When Flight International last flew the Citation X in 2002, Boeing was still wrestling with the Sonic Cruiser concept, an effort to field a nearly supersonic transport aircraft. In the intervening 12 years, much has happened. The Sonic Cruiser was shelved in favour of the successful 787.
The Citation X, which held the record for the fastest civil aircraft from its certification in 1996 with a maximum operating speed of Mach 0.92, was displaced by the ultra-long-range, large cabin G650 with a top speed of M0.925.
With the 2010 launch of the X+, Cessna set out to recapture the high speed crown. While it is doubtful the G650 and Citation X are direct competitors, Cessna inched ahead of it when the X+ was certificated in July with a top speed of M0.935. While increased speed alone might be adequate justification for the fielding of an improved aircraft, the X+ brings much more to the table than that. Propulsion efficiencies and standard winglets offer real world performance enhancements. While a state-of-the-art flightdeck suite is bound to further please X+ pilots, an enlarged cabin will coddle those fortunate enough to ride in its cabin.
Craig Hacker Photography
New fan and winglets
The X+ mounts two Rolls-Royce AE3007C2 engines, a derivative of the AE3007C. According to R-Rs, the C2 features a more efficient higher flow fan with 39in (99cm) compound swept blades. The bigger fan yields a 1.4% improvement in thrust specific fuel consumption, meaning less fuel and fewer green house gases. The C2 also puts out more thrust, with increases ranging from 4% at take-off to 9% during climb. At cruise, where the X+ spends the vast majority of its time, R-R advertises a 7% increase. Changes were also made to the full authority digital engine control (FADEC) to incorporate an autothrottle (A/T) and improve diagnostics.
Winglet Technology offers elliptical winglet retrofit supplemental type certification kits for the Citation X, an option also available directly from Cessna. For the X+, they are now standard. The 48in high winglets increase the wing’s area while reducing drag. With the exception of structural modifications needed to mount the winglets, the X+’s wing is identical to the non-wingleted X’s, retaining its aggressive 37º sweep back angle. Wing span remains the same, but the effective wing area is increased by 1.49m2 (16ft2) to 50.45m2. They do, however, add 67kg (148lb)to the X+’s weight.
Interior improvements
In addition to the above enhancements, Cessna has also improved its cabin. The X+ retains the same fuselage tube as its predecessor, but a 15in plug has been inserted immediately aft of the cabin entry door. As with the X, the X+’s cabin features a dual club arrangement with an aft lavatory. The primary beneficiaries of this increased space are the forward club seats, where more legroom has been provided. The room behind all cabin seats is also increased, giving better recline capability. Individual seats are also improved which, according to Cessna, are more comfortable. A belted lavatory increases standard seating to eight.
As of late, cabin management systems (CMS) have come to the forefront in efforts to further enhance the passenger experience. Gulfstream has gone so far as to develop its own “Gulfstream cabin management system”. Cessna has elected the use of a fibre optic based system developed by Heads Up Technologies. Bespoke fixed touch-screen panels are available at each seat, giving control of cabin temperature, lighting, window shades and entertainment. Wi-fi comes standard and an iPhone app is under development.
The Garmin G5000's three large displays can present untold amounts of information
Craig Hacker Photography
Garmin G5000 flightdeck
For the X+, Cessna has fielded a totally new flightdeck. A new centre pedestal is flanked by new control yokes and columns. In a change from Cessna practice, there is a small overhead panel that controls lighting. While these are noteworthy, it is the new avionics suite that shines. Gone are the five Honeywell Primus 20cm x 18cm cathode ray tubes, replaced by three 356cm (diagonal) Garmin wide format displays. There are four Garmin GTC570 high definition control and display units (CDUs), one outboard on each pilot’s panel, for respective primary flight display (PFD), and two forward of the pedestal for the centre multi-function display (MFD). The flight guidance panel (FGP), to control the auto pilot (AP) and flight director, is mounted on the centre of the glare shield. A single gauge electronic standby flight display is mounted between the FGP and MFD.
In-flight evaluation
For my preview flight, I was accompanied by Textron senior demonstration pilot Alan Pitcher. I followed him as he conducted the pre-flight walk around inspection. Pitcher pointed out the X+’s flight primary control surfaces, which with the exception of the upper rudder, are non-reversible and hydraulically actuated. Like the Boeing 727 I had previously flown, the X+ has an upper and lower rudder. The larger lower rudder is hydraulically powered, while the upper rudder is electrically actuated. While the lower rudder has dual independent hydraulic power control units and a standby hydraulic power source, for redundancy purposes Cessna installed the upper rudder. Both rudders have independent dual channel yaw dampers, a needed feature given the natural Dutch roll tendency of this highly swept wing aircraft.
Overall the inspection was straight forward, with only a few access panels needing opening. Before we entered the aircraft I glanced at the swept wing with its large elliptical winglet. As I had noted during my prior flight, I was again reminded of how impressive the wing’s design was. The upper surface is smooth with no protuberances, done to promote laminar flow. Only a small stall strip blemished the highly polished wing’s leading edge. The wing looked like it was built for speed, and once airborne I would see for myself how the Citation X+ handled M0.935.
Touchscreen interface
Once seated comfortably in the left seat, Pitcher guided me through the pre-flight initialisation of the Garmin G5000 avionics suite. I was somewhat familiar with the G5000, having conducted two preview flights with it; one in a Bombardier Learjet 40XR avionics testbed and the other in a production Learjet 70. Interface with the flight management system (FMS) was via the touch screen CDUs, which used infrared positioning to locate button actuations. No tactile feedback is provided for button pushes, yet as I found before, “buttons” were easy to actuate. Audible feedback is selectable that sounds a “Clunk” when a screen touch is not recognised as a button push. Data and pre-programmed routes can be loaded into the FMS with a SD card, reducing required button pushing, but Pitcher manually loaded our short flight plan.
Pre-flight checks were expedited by the “system test” page, only one layer down from the CDU’s home page. Aircraft system tests were stepped through automatically, with limited pilot involvement. While an electronic checklist is available, being somewhat old school, Pitcher and I used a paper checklist to accomplish required pre-start checks. Both engines were started using bleed air from the tail mounted auxiliary power unit. Pilot involvement for the FADEC controlled starts was two button pushes per engine and of course monitoring start progress. Only a slight thrust lever (TL) increase was needed to start the X+ rolling when cleared to taxi.
SafeTaxi
While I am familiar with Wichita Mid-Continent airport, Pitcher showed me two ways the G5000 avionics can increase airfield positional awareness. He put his primary flight display into the “Split” mode, with two-thirds devoted to the PFD and one-third for a Jeppesen Airfield diagram. On the MFD’s map display, he ranged down until the airfield runways and taxiways were clearly visible. The more he zoomed in, greater detail appeared to include airport markings and buildings. Garmin calls this position awareness tool SafeTaxi. As we taxied to runway 19L for takeoff, our own ship position was clearly presented on both displays. Both the Jeppesen chart and Garmin displays enhanced positional awareness, but SafeTaxi’s finer detail made it the go-to display.
The flaps were set to “2” in preparation for take-off (TO). Cessna no longer uses degree positions for flap/slat setting; the old 5º, 15º, 35º positions are now labelled 1, 2, and FULL. Prior to taking the runway I armed the A/Ts with their TL mounted switch. Cleared for take-off, advancing the TLs about midway up the quadrant caused the A/T clutch to engage. The A/T mechanism then moved the TLs to the test day take-off power setting of 84.1%N1. Acceleration in the 13.46t (363kg of crew/passengers and 3.2t fuel) Citation was brisk. Pitcher called “Rotate” at 129kt (240km/h) indicated air speed (IAS), and the X+ leapt off the runway. The X+ is a two-pilot aircraft and once airborne, Pitcher retracted the gear, with flap retraction at 400ft above ground level. Passing 4000ft mean sea level (MSL), I engaged the autopilot for the climb to FL400.
Direct climb and dash
One of the major improvements the winglets and more powerful engines bring to the X+ is increased climb performance. At maximum take-off weight and standard day conditions, Cessna advertises a direct climb to FL450 in only 23min, a marked improvement over the 28min to reach FL430 in the X. Pitcher recommended we climb at 285kt IAS, which is M0.85, slightly faster speed than the minimum time to altitude profile of 270kt IAS/M0.78. We burned 480kg (1,060lb) and reached FL400 in about 23mins. At our weights the X+ could have easily reached its ceiling of 51,000ft.
Once level, I let the Citation accelerate at cruise power (90.1%N1) until the speed stabilised at 280kt IAS, representing high speed cruise. A total fuel flow of 2,520lb/h (PPH) gave a Mach of 0.907, with a resultant true airspeed (ISA+1º) of 520kt. Next I slowed the X+ to a long range cruise Mach of 0.82. A total fuel flow of only 1,640PPH was needed to hold 250kt IAS. On the nearly standard day, the X+ was maintaining 472kt IAS. These two cruise points show the non-linear relationship between speed and fuel usage. A 49kt increase in true airspeed comes at the expense of a greater than 50% increase in fuel consumption. That being said, the X+ still offers transcontinental performance with four passengers and National Business Aircraft Association instrument flight rules (IFR) reserves. A 3,000nm (5,560km) segment can be flown at high speed cruise with a total en route time (take-off to touch down) of 6h 14min, an average speed of 480kt.
While at altitude I had the opportunity to sample the X+’s cabin environment. Before I left the flightdeck, I noted that the 9.3psi (0.64bar) differential pressurisation system held the cabin at 5,900ft. At FL510 the cabin altitude would be 8,000ft, on par with that most airliners have cruising 10,000ft lower. The preview aircraft had the standard double club arrangement. While not a true stand up cabin, I had no difficulty moving about the plush passenger compartment. Ambient noise level was fairly low, allowing conversation at normal voice levels with others in the same club section.
High-altitude safety
Once seated on the flight deck, Pitcher and I discussed the emergency descent mode of the X+’s auto flight system. Like the X, the emergency descent mode is triggered at high altitudes if the cabin altitude exceeds 14,500ft. If triggered, the AP will start a descent and allow the aircraft to accelerate to maximum operating Mach number or speed (MMO/VMO). The X+, unlike the X, has A/T, and this will retard the engines to idle to speed the descent to 15,000ft, where the autopilot will level the aircraft. Several high profile accidents, such as Helios Airways Flight 522, may well have been avoided had this AP safety mode been installed.
With air traffic control clearance to 15,000ft MSL, I used the Vertical Speed mode of the AP to initiate the descent. I set the TLs to mid-range to speed the acceleration to M0.935 (MMO) and disengaged the AP. Approaching M0.92 an amber “A/T AUTO ON” caution was presented on the PFD, signifying the A/Ts had woken up and been retarded to idle in an effort to slow the aircraft. I lowered the X+’s nose further, accelerated it to MMO where a slight airframe burble and prominent "barber pole" on the PFD announced our high speed. As the 4,000ft/min descent continued, the 350kt IAS VMO limit was reached, where the aircraft was responsive to small control inputs in all three axes. Extension of the Speed Brakes increased the rate of descent by 2,000ft/min and caused a slight pitch up. About 5kg of forward yoke pressure was needed keep the desired pitch attitude.
Automatic help
As we levelled at 15,000ft, Pitcher cancelled our IFR clearance so I could freely manoeuvre while evaluating the X+’s low speed handling. The X+’s instrumentation presents angle of attack (AoA) in two places. One is in an indexer mounted on the glare shield left of the centre windshield pillar. This indexer is reminiscent of the one in the Northrop T-38s I flew in the US Air Force, handy for monitoring speed during visual landing patterns. The other is a colour segmented arc display that is pilot selectable for presentation in the PFDs. I primarily referenced the PFD gauge as I accomplished two approaches to stall, the first in a clean configuration and the second in a TO configuration, gear UP and flaps 2.
I set the TLs to idle and slowed the X+ at about 1kt/sec. At about 0.6 units of AoA, just below the gauge’s yellow band, the leading edge slats were automatically extended to reduce stall speed. When AoA reached the bottom of the red band, about 0.84 units, the A/Ts woke up and advanced power to prevent further slowing. As was the case at altitude, an amber “A/T AUTO ON” message was presented on the PFD. This feature stabilised the aircraft at a safe speed above the stack shaker actuation point. For the flap 2 stall Pitcher pulled the A/T circuit breaker, to prevent them from waking up. As before I slowed the aircraft at about 1kt/s. I ignored the slow speed warnings, holding yoke back pressure until the stick shaker activated. At shaker onset I relaxed yoke back pressure to hold 106kt IAS/0.9units AoA. At this slow speed the X+ didn't drop a wing and was responsive to small control inputs. I advanced the TLs to mid-range and relaxed yoke back pressure to recover the X+ to the normal flight envelope.
The type features an enlarged cabin
Craig Hacker Photography
Polished glass
With the slow speed manoeuvres complete, Pitcher retracted the flaps and we turned towards Mid-Continent for some pattern work. During this lull in activity I reflected on the powerful capabilities of the G5000 avionics suite. Synthetic vision (SV) comes standard as does a terrain awareness and warning system (TAWS) and a traffic collision avoidance system (TCAS II). In the X+, as well as in prior G5000 flights, I found the sizeable PFDs provided a large palette for the SV system. While Central Kansas is very flat, SV terrain depictions should be quite useful during approaches in low visibility conditions, while TAWS should reduce the likelihood of a controlled flight into terrain event. TCAS II derived targets are displayed in "God’s eye view" format on a PFD insert box. Within the ADI’s field of view, TCAS targets are presented at relative azimuth and elevation, facilitating visual acquisition. We had clear skies for our preview flight, but thunderstorms are a constant threat in the summer over much of the USA. A Garmin GWX 70 weather radar comes standard as does NEXRAD satellite weather. In the past I have found satellite data aids in long range planning, while the onboard radar helps thread the needle of close-in buildups.
Recovery to the airport was via radar vectors to an instrument landing system final approach segment. I hand flew the aircraft to get a feel for Cessna’s fastest aircraft. Pitcher loaded the approach to runway 19L, with the localiser frequency auto tuned. I found it easy to maintain course and glide path, with manual airspeed control facilitated by the responsive FADEC controlled engine. The trailing link main landing gear absorbed my less than perfect touch down, making me look like a pro. Moderate pressure on the toe actuated wheel brakes and thrust reversers quickly slowed the X+ to a stop.
After the full stop landing, we taxied back to the approach end of 19L for our last event, a simulated engine failure after lift-off and one engine inoperative approach. Once we were airborne, Pitcher retracted the gear and immediately pulled the right TL to IDLE, simulating an engine failure. During climb out, about 30kg of pressure on the left pedal was needed to maintain coordinated flight with take-off power (84%N1) set on the left engine. Flaps were left at 2 for the approach, and available rudder trim was sufficient to zero out pedal forces once level at pattern altitude. Prior to turning final, I zeroed out the rudder trim and Pitcher extended the landing gear. On final only a small amount of rudder was needed to counteract the asymmetric thrust condition. After a smooth touch down I applied wheel brakes and left the engine thrust reverse alone to slow the aircraft. Pedal-actuated nose wheel steering (+/- 6º) was sufficient to maintain runway centreline as I slowed the X+ for runway turn off. Taxi back to the Cessna ramp was uneventful, with post flight checks completed in a snap.
Fast and docile
Since its certification in 1996, Cessna has sold over 300 Citation Xs. Its third iteration, the X+, brings real world improvements to the marketplace. At speeds in excess of M0.9 the X+ can comfortably carry four passengers over 3,000nm. Not mere fashion statements, its sizeable winglets significantly enhance performance throughout the flight envelope. While passengers will no doubt enjoy the comfortable lengthened cabin, the pilots will surely appreciate the updated paperless flightdeck. The X+’s Garmin G5000 avionics suite’s three large displays can display untold amounts of information in a multitude of pilot friendly formats; an upgrade that significantly enhances pilot situational awareness and flight safety. And in a first for Cessna, the X+ even has an auto-throttle system. It is not often that you can buy the best of anything for less than the dearest price, but for a price of $23.5 million you can get the world’s fastest business jet.
Source: Flight International
