Extreme Versatility – PAC750 XL
The Kiwis have coined a new acronym for their latest and arguably best-yet “airborne ute.” The PAC750 XL is now designated an “XSTOL” aircraft, standing for “EXTREMELY short takeoff & landing.” And it’s well described!
It all started well over 50 years ago when NZ started exporting its Fletcher FU24 superphosphate spreaders into Oz, along with emigrant ag pilots to fly them.
The Fletcher kicked off in 1954 with a minuscule 225 hp Continental O-470, which hadn’t presented much of a problem in New Zealand’s cold, dense atmosphere with its steep airstrips and strong winds. Australian ag pilots however believed it relied on the curvature of the earth to get airborne in our hot and high conditions, and that only an unusually muscular young pilot could comfortably spend all day dodging gum trees in such an underpowered craft.
“It’d be a bloody good aircraft with about another 500 horsepower,” commented one expat Kiwi pilot.
So the factory kept adding horsepower, starting with 250 and 300 hp Lycomings and working up to the horizontal eight cylinder 400 hp IO-720, which gave the massively strong airframe enough grunt to demolish almost anything it couldn’t circumnavigate.
In one classic case a Fletcher returned from an early morning mosquito swamp spraying session with no visible damage, although it seemed to be flying slower than expected. Legend has it that the operator didn’t know it had hit anything until he got a bill from the power company for the replacement of a kilometre or so of cable and several poles. The legend goes even further, alleging the complaint originated from a farmer resident who’d been reaching out for his slice of toast when the electric toaster disappeared backwards through the wall!
After-market modifiers tried some huge American V-8 truck engines, all of which were amply accommodated by the original Fletcher’s robustly over-built structure, but the real breakthrough only came with the venture into turbine power. So the aircraft finally got the extra 500 hp it needed.
Hamilton (NZ) based Pacific Aerospace Corporation (now officially PAC) had already identified a need to improve payload by also adding more structural strength and cubic capacity. First came a factory-built 500 hp PT6-powered model in 1967, then two Garret-powered versions in 1968. After-market conversions then began to appear, powered by PT6 and Walther 500 hp engines.
At this point PAC completed a major redesign, transforming the basic aeroplane into the Cresco, still a dedicated agricultural aircraft primarily used for super-spreading, and initially powered by a Lycoming LTP-101 turboprop, but after the first nine units had been delivered the manufacturer finally switched to Pratt & Whitney’s PT6A-34, developing 750 (560kW) shaft horsepower.
Happily the Cresco makeover had also included a major aerodynamic tidy-up. Ailerons were re-worked to include shielded horn balances at the outboard ends, with 15 vortex generators ahead of each aileron. Rudder and elevators also had shielded-horn balances, and the handling enhancements were noted and widely appreciated by the ag pilots whose gripes had driven the improvements.
Bulging biceps were no longer a prerequisite thanks to the aerodynamic re-work, and the tough, simple PT6 was perfectly matched to the enlarged weight lifting capacity.
Next came the larger cabin area – once the hopper was removed – attracted the interest of skydiving operators, some of whom began using the Cresco in that role. In the US and Europe, parachuting operators were also interested but wanted an aircraft capable of climbing to 13,000 feet in 15 minutes carrying 18 jumpers. PAC promptly responded by transforming the Cresco into the PAC750XL, a passenger/utility aeroplane with an even roomier fuselage that could accommodate 18 thrill seekers, and a big enough jump door to get them overboard in just a few seconds.
So although the PAC 750XL may look a lot like its grandparent FU24, the genre has worked its way up the capability ladder through all those re-births, carving out notable niches in new markets that are far removed from its agricultural origins.
The first nine aircraft were sold into the skydive market, until then dominated by elderly Piper Chieftains, DH Twin Otters, Shorts Skyvans, Nomads, and (in Europe) Pilatus Porters.
The new model had also gained all the attributes it needed to launch itself as a utility transport anywhere there are high-altitude, short and rough airfields. Papua New Guinea, Africa and South America were seen as potential markets and proved to be just that.
Other qualities such operators seek include task versatility, reliability in a high utilisation environment, and ease of maintenance – all of which are assured by the PAC 750 XL’s design philosophies.
An optional cargo pod was added to the expanded cabin, changes to the flap system improved STOL performance even further, as did added tailplane and fin area and other aerodynamic refinements, along with factory-added skydive steps, toe-holds and grab rails, and a jumping door that can be closed by the pilot for high-speed descents. All that came with a wide CofG range to make the aircraft tolerant of CG shifts despite low para-dropping speeds, single-point refuelling for fast turnarounds, and docile low-speed handling. The cabin is now over 1.37m wide and more than 4m long from the back of the pilot seat to the rear bulkhead. The skydive version also has a mike jack near the rear door for jump controllers, external & internal grab rails, and external lighting for night jumping. All this adds up to the world’s first purpose-built para-jumping aeroplane.
Interstingly the 750XL has since proven to be an even more capable super-spreader than the Cresco, and PAC now offers it in that configuration, with some already out there spreading. But its built-in qualities have also made it a popular utility aeroplane, a prospect that PAC had planned for right from the start. The cabin provides 6.8 cubic metres of cargo space with a further two in the cargo pod, which has three large drop-down doors on the left side. Variously sized cargo doors are offered, the larger (parachuting) door measuring 127cm high by 119cm wide, with a sill height of 112 cm. The aircraft comes with eight quick-install seat attachment points in the floor, along with cargo attachment points and a “skydive partition” behind the cockpit.
The 13m-span constant section wing is a single-piece structure, with the spar running right through the forward cabin just behind the crew seats. A 3.5° dihedral on the outboard sections enhances stability and also provides ample wingtip clearance on narrow unmade strips. Single-slotted electric flaps, spanning 7m, now offer two positions: 20° and 40°. The three-bladed constant-speeding Hartzell prop is fully feathering and reversible, and static propeller clearance is a generous 380-460mm.
Safety cables secure the nosewheel and strut against dropping out if a torque-link fractures. The outer engine cowl is of Kevlar with glass fibre-resin forced plastic ducting, and engine foreign-object damage protection is comprhensive. In the lower cowl, the filtered engine-air intake faces aft into an “inertial separator” plenum chamber, which is fed from ports on both sides. Four fuel tanks, fore and aft near each wing root, all feed into respective fore-and-aft sump tanks, and automatic selective pumps are used to empty the aft tanks first for automatic CG optimisation. Total fuel is 861 litres with 841 useaable, providing for three hours of productive flying with reserves. Five drain points protect the fuel system from contamination risk, and single-point refuelling is standard. There’s also a ground power socket on the left hand rear fuselage.
Veteran demo pilot and instructor Roger Cruickshank from the nearby Hamilton Aero Club shows me around the very professionally laid-out cockpit.
There’s more than ample panel space for all the instrumentation and spare space if needed. Two upwards-opening crew doors, great for ventilation, are supported by gas struts and closed by operating a very firm over-centre lock. There’s also a 4-step removable ladder that can be fixed to the skydive step for entry through the main cabin door, and this is the easiest way for crew to board unless a portable step-up is available. Our demo aircraft was fully IFR with dual flight instrumentation including autopilot, radar altimeter, transponder, dual GPS VHF Nav/Comms and weather radar. An engine, electrical and fuel systems annunciator panel sits right above the primary flight panel. A blue light indicates beta-range operation; the generator warning is a ‘low voltage’ light which illuminates at about 25v. The fuel-filter warning-light operates if differential upstream and downstream pressure is about to initiate a filter bypass. A separate fuel-pressure warning operates below about 0.l4bar (2 1b/in2). Warning lights are also provided via a magnetic-chip detector in the forward part of the gearbox, and by a differential air-pressure sensor on the main intake filter, which is designed to detect clogging by dust, fertiliser or indigestable (for a PT6) birds. The standard avionics panel has the minimum avionics equipment required for flight, but this demo aircraft has considerable additional optional equipment.
Stall warning is provided by a conventional leading edge lift detector providing an audible warning squawk about 5 -10 knots above stall speed.
Power management indicators (all digital) indicate torque, Np (Turbine RPM), inter-turbine temp (ITT), compressor RPM (Ng), Oil temp/pressure and fuel pressure/flow. Indicators are provided for (manual) aileron and rudder trim, electric elevator trim with a manual backup, and flap position. The aircraft comes in a range of optional IFR and VFR specs and in any of three optional configurations – skydive, passenger/cargo, and agricultural. Certification is under the standards set by the FAA FAR 23 utility category.
Having flown the Cresco many years earlier I was curious to see how its handling characteristics could possibly have been improved. The improvements started with a considerably widened fuselage that provides ample lateral space for two crew. The tri-gear configuration along with pilot seating well forward of the leading edge give excellent forward and downward static and ground manoeuvring visibility with the surface visible about 8m directly ahead. However there is full forward visibility a few degrees either side of the long narrow nose. Dual controls with dust-proof electric trim and press-to-talk radio switches are on either control-stick grip, with a manual trim-override within easy reach. Intercom is of high quality when using headsets.
Pilot seat positions are adjustable for rudder pedal reach, and the checklist is organised logically in relation to the check items’ location.
Ambient temperature at Hamilton was 22°C (ISA+7). Engine handling is not FADEC-controlled and starting is manual, with fuel in at 15-18% N1 (RPM) and ITT peaking at about 620°. The engine becomes self-sustaining at about 52% N1 and settles to ground idle at 500°C.
The idling prop gradually changes pitch to pick up a little thrust, and taxying on flat ground can mostly be done without advancing beyond idle power. Nosewheel steering is via the pedals up to 20° to the left and right of neutral, and beyond that point the aircraft can be turned by moderately braking the inside wheel. Braking throughout the entire operation was even, grab-free and extremely smooth. The power lever is rigged for a wide beta-range movement arc to avoid inadvertent reverse operation, but reverse is also available for emergency stopping and tight manoeuvring.
First we fly two circuits – one off Hamilton’s main runway and one off the adjacent grass strip which is equally popular with local pilots. All controls are well-balanced and effective through the entire speed range, the aircraft is stable but highly responsive and a pleasure to fly, with ego-friendly landings made easy by those factors plus the long-stroke oleo legs.
Cruickshank suggests a “maximum performance takeoff,” which at this modest weight was going to be fun. Following his instructions and holding the aircraft on the brakes, I bring up the torque until they can no longer stop forward motion, then release them and almost simultaneously rotate on his command. This immediately achieves a very positive climb, initially at about 35 KIAS, but Cruickshank urges a lowered nose for a less exciting airspeed. By that time we already have about 300 feet of sky under our wings, but even at best-rate speed (91 KIAS) we’re still climbing at around 3,000 fpm.
At a sensible altitude we further explore the 750’s general handling. Stick and rudder pedal forces remain light and positive through all working manoeuvres, including 70 KIAS with flaps 20. We then fly a simulated parachute dropping run at 80 KIAS. At this speed, fuel flow was a modest 140 lb/hr, suggesting all-day endurance in roles such as survey and surveillance operations. Once the aircraft is stabilised in any regime with appropriate flap, power and trim, control remains positive and easy even at 60 KIAS. The aircraft is quite trim-sensitive to all power, flap position and airspeed changes, but compenspatng with electric trim soon becomes automatic once you know what to expect from a particular configuration change. The landing flare in particular requires considerable back stick, and a little up-trim bias helps rotation in the flare. It’s necessary to forget about holding the nose up while braking, as this reduces the steering effectiveness, but the nosewheel is rugged, forward speed is low, and reverse is always available if needed – which it wasn’t except for a demo of its effectiveness. When rolling backwards, stopping is achieved by moving the prop control to forward thrust, because braking at that point may otherwise cause the nosewheel to leave the ground and an aft-loaded aircraft’s tail protection skid to contact the surface.
It’s all been amazing performance and great fun so far but to explore the aircraft’s real-world capabilities we return to Hamilton to load some ballast for a few more realistic operations.
Back to the real world
At first glimpse the nearby farm airstrip looks worryingly short, but we are easily able, after a 60 KIAS approach, to stop in less than half its length with a little reverse thrust. “That’s plenty for a takeoff without backtracking,” says Cruickshank. “Flaps and trim set for takeoff.” And amazingly it was, with positively eager lift-off at around 60KIAS . At MTOW and max continuous thrust this time we could see that the 750 would have easily reached 12,000’ in under 10 minutes from brakes release (from an airfield elevation of 2,500’.)
All of this confirms PAC’s claims of takeoff & landing in less than 244m at MTOW, even when you’re hot and high, and a load carrying capability of over 1.8 tonnes, or more than the aircraft’s empty weight.
We stayed below a cloud layer at 8,000’ to see how a relatively inexperienced pilot might handle a paradrop in moderate turbulence. First, Cruickshank suggested some exploration of stall behaviour. There is no wing washout, and it is not missed in the stall, which is almost a non-event, full recovery being achieved by simply easing back on the elevators.
A deliberate stall in a steep climbing turn demonstrated the extreme degree of mishandling that would be needed to get into that situation, but the 750 controls simply became mushy without any tendency to drop a wing, and full recovery was also achieved by relaxing back pressure and adding power, of which there is always plenty in reserve.
In standard conditions an innocuous full-flap, power-off stall occurred at about 37 KIAS at minimum weight, later in the day increasing to 58 KIAS at gross weight. Flapless, those figures increase by about 10 knots. Stall characteristics remain mild in all configurations; the aircraft is fully controllable down to and below the stall with both aileron and rudder, producing a very gentle nose drop and no unpleasant surprises, even as I further tested post-stall controllability with rudder and aileron. A moderate power application flew us straight out of the stall without wing-drop.
Trying my luck as an ag pilot, I found the 750 just as easily handled on lower-level tasks, with precise and positive rudder response for skidding around obstacles, and crisply prompt elevator control for flying over them.
Skydiving skills
There hadn’t been any skydivers around to test the 750 on during our NZ flight, or maybe they were hiding, so we spoke to Mark Hogan, senior pilot at Skydive Nagambie (Vic) for an operator’s viewpoint. Hogan gives the aircraft top marks for economy and utility:
“We normally go to 14,000’, and with a standard load of 12 people we probably get up there in about 12 minutes, so we’d normally do a full turnaround in about 16 minutes, or with 17 people it can take 17 minutes. Our strips’s 900m long but from the threshold to the refuel point’s 700’ and you an always pull up by the fuel point. I’ve never bothered to pace out a takeoff run, but it’s a lot shorther than a [Cessna] 182.
“We can launch an entire load in one go basically. We can fit up to 17 people in, plus pilot, and depending on the way they stack up we can launch that so they all leave the aircraft pretty much together. We can start with eight to ten people on the door alone, with a couple up front to keep in trim, and it’s still very stable, and even then we’re still nowhere near MTOW. We can pretty much take the same load as a Shorts Skyvan and with only one turbine we’re about one-third of the fuel burn. So they’re pretty economical, and if they’re doing a formation jump everybody can be out of the plane in something like five seconds.
“For tandem jumping with sport jumpers we’ll do a two mile long run-in and they leave a gap between stages. For descent we use 12 PSI, 82% on the prop, and 160 KIAS.”
Bush operations
All the capabilities we’ve observed so far make this aircraft a highly useful passenger/cargo carrier, particularly on high-frequency short-haul and short-field operations that are typified by Papua New Guinea. At MTOW with ambient temperatures at ISA + 5 and at MTOW of 3,405 kg at an airfielf height of about 2,000’ AMSL, the 750XL demonstrates it can meet or beat all flight manual figures. Those figures include a 315m takeoff ground roll, 110m stopping distance, climb to 12,000’ in 12 minutes from brakes release, and 168 kt cruise at 10,000’ at ISA.
PNG operators see the 750XL as a BN2 Islander replacement. Goroka-based 7th Day Adventist Aviation gave the aircraft full marks after six months of operation and is now operating its second unit on a variety of mixed passenger and cargo type operations, configured in the utility role with eight removable seats and an under-belly cargo pod. One of the primary crops in the region is coffee which comes in 50 kg bags, and the operator gets close to a Twin Otter payload on shorter legs – up to about 1200 kg with approximately three hours fuel, and being single-engined they’re obviously cheaper to run. The aircraft airborne with a similar ground roll to a turbo C206, but carrying between two and three times the 206 payload so it’s a very effective machine. It’s also a very rugged aeroplane, which the demanding short, steep and rough strips require.
A typical strip is about 400 metres long on a 14° slope at a little over 5,000 ft. Average stage lengths are low, and it’s not uncommon for a pilot to finish a day’s work with a dozen more landings on that kind of strip, so until now it’s always been Islander and turbo C206 country:
The manufacturer now has the aircraft approved by the major oil companies operating in Africa as the only single engine aircraft they permit on their contracts (including helicopters).
11 PAC750 XLs in skydive configuration are now in service in Europe and 13 in the USA . In Africa there’s one skydiver and 11 in utility configuration. In the Asia region there are 7 utility aircraft, and about a dozen in Oceania – nine on survey work and four skydiving. The majority of forward-ordered aircraft are in the utility configuration with a few orders also from skydiving operators in Europe and the USA, and two more for aerial survey work. Current price in skydive configuration is about US$1.65m.
Other military, tourism and survey applications – and possible floatplane operations – suggest further unexplored markets and there seems to be no reason this remarkable performer shouldn’t find even more eager customers in markets where honest performance, reliability and cost-effectiveness are still demanded.
