The single-engined Pilatus PC-12 offers plenty of cabin space, is economical to acquire and maintain, and is versatile to operate
While the Swiss are renowned for their versatile knives and finely crafted watches, Stans, Switzerland-based Pilatus has made its name building rugged single-engined aircraft. From a background in utility and training aircraft, the company launched into the business-aviation segment in 1991 with the PC-12 - the aviation embodiment of those renowned Swiss qualities of versatility and craftsmanship.
The single-turboprop PC-12 has been successful, and Pilatus will deliver its 500th example before the year-end. Flight International was invited to evaluate the aircraft by the company's US arm, Pilatus Business Aircraft.
On paper, the PC-12 may be a single-engined aircraft, but on the ramp it is a big aircraft. Dwarfing the upcoming crop of light jets, the PC-12 has a wingspan of 16.2m (53.25ft) and sports a cabin volume (excluding cockpit) of 9.34m2 (330ft2). At 5.16m, the cabin is longer than that of its most direct competitor, the twin-turboprop Beechcraft King Air B200.
All the more remarkable, then, that this spacious aircraft is propelled by a single engine. Not just any engine, though - a Pratt & Whitney Canada PT6A-67B, a member of one of the most reliable engine families ever produced. Thermodynamically rated at 1,200kW (1,605shp), on the PC-12 the -67B is flat-rated to 900kW for 5min (up to 51°C at sea level) and to 750kW for continuous operation. These power levels are significantly lower than the engine's design maximum, and should contribute to enhanced reliability.
According to Tom Aniello, vice-president and chief marketing officer, the aircraft most frequently evaluated by PC-12 purchasers are the single-turboprop EADS Socata TBM700 and Piper Meridian, the twin-turboprop King Air B200 and the Cessna Citation CJ1 light jet. Of this diverse group, only the B200 offers a cabin anywhere near as large as the PC-12's.
The large cabin does come at a price, however, as the PC-12's high-speed cruise is only slightly faster than the smaller Meridian's, and some 20kt (37km/h) slower than the other turboprops. While the PC-12 is more than 100kt slower than the CJ1, its cabin is 75% larger. On shorter-duration flights, actual time savings may be minimal, and the PC-12's longer range may even allow it to arrive at its destination sooner if the CJ1 needs an en-route fuelling stop, says Pilatus. The PC-12 is also more frugal with fuel than either the CJ1 or the B200.
Although trip fuel burn is a significant part of operating costs, it does not tell the whole story. Compared with a twin-turboprop, the PC-12 has half the engines and propellers to maintain. With engine and propeller overhaul reserve expenses approaching $100/h, a single engine can significantly reduce operating costs. The other significant economic benefit of a single compared with a twin is acquisition cost. Comparably equipped, a PC-12 costs $1.7 million less than a B200.
The perception that a twin-engined aircraft is safer than a single is perhaps the one reason why the PC-12 has not enjoyed even greater success. Multi-engine designs were driven by several factors, overall power required and engine reliability among them. The PT6 family offers engines with up to 1,500kW - more than enough for a single engine to power a fairly large aircraft.
Having two engines does offer redundancy, because a single-engine failure may not dictate a forced landing. But whereas commercial airliners are operated so that a flight can be recovered safely after the loss of a single engine, some twin-engined aircraft operate at gross weights that may not guarantee a climb capability on one engine. Even if there is sufficient thrust to continue flight, an engine failure in most aircraft leads to an asymmetric thrust condition to which the pilot must react immediately to recover the aircraft. Having amassed nearly one million flight hours, the PC-12's accident rate, both fatal and non-fatal, is statistically equivalent to twin turboprops, says Pilatus.
According to Pratt & Whitney Canada data, the PT6 family has demonstrated exceptional reliability. Up to October 2003, 31,606 delivered engines have flown more than 252 million hours with an in-flight shutdown rate of one per 333,333 flight hours. Like most business aircraft, the typical PC-12 flies about 300h/year. Statistically, at least, there is only a one in 1,000 chance each year that a PC-12 operator will lose an engine.
Should the engine fail, the PC-12 becomes a glider with a 16 to one glide ratio. At 30,000ft (9,150m) and its 4,500kg (9,920lb) maximum take-off weight (MTOW), the aircraft can glide for 32min and cover 140km (77nm). This leisurely descent rate assumes sufficient oxygen for the crew and passengers. The PC-12 is certificated for operations on dirt and grass strips, increasing the odds of finding a suitable landing field. Should a prepared field be unavailable, the PC-12 has a stall speed in landing configuration of only 64kt at MTOW. The cockpit seats, rated at 26g, and the cabin seats, at 19g, act as impact absorbers in a hard forced landing.
Although the PC-12 at first appears relatively small, as your mind scales it to match typical single-engined aircraft, the cabin entry door with its four integral steps brings home the fact that this is a big aircraft, larger than a Cessna 208B Caravan. During the pre-flight inspection, chief pilot Randolph Schneider pointed out some of the PC-12's safety features. Because one of the main causes of engine failure in the PT6 family is loss of oil, sometimes from an improperly secured oil cap, engine oil level is checked via a sight gauge, allowing the oil cap to be left in place. If the oil requires servicing, the cap's design makes incorrect installation almost impossible.
The PC-12 is certified for flight in known icing, and each of the four blades on the constant-speed propeller has an electric de-ice boot near its root. The entire wing leading-edge is de-iced by sequentially operated pneumatic boots. Unlike most aircraft where the boots protrude slightly, the PC-12's are flush to the leading edge to smooth airflow over the wing.
Mounted on the right wingtip, the standard weather-radar pod provides a slight right yaw moment in flight, countering the leftward moment generated by the propeller. Unlike most propeller-driven aircraft that have a canted vertical stabiliser to counteract "p-factor"-induced yawing, the PC-12's is mounted true to the aircraft's longitudinal axis.
A large cargo door on the left side of the aft fuselage is 1.35m wide by 1.32m high and hinged along its upper edge, with the door rotating upward to allow clear access to the large cabin compartment.
The cockpit is quite large, with upright seating. Field of view out of the four cockpit windows is good, allowing a direct view of the entire wing leading-edge. A simple overhead panel contains controls for the electrical system and lighting. Circuit breakers, colour-coded by bus, are arranged on sidewall panels. The forward instrument panel is arranged logically, with flight director and autopilot controls mounted beneath the glareshield. Primary flight instruments comprise electronic attitude director and horizontal situation indicators, with conventional round-dial airspeed, altimeter and vertical velocity indicator.
The centre radio stack houses dual Garmin 530 nav/comm/GPS transceivers and an LCD engine instrument system. The centre pedestal had a Honeywell Bendix/King KMD-850 multifunction display forward of the single power control lever. Located on the forward panel just to the left of the pedestal, the landing gear lever is an easy reach from the left seat. Overall the cockpit is fully functional, yet seems dated. With glass cockpits available in piston singles, Pilatus would do well to offer an updated cockpit.
The aircraft battery - a second is optional - was used to start the engine. As gas-generator speed (Ng) stabilised above 20%, the condition lever was brought to the ground idle position, supplying fuel to the engine. Light off was immediate, with inter-turbine temperature (ITT) peaking at 650°C before the engine stabilised at 65% Ng. Post-start checks were accomplished easily, the main item being a check of the stall shaker/pusher system.
Idle power was sufficient to leave the chocks and the rudder pedal-controlled nosewheel steering was excellent. Wheel brakes were actuated by depressing the tops of the rudder pedals. Although they were effective, even with my heels on the floor I often inadvertently tapped them. I got the hang of the brakes by the end of the flight, but a different design would make ground operations even easier.
Before taking Jefferson County's runway 29R for departure, I set the electrically operated flaps to 15° and power to flight idle. I used the power lever-mounted switch to set the rudder trim to 6° nose right. Once cleared, I released the wheel brakes and advanced the power lever to the forward stop, which gave 40.5lb/in2 of torque (indicated in lb/in2 in Pilatus aircraft). Acceleration was brisk and at 80kt indicated airspeed, light yoke back-pressure was required to rotate to a 5° nose-high attitude where the wing's incidence angle, about 3° up relative to the fuselage, allowed for lift-off. The initial climb out was at 100kt, with the landing gear extended. Once there was no usable runway in front of us, landing gear was retracted, followed shortly by the flaps.
The first part of the climb was hand flown, as we followed vectors along our route of flight. Roll control forces were heavy at all speeds, and increased as speed increased. The PC-12's yaw damper incorporates an auto-trim feature that automatically centres the ball as power levels change. The system did a good job of keeping the aircraft in trim, but was unable to keep up with rapid power changes.
For the latter part of the climb the autopilot was engaged, while a constant ITT of 725°C was set manually during the 140kt climb. From brake release at the 5,680ft-high airport to levelling off at FL280 (28,000ft) took 21min. Total fuel burn was 76kg. Once level, the autopilot did an admirable job of maintaining altitude and following flight-management system lateral navigation commands.
With 720°C ITT set (23.4lb/in2 torque), the 3,748kg aircraft accelerated to and held 162kt. A fuel flow of 150kg/h gave a true airspeed of 255kt on an ISA +10°C day. Retarding the power lever to an ITT of 635°C (19.1lb/in2 torque) slowed the aircraft to a long-range cruise speed of 152kt. True airspeed dropped to 240kt and fuel flow decreased to 127kg/h. With our two pilots and one passenger, Pilatus projects an instrument flight rules (IFR) range of 3,520km at FL300 on a standard day. Four more passengers decreases IFR range to 3,300km.
With headsets on, cockpit ambient noise level was fairly low. With headsets off it was possible to talk cross-cockpit, but at elevated voice levels. Although the turbine engine is smooth, it is in front of the cockpit and the optional noise-cancelling headsets would be a wise purchase.
At FL280 the PC-12's pressurisation system, with a maximum pressure differential of 0.4 bar (5.75lb/in2), maintained a cabin altitude of 9,000ft. The cabin was in a six-seat executive configuration, the four forward seats in a club arrangement and two aft seats forward-facing. Noise in the passenger cabin was low, but louder than in a comparable jet. Pilatus claims the PC-12's cabin noise level is 4dB lower than the King Air's. Cabin noise was reduced somewhat when the divider doors for the forward lavatory, just aft of the cockpit, were extended. Given the PC-12's long range, the lavatory is a popular option, and is standard in the executive configuration chosen by 95% of buyers.
With the high-altitude portion of the flight complete, Schneider and I used the radar to carefully pick our descent route for approaches at Greeley, Colorado. During the first part of the descent, the aircraft was accelerated to its maximum Mach number (Mmo) of 0.48M (198kt indicated) at FL255. Aircraft response to sharp control inputs in all three axes was well damped. Should a maximum-rate descent be required, the recommended procedure is to slow below 177kt and lower the landing gear.
Once the gear is extended, the aircraft can be accelerated to Vmo/Mmo. Descent rates approaching 9,000ft/min were achieved during flight testing, says Schneider.
Following ATC vectors, I engaged the autopilot in heading mode and armed it to capture the localiser and glideslope to runway 34 at Greeley. The autopilot accurately tracked the ILS while landing gear and flaps were extended for landing.
A final approach speed of 85kt for the 3,700kg aircraft was held. As the winds were gusting to 16kt out of the south-east, I disengaged the autopilot at 500ft and circled to the east of the field to land on runway 16. I flew a 4° glidepath for the visual final approach. Airspeed was easy to control on final, and I slowly retarded the power to idle about 15ft above the runway. As the power was reduced, with the yaw damper off for landing, left rudder was required to maintain co-ordinated flight. The flare was started at 10ft, with constantly increasing back-pressure until touchdown.
The trailing-link main landing gear absorbed the residual sink rate, giving an extremely soft touchdown. Wheel brakes alone were used to slow the aircraft for runway turn-off and taxi to runway 09 for a simulated short-field take-off.
Flaps were set to 30° for the short take-off, and the power lever advanced to the forward stop. Once the torque stabilised at 4.63Nm. I released the wheel brakes. At 68kt and just 275m down the runway, the PC-12 was rotated to an 8° take-off attitude. With a 10kt headwind, the aircraft leapt off the runway after a ground roll of only 305m. At sea level on a standard day at the 4,500kg MTOW, Pilatus publishes a ground roll of 450m, with 700m required to clear a 15m obstacle.
Staying in the visual pattern, we next set up for a short-field landing. With flaps set to their 40° maximum, a final approach speed of 75kt was maintained. Again a 4° glidepath was held to an idle-power touchdown with minimal flare. Once again the trailing-link main gear did yeoman service, yielding a soft touchdown.
Reverse thrust and moderate wheel braking stopped the aircraft after a 230m ground roll. During the roll, the left wheel locked momentarily, forcing me to release the brakes. Short-field landing performance on hard-surface runways could be improved by adding an anti-skid system.
After a normal take-off from Greeley's runway 09, the aircraft was cleaned up and climbed to 7,500ft for a stick shaker/pusher test. With idle power set, the gear was extended and flaps lowered to 40°. Slowing the aircraft in a slight climb at 1kt/s, the dual stick shakers activated at 61kt. No aerodynamic indications preceded the stall, and the aircraft was rock steady as it slowed to 56kt, where the stick pusher fired. Releasing back-pressure and advancing the power lever returned it to normal flight.
Gear and flaps were then retracted and the aircraft accelerated to 100kt. Schneider then pulled the power lever to near idle to simulate engine failure.
Immediately I turned the aircraft towards the field and established a 110kt glide. In the event of a dead battery, the gear would have to be pumped down and a no-flap landing made, but in this case electrical power was available.
On reaching the field, the landing gear was lowered and flaps extended to 40°. My aimpoint was just short of the threshold of runway 09, and 90kt was held on final. Once in ground effect, at about 50ft, I slowed the aircraft for a 70kt touchdown. Wheel brakes alone brought it to a stop less than 460m from the threshold.
The last portion of the 2h flight in the PC-12 was the short hop from Greeley back to Jefferson County, where a visual approach and full-stop landing was accomplished. Shutdown and post-flight checks were easily accomplished.
The PC-12 is a unique aircraft that offers exceptional payload/range capability. Competing turboprop aircraft may be slightly faster, but come up short in available cabin volume. The PC-12 also offers good short-field capability, yet can climb above most icing conditions and cruise comfortably at FL300.
Slower, but cheaper
Although the PC-12's cruise speed is slower than a jet's, time is money and jet operators will pay a hefty premium for each minute saved.
The PC-12 can perhaps be best summed up in a word - one. One engine to reduce acquisition and operating costs. One pilot to allow owner-operators to stretch their wings. One versatile aircraft with a large cabin. Available to anyone willing to contemplate that one engine may be as good as two.
MICHAEL GERZANICS / DENVER, COLORADO
Source: Flight International