nose-leg retracts backwards into the fuselage and the mainwheels inwards, to wells in the wings. Consequently the wheel track is quite wide. The nosewheel is completely covered when retracted but the mainwheels are not. Leaving off the wheel-well doors probably contributes to the G120TP’s impressively high VLE (maximum landing gear extended speed).

The wings feature impressively large upswept winglets and powerful LED landing and taxi lights built into each wingtip, but what really caught my eye was the size of the pitot (the thing is huge!) and the very neat, direct-reading analogue fuel gauges flush-mounted in the top of each wing, near the root.

Elevator, aileron and rudder trim can all be adjusted in flight: the elevator uses trim tabs while the ailerons and horn-balanced rudder use a spring-bias system. The electrically actuated flaps are of the slotted type, but unlike the Grob G115E there is no wash-out in the wing.

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

Build quality is very high. While visiting Grob’s base near Munich, I went on a factory tour and saw exactly how the aircraft is constructed. It’s noteworthy how many of the components are built in-house, greatly reducing the company’s reliance on external suppliers. Split vertically, the semi-monocoque fuselage is constructed from two halves made almost entirely from composite materials, primarily carbon-fibre reinforced polyester (CFRP) covered with a gel-coat and then painted with ‘two-pack’ polyurethane paint.

The wings and tailplane are of the ‘sandwich’ type, having a honeycomb core covered with a CFRP skin, while the flying controls are made from rigid foam skinned with conventional GRP. As composite aircraft do not conduct electricity very well, and the 120TP is designed to be IFR-certified, lightning strikes are handled by a special conductive layer underneath the paint.

The result is a beautifully made, corrosion-proof airframe stressed for aerobatic manoeuvres up to +6/-4g that has a service life of 15,000 hours and no Fatigue Index (the bane of all-metal military trainers like the SA Bulldog) to worry about. I ask Tom if very high ambient temperatures ever cause problems, but he just smiles and says no. The United Arab Emirates Air Force has never had an issue with its G115s, and nor has the German air force with the G120As it uses in Phoenix, Arizona−and summer temperatures at both Al Ain AFB and Luke AFB regularly exceed 40ºC.

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

Military philosophy

In keeping with military philosophy (and also for pilots who may eventually be streamed on to helicopters or become co-pilots on transport aircraft), the cockpit is configured to be flown solo from the right seat, so Tom suggests I fly from there. With the big bubble canopy slid right back, access to the cockpit via sensibly-sized non-slip wingroot walkways is excellent, so I shrug into my parachute, set myself down in the right seat, adjust both it and the rudder pedals and then strap myself in. The five-point harness is very good−possibly the best I’ve seen in an aircraft in this class, as the lap and crotch straps offer plenty of adjustment and the shoulder straps are of the inertia reel type which can be locked for take-off and landing. The seat shells are also very strong, being rated for up to +26g.

I then begin to assess the overall layout of the controls and instruments while Tom settles into the other seat. An ex-Bundeswehr major and Tornado pilot, Tom is chief instructor and also a test pilot for Grob, and exudes an impressive combination of calm confidence, wide experience and deep knowledge. I’m very much looking forward to flying with him.

The cockpit is sufficiently wide to avoid constantly rubbing shoulders with the other occupant and the generous baggage bay behind the seats is accessible in flight. Two features instantly indicate that, despite its ‘D’ registration, this aircraft normally operates in a military environment−there is a small UHF radio next to the standby EFIS (Electronic Flight Information System), and there are no ignition keys!

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

The instrument panel is fitted with a sophisticated Genesys Aerosystems avionics suite, which consists of four identically-sized liquid crystal display screens mounted vertically. These are arranged as a PFD (primary flight display) in front of each pilot, with the other two functioning as MFDs, (multi-function displays), one for systems and the other for situational awareness in the centre of the panel. Analogue instruments are entirely absent; the back-up instrumentation consists of a small, self-contained Emergency Standby Instrument System in the centre of the panel which displays attitude, altitude, speed and heading. In the unlikely event of a total electrical failure it has its own integral battery which provides power for two hours in normal conditions and one hour in extreme cold.

The curved control sticks are topped with comfortable pistol grips and fall nicely to hand, as do the power and condition levers (henceforth referred to as PL and CL respectively) which are mounted in an engine control quadrant in a neat central console that extends aft from the base of the instrument panel back between the seats. There isn’t a dedicated prop lever; instead the CL controls both the fuel and prop. If the CL is out of cut-off/feather the fuel is on. When the CL is full forward and the N1 (low-pressure compressor RPM) is high enough to have sufficient oil pressure to put the blades to fully fine, prop control is automatically governed at around 2,030rpm. Setting the CL to low rpm moves the blades to coarse pitch, but this is only used for practising forced landings.

The instructor also has a PL on the left cockpit wall. Apart from the turbine control levers and a lever for the friction lock, the console also carries the elevator trim wheel and its adjacent indicator aft of the quadrant, while the fuel valve and flap selector are in front. A fascinating feature is that the flap switch has four settings: UP (0°); TAKE OFF (20°); LAND (40°); and FULL (60°). I found this rather curious, as almost all aircraft in this category land with full flap.

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

A row of mostly toggle switches along the bottom of the panel controls the electrical services, with two rows of circuit breakers (CBs) directly underneath. The more important CBs (including those for the flaps and hydraulics) are situated in a column marked BATTERY BUS next to the undercarriage selector. The Crew Alerting System (CAS) consists of red ‘Master Warning’ and amber ‘Master Caution’ annunciators, specific messages being shown on the PFD. One feature I definitely approve of is the parking brake, which is easy to both reach and operate. This may sound trivial, but I recently flew an aircraft in which, once the seat was set and my harness tight, I found it was impossible to reach the parking brake.

Good field of view

When I flew the Grob G115E many years ago, probably my least favourite facet of the aircraft was the considerable blind area caused by the combined thickness of the arch of the canopy bow and the windscreen frame. As Tom slides the canopy shut I wonder if the G120TP will be the same, as the windscreen frame and canopy bow seem similarly proportioned. In fact, the field of view is very much better, so much so that I never notice either of these obstructions in flight.

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

Engine start is very straightforward: battery on, select Start (Auto Ign is selected automatically as part of the start cycle), and between 12-15%N1 move the CL fully forward. We get a nice cool start, and as soon as the engine is stable Tom brings the powerful air conditioning on line and I begin to study the PFD and MFDs. Interestingly−and confirming CAA advice not to wear them−the polarised sunglasses I am testing are incompatible with the Genesys screens and I quickly realise I’ll have to fly using the standby EFIS, while whenever Tom points out something of particular interest on the MFD, I have to peer over the top of my shades like an old fogey. One feature I really like is that colour coding is used extensively.

Consequently you don’t need to know (for example) what the oil pressure or temperature should be. If it’s green, it’s good. The primary power indication is torque (TQ), displayed as a percentage. Maximum power is 456shp, while MCP (maximum continuous power, and 92%TQ) is 380shp. However, at the early stages of a pilot’s training a simple mechanical stop can be used to artificially constrain power to 312shp. This is known as ‘Reduced Power Rating’. Tom recommends increasing power by moving the PL with the flat of the hand, as when the engine is at about MCP the PL lines up with the CL and provides a useful tactile clue.

This is very neat, but I’m not quite so enamoured with the fuel system. The aircraft is approved for up to thirty seconds of continuous inverted flight, but as the inverted supply is mounted in the left tank, the fuel selector is usually left on that tank. When an imbalance is seen on the MFD you just flick on the transfer pump switch and it automatically pumps fuel from the right tank to the left for two minutes. Although the fuel quantity tapes are clear and unambiguous, I’m still slightly surprised fuel transfer isn’t automated, using a similar system to the TBM900. Tom explains that while it could be, the rationale is that student pilots should be aware of fuel balance and transfer issues, as the aircraft they go on to fly operationally may not have automatic transfer systems.

Flight Test: Grob G120TP  Photo: Christoph Schubert/GrobFlight Test: Grob G120TP Photo: Christoph Schubert/Grob

Taxying out reveals the G120TP to have very pleasant ground handling characteristics. The field of view is excellent, the toe-operated hydraulic disc brakes are powerful and progressive and the nosewheel steers through the rudder pedals up to 10º either side of neutral. Rather than riding the brakes, ‘beta’ is used to maintain taxi speed at a sensible pace.

With all the checks complete, flaps set to take off, ignition to continuous and full right rudder trim, Tom lines us up with the centreline of Runway 15 at Grob’s private airfield at Tussenhausen Mattsies, stands on the brakes and opens up the engine to 75% torque. With an outside air temperature of 25ºC and an airfield elevation of 1,800ft, we have a density altitude of around 3,500ft−while with 150kg of fuel and no baggage we are about 75kg below the maximum takeoff weight of 1,515kg. Tom has ensured we only have half fuel as the maximum weight for aerobatics is 1,440kg and the aircraft we’re flying is the prototype, which has practically every conceivable option installed. Consequently its empty weight is greater than production aircraft.

Tom releases the brakes and smoothly goes to 95%TQ while I follow through on the controls. The acceleration is strong and we rotate at 75kt after a very short ground roll of around 400m. Gear up, flaps up and Tom says, “You have control.” The airspeed increases rapidly towards our Vy (best rate of climb speed) of 116kt and we race skyward with the vertical speed indicator indicating just over 2,600fpm. It occurs to me−and not for the last time−that the G120TP feels and flies more like a jet than a propeller driven aircraft. To the uninitiated this may seem unlikely, bearing in mind there is a great big five-bladed prop churning away−but Grob’s engineers have done a great job in making the G120TP well… jet-like. From the single power lever operation to the absence of P-factor (asymmetric blade effect), it just doesn’t feel like a propellerdriven aeroplane.

This aircraft talks to you

Having turned towards the west in order to keep clear of Munich, I continue the climb to 8,000ft. I’m keen to try some aerobatics, but first need to assess some of the basic handling characteristics. Stability seems good and control around all three axes equally satisfactory, with light, powerful ailerons, a slightly heavier though no less authoritative elevator and an effective rudder. Control harmony is also spot on, with the ailerons being the lightest control and the rudder the heaviest. Adverse yaw is practically non-existent, and I’m slightly surprised at how little rudder is required in flight. I wonder if perhaps there is some kind of interlink between the ailerons and the rudder (the G115 has such a system) although it doesn’t feel like it. Tom explains that the aileronrudder interconnect system fitted to the G120A was discontinued on the G120TP as the new winglets are so efficient. As well as helping to counter adverse yaw, they also improve the slow speed handling, yet do not have a negative effect on the high-speed handling qualities, nor diminish the roll-rate.

The controls feel extremely taut, and this is because pushrods are used for the elevator, ailerons and even the rudder. Consequently−and unlike designs that use cables for the primary controls− there is no ‘slop’ in the control circuits at all. This is one of the great advantages of using pushrods over bellcranks and cables, and is almost certainly a byproduct of Grob’s long association with sailplane design and manufacture.

A look at the slow end of the speed envelope reveals no surprises−indeed the 120TP has exemplary stall characteristics. Even with the flaps fully retracted there is plenty of pre-stall buffet before the wing finally quits at about 70kt, while with full flap the airframe really shakes! There are also audible and visual stall warnings, but they’re pretty well superfluous. This aircraft talks to you. The difference in the stall speed with flaps fully extended is about ten knots slower, and the ailerons remain effective even deep in the stall. When stalled in a simulated ‘base to final’ turn, it rolls wings-level.

From stalls we move to spins, where the G120TP’s behaviour is equally predictable. Tom demonstrates a three-turn one to the left and then I try a couple. I like how it spins, with a nice crisp entry and the nose well down.

Initially I find the rotation rate quite quick, and wonder if it is really going to start ‘winding up’ by the third turn, but it simply settles down. Recovery is prompt and effective. Tom recommends that when recovering from an inadvertent or incipient spin to just use the standard spin recovery procedure of PL to idle, full opposite rudder then stick forward with the ailerons neutral. However, with a deliberate spin that has been allowed to develop past two turns, recovery is actually quicker with just a hint of pro-spin aileron. Spins always consume altitude, but one of the great things about the G120TP is all you have to do is push the power up, climb at Vy and you’re back to where you started in less than a minute.

I’m keen to move on to some aerobatics, but first Tom wants to check whether I can still take some ‘g’ without a G-suit, so he opens the PL back up to MCP, lets the speed build to 200kt and then reefs the little prop-jet round in a tight 360. This generates a sustained 4g, but I am still able to function, despite sinking down in my seat. Even with my advanced age and increasing girth, I still seem to be impervious to ‘G-LoC’ (and this machine has enough power that it can be an issue) so we can safely sample some aerobatics.

Tom suggests I try and replicate manoeuvres he first demonstrates. This sounds good to me, as I’m more than a little rusty. By coincidence the last time I flew aeros was also in a fine German flying machine, a Bücker BU-131 Jungmeister (Pilot, January 2015). But as you might imagine, a radial-engined, open-cockpit, single-seat biplane is just a little different from a glass-cockpit turboprop.

Tom starts off with a loop, which I copy quite successfully. The trick seems to be set MCP, let the speed build to 200kt and then initiate a smooth 4g pull-up. This should be where the outstanding field of view through the big bubble canopy really comes in handy. Unfortunately, it’s a bit hazy, which is a shame as a decent horizon would really help. We then move on to an Immelman turn and−as you’d expect−Tom’s is an exercise in precision. For some reason, though, I just can’t do a good one, and despite Tom’s patient instruction they’re all rather untidy affairs. I also try my hand at inverted flight, and once I realise that it’s a lot easier if you get the nose well up before rolling inverted, this goes quite well. I never do get a decent Immelman though.

Smorgasbord of digital delights

The fully integrated glass cockpit contains a veritable smorgasbord of digital delights, including a graphical FMS (flight management system) and synthetic vision with Highway in the Sky (HITS) presentation for both precision and non-precision approaches, Terrain Awareness and Traffic Avoidance Systems (TAWS/TAS) and integrated audio/radio. Eventually it will even be able to synthesise some aspects of a modern warplane, as the MFD will display virtual radar, virtual radar warning receiver and virtual stores management systems.

One aspect that Tom is particularly keen to show me is the ‘blue ring’. This is an instantaneous glide path calculator that clearly displays exactly how far you can glide at any given moment. Basically, from your present position you can reach anywhere within the ‘blue ring’ should the engine flame-out, while the line shows exactly where you’ll be at 200ft AGL and 100kt. To show just how accurate it is, Tom pulls back the PL, trims for 100kt and I study the PFD. The ‘blue ring’ isn’t a circle or even an oval but is actually quite ragged, as the computer also takes into account both the terrain elevation and the relative wind.

Tom then pulls the CL back to low RPM, the prop blades coarsen (which reduces drag) the nose pitches up and the ‘blue ring’ instantly expands, as the computer is set for a specific glide ratio that is achieved at ‘best glide speed’ with a feathered prop. To say I’m impressed is an understatement.

255 kt cruise

On the way back to TussenhausenMattsies I ask Tom what the G120TP is like in the cruise, and he replies that at MCP and 10,000ft the typical true airspeed is around 225kt for a fuel flow of 80kg/hr, while the range is in excess of 700nm. As for limiting speeds, below 13,000ft the 120TP’s VMO (maximum operating limit speed) is 238kt IAS, while above 13,000ft it is Mach 0.45.

Back at base Tom demonstrates the first touch and go, talks me round the second and then just lets me get on with it. It’s a very easy aircraft to fly−the trick seems to be to fly the downwind leg at about 120kt and 1,000ft AGL with the wingtip visually just touching the runway. At about the midfield point, set the flaps to TAKE OFF, then pull the power back to around 35%TQ. Abeam the numbers lower the undercarriage and when you’re at about 45 degrees to the threshold turn base and ease off another 10%TQ. Tom says the picture should be “one-third sky, twothirds ground” and I find this works as advertised, and the speed bleeds back nicely to 100kt. Turn final, flaps to LAND and by gently modulating the TQ the 120 simply slides down the slope towards the runway. A smooth easy flare, pinch the last of the power off and it practically lands itself. Flaps back to TAKE OFF, smoothly bring the power back up (there’s no need to check the TQ indicator, your thumb hitting the CL and the insistent push in the back tells you all you need to know) then rotate at 75 and you’re flying again.

Two interesting observations are that despite all that torque there’s no tendency to swing, and there are practically no pitch trim changes when the flaps or undercarriage are lowered. There might just be a suggestion of pitch-up as the flaps transition from TAKEOFF to UP but it’s very subtle. For our final landing Tom resumes control and demos a short-field landing with FULL (60º) of flap. The float is noticeably reduced, and as soon as the nosewheel is on the ground he pulls the PL into reverse, the engine bellows hoarsely and we stop extremely quickly. Grob claims that even at maximum landing weight the G120TP can land over a 50ft obstacle and still stop within 455m, and I can well believe it.

A fine trainer

In conclusion, I think that the G120TP is an excellent aircraft. It possesses great performance and excellent handling, combined with good fuel economy; a relatively low-maintenance airframe with a large comfortable cockpit and advanced, integrated systems. I also think it’s a fine trainer. It’s easy−but not too easy to fly, while the integrated CVR/FDR (cockpit voice recorder/flight data recorder) is a powerful MDS (Mission Debriefing System) tool. The arguments for side-byside as opposed to tandem seating both have their pros and cons, but having been instructed−and given instruction−in aircraft featuring both configurations, personally I’d say that the pros of side-byside outweigh the cons. And of course, the G120TP is much more than just a training aircraft−it is part of a complete training package that incorporates ground based training systems (simulators and cockpit procedure trainers) along with computer based training for theoretical instruction.

Finally, while turbine-powered aircraft are thirstier and more expensive both to buy and maintain, as there are practically no operational piston-powered aircraft in both the commercial and military worlds today, starting off in a turbine-powered aircraft seems to make perfect sense. In fact, as the G120TP can also be fitted with HOTAS (hands on throttle and stick) systems and lightweight Martin-Baker Mk17B ejection seats it’s an ideal introductory aircraft for the world of military flying. I really liked it, and am not surprised that it won the UK Military Flying Training System competition. I’ll bet that when the G120TPs are eventually pensioned off there’ll be a queue of eager sport pilots waiting to snap one up. Its got a fine field of view, great handling and plenty of power. It can even operate from less than 500 metres, and it looks cool too. I’d love one!

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