Liberty / Discovery XL-2 (Oct 2016)

Liberty1

On the parking stands of any PPL/IR AGM, one aircraft with ‘junior Cirrus looks’ seems oddly modest; the Discovery (née Liberty) XL-2. Yet it is safe to claim, with 10 years experience, that the XL-2 is a credible synthesis of training and long-range touring aircraft. Evolved from the aerodynamics of the tricycle Europa, designed by Ivan Shaw with wings by Don Dykins (BAC/Airbus), this two-seater is rooted in a strong British gene pool. It started with the desire to make a certified Europa at 1.25 scale for every dimension – fuselage, flying surfaces and even engine, replacing the 100 hp Rotax engine with a 125 hp Continental.

In a bold move, the XL-2 became the first and still is the most popular, certified aircraft with FADEC ignition technology instead of magnetos. The XL-2 is also the first two-seater to be IFR certified since the Cessna 152, which says much about its potential for instrument training. With a fuel burn under 20 litres avgas per hour, a range of 522 nautical miles (5+ hours, less reserves), this is the kind of “go-places” aircraft that takes you from Greece to the North Cape in comfort and safety. It has even been known to the cross the North Atlantic with no modifications.

History and configuration

Back into 2008, HB-KOV was the first EASA-registered IFR certified XL-2. Since then, a dozen XL-2’s have found their way into European airspace, a little less than 10% of the current worldwide airworthy fleet. The manufacturer, now called Discovery Aviation, supports the aircraft efficiently thanks to a large inventory of spare parts, dispatchable on a day’s notice; in addition, they are reportedly ramping up for the production of their next twenty aircrafts.

HB-KOV has recently been fitted with an Aspen EFD 1000, in addition to the original dual GNS 430, GTX 330 transponder and a Bendix/King KN-62A DME. One day, all approaches will have GNSS overlay approved for the missed approach segments. [Editor: we can but so hope!] In the meantime, we are either restricted to approaches where no ADF is required or where we can declare a simulated approach remaining VMC for the missed approach segment and use the NDB waypoint on the GNS 430. On the bright side, the aircraft is EASA approved for IFR (single pilot), LVO approach and landing CAT-1, PBN RNAV 5 (B-RNAV) and RNP APCH LNAV. The absence of an autopilot is a choice, one that again spares the weight of the install and focuses on hand-flying the aircraft. As instructors, we very much depend on the student as autopilot!

The main design objective for our avionics configuration is efficient training for the EIR (en-route) and CBM (Competency-based modular) instrument rating, both for FAA/ICAO IR conversions and new candidates. Following the example of Rate One Aviation, we also operate a Redbird FMX simulator undergoing EASA FNPT-II approval with commonality in the instrumentation, DME and GPS NAV/COM setups.

Operation

Preflighting the XL-2 is quite standard; one will notice that all flying surfaces are articulated with pushrods instead of cables and hence are very responsive. There is a single fuel tank located at the back of the seat, near the center of gravity, containing 106 usable litres. Entry begins with sitting on the wing. Once seated, the cockpit simplicity is striking and ideal for training: single lever power control, FADEC engine/absence of mixture, fixed pitch propeller and fixed gear free much time for proper execution of instrument procedures.

Performance and power settings

Liberty2With wide open throttle and 80kts IAS, the rate of climb at MTOW on a standard day at elevation 1400’ exceeds 500 fpm, hence complying with the LSGG SID requirements for R/W 05 and 23. Other typical power settings would be:

  • high-speed cruise: 65% power, giving 115 kts
  • TAS economy cruise 53% power, giving over 100 kts
  • IAS 3° approaches: 2750 rpm giving 127-131 kts on a 3º glide – a good compromise between the operational needs of busy Geneva airspace and the aircraft’s 2800 RPM red-line.

Comfort

When Ivan designed this aircraft, he had the seat designs inspired by his Range Rover seating; the plush resemblance lives up to this day. Factor in a cabin width of 122 cm / 48 inches and one can fly for hours without rubbing shoulders with the student nor feeling any discomfort.

Stability

Rate one turns can be established and maintained with minimal use of the control column: Steep turns, most uncommon in instrument flight, do require more input but are very manageable without nose dip. Thankfully, the airframe is very stable outside of cloud. In cloud, it is wise to pull back well below the 100 kts Va in order to smoothen the flight experience, as the light airframe would otherwise tend to be bounced around – think of this as another great learning feature.

With only the simplest icing protection (pitot heat, windshield heat, alternate air, alternate static), one is precluded from training in forecast icing conditions. Given the feeble power reserve at altitudes above 11,000ft, descending is the most likely option to exit unfavourable levels.

Redundant systems

Approaches are conducted using the conventional CDI’s (course deviation indicators) rather than the Aspen, again to reduce complexity and to resemble the experience in the simulator. The beauty of this arrangement is that one can toggle between traditional instruments scan, limited panel with the Aspen turned off or glass panel only navigation by covering the traditional gauges. Add in to this the redundancy of having two airspeed indicators, two attitude indicators, two vertical speed indicators, two slide and slips and two altimeters.

The electrical system is designed so that the FADEC engine draws its required power from the alternator first. If the alternator fails, they are powered from the main “A” battery. If the main “A” battery fails, a secondary “B” battery is certified to produce one hour’s worth of power to both FADEC and electric artificial horizons and directional gyro. The Aspen has also an uncertified back up power as well for 30 minutes in case of electrical failure; these mitigation measures are comforting while doing maritime crossings.

And on the flip side

Surely there must be disadvantages as well. In order to remain within the allowances of useful load, two heavy individuals would need to limit their fuel and/or remain below the 100 pounds/45 kg of baggage allowance. An optional maximum takeoff weight increase of an extra 97 pounds is available as an STC from the manufacturer. Personally, we prefer the original version of the aircraft as the modifications required by the STC change the initial flap setting from 20° to 10°, with the original 320 meter take-off roll then substantially increased.

The finger-actuated brakes may appear peculiar. Whenever there is enough power to provide rudder authority, we suggest using the adjustable rudder pedals for macro movements on the ground and use only the finger brake for small movements and fine-tuning course on the yellow line. This way, brake pads are spared; and as the fingerbrakes are located in the middle console, the instructor can monitor usage at all times and intervene if needed.

One would not be as enthusiastic in recommending this aircraft if its primary intention was a basic trainer with repetitive landings. The landing technique does require some flare finesse and avoidance of three point landings. Unlike a Cessna 172, the XL-2 can only be a first choice aircraft in a multi-role capacity (some basic training, advanced ratings, IFR, recurrency etc) rather than as a primary trainer only. With relatively small wheels, taxiing on grass is not its strongest point but grass landings and takeoffs are quite acceptable and routinely used in Geneva.

A remarkable compromise

Liberty3It is hard to emphasise the importance of simplicity for basic instrument training; one can focus on the activities that need to be performed methodically during procedures – a great advantage for the subsequent skill test. After the building blocks are solidly in place and possibly maintained with hour-building at the XL-2’s affordable operating cost, it is always possible to upgrade to a bigger iron.

The benefit of flying an aircraft keeping up with the flow of traffic at 160 kts as against 130 kts at two or three times the hourly cost is a choice that individuals will have to make for themselves. The energy consequences of additional weight and speed are squared and fuel consumption in a heavier aircraft can be over three times what the XL- 2 uses. In Switzerland, this cost implication is increasingly important.

The combination of operating the lowest carbon-emitting IFR aircraft available and extensive use of a full-motion simulator mean that we can deliver an instrument training product in a fraction of the ecological footprint previously required. The Continental engine is well managed by the FADEC system, which compensates for differences in cylinder temperatures and optimizes mixture 240 times per minute.

The best way to try out the XL-2 is during an introductory or initial assessment flight before a Competency-Based instrument rating or FAA/IR conversion course. We have found that in under two hours, most pilots are able to assimilate the specificities of the aircraft.

Events

    No Posted Events

More Events »

Latest Forum Topics

  • Please login to view posts.

More Topics »