STEWART PENNEY / STANS
Pilatus believes its PC-21 turboprop is a new breed of basic trainer capable of blurring the boundaries between the training phases
Pilatus has just begun the third of four PC-21 flight-test phases and is aiming to certificate its newest turboprop basic trainer at the end of next year.
The PC-21 has been flying since 1 July 2002 and, by the end of June, Pilatus had concluded the second of four phases in the flight-test programme, the stage being used for envelope expansion and "proof of design" work, demonstrating the feasibility of a high-performance turboprop. Phase 3 will "tune" the aircraft for its training role, while phase 4, to begin in the second quarter of next year, will focus on certification. By the end of June the aircraft had completed 160h in 135 flights.
Initially, certification will be to civil visual flight rules (VFR), but will be extended by mid-2005 to instrument flight rules (IFR). Pilatus had only planned VFR clearance, but the substantial changes in civil airspace management now mean it is appropriate to clear the PC-21 for IFR from day one.
The PC-21 is the latest in a long line of Pilatus trainers. The Stans, eastern Switzerland-based company has sold more than 520 PC-7/PC-7MkIIs and 262 PC-9s, the latter also being the basis for the Raytheon T-6 Texan, winner of the US Joint Primary Aircraft Training System (JPATS) competition. Around 780 T-6s are expected to be ordered by the US forces.
Despite the continuing sales of the PC-7/9, Pilatus managing director strategic projects Kevin Smith says the company elected to start its next product with a clean sheet. "We did look at a jetas well as the high-end turboprop," he says. Pilatus also built a high-performance proof-of-concept (PoC) aircraft - a PC-7MkII with the wingspan reduced by 2m (6.6ft) and spoilers added to increase the roll rate. Engine power was also more than doubled, from 750shp (560kW) to 1,600shp. These changes gave the PoC aircraft a "Eurofighter-like roll rate and very fast acceleration to 200kt [370km/h]", says Smith.
The PC-21 roll rate is 180°/s using the spoilers. These also allow shorter span ailerons, which means the flaps are large enough to reduce stall speed to 80kt. The ailerons are still powerful enough to control the aircraft if the spoiler system is inoperative. Bill Tyndall, PC-21 project pilot, says the pilot does not notice the spoilers in the air.
Aileron buffet caused by the spoilers was an issue with the PoC aircraft, but "it's not an issue now", Tyndall says. Bruno Cervia, engineering director new aircraft projects, says the spoilers only deploy after 5° of aileron has been applied. Tyndall adds that this eliminates any over-control around the mid-stick position.
The PoC aircraft also removed risk from the high-performance turboprop concept, says Smith. It first flew in November 1997 and had completed 210h by December last year when its test programme ended. The PoC aircraft was used to test PC-21 software in the air before the latter flew and was instrumental in clearing the new aircraft for first flight soon after roll-out, says Smith.
The combination of the PoC tests and market studies led Pilatus to the conclusion that it had to be an innovator or leave the market, says Smith. This provided the impetus for a November 1998 decision to fund development of a new - rather than derivative - turboprop trainer, at the time known as the PC-XX.
Unlike many aircraft development programmes, the PC-21 is self-financed by privately owned Pilatus, says Smith. The programme is budgeted at SFr200 million ($150 million). Smith says: "We write off the development costs on an annual basis against profit. There is no goodwill charge either."
This amortisation of development funds against existing products such as JPATS licence fees and the PC-12 utility aircraft meant that in 2002 Pilatus reported profits of SFr25 million on sales of SFr481 million. Profit would have been SFr66 million without the R&D spending. "We spent SFr41 million on R&D. We're small so we have to stay ahead. We can't be a 'me-too' company, we have to be out front," says Smith.
The PC-21 will improve training efficiency "through the intelligent use of technology, not using technology for technology's sake", says Smith. The PC-21 is designed to meet a specification so technology is used "where it gives substantial and tangible benefit".
Training efficiency
As it is cheaper to operate a turboprop than a jet, training efficiency is boosted by using the former for more of the syllabus. If mission management is a key part of pilot training, then the turboprop should be equipped with suitable avionics. It can then be used for handling and systems management training while the jet trainer is used for advanced training, says Smith, noting that the Swiss air force has retired its relatively new BAE Systems Hawks and now uses two-seat Northrop F-5s for high-energy manoeuvring training and other work outside the PC-7's performance envelope.
There is now a fundamental question - where does the jet replace the turboprop, if at all, in the syllabus? These requirements, driven by cost and the changing nature of combat-aircraft operation, require new thinking in air forces and aircraft manufacturers, says Smith.
PC-21 design drivers include low through-life costs, coupled with "driving up" user value. Acquisition and life-cycle costs must not exceed Pilatus's turboprop benchmark. As a result, the PC-21 is faster, has a higher rate of roll and has more equipment to emulate frontline aircraft, says Smith. "A core objective is an aerodynamic capability greater than any other turboprop for training". However, the company's predictions mean "PC-21 acquisition and life-cycle costs are the not the most expensive for a turboprop", says Smith.
Pilatus benchmarks the PC-9 as 1, with jet trainers being 3-6:1 in cost terms. A frontline fighter is 30-60:1, with the odd example reaching 100:1. "We can argue that you can do 5h on the turboprop for the same cost as an hour on a jet," he says.
Jet trainers now in development are aimed at pushing capability into the operational conversion unit training role. But these aircraft will need an enhanced turboprop to improve the interface with the jet.
Pilatus believes the PC-21 can be used for much of the traditional six phases of training, promoting the turboprop for Phase II basic, Phase III advanced and some of Phase IV fighter lead-in. For Phase IV, a typical ground-attack profile includes a1-1.5min target run, regardless of whether it is flown at 300kt or 600kt, says Smith, adding: "There is a difference between the two, but it is not worth worrying about." What is not possible to train in a turboprop, he acknowledges, is advanced air defence where high specific energy manoeuvres are part of the course. He says some air forces are considering removal of Phase I ab initio and replacing it with a combination of synthetic devices and the basic turboprop.
Inclusion of advanced avionics will allow air forces to start instilling frontline ethos - such as setting the correct jettison package in case of engine failure on take-off, or turning the master-arming switch on - from the first day. "Training is not for fun, but to learn frontline skills," says Smith.
Development of the PC-21 officially began in January 1999, although the team had been put together in the last days of the preceding year. On 1 May 2002, an "internal roll-out" of the first aircraft, PO1, was held in Stans and, after two months of ground tests, it made its first flight on 1 July at the hands of Tyndall.
Smith says the intention had then been to accrue 50h and attend the Royal International Air Tattoo and Farnborough air show in the UK. Weather problems restricted flying to 10h, "all fault free". As a result, FOCA, the Swiss airworthiness regulator, permitted Pilatus to fly to the UK provided relights and other safety-critical functionality were demonstrated. This was done and the aircraft flew to the UK that day.
Flying has continued since the aircraft returned from Farnborough, "with no show stoppers". As with all flight-test programmes, problems have been found, including too-light rudder pedal forces. Although Pilatus concentrated on problem identification during phase 2 testing, this was changed while the aircraft was grounded by bad weather. Another "safe change" introduced in this way was the raising of the rear cockpit floor.
Pilatus plans a 450h flight-test programme to achieve FOCA VFR certification by late next year. Smith says solutions have not been introduced as soon as problems have been identified because the company wants to understand all the issues and make sure that by fixing one problem it does not create others.
Phase II testing included flow visualisation up to 25,000ft; flutter up to VD and M0.76 at 18,000ft; stability and control including low and high weight aft centre-of-gravity; and load factors up to 8g. The aircraft is due to be cleared for +8/-4g.
A key system allowing the PC-21 to be used over the broad expanse of the training cycle is the power management system, which controls the output of the 1,600shp Pratt & Whitney Canada PT6A-68B. This was proven on the PoC aircraft and used unmodified in the initial phase of PC-21 flight testing. The system restricts available power at the end of the runway to 900shp, steadily increasing to the full 1,600shp at 230kt, making the take-off and initial acceleration easier to manage and reducing the number of rudder changes.
"It gives straight line acceleration similar to the Hawk," says Smith. It also allows use of automatic rudder compensation, eliminating the need to use the rudder as thrust and giving "jet-like" handling. This system can be switched off to allow yaw-effects demonstration.
In September the power management system profile will be altered to provide 1,080shp on take-off up to 70kt, with full power available at 200kt, says Pilatus chief test pilot Andreas Ramseier, who shares testing duties with Tyndall.
John Senior, Pilatus vice-president research and development, says the extra power can be made available because the aircraft is "docile". Pilatus can now offer an aircraft with benign take-off and landing characteristics that can also undertake a low-level navigation exercise at 300kt, or a target run at 330kt. Ramseier says the PC-21 "handles like a light jet such as the Hawk, but take-off is like the PC-7Mk2".
Design improvements
As a result of the changes, diameter of the Hartzell five-blade scimitar-propeller will be increased by 50mm (2in) to 2.39m. The propeller was originally aluminium, but this reduced the engine time between overhauls, and limited aerobatic performance. The material was changed to aramid, which would have been cheaper than carbonfibre, but cannot be non-destructively tested, so propellers would have to be scrapped if overstressed, although they may still be serviceable. The unit is now carbonfibre with titanium erosion strips. "The [propeller] fatigue life is about the same as for the aircraft," says Smith.
The turboprop's 8g-capable wing has a modified NACA 6-series high-speed aerofoil section, a moderately swept leading edge and the spoilers tested on the PoC aircraft to provide a fighter-like roll rate. Maximum operating speed (VMO) has been cleared to 370kt/Mach 0.72 while maximum dive speed (VD) is cleared to 420kt/M0.8. Ceiling is set at 25,000ft, although PO1 has been flown to 36,000ft. During phase 2 flight tests, Pilatus fitted PO1 with winglets to evaluate the effect on performance. "Aerodynamics studies say they give benefits, so we had a look," says Smith.
Senior says flight testing has proven the PC-21 to be a "well-handling platform. We could start certification soon." Pilatus is, however, "focused on an operationally perfect platform" in terms of stick-force-per-g and lateral forces. Phase III flight testing will tune the aircraft - altering gearing ratios - to meet this desire, says Ramseier. He says lateral stability will be improved because Pilatus has kept the gearing "quite stiff to avoid pilot-induced oscillation", maintaining a safety margin during envelope expansion.
Smith adds that "getting the gearing right is making sure the aircraft is fit for its purpose. It is subjective and it takes time as it is fine tuning." The PC-21 has traditional flying controls, with primary controls being mechanically linked and secondary controls - such as spoilers and flaps - being hydraulically driven. UK-based company Claverham supplies the hydraulic system. It is similar to that in the PC-9 and PC-12, says Cervia.
Senior says solutions to the problems identified in the first year of testing are "all qualified as doable without significant changes". An engineering flight simulator has been used to study the trends and effects of the modifications before they are introduced to the aircraft.
The aircraft carries enough fuel for a 3h hi-lo-hi sortie. "It will do standard UK Royal Air Force sorties back-to-back," says Smith. The UK's Military Flying Training System contest is a prime focus of Pilatus's PC-21 marketing campaign. Refuelling is single point, and can be performed "hot" without shutting the engine down.
The two pilots sit on Martin Baker Mk16L zero-zero ejection seats in a pressurised cockpit and plugged into an anti-g system. In front of each pilot are three Barco 150 x 200mm (6 x 8 in) active matrix liquid crystal displays. The centre unit is the primary flight display, regarded as safety critical, while the outer multifunction displays (MFD) are less critical, allowing Pilatus to use rapid prototyping tools to develop software. The PFD is a smart display with its own processing. The MFDs are dumb, driven by the mission computer. Meggitt provides the standby instruments.
The cockpit is reminiscent of the Saab/BAE Gripen and as with most modern aircraft the PC-21 crew can select whatever display is desired on the MFDs, although the central PFD screen is for flight-critical information. The avionics and mission system are open-architecture. Aircraft PO1 recently received its CMC Flight Visions head-up display (HUD), and a HUD repeater is fitted in the rear cockpit. Night vision goggle-compatible lighting will be added in October.
Navigation upgrades
The navigation system has also been upgraded to an inertial measurement unit (IMU) from an attitude and heading reference system, used as Pilatus originally planned to install BAE Systems GPS-satellite/inertial navigation equipment.
Pilatus is developing synthetic radar for the aircraft, with the first version available less than a month after the company's board gave the system the green light. The system will allow the PC-21 to be used to teach mission systems management and for other advanced training roles, perhaps including operational conversion topics not possible with today's turboprops.
Nigel Wainwright, Pilatus new aircraft projects training systems consultant, says the company was able to develop the radar simulation quickly using the VAPS rapid avionics prototyping system. It allows a simulated radar picture to be displayed on one of the two MFDs in each PC-21 cockpit. Typically, another embedded simulation - the stores management system (SMS) - would be shown on the other MFD, says Wainwright.
Pilatus is also considering a datalink - probably based on the improved data modem - allowing two or three aircraft to be networked. It would be compatible with the European NATO RAIDS rangeless air combat manoeuvring instrumentation system and be installed internally, eliminating the need for a pod. If a customer requires a pod or other stores, the PC-21 has five hardpoints.
The synthetic radar display is based on the US APG-series of radars and Pilatus is in the process of integrating it with the HUD, allowing operation as in a frontline fighter. The instructor controls the synthetic-radar profile, using the split cockpit functionality. The number of bogey aircraft, plus their speed and altitude, can be modified.
A datalink would eliminate the virtual targets and allow teaching of dynamic manoeuvring to the system, says Wainwright. The SMS can be programmed so it can show, for instance, the warload for a Boeing F/A-18-class fighter.
The VAPS-developed software is sent to UK-based Datel for test and certification and then used for non-flight-critical functions, such as driving the MFDs, which means customer requirements can be quickly implemented. The single primary flight display in each cockpit is flight-critical and software writing procedures reflect this.
Wainwright heads a cockpit-working group and he says VAPS has permitted rapid improvements to be made to the presentations on the MFDs and HUD as the group defines the system. The only flight-critical software is that driving the PFD and the aim is not to alter it. Smith adds: "Design it once as it is expensive to re-certificate." This architecture separates flight-critical from mission-critical systems such as the MFDs.
The cockpit working group includes test pilots, human factors experts and relevant engineers as well as those like Wainwright with relevant operational experience. They drive cockpit development, which is then rapidly prototyped using VAPS. The group then evaluates the results, asking for iterations of the design when necessary. This process also allows rapid response to customer needs, says Wainwright.
General Dynamics UK is responsible for the mission computer. The system is on its third processor type as the open architecture allows frequent upgrades to remain state of the art. Remote interface units (RIU) will be added next, says Smith. This will reduce wiring complexity and weight while improving reliability.
Extra capacity
Cervia says RIUs will be fitted in the front cockpit, rear cockpit and avionics bays, meaning only 1553 databus wiring is needed to link the three. The computer is bigger than needed, but easy to write software for, he adds. It will also provide additional capacity for customer-driven additions to the system as there are five spare card slots in the box. It drives the MFDs and HUD repeater and receives GPS/IMU data.
The second prototype, PO2, will be fitted with a CMC CMA-3000Mk2 flight management system (FMS), says Ramseier. The pilot controls the mission system and displays with the hands-on-throttle-and-stick inceptors, the upfront control panel on the HUD mounting and the soft keys around the screens. The mission computer is not flight-critical as there is a separate engine instrumentation and crew-alerting system in the front cockpit.
A student will enter a PIN number, which will track his progress and help configure the aircraft for that student. The airborne activities will also be linked with the student's laptop, which will include computer-based training for the aircraft systems. The airborne sortie is recorded digitally rather than via video, so that display pages not used by the pilot in flight can be considered during debriefing.
A switch in the cockpit allows it to be selected for instructors or students. Instructors can also split the cockpits so their displays either mimic the students' or the instructors can use their screens to set up mission profiles and drive the embedded simulation system leaving the students unaware of the changes.
Senior says the open architecture system brought challenges, "but we coped, and we've been very successful". He adds, however, "one drawback is open architecture is too flexible, too easy to keep updating".
To improve turnaround times when maintenance is required, Pilatus has ensured good access to the line-replaceable units. For instance, the avionics bay behind the cockpit has an access door underneath the aircraft and on the starboard side. This allows a technician to stand upright in the bay and hand the faulty box through the aircraft's side to a colleague.
The bay is also served by the cockpit's Honeywell Normalair Garrett (NGL) air and vapour-cycle system. This, coupled with anti-vibration racks, pushes the mission computer's predicted mean time between failures to 4,500h, says Smith. Although the bay is not fully pressurised, there are narrower pressure and temperature differentials during a mission than an ambient bay would provide, sonegative effects are reduced. Conditioning the bay allows use of plastic, rather than military-specification metals, which reduces weight. Engine-mounted accessory gearboxes driving two 300A DC generators provide electrical power. Distribution is via dual Pilatus-developed buses, each capable of driving all essential systems. Batteries provide engine start and drive the mission system before engine start.
Pilatus expects many of its PC-21 deals to involve service provision - it will be contracted to provide a fixed number of aircraft or annual flying hours. Finite element modelling has been used to consider through-life costs, as well as a design tool and for virtual static and fatigue testing. PO1 is equipped with strain gauges to allow load measurement and ensure that predicted loads match reality. Two more test airframes will be built: SO1 for ultimate load trials and SO2 for fatigue testing.
Pilatus has applied for a patent for the PC-21 wing leading edge, which is designed to dissipate the energy of a birdstrike spanwise, preventing birds from penetrating deep into the wing structure, which reduces the need for costly and lengthy repairs.
Birdstrike requirements and a desire to have no forward canopy arch to impair the pilots' forward view meant the canopy was redesigned using tapered stretched acrylic and a slightly altered shape. The high-speed sledge test of the crew escape system is planned for next month at Martin Baker's Northern Ireland facility in the UK.
In another move to identify life-cycle costs early, Pilatus assembled the first wing by fitting all the internal systems, then removing them, adding the top skin and reassembling the system through the access panels to prove the maintenance concepts.
Aluminium structure
The PC-21 is predominately aluminium, with carbonfibre used "where there is an advantage". The material has not been used in the primary structure, but has been used in the wing/fuselage fairing, around the engine cowling and ahead of the cockpit where the ability to create three-dimensional composite structures is an advantage, says Cervia. A PC-21 wing can be built in one-fifth of the time it takes to build a PC-9 wing, says Smith. Pilatus has an annual production capacity for 40 PC-21s in place.
Computerised design tools also helped reduce the cost of prototype aircraft as digital mock-ups and dynamic models were used, allowing Pilatus to delay the start of the first aircraft production until later in the design evolution. PO1 parts manufacture started in April 2000 with assembly beginning in January the following year. "That makes it more [production] representative and therefore cheaper to build," says Smith, adding that PO1 was built on production tooling. PC-21 build time is expected to be half that of the PC-9.
Computational fluid dynamics (CFD) was coupled with windtunnel tests to develop aerodynamics. The CFD code was used in an engineering flight simulator so pilots could evaluate stalls and spins two years before metal was cut, says Smith. Data from the PoC aircraft was used to prove the engineering simulator and fine-tune it, he adds.
Pilatus plans the first flight of the second PC-21 towards the middle of 2004. The company decided last year to delay the first flight of PO2 as it felt it would gain little in terms of manufacturing knowledge by building the second aircraft early. Senior says: "We have kept the first aircraft flying constantly. We wouldn't gain a lot from the second aircraft."
The second aircraft will now be built close to or after final design freeze and will be "more or less a production aircraft", he adds. This removes potential certification issues with two non-production representative aircraft, says Senior. PO1 is essentially an aerodynamic test airframe, while PO2 is the systems test prototype. Delaying PO2 means the "impact on development costs is dramatic", adds Smith.
He says PO2 will have minor differences from the first aircraft, including some structural strengthening as "PO2 is to last longer than PO1". Ramseier adds that the second PC-21 will have an autopilot and FMS, not fitted in PO1. The autopilot will allow single-pilot IFR operation modern air traffic management regimes.
Ramseier says these aids will be useful when air forces deploy the aircraft any distance that requires operation in civil airways. Wainwright adds that flying training is not just about producing fast jet pilots, and that PC-21s will train pilots destined for multi-engined aircraft. "We have to be smart and reflect the wider requirement," he says, noting that modern transport and maritime patrol aircraft are fitted with HUDs, FMS, and glass cockpits. Smith says two customer evaluations have "substantially confirmed the state performance and its fitness for purpose" for Phase II, III, and some elements of Phase IV.
The company believes it can sell at least 300 aircraft over 20 years.
Source: Flight International
