Peter Henley/WICHITA
Strong short-field performance by Cessna's Citation Excel allows the aircraft to use small, less busy and perhaps more conveniently located airfields than those needed by bigger business jets.
Flight International flew a production Excel, based at the Cessna facility at Mid-Continental Airport in Wichita, Kansas. Product marketing manager Michael Pierce describes the aircraft as having "light-jet features with mid-size comfort". These light-jet features - the unswept wing, cruciform tail and simple systems of the Citation Ultra - give it good short-field performance and easy maintenance.
Certainly, the wing platform and the Pratt & Whitney Canada PW545A turbofans - rated at 3,800lb (16.9kN) sea-level take-off thrust each - give the Excel an impressive runway performance and an excellent initial rate of climb. Although this flexibility is a clear advantage, it does penalise the cruise at altitude, where the Excel is nearly 100kt (185km/h) slower than its bigger, swept-wing brother, the Citation X (Flight International, 11-17 June, 1997).
But this penalty probably poses little disadvantage because the Excel is designed for ranges of just over half those of the Citation X - 3,300km (1,780nm) against 6,300km. So, unlike the X, which is capable of global operations, the Excel is aimed at intracontinental, relatively short-haul flying. In the final analysis, the customer must weigh the importance of runway performance against transit times.
To create the Excel, Cessna's logic was to combine the wing, tail and systems from its Citation V Ultra light jet, with the fuselage cross-section of its mid-size Citation VII and X. The Ultra and Excel cabins are therefore the same length, at about 7m (23ft), but the Excel's is nearly 0.3m higher and about 180mm (7in) wider.
The Excel flown by Flight International did not have the optional Honeywell RE100 (XL) auxiliary power unit (APU) or the Honeywell enhanced ground proximity warning system (EGPWS). About 50% of Excel customers choose the APU, but that figure is expected to rise to 80%, says Pierce. The need for EGPWS depends on the terrain above which the aircraft is expected to operate.
Service accessibility
Because business jet pilots have to refuel and inspect their aircraft at technical stops, Cessna demonstration pilot Tony Mahoney and I checked the accessibility of service points before our evaluation flight.
The avionics are compactly stowed in the nose and easy to access via two large, flush-fitting, latched doors.The fuelling point is simple and ideally placed just forward of the starboard wing. It is a pity Cessna did not fit another hatch beneath the starboard engine, because it would have provided access to the APU. Engine oil level can be checked on a sight-gauge rather than having to fiddle with a dipstick. Each wing has 26 vortex generators and 11 boundary-layer energisers, all of which must be intact for dispatch.
All these turn-round tasks can be undertaken while passengers and their baggage are being loaded on the aircraft's port side. The baggage hold is large at 7.4m³ (80ft³), and of a shape that can take skis or hanging suit-bags.
I took the left seat and Mahoney the right. Dale Carter, another demonstration pilot, acted as safety pilot behind us. The cockpit is small but comparable to other business jets in the same class. The only way to climb into or out of the seat was by bending and easing one leg at a time over the centre console and the seat cushion. This was not difficult, and with practice would become easier. The seat has a five-point harness and is adjustable fore and aft and for recline. It was comfortable for its size.
There is an eye-position indicator and the rudder pedals are adjustable by lifting a locking lever beside each foot pad. The Excel benefits from the raked, curving windows of the mid-size Citation range, and the field of view was excellent. Each pilot could easily see the outboard half of the wing on his side. The small rearmost side-window can be opened.
A slim pocket for documents is fitted on the cockpit side wall beside each pilot. Two cup- holders sit in each cockpit sidewall above the document stowage. An emergency oxygen mask is located by each pilot's outboard thigh. Each pilot has a sun visor, which could be positioned easily on a continuous rail which runs round the top edge of the windows.
The Honeywell Primus 1000 flight guidance system is neatly installed, with a vertical 200 x 180mm primary flight display (PFD) for each pilot and a third, centrally placed multifunction display (MFD). The crew alert and warning system panel is beneath the glare shield, and underneath are the vertical-scale engine instruments on two smaller cathode-ray tube scopes (CRTs). Because these indications for N1, turbine gas temperature (TGT), oil press re and oil temperature are displayed side-by-side horizontally, the compelling cues for left engine and right engine are lost. With vertical columns of displays, an indication in the left column is clearly for the left engine, and vice versa. The possibility of misidentifying a malfunctioning engine during heavy workload would appear greater with this horizontal display. The panels for controlling the aircraft systems use toggle and rotary switches rather than push-buttons with integral indicators.
Circuit breaker
Circuit breaker and fuse panels are neatly fitted on the cockpit side walls outboard of each pilot's seat. Inevitably, there are some panels, such as the engine-starting controls by the captain's left knee, which are not conveniently placed for both pilots to reach. This cannot be avoided in a small cockpit without overhead panels, but in this case there seems to be nothing that either pilot cannot reach at a stretch or see reasonably easily.
The PW545A turbofans have electronic engine control, but not full authority digital electronic control. Because this particular Excel had no APU, the engines were started with aircraft battery power. Pressing the appropriate engine start button (next to my left knee) rotated the turbine. At 89% N2, the latch was lifted on the associated power lever and the lever moved to idle to introduce fuel to the engine. Despite using the aircraft battery to start the engines, the maximum interstage turbine temperature (ITT) was a reassuring 93°C less than the maximum permitted. As well as monitoring the ITT, N1 had to be checked by 25% N2 or a manual abort of the start would have been needed. The vertical-scale engine instruments were easy to read and interpret.
During taxiing, nosewheel steering via the rudder pedals was positive and responsive. Mahoney warned the toe-operated, multiple carbon-disc wheel brakes might be fierce when cold, but gentle pedal pressure gave smooth and progressive braking. While taxiing, operation of the Nordam target-type thrust reversers was checked and the take-off speeds and engine N1 obtained from the abbreviated checklist.
Mid-Continental Airport is about 1,300ft (400m) above mean sea level, the temperature was 2°C, the surface wind about 15kt from 030° and the runway in use was 1R. The aircraft take-off weight was 7,730kg (17,000lb) against the maximum permitted 9,070kg, giving reference speeds of: V1 102kt, VR 106kt and V2 118kt with a flap setting of 15°. The speeds were set and displayed on the PFD airspeed "tape".
The N1 for the maximum performance take-off appeared at the top of the engine instrument display. The aircraft was held by the foot brakes and the power levers advanced to the take-off detent. All three detents for take-off, climb and cruise are clearly marked on the power lever quadrant and can be felt by the pilot, but the power lever position is not indicated on the instrument panel. This would be a useful addition and will be incorporated on later production aircraft, Cessna says.
Brisk acceleration
The engines accelerated smoothly and the achieved N1 was checked. When the wheel brakes were released, the Excel accelerated very briskly. The nosewheel steering kept the aircraft straight until the rudder became effective at about 60kt. A large rearward movement of the control column was required to rotate the aircraft and achieve the 12° pitch angle demanded by the attitude director indicator. When we were safely airborne, Mahoney raised the undercarriage and then, at 140kt, retracted the flaps. The power was reduced from take-off to climb thrust by repositioning the power levers at the climb detent.
For about 30s from the time the climb attitude was achieved after take-off, it was necessary to trim continuously nose down - using the trim pole switch on the control yoke - to counter the longitudinal trim change as the flaps were retracted and the aircraft accelerated.
Two things happen in the Excel when the flap lever is moved from the 15° position in the gate to the 0° position. Firstly the flaps retract, and secondly the horizontal stabiliser, which has two predetermined angles of incidence, repositions itself automatically. This achieves some of the benefits of a fully variable-incidence tailplane in terms of longitudinal trim and drag over a wide speed range. A variable-incidence tailplane, on the other hand, can be moved progressively in response to longitudinal trim change: this has only two pre-set angles of incidence - 2° nose down for take-off and landing and 1° nose up for climb, cruise and descent.
Mechanical controls
The change of incidence is triggered by a microswitch in the flap gate quadrant. It takes about 25s for the tailplane to move from one position to another. The effect of the tailplane moving must be countered by the pilot through the elevators. The out-of-trim force on the elevators has to be alleviated via the elevator trim tabs. Consequently, the pilot has a lengthy task of re-trimming whenever the flap lever is moved down or up between the 0° and 15° flap positions. No doubt Excel pilots become used to re-trimming instinctively, but the pronounced trim change with configuration change remains a characteristic of the aircraft.
The primary flying controls are all mechanically operated by cables. Routine re-trimming was easy via the electric trim switches or the mechanical wheel on the centre console for the elevators, and via mechanical wheels at the rear of the centre console for the ailerons and rudder. The single rudder tab acts as both a trim tab and a servo tab. There is a rudder-aileron interconnect. Although the control break-out forces were higher than with a power control system, the controls were well harmonised. Roll control is by ailerons only - there are no spoilers - and a full aileron deflection at 45° of bank to 45° the other way produced good roll acceleration and rate of roll.
The Excel has good natural damping: a stick slap (pitch bump) at 200kt and 21,000ft gave dead beat or immediate damping. A rudder doublet at the same speed provoked dutch roll with the yaw damper off, which was naturally damped in about two cycles. With the yaw damper on, the roll was damped in one cycle.
Using the autopilot, the climb was continued to the maximum operating level of 45,000ft. Passing 43,500ft, the indicated airspeed was 160kt and the rate of climb 2.54m/s (500ft/min). At 45,000ft (surprisingly warm at ISA + 7°C) and an all-up weight of 7,200kg, the power was kept at "climb" so that the aircraft could accelerate for a quick look at the cruise. The speed stabilised at Mach 0.684, at cruise power, with a total fuel flow of 470kg/h.
A brief descent was then made to 44,000ft, to stay within the altitude limits, for a couple of 45° bank turns. At M0.725, unmistakable buffeting occurred, but releasing back-pressure on the control column stopped it instantly.
The power levers were retracted to flight idle and the speed brakes extended for a descent to middle altitudes for some stalls. The hydraulically operated airbrakes consist of a panel above and below each wing. Selecting them via the electric switch on the power lever quadrant resulted in little pitch change but considerable aerodynamic burble and noise.
The first stall was clean, power off, at 7,020kg. Speed was reduced, wings level, at 1kt/s from the trim speed of 117kt. Stick shake occurred at 103kt. Buffet onset gave good natural stall warning while the stall was defined by a gentle g break and nose drop at 95kt. The left outboard wing stalled first, causing a gentle roll to the left, but this was easily contained with opposite rudder. The same technique was used to approach the second stall with undercarriage down and the flaps at 35°. Stick shake occurred at 95kt, followed by good natural buffet and a rather more pronounced nose drop with a minimum speed of 86kt.
No pusher needed
The Excel does not have a stick pusher and clearly does not need one. To meet the certification requirements for the stall without recourse to a pusher, the distinctive ventral fins were added under the fuselage's rear.
Next, radar vectors were flown for a coupled instrument landing system approach to runway 1R back at Wichita. The Excel has a recent version of the Primus 1000 avionics. Each pilot's PFD has an attitude director indicator (ADI) above a horizontal situation indicator on the one screen. The ADI demand presentation can be either single bar or cross-pointer, chosen via a selector switch.
The MFD is used for the weather radar and navigation display. It can be used as a PFD if a display fails. Traffic alert and collision avoidance system and EGPWS displays appear on the MFD when these optional items are fitted.
The autopilot flew the aircraft smoothly and precisely. Airspeed was controlled manually with power. The glideslope was flown at 5kt above the threshold speed (VAT or VREF) because of the surface wind. At 200ft above ground level, the autopilot was disconnected and a full stop landing made.
Easy to fly in ground-effect, the Excel had a tendency to float if an attempt was made to "grease it on". The trailing-link main gear was compliant and generally the aircraft flattered the pilot, being almost impossible to land badly. Had runway length been critical, there would have been no problem in placing the aircraft on the ground near the runway threshold. Reverse thrust was easy to select, effective, and easy to cancel at 60kt.
Two visual circuits were then flown. It was here that the Excel was at its nimble best from a handling point of view. The weight was now 6,820kg, temperature 2°C and the performance decidedly lively. Because the limit speed for 15° flap is a useful 200kt, it was possible to take off, fly the circuit and perform a touch-and-go without altering the flap setting and therefore the requirement to re-trim was avoided.
The straight wing and relatively simple high-lift devices (trailing-edge Fowler flaps in two segments per wing operating through 0-35° of travel), together with a general tautness of handling at circuit speeds and altitude made the aircraft feel safe and predictable at 45° of bank.
Impressively quiet
Throughout the flight, the cockpit was impressively quiet, in terms of both engine and aerodynamic noise. Mahoney and I did not wear headsets and could converse easily. On the final landing, I tried the wheel brakes at maximum application and found them exceptionally powerful and smooth, and the anti-skid system's operation could only just be discerned.
After the first take-off into the circuit, Mahoney simulated an engine failure (by which time the airspeed was 140kt - 20kt above safety speed) and the foot forces to keep straight were pleasingly light. To date, the Excel has carried a penalty in take-off performance from a wet runway under the US Federal Aviation Administration's Part 25 certification requirements for minimum control speed on the ground and an associated maximum foot force for full rudder deflection of 68kg. In future, Cessna says, a simple rudder bias will become standard to bring wet runway performance more in line with that for a dry runway.
The Excel is a clever blend of a simple, proven wing and a roomy fuselage. Some compromise has been required to combine the requirements for good short-field performance and adequate high-altitude, high-speed cruise. The result is a charismatic business jet with considerable customer appeal which is reflected in its current market demand.
Source: Flight International
