Cessna Citation X business jet is the fastest commercial transport in production, yet is designed to be flown from small airfields.
Cessna aircraft is responsible for an unusually wide range of types. Its current Citation family consists of six business twinjets - designed to meet the differing needs and resources of any organisation requiring a business jet. The six Citations range from the entry-level CitationJet, a four seater certificated for single pilot operation, to the Citation X - an eight seater capable of speeds up to Mach 0.92 and operating levels up to 51,000ft (15,500m).
At the top of the range, and the subject of this Flight International flight test, is the Citation X. This is the largest, fastest and most sophisticated of the Citation family and was conceived about ten years ago when Cessna identified the important parameters for future business jets as better field performance, higher cruise speeds and better despatch reliability, combined with lower capital costs and operating expenses. An aircraft derived from an existing Ciatation, the company decided, was not viable because it could not meet all of these parameters.
A different design
The Citation X is dramatically different from the rest of the Cessna range in its aerodynamic design. Both the wing and tailplane have pronounced sweep (37 degrees and 40 degrees respectively); the wing is unusually clean, having no fences or vortex generators, and it does not have any winglets. To reduce drag at high cruise speeds, the Citation X's rear fuselage has been heavily "area-ruled", leading to a pronounced waisting to compensate for the large engine nacelles.
The fuselage is mounted on top of the wing (to avoid an intrusive step in the cabin floor) and the aerodynamic blending between wing and fuselage is achieved through a large, carefully profiled, ventral fairing. This fairing serves also to hold some fuel and to provide stowage room behind the wing. Cessna worked with Boeing computational fluid-dynamics experts in Seattle to refine the airframe design, and then tested the validity of the final configuration in Rockwell's trisonic windtunnel in Los Angeles.
Ingenious design
The overall result is an aircraft which looks sleek, elegant and svelte in the air. On the ground, however, it becomes more an icon for the ingenuity of computer-aided design and the versatility of modern production techniques. The almost-constantly varying section and incidence of the wing can be seen clearly, as can the scalloping of the rear fuselage and the oddly flat-sided cross-section of the ventral fairing. Up close, it is a practical, purposeful, technological manifestation, in the same mould as a modern Formula One racing car.
Flight International had the opportunity to find out what all this amounted to in the air when a Citation X was staging through the UK's Luton Airport during a European sales demonstration tour. A thorough walk-round with demonstration pilot Mark Snider served as a technical briefing on the aircraft.
The windscreens meet the bird-impact resistance required for certification when cold, and electrical heating is used for anti-icing and demisting only, and not to warm the windscreens to increase their birdstrike resistance. Cessna claims that the curved shape of the windscreens causes rainwater to be swept away by the boundary layer airflow, which can be augmented, if required, by an electrically driven blower fan forcing ambient air through ducts at the base of the windscreens.
Some of the avionics hardware is housed between the elegantly pointed nosecone and the cockpit, and the access doors on either side of the nose are reinforced to protect these avionics from bird-impact damage. The central refuelling panel is on the right-hand side of the ventral fairing, while sequencing is automatic when filling the wing and ventral tanks. Inflight fuel is drawn first from the tanks then moved into the wing from the centre tank. If neccesary, fuel can be cross-fed in flight by the crew. Overwing refuelling points are provided.
Anti-icing uses engine bleed-air, which is ducted to the wing and tailplane leading edges and to the engine-intake cowls. The wing has leading edge slats which open automatically at 12 degree angle-of-attack (AoA), and these remain anti-iced, even if deployed, via a flexible pipe between the fixed leading edge and the slats. Access to the duplicated hydraulic-system reservoirs, and for servicing the lavatory and changing the batteries, is commendably easy via hinged panels. All these external panels have locks and latches - the inner surfaces of the latches are finished in Day-Glo orange so that any improperly closed hatch is immediately obvious. The AlliedSignal GCTP36-150 auxiliary power unit (APU) is housed in the tailcone.
The cabin door, which is on the left hand side of the fuselage behind the cockpit, opens outwards and downwards, and has built in steps and a handrail. It is counterbalanced and light in operation. Cabin and cockpit furnishings are completed to customer requirements: this demonstrator has been attractively fitted out with light-coloured carpets, trim and leather seat coverings. There is an abundance of polished wood veneer and gold fittings.
The cabin, which is just over 8.5m long, has eight seats and a small, but well thought out, lavatory at the rear with hanging space for clothes and sliding doors to isolate it from the cabin. A lowered aisle gives headroom of almost 1.73m, and the cabin is just under 1.7m wide. Equipment which can be specified by customers includes galleys, microwave ovens, entertainment centres and complete airborne offices. There is 2.54m3 (90ft3) of heated and pressurised baggage space in the rear fuselage.
Sweeping displays
Attention to detail is also evident in the small, but crisply designed, cockpit. Five cathode ray tube (CRT) displays, part of the Honeywell Primus 2000 avionics suite, sweep across the width of the instrument panel, with supplementary instruments and control panels above and beneath. There is no overhead panel. Systems controllers are ranged across the bottom of the instrument panel, while fuse panels and the pressurisation controller are consigned to the cockpit sidewalls beside each pilot's thigh. This layout results in the control panels being neatly and symmetrically contained in the restricted space available, but limits access to certain control panels by both pilots. For instance, the captain can neither reach nor readily see the pressurisation control panel on the co-pilot's side, nor can the co-pilot easily reach or see the lights and fuel control panels which are in front of the captain's left knee. The centre console houses the two control-display units for the flight management system (FMS), the two radio-management units (RMUs) and the weather-radar controller.
Access to the seats is not easy because of the limited headroom and the absence of any space between the seats and the centre console - but doubtless a technique requiring less contortion could be developed with practice. The seats are leather-covered and fitted with five-point harnesses. They can be adjusted easily fore and aft and for height, while the whole seatframe tilts when the seatback is reclined, giving a tilt also to the seat cushion. There are slim armrests which could be stowed or deployed, but which could not be readily adjusted for height. I instinctively did not like the seats and would have preferred independent back recline and the means of lowering the armrests. There is no room to stow flight bags in the cockpit, but there is good stowage behind the seats for in-flight documents such as the standard Jeppesen volumes. There is a cupboard space immediately behind the cockpit.
Good views
The field of view from the cockpit is good: the windscreen pillars are reasonably slim and not too intrusive, and each pilot can just see the wingtip on his or her side of the aircraft by craning to look aft through the side window. Each rear side window can be opened to pass documents to and fro on the ramp.
The Primus 2000 dual digital autopilot, flight director, and engine0indicating and crew-alerting-system (EICAS) are integrated with the dual FMS, which provides lateral and vertical flight management and a worldwide navigation database. Attitude and heading information comes from dual attitude-and heading-reference systems and there are two micro air-data computers. Each pilot has two 200x180mm CRTs. One is for the primary flight display (PFD), which displays attitude, heading, air-data, flight direction and navigation data. The other is for the multi-function display (MFD), which shows lateral and vertical navigation data and can be used for checklist displays.
The centre CRT is the EICAS display, giving engine indications and systems information. In the event of a screen failure, the display can be moved to another CRT.
The APU was started using aircraft battery power - although there is an external electrical power connection point for use when required. The EICAS can also be powered from the aircraft batteries so that systems status, indications and crew alert functions are live before any of the starting checklist is tackled. Once electrical power and bleed air became available from the APU, all displays were selected on, and the flight-plan details were entered in the FMS before the engines were started. As the two Allison AE3007C turbofan engines use APU bleed air for starting, there was no electrical power interrupt for the EFIS.
The AE3007Cs have dual-channel full-authority digital engine control (FADEC), while the start control panel is in front of the captain's right knee. The master switch is selected "on" and the appropriate start switch depressed. Fuel is introduced by moving the power level from a fuel-off position behind the idle gate after releasing a small hook-shaped latch on the back of the power lever. Thereafter, the FADEC takes control of the engine start and the interstage turbine-temperature (ITT) is indicated by vertical ribbon displays on the EICAS. Should a malfunction occur, the FADEC would abort the engine start.
The undercarriage is electrically controlled and hydraulically operated, and has twin wheels on each of the trailing-link main legs, which have hydraulically operated anti-skid carbon brakes, and a twin-wheeled nose gear which is hydraulically steered. The nosewheel tyres have chined sidewalls to deflect runway water away from the engine intakes. The parking brake is controlled by a small T handle on the aft end of the centre console.
A little power was needed to start the aircraft rolling, after which speed could be regulated using touches of reverse thrust. The Dee Howard target-type thrust reversers are controlled by "roll-over" levers on the front of the power levers. The front-deflector position is indicated by lighted legends beneath the glareshield. Normally only left-engine reverse thrust is used during taxiing, to avoid APU exhaust entering the cabin-conditioning air.
Nosewheel steering is controlled by a flat, solid, wheel with finger grips around its rim, on the captain's cockpit sidewall. The steering is positive with a strong self-centring component, and the aircraft is straightforward to taxi.
At the runway holding point, pre-takeoff checks were completed using the paper checklist. The flap was set for take-off at 5 degrees and a full control check carried out by reference to the systems display on the EICAS CRT. The EICAS screen is divided horizontally into two segments, the upper half displaying fan speed, ITT, oil temperature and pressure, and fuel contents and flow. The lower half is divided again, into three vertical segments, showing tailplane incidence and flap position on the left. Crew-alert messages in the centre, and systems information on the right. Although the EICAS displays a large amount of information, it is easy to read and scan. In fact, the concentration of so much information facilitates a last moment confidence check of the aircraft's health and configuration, with a glance before take-off.
Take off conditions
Luton Airport is about 500ft above mean sea-level and runway 26 was in use. The surface wind was from 220 degrees at 14kt (26km/h) and the temperature was 12 degrees C. There was no adverse weather, but extensive middle-altitude broken cloud and visibility was good. Our Citation X, N715CX, had 2,600kg of fuel in the wing tanks and the centre tank was empty. The brakes-off weight was 12,600kg: the maximum permitted all-up weight is 16,200kg. The relevant reference speeds, clearly displayed on the PFD as an aide memoire, were: V1 and VR 114kt; V2 127kt; with approach and threshold speeds of 118kt and 113kt should an immediate return to landing have become necessary.
The recommended technique for take-off was conventional. At brakes-off, the power levers were moved smoothly forward in their quadrant through two detents labelled CRU (cruise) and CLB (climb) into the third detent, TO, for take-off power. These detents can be readily felt through the power levers, but the position of the levers is helpfully shown on the EICAS using the same legends.
The AE3007C's high-bypass ratio, two spool, axial-flow turbofans, rated at 28.7kN (6,440lb) thrust, spooled up rapidly to take-off power. The aircraft accelerated quickly - a little into-wind roll control was applied and it was easy to keep straight using nosewheel steering initially and then rudder. The Citation X rotated easily and unstuck cleanly, and gear and flap were retracted by Snider with no noticeable trim change. Acceleration to 250kt was rapid and the aircraft settled comfortably into the climb; initial climb-rate was about 3,00ft/min (15m/s). Climb power was selected by moving the power levers back one detent, confirmed by the EICAS display of CLB. Throughout, the FADEC produced smooth, synchronised responses to the power changes demanded. Air-traffic control provided radar direction for the climb to 41,000ft. The APU was kept running during the climb to about 10,000ft. This is a Cessna recommendation so as to have an extra source of bleed air available should both the engines flame out.
During the climb, it was possible to look at the Citation X's handling qualities. Much of the flight-control system is conventional, in that the primary control-surfaces are hydraulically powered with redundancy provided by dual independent hydraulic systems and manual reversion if all hydraulic power is lost. Roll control is via the ailerons and two roll spoilers for each side. Movement of the outboard spoilers stars after about 3 degrees of aileron deflection and after about 6 degrees for the inboard spoiler. There are also six hydraulically operated airbrake panels (three per side) on the wing and the electrically operated flaps are composed of three segments per side, which move simultaneously.
Unconventionally, the CitationX has dual rudders - a large, main, lower panel and a second smaller upper panel - both having different hydraulic power sources. Longitudinal trim is achieved by the electrically operated variable-incidence tailplane, which is controlled by the pilots through split switches on each control wheel and by a Mach trimmer.
If an aileron or elevator becomes jammed control runs can be "split" by the pilots using a small control on the centre console. Control would then be exercised by the pilot on the side of the aircraft with the unjammed control-surface. The controls can be reconnected in flight should the jammed surface become free.
Control forces
The Citation X has pleasantly harmonised controls in all three axes and the control forces required were light while the system feel was pleasant. When applying roll control, a distinct notch could be felt as spoiler deployment was initiated by the hydraulic "mixer box". Stick force with g was comfortable and the aircraft easy to trim - although I found the electric pitch-trim a little slow in operation. Lateral trim is via split rocker-switches towards the rear of the centre console, and rudder trim via a large knob behind the lateral trim switches. Trim is electrically controlled in all three axes.
From an entirely arbitrary snap-shot look, taken at 250kt in the climb, stability appeared positive laterally and longitudinally, while rudder deflection was met with dead-beat damping. The Citation X has dual yaw-damper servos linked to the lower rudder to provide dutch-roll damping and turn co-ordination. The aircraft cannot be flown without the yaw dampers in operation.
At 41,000ft, the buffet boundaries were pleasantly generous, allowing 40 degrees banked turns at M0.87 without onset of either Mach or stall buffet. A climb towards 51,000ft was then started. I continued to fly the aircraft manually - although the autopilot could have done the job much better - to judge the handling as the maximum operating level of 51,000ft was approached. By 46,000ft, the climb rate had reduced to about 150ft/min (0.76m/s) at M0.8. By 50,000ft, the handling had become noticeably less crisp and in fairly gentle sustained turns at M0.75, light pre-stall buffet could be provoked. As is to be expected, the Citation X would have been easier to handle below its service ceiling. The highest cruise level chart published in the manual is for 47,000ft.
Economical performer
In conditions similar to our test (weight about 1,300kg and temperature ISA+10 degrees C), the Citation X could be expected to cruise at M0.8 using an impressively meagre 625kg fuel per hour. The recommended techniques for greater range are to reduce thrust to maintain a constant indicated airspeed as aircraft weight reduces or, for best range, to reduce thrust so that airspeed reduces with decreasing aircraft weight.
On the day of the test, we briefly established the aircraft in a cruise at 38,000ft. It accelerated rapidly, just failing to reach M0.92 (probably because of the outside air-temperature of ISA +7 degrees) but settled at M0.91 and, when established in the cruise, was consuming just under 700kg an hour at about M0.76.
A descent was initiated to return to Luton. With the power levers closed, airbrakes out at M0.83, the descent rate was about 4,000ft/min. Use of the airbrakes at speeds between M0.75 and ).8 produced barely discernable pitch changes and only the slightest trace of burble; the airbrakes are selected by a convenient lever on the centre console.
A particularly impressive aspect of the Citation X's handling was revealed with the selection of flap near the limiting speeds: 5 degrees was selected just below the limiting speed of 250kt, 15 degrees at 210kt and full 35 degree flap at 180kt, each selection provoking only the slightest trim change. The small out-of-trim force was easy to hold and easy to trim out. The Citation X does not have automatic trim compensation.
Behaviour approaching and in the stall was also impressive. Roll control remained crisp and responsive down to the stall. The Citation X has a stick-shaker but, unusually for a T-tailed aeroplane, no stick-pusher. Typically, at about 5-10 kt before the onset of stick-shaker, the slats deployed automatically. At 5 degrees flap, power off, stick-shake occurred at 109kt and the stall (defined by natural buffet) at 105kt. Holding the aircraft in the stall resulted in a high rate or descent, but no nose drop. Similarly with full flap and gear down, the speeds were about 94kt for the stall warning and 88kt for the stall, this time at a slightly higher angle of attack. Recovery from the stalls was straightforward using the standard technique of pushing the nose down and putting power on.
A coupled approach was flown to the instrument landing system for runway 26 at Luton. During this recovery, an imbalance of about 180kg between wing tanks was clearly shown on the EICAS fuel panel. While Snider could not explain why the imbalance had occurred with the Citation X's automatic fuel system, manual cross-feeding corrected the situation. There were also, during the descent and recovery, two red crew-alerts for hydraulic-system overtemperatures. These were fairly obviously caused by a detector fault as the hydraulic system kept in use. The avionics provide the ability to record airborne faults for analysis on the ground to help with rectification. I hope that the two minor systems faults which I encountered in a flight of less than 2h were unlucky rather than commonplace.
Quiet throughout
The Citation X flew the coupled approach well, all changes to heading and altitude being smoothly executed and accurately maintained. Sound levels were pleasantly low throughout and there were no intrusive aerodynamic, engine or system noises. During the approach, the aircraft passed through a light shower. The water was blown away across the windscreens (in a way that looked as if the external surfaces might have been treated with a rain repellant) and it remained possible to see clearly throughout. The blower was not used.
The Citation X remained pleasant and conventional to control longitudinally in the flare and into ground effect. There was a crosswind component of about 10kt from the left for both the full-stop landings I made and the recommended technique was to crab the aircraft and kick it striaght in the flare, hopefully just before touch down (a wing-down technique was not recommended because of the slight risk of wingtip contact with the runway in severe conditions). In both instances, however, yawing the aircraft just before touchdown led to the into-wind wing rising rapidly - possibly an effect of the marked sweep. This was readily countered with roll control, but lateral control remains crisp down to the stall and it was possible to provoke a lateral oscillation through over-controlling. Snider remained remarkably sanguine during my touchdowns, but I was fully occupied with putting the aircraft on to the ground and had to delegate the selection of ground spoilers and reverse thrust to him; the X does not have automatic ground spoiler deployment.
The Citation X is a pleasure to fly, has good handling qualities and safe characteristics at low speeds down to the stall. Familiarity with its roll sensitivity in crosswind landings would doubtless eliminate any tendency to overcontrol. The Citation X is the epitome of how to design and build an aeroplane which is highly competent in its operational role, without significant compromise, yet at a price which makes it keenly competitive in the market. Cessna's chosen field in which to make the Citation X a leader among business jets was speed. The aircraft has the capability to cruise at M0.92 and to achieve that sort of speed has been designed with drag reduction as a paramount goal. When range is an operational requirement, cruise speeds will have to be reduced; cruising at M0.82, typical visual flight rules ranges with 45 min reserve are about 5,000km (2,700nm) with 1,180kg of payload or about 6,100km with 180kg of payload.
Source: Flight International
