Since the Pilatus PC-XII high-powered, single-engine turboprop had its first flight at Stans, Switzerland, on 31 May 1991, it has undergone a series of radical modifications. It now offers an almost unique blend of short-field performance and high-climb and cruise capability, combined with sturdy handling.
Looking over the aircraft with Pilatus test-flight director Hans Galli, on a crisp March morning at Stans airfield, 15km (9 miles) southeast of Lucerne, Switzerland, it was easy to see where the main external modifications have been made. The wing span has been increased and winglets added to improve stability margins at low speed with full flap, while large ventral strakes and an extended dorsal fin ensure directional stability. Originally, there was a single full-width elevator, but now separate elevator panels remove airflow interaction from the rudder at large angles and new swept tail-plane tips have been added to counter elevator snatch.
A top-hinged cargo door is balanced by air-struts, or is electrically driven. It keys down into the side frame and two hooks engage heavy bolts in the sill. Locking is clearly shown by four large green discs in the door face. The downward-opening passenger door has simple, well-engineered, drop-down integral steps. There are cockpit indications for the correct operation of both cargo and passenger-door locks.
THE COCKPIT
The cockpit is well thought out. There are circuit breakers on each sidewall, which are stepped, angled and colour coded for clear identification. A ledge of switches below each instrument panel is set at a good angle and a digital stop-watch, outboard of this panel, is adequate for timing engine starts (but inconvenient for in-flight timing). Large flight-case bins are to hand outboard of the pilots' seats.
A small overhead panel is dedicated to colour-coded electric switches, with a wide red gang-bar for "all-off" in an emergency landing. A grab-handle on the centre windscreen pillar is of more help to a pilot settling into his seat than is one over the screen or on a side-pillar. Access is made easier by dashboard mounting of the control column.
In all small aircraft, eye height, knee clearance with full control-wheel angle and leg extension at full rudder demand tight seat adjustment, but it is difficult in the PC-XII. The new high-strength seat is comfortable, but the adjustment-levers under it are tricky: they are to be changed. Rudder-pedal adjustment is adequate.
AVIONICS
The avionics are good for the size of aircraft. The Bendix 40 electronic flight-instrument system (EFIS) runs from a single attitude- and heading-reference system, with an option for the co-pilot EFIS. A multi-function display (MFD) option on test aircraft HB-FOE - with electronic checklist, and EFIS reversionary mode - has replaced the radar display. A "joy-stick" moves a cursor on the screen. EFIS and MFD controllers are neatly sloped ahead of the engine-control levers.
Twin avionics busbars branch out from the 300kvA main generator and the secondary 100kvA generator, which cover essential services. A central advisory and warning system (CAWS) includes a red warning of left and right air/ground switch disagreement and an engine-oil-level detector.
The engine instrument system (EIS) has vertically arranged liquid-crystal displays. A new horizontal array will show standby digital data below each gauge. The EIS alerts are well designed, with delays for momentary exceedances. Gauges flash at 40 characters a second (cps) for a caution and 80cps for a warning.
STARTING UP
One switch starts the Pratt & Whitney Canada PT6A-67B engine - the condition lever is set to ground idle at 13% Ng (gas generator speed) and the starter reverts to generator mode at 46%Ng, or cuts out after 30s. In sub-zero temperatures, high idle is temporarily set until Ng reaches 50%.
The autopilot self-test after the start is important, and a valuable guard, as test engagement in flight is inhibited.
Our flight ramp weight, with the fuel tanks three-quarters full at 900kg, was just over 3,300kg - 800kg below maximum take-off weight. Wing-fuel-balance pumps start automatically. For redundancy, there are jet pumps, electric booster pumps and engine-driven pump.
Ground-idle power urged the PC-XII forward. Braking at intervals contained the speed. A strong reverse-thrust detent, overcome by triggers under the power lever, at first encouraged a return into too much forward thrust. There is no other guard, but in-flight safety testing shows no slipstream interaction with the high-set tail-plane and elevators.
The low-idle figures at 6°C and 1,500ft (460m) altitude, were 64.5% Ng, 495°C inter-turbine temperature (ITT), with 95kg/h fuel flow. Propeller speed (Np) at 1,020RPM was above the restricted range of 350-950RPM. High idle was 78.5% Ng, 540°C ITT and 1,485RPM Np.
Intercom buttons, on each control-wheel inner arm, are hard to use when your hand is on the power lever, so Pilatus intends to fit an open-microphone intercom. A guide to all control-wheel switches is printed on the wheel boss, where there is room for a chart clip.
An easy dab on an inside brake takes the nose-wheel smoothly from its 12° mechanical steering limit to caster up to 65° on tight corners. The main gear turning circle is 9.5m. Pilots should beware that the wingtips arc 5.5m wide of the nose.
TAKE-OFF
Stans airfield lies between mountains rising to 1,500m and so Galli proposed familiarisation by manoeuvring within a limited airspace.
Take-off flap was 15°, indicated on a simple clear gauge, conveniently located just above the airspeed indicator. A triple trim indicator is small, but clear, with thick white needles on a black background. Take-off settings are shown by thick green lines. A green lozenge marks the aft pitch limit, and the rudder-trim bias is to the right.
The power lever was at arm's stretch at an indicated 44.4lb/in2 (3.06bar) maximum take-off torque (PT6 torque is measured in terms of engine-oil pressure, shown in lb/in2) - a limiter restricts it to 46lb/in2. Power is flat-rated at 895kW (1,200hp) to 51°C at sea level and at 745kW to ISA (International Standard Atmosphere) +30°C in climb, which is the performance ideal for short fields in hilly terrain. Ground roll at maximum weight under ISA, sea-level conditions is just 310m, with a take-off distance over a 50ft obstacle of 555m.
Directional control felt smooth. Rotating at 80kt (150km/h), the nose was raised to 10° and the gear selected up. Its dedicated hydraulic system howled for about 8s. The best gradient airspeed is 95kt, when a 2,000ft/min (10m/s) rate of climb is reminder enough to reduce power. The PC-XII has the highest power-to-weight ratio among similar single-engine aircraft, says Pilatus. Flap is retracted at 100kt.
The rudder-trim control is a neat rocker-switch in the power-lever knob. Even better, when the yaw-damper is engaged, it automatically trims the aircraft directionally - reducing workload on power changes. Pitch and lateral trim, using a separate arming trigger as well as a "coolie-hat" switch on the control wheel, need getting used to, however.
In a climb to 1,500ft, before turning downwind, the aircraft cleared near terrain. A 25° banked turn at 110kt, ran on into an S-turn, to bring the PC-XII parallel to the runway, downwind at 2km.
FAMILIARISATION
I selected flap and gear late and exposed my "heavy"-aircraft background by keeping 100kt and circuit altitude until just before turning to clear the hills by the town. With an aircraft of the size of the PC-XII, this is too closed in and I left myself in a classic "too-high, too-fast" situation.
The flight-idle drag of the 2.67m-diameter four-bladed Hartzell propeller on the PC-XII is not as high as on many aircraft and I had to dive towards touchdown, only allowing the speed to bleed off in the last few seconds. About 15lb/in2 torque gives 100kt with 15° flap. I was able to be more precise with my hand on the base of the power lever, but Galli said that this was because I was sitting slightly forward of the best position.
The view over the long cowling is good and, at an 80kt approach speed, the cowling's upper surface points nearly along the line of flight, giving a good view of the runway. This flat attitude was clear when, again touching down too fast, I nearly three-pointed on nose and main wheels.
Two more "touch and goes" produced an acceptable final "product" - descending early and close by the hill, at 85-90kt with 30° flap and taking 40° flap early. Flare from 75kt, raising the nose to the horizon, gave a prompt, yet smooth touchdown on the main-wheel trailing-arm suspension.
A flight to Grenchen airfield, 80km to the west, left the 2,100m-high Mount Pilatus, from which the aircraft manufacturer takes its name, behind us.
APPROACH AND LANDING
At 3,100kg on final approach, the fast/slow scale on the attitude indicator showed speed at just under 80kt in the bumpy weather - the stalling speed is only 57kt.
Galli noticed that Grenchen was busy and said that only a few landings might be made. I made an easy touchdown, less than 100m along the 710m runway, with a short murmur from the stick-shaker. As I retarded the power lever, Galli heaved it back for full reverse and stood on the brakes. We stopped in under 150m ground roll - an impressive demonstration.
In a 30°-flap take-off, I rotated at under 75kt into a towering 90kt climb, to cross the runway end at 250ft. For a touch-and-go, I flared from 70kt, to a target touchdown. It pays to be precise on the final approach and landing in all low-speed aircraft - even 5kt is a significant error, when stalling speed is less than 60kt and approach speed is 80-75kt.
Full power and a hint of rearward control lifted the aircraft back into the air. There is ample climb performance with 40° landing flap. Slam accelerations can be made with impunity. If the pneumatic element in the fuel controller fails, a separate "manual" power lever bypasses it: this is just as responsive, using steady movement.
HANDLING AND STABILITY
Maximum operating indicated airspeed (Vmo) is 236kt at low altitudes, but 170-180kt gave us a better ride in medium turbulence at 3,500ft. The wing is lightly loaded and responsive to vertical gusts, but pitch, roll and yaw responses are low, so the attitude remains steady.
A stick-slap or a push-pull on the controls produces little reaction, as on a big aircraft: rudder-only turns need a firm foot load and roll initiation is hesitant. Dutch roll is an equal balance of rolling and yawing, which damps to half amplitude in three cycles. Reversals from 30° bank left to right take about 7s.
This "stiff" directional and lateral stability - "airliner-type" handling - is preferable to the liveliness of some smaller aircraft, which are manoeuvrable, but fidgety in turbulence and unforgiving to inattention. A sideslip at 100kt with 15° flap confirmed the high stability - with full rudder and a high foot-load 8° of bank was reached.
Cruise-climb is at 160kt IAS, reducing to 115kt by 30,000ft. Maximum-weight rate of climb at 5,000ft is 1,500ft/min and 30,000ft can be reached at over 500ft/min within the half-hour.
In the Swiss Alps, however, one thinks of gradient climb - at 120kt, reducing to 110kt, with an initial nose-up attitude of 7./5°. Up to 10,000ft, torque remained at 37lb/in2. It then decreased and, by 18,000ft ITT, became limiting at 760° with 102% Ng. Fuel flow eased from 225kg/h at 20,000ft to 155kg/h on leveling at 30,000ft in ISA - 5°C conditions.
The icing indicator is a simple blade on the port leading edge. De-icing was switched on for cloud tops at 15,000ft. Unusually, the dual-rate boots can be used on take-off. Should ice persist, a landing with 15° flap is predicated, but threshold speed is still below 100kt. Ice build-up was simulated in test by blocks on the stabiliser leading edge: approaches to a go-around from 200ft were tried in this configuration.
Engine run-down, is guarded against by governing Ng, above a minimum of 50% at all times. The engine igniters also come on automatically when the ITT drops below 500°C. This is noticeable in descent and on the ground, with the condition lever at ground idle.
The autopilot was engaged, and it smoothed the ride in comparison to manual handling, although the attitude is so stable that I could write my notes, even while hand flying. A blue light on the CAWS signals that autopilot pitch-trim is operating. A red light warns of a runaway, which is accompanied by automatic reversion to alternate trim operation and autopilot disconnect.
Autopilot modes can be re-programmed, most easily by manual attitude-adjustment while holding a button on the inboard control-wheel arm, or by an increase/decrease switch at the left of the mode-selection panel. Vertical-speed hold operates separately through the altitude-acquire controller.
The mode panel, set back under the glare-shield, is not prominent, but the modes selected and armed are shown small, but clearly, on the attitude display - vertical modes to the left, lateral to the right.
The engine purred like a sewing machine in level cruise at 169kt IAS (260kt TAS). With 26.3lb/in2 torque, fuel flow was 165kg/h. The propeller speed is governed to 1,700RPM in flight. A lower propeller speed has been considered for the cruise, although most PT6A-driven propellers turn faster.
In maximum-speed cruise (265kt at 30,000ft) six passengers and bags can be carried 1,850km with reserves. At 3,200kg mid-weight, in long-range cruise at 205kt and at 25,000ft, fuel consumption is just over 100kg/h giving a specific range of 3.4km/kg in ISA conditions.
Tight level turns, at 60° of bank and steady altitude, to loads over 2g, made the fast/slow speed scale show a reducing speed margin. As a stick-pusher identifies the stall point, there was no attempt to explore natural g-buffet, but the wing obviously has good qualities in manoeuvre.
EMERGENCY DESCENT
An emergency descent is made, simply by selecting gear down and closing the throttle. Maximum descent speed is 176kt, so little, if any, speed reduction is needed in high-altitude cruise. The maximum gear-down speed is the lower of Vmo or Mmo (236kt or Mach 0.48). The drag from the gear gives a spectacular descent rate - 6,000ft/min at first, with the nose pointing 15° down, reaching over 7,000ft/min at 236kt, pointing nearly 20° down. (These odd limit speeds, result from the perverse, but universal, practice of adopting calibrated airspeeds as round numbers.)
I kept my eye on the standard three-pointer altimeter, which is not really suited to the high-altitude capability - a drum/pointer altimeter would be a much better choice for such a high-performance aircraft.
With the propeller feathered, the gliding gradient is only 1/17 at 110kt and high weight (at 3,000kg, best speed was a mere 95kt); at an average descent rate of 800ft/min, 100km can be covered in a glide, even from 20,000ft. This gives ample time for rectification or decision on a suitable landing airfield. The test aircraft was fitted with a global-positioning system, and Galli called up the display of the five nearest airfields in the database, saying that "dead-stick" landings had been made during the test programme.
The reliability of any PT6A makes gliding an unlikely prospect, but it is comforting to know that it is not difficult to handle. Normal rapid propeller feathering does require electrical supply from the single battery, but tests have shown that the blades will slowly feather without even that. Maximum declared altitude for relighting with starter assistance is 20,000ft.
Emergency fuel and bleed-air shut-off flaps, flush with the rear face of the centre pedestal, are easy to pull up. An engine-fire warning is fitted, but not fire extinguishing. Fuel lines from tanks to pumps run outside the pressure shell.
Gear control is electrical, so loss of the main generator or engine entails manual gear-extension, which is by free-fall, backed by an emergency hand-pump for lock-down where needed. The pump arm, which pulls neatly out from the rear of the centre console, should only be needed after a cold soak has thickened the hydraulic oil.
The gear relies on hydraulic pressure to remain retracted, with no up-locks. This pressure is backed by a nitrogen charged accumulator, unusually dedicated to retraction, rather than assisting extension.
Both windscreens are heated in part, to give a clear area, but only modest electrical loads. During a rapid descent, a frame of frost soon formed. Crystals also grew on the unprotected side windows, but cockpit-ventilation modifications will reduce this. The pilot has a small direct-vision panel, well placed in a forward position. Twin exhaust stacks take the hot gases away from the fuselage and windscreen.
Narrow tinted sunshades, which pivot down from above each windscreen, can be only angled to give limited protection from the Sun when it is directly ahead.
Warning of exceeding flap speed limits (163kt or 130kt), is given by both a red light in the flap gauge and a tone. Flap and power changes have little effect on pitch trim, and change with speed is modest. The electric trim is adequate, but slow (often because of the runaway trim certification requirement). It can be interrupted rapidly by a guarded switch, close to the pilot's right side. The flap-interrupt switch is alongside it, so any unusual attitude trend can be quickly dealt with by operating both together - very neat.
The alternative stabiliser drive is that used by the autopilot. The alternative flap switch is useable after an interrupt caused by main unit failure. The flap geometry - initial run-out and then deflection - ensures a 12kt improvement in stall speed with 15° flap, and an equal amount more with 40° full flap. Roll rate was 30% higher - 10kt above stick-shaker speed.
A stick-pusher is fitted, as the wing could roll or pitch greatly in an aggressive stall at high power in the landing configuration. A small stall-breaker strip, at the leading-edge root of the right wing, keeps the maximum allowed wing drop to 30°. It counters asymmetric slipstream-lift effect at low speed and high power.
Stalls were made at between 10,000ft and 8,000ft. Rosemount miniature angle-of-attack vanes, on probes near each wingtip, seem exposed and fragile, but work smoothly and accurately. Apart from driving the fast/slow indicator, either operates the stall warning (shaker and tone) and both trigger the stick-pusher.
Stick-pusher operation is short acting and without snatch. The electric ram's 23kg push can be resisted, if need be, without too much effort, and the pitch-down and release are well balanced for both speed recovery and minimum height loss. One expects a small height loss with low airspeeds, but recovery is still remarkable.
At 3,000kg, stick-shaker speeds ranged from 85kt clean to 62kt with full flap. Pusher operating speeds ranged from 74kt to 52kt. The pitch angle was typically reduced by nearly 20° in three stalls, but height loss was only 150-200ft. At 4,100kg, the pusher would operate at 59kt with full flap and with any future weight of 4,500kg at 63kt.
Tests have already been made at take-off weights of up to 4,500kg. Structurally, this is easily accommodated and it is being considered as an option. The present power would probably not have to be changed, but, if a customer wanted ultra-short-field performance, the engine thermodynamic capability is 1,195kW.
EASY AND FLEXIBLE
The landing at Stans confirmed an impression of easy behaviour, competent handling, impressive performance and flexibility. Speed can easily be kept at 170kt, to keep in a flow of faster aircraft - albeit with anticipation of slowing by nearly 100kt.
Addition of a radio altimeter will enable the full capability of autopilot-approach coupling to be used to major airports, completing the PC-XII's capability to handle anything from grass field to major runway operation. The trailing-arm gear, good clearance of the sturdy aluminium propeller from the ground and intake protection are well suited to grass-field operation.
The PC-XII is a 4t aeroplane with light-aircraft performance. It is a 150kg/h aircraft in economical cruise, so full tanks will give some 8h endurance, if required.
Pilatus' decision to fit a standard, large cargo/baggage-loading door offers flexibility of ownership, appealing to a wider range of second owners. The current price is around $2 million - about 10% more in the executive configuration flown, with all normal options. Orders and options to date total 44.
Source: Flight International
