Double act

As the new frontier of modern trainers enters the market place, the traditional range of single-engined aircraft face a frontal assault from a new breed of machines. The likes of Diamond and Cirrus have on offer a fresh generation of aircraft that incorporate the best technology, materials and equipment that are available today. And yet, the light twin trainer market has been all but overlooked.

The likes of the Seminole, Partenavia and venerable Beechcraft Duchess still ply their trade in the demanding realm of multi-engine and IFR training, decades after they first took to the sky. To date, the growing revolution of aircraft manufacturing has effectively stalled in this area of design. Larger twins are involved in their own grand battle against the rise of the single-engined turbo-prop, while the smaller twins can boast revamped versions of the stalwart Beech Baron at best.

The DA42 is one of the few options that have diverged from this path. With two engines, four seats and a True Airspeed of around 150 knots, it is a design category that has not really shaken the tree of private aircraft owners since the Piper Twin Comanche in the 1960s. However, as a trainer, the DA42 is set to cause a stir as flight schools seek to modernise their fleets. There are numerous new types available in the basic training phases, yet precious few for the advanced stages. This can lead to the disappointing situation where a student is exposed to top-end equipment from their first flight, only to regress to a very tired multi-engined trainer. The DA42-L360 offers not only an advanced modern fleet option, but the genuine continuity offered by the Diamond fleet.

A Power Struggle.

To fully understand the challenges that Diamond have faced with the L360 variant, it is vital to recount its heritage. Born of the Diamond blood-line, the original DA42 carried its modern philosophy beyond its airframe and onto its powerplants by being equipped with Jet A-1 sipping Thielert turbo-diesel engines. These miserly powerplants offered not only tremendous range and economy, but simplicity of the highest order. The liquid-cooled diesels were FADEC controlled and called for only a single lever per engine in the cockpit for control of both power and propeller. It seemed that the face of 21st century light twins had arrived.

Unfortunately, there are sometimes hurdles associated with forging a new frontier and unfortunately for Diamond, the Thielert engines were one such case. The engines were plagued by a relatively poor service record and ongoing parts replacement, which resulted in both added cost and lowered reputation and ultimately, in 2008, Thielert Engines entered into insolvency proceedings. Diamond and the owners alike had been let down by one aspect of a very promising aircraft, but rather than abandoning the project, Diamond set about seeking a solution.

The result was the DA42-L360. Powered by two very tried and trusted Lycoming IO-360 fuel injected engines, Diamond sought to mount an old friend on the new generation’s wings. It is the same powerplant found on the DA40 and is similarly equipped with ‘PowerFlow’ tuned exhaust system. This re-fitting was a monumental task when one considers that the replacement involved much more than merely bolting on a new engine type. The associated piping, wiring, cooling and FADEC issues ran all of the way from the firewall to the cockpit and yet, the problems were overcome. Diamond should be congratulated on offering a new solution and a retro-fit option rather than just walking away from the problem, yet the strengths and occasional shortcomings of this aircraft can still be traced to the original DA42 as we will see.

A New Look.

As with so many new types, the DA42 attracts attention from quite some distance. At first glance, its form bears a slight resemblance to the British Beagle B.206, but it soon becomes apparent that this is a significantly different breed. Its smooth lines, winglets and narrow aft fuselage are all traits of the growing line of the next generation of light aircraft. Yet Diamond’s ‘twin’ calls for closer inspection as the presence of two engines ensures that it stands out from the pack.

The appearance of the entire Diamond range suggests a design philosophy that speaks of efficiency. Streamlined and highly finished, the use of modern materials in these aircraft provide a strong yet minimalist looking airframe and the DA42 is no exception. From its sleek nose and streamlined cabin to its tapered aft fuselage and swept T-Tail, the DA42 possesses very clean lines. Similarly, its three-bladed composite propellers, high aspect ratio wing and dominant winglets are all well in keeping with the overall appearance and philosophy of the aircraft. The empennage consists of a large elevator fitted to its T-Tail and a substantial strake beneath the aft fuselage. The latter would suggest both an effort to enhance longitudinal stability in the face of the tapered fuselage and a mounting station for an adequately sized rudder.

Interestingly, the ‘T-Tail’ does not seem out of place on the ‘Twin Star’. When Piper and Beechcraft went down the T-Tail road in the late 1970s, it was an aberration that was not overly well-received and generally spoilt the look of types like the Piper Arrow. However, this tail seems to be in harmony with the overall look of the DA42 and its small downward set strakes balance the look of the large winglets. Aerodynamically, the T-Tail was sometimes vulnerable to reduced airflow at high angles of attack as fuselage shielding took place. The generous, full-span elevator of the DA42 boasts dimensions to seemingly nullify this concern. Additionally, an electrically powered Variable Elevator Backstop (VEB) limits the elevator ‘up travel’ above certain engine power settings.

The engine cowlings have called for a major re-jig as the 180HP Lycomings have replaced their liquid-cooled diesel predecessors. The large lower intakes are gone, now replaced by conventional intakes either side of the spinner, common to horizontally opposed piston engines. However, rather than the airflow ultimately being dumped beneath the engine, it escapes via broad, finned vents on the upper side, ensuring a fair share of cooling for all of the cylinders.

The long slender wings house a series of main and auxiliary fuel tanks for a total useable capacity of around 76 US Gallons, or just fewer than 300 litres. However, herein lies one of the issues for the aircraft as this fuel capacity was designed around the highly economical diesel engines, but when paired to the Lycoming engines represents a 90 litre handicap when compared to the 180HP carburetted O-360 engined Beechcraft Duchess. This equates to greater than an hour less endurance for the DA42 and IFR instructors have expressed that this can effectively limit the duration of their IFR navigation exercises, particularly when weather and traffic holding come into play.

The other aircraft limitation that relates directly to the re-engining of the DA42 lies in the weight and balance envelope; or more accurately, outside of the envelope. The new Lycomings are lighter than the previously fitted diesels and sit ahead of the datum, essentially moving the centre of gravity further to the rear. Consequently, with full fuel and three persons on board, the aircraft’s CofG moves out of the aft limit of the envelope. To facilitate the third person, a rectification has been designed involving ballast in the nose, however, this then forces the aircraft out of the front end of the envelope with only one person on board. Ultimately, the industry feedback suggests that operationally, the DA42 is best considered as a two-seat trainer. Diamond have definitely done their best in the face of a very challenging situation and certain trade-offs were always going to be called for, however, the DA42 is still a very fine aeroplane.

Beneath those wings, the DA42 sits proudly upon a heavy duty undercarriage. Surrounded by modern composite materials, the metal tricycle gear features substantial oleos while the main gear employs sturdy trailing arm suspension. Even though this aircraft feature may not display the same aesthetics as the rest of the airframe, it is a practical philosophy for a training aircraft that is set to land a few more times than it actually arrives.

Entry to the cockpit is available to the front seats on either side via an overwing walkway. In keeping with its siblings, the cockpit features a large bubble canopy hinged at its leading edge while the rear row is only accessible on the port side. Stepping down into the cockpit, many of the strengths of the Twin Star become evident. Its fixtures and finish are of the highest quality and the Garmin G1000 dominates its clean instrument panel, however, it is the commonality with its Diamond brethren that is immediately apparent. If one were to ignore the presence of dual levers on the throttle quadrant, this cockpit is a DA40 for all intents and purposes; from the control column between the legs to the high pitched canopy and the great visibility it offers.

The throttle quadrant warrants special note as Diamond have done an excellent job of fitting a full set of dual levers for the variable pitch propellers, mixture controls and throttles where single levers once resided in the ‘FADEC diesel’ cockpits. The new levers are of substantial size and well differentiated in both spacing and ergonomics. Easy to overlook, this is just one challenge that Diamond have countered with both skill and style.

Getting Started.

The seat is fixed and the pedals are adjusted to suit the individual, removing the chance of a sliding seat on take-off and also maintaining a constant position between the seat, control column and throttle quadrant. With its standard seating position quite low and aft, the use of cushions will be prudent for shorter pilots. Otherwise, the overwhelming feeling of commonality with the other Diamond aircraft is the dominant sensation.

Prior to start, the Variable Elevator Backstop is checked by advancing and retarding the throttles with the control ‘stick’ held fully aft. Otherwise pre-flight sequences are straightforward and enabled by the presence of the wonderful Garmin G1000. Curiously, the aircraft has a key for each engine in the absence of merely magneto switches. It may be an idea to keep a spare key or two concealed on-board to avoid an unscheduled overnight due to the keys being lost. However, the engine start technique is conventional for a fuel-injected engine, with a short prime, followed by a mixture lean start; advancing the mixture when the engine springs to life. The noise levels within the aircraft are quite low, mainly by virtue of the three bladed props one would suspect. It’s important to monitor the RPM on the ground in a pleasantly quiet aeroplane as the tendency is to think that the revs are somewhat lower than they actually are.

Taxiing is very simple and minimal inputs are required to guide the aircraft along the taxiway. However, keep an eye on those long sleek wings and winglets when manoeuvring in confined areas such as parking areas and run-up bays. Before take-off procedures are conventional, including a verification of the cross-feed system before flight, additionally, the auxiliary tanks feed into the main tanks by virtue of transfer pumps, whose switches are located at the rear of the centre console. There is one further aspect of the fuel system that does seem a little challenging; there are no quantity gauges for the auxiliary fuel tanks. The quantity can be verified visually on the ground if the tanks are full, or through a fuel pump test that will flag a pump fault when the tanks are empty. However, there is no way of verifying the contents of the auxiliary tanks between these two extremes.

Checks completed, we move to the holding point and set our sights down the centreline.

The Magic Carpet.

For our sortie, the aircraft had full main tanks, me and the experienced Richard Tomlin from Hawker Pacific on board. At this standard training weight, the aircraft accelerated and lifted-off with impressive gusto. Equally impressive was the commonality of layout, control inputs and attitudes with the single-engined DA40. In addition to best rate and angle of climb scrolling down the speed tape, the critical ‘engine out’ rate of climb (Vyse) was highlighted at 82 knots and well above Vmca of 65 knots.

Like its DA40 sibling, the aircraft climbed out like a magic carpet ride with the great visibility afforded in all directions. The control feel was almost identical to the DA40 and had that substantial feel that catches one off guard if any misleading preconceptions are held about how a composite aeroplane should feel. Throughout basic flight manoeuvres, the controls were well harmonised with rate of roll and pitch corresponding nicely to the control input throughout a range of speeds. It trimmed out at a little over 150 knots TAS with an intermediate power setting of 22”/2300rpm set at only 3,000 feet. This speed would undoubtedly sit a little higher when allowed to stretch her legs at altitude.

Electric trim is available through the control column, but a tidy little manual trim wheel is also located between the seats and at times old habits took over and the aircraft was manually trimmed. It is hard to express too greatly the wonderful ergonomics of this aeroplane and the ease of transition it would offer DA40 pilots moving up to the next level and from this perspective, Diamond have exceeded their training goals.

Prior to the flight, I was aware that this would be the first ‘multi’ that I had flown with a control column in place of a yoke. Such thoughts were wasted effort as the control inputs were extremely intuitive and the asymmetric handling proved to be very conventional. The rudder inputs were possibly of a slightly higher loading than other ‘twins’ but still well within the realms of an aviator. Once feathered, the aircraft performed admirably on one engine in the cruise with an effective trim relieving the rudder loads, resulting in tremendous flying characteristics. An unfeathering accumulator permits the re-establishment of power on the ‘failed’ engine with genuine ease.

As suggested by a Vmca of only 65 knots, slow speed flight is a non-event in this aeroplane and wandering back to those speeds would call for some genuine inattention on behalf of the pilot. A reasonable section of the air-to-air component of the flight was flown in the low speed regime with certain sections in the ‘dirty’ configuration of gear down and flap extended. Throughout, the aircraft handled marvellously for the photographic session and while lower speeds equate to lesser control responsiveness, the DA42 could not be described as ‘sloppy’ at any stage. In fact it felt very stable at the lower end of the speed envelope, which offers great comfort when sharing the circuit with smaller, slower single-engined types.

In fact, entering the circuit in this aircraft is a breeze and surprisingly so considering its very aerodynamic form. Assisting deceleration are two stages of conventional flap which extend beneath the fuselage as a section of split flap. With the first stage limited to 137 knots and full approach flap to 111 knots, the process of arrival is very simple and again aided by fantastic visibility. Ultimately, with a maximum landing gear extension speed of Vne of 194 knots, there is no excuse for not ‘dangling the Dunlops’ if absolutely needed.

Around We Go.

Within the circuit, the DA42 is as simple as it gets. Its generous extension speeds allow for good speed management, while the associated pitching with configuration changes is mild and easily anticipated. The great visibility affords a clear view of other traffic and the runway, even when flying circuits from the ‘far’ seat.

If an almost boring routine of gear extended mid downwind, then first stage of flap approaching base and full flap established on base is employed, not too much can go wrong. Power settings are conventional and the speeds flown are very generous at around 90 knots on base and 85 knots on final showing due respect for Vyse. Speed will wash off when the three-bladed props are set to full fine on final, but this is easily anticipated. The approach perspective is reminiscent of the DA40 and the aircraft is very stable on final. Speed management is straightforward right into the flare and the landing attitude and trailing arm undercarriage can make anyone look good. The wrap around windscreen also offers a wonderful degree of peripheral vision in this phase of flight.

With an engine out, the lighter weight of the composite form and sleek lines allows the DA42 to climb away with a little more ease than it ‘blunter’ metal predecessors. The rudder inputs are again a little heavier and there is a degree of relief that is always associated with securing the failed engine without undue delay. With zero thrust set and two on board, the single engine climb performance was quite impressive.

Having sampled the DA42 at most ends of its envelope, it was with some regret that it was time to return to land. This aircraft is like its siblings, from line-up to landing, the DA42 is a pleasure to fly.

A Job Well Done.
The DA42-L360 was borne of an awkward re-engining due to circumstances beyond Diamond’s control. Unfortunately, this is the price sometimes paid when new boundaries are pushed, but to Diamond’s credit, they were brave enough to not only venture down this path but continue when adversity raised its head.

Through the DA42-L360 they have offered an alternative to DA42 owners past and present while still seeking to set the pace in twin training. And they have not finished yet with a new diesel DA42NG promising to fulfil the destiny of this ground breaking aircraft.

For the moment, the DA42-L360 represents the head of the Diamond training range and builds upon a tremendous philosophy of commonality and innovation. As the competing twin trainers continue to tire from the demands of asymmetric training, the DA42 is undoubtedly a diamond in the rough.

(c) Owen Zupp July 2010