MICHAEL GERZANICS / TOULOUSE
Airbus's latest A320 derivative, the A318, is a highly capable addition to the family. But is it good enough to win over non-A320 family operators?
Airbus's single-aisle fly-by-wire (FBW) family is centred on the 150-seat A320. Launched in 1984, this highly innovative aircraft entered service in April 1988, since when 1,169 have been delivered, with outstanding orders for 491.
The first derivative, the A321, was created by stretching the A320's fuselage by 13 frames, or 6.93m (22ft 9in). Essentially identical to the A320, the A321 entered service in January 1994, seating up to 185 passengers. The next addition to the single-aisle family was the 124-seat A319, a 3.73m (seven frames) shrink of the A320, first delivered in April 1996. These larger and smaller siblings have further extended the A320 family's success by garnering a further 1,306 orders, 778 of which have been delivered.
However, several airlines expressed interest in an even smaller aircraft to fill the void in the 100-seat range. Launched in April 1999, the third and perhaps final addition to the A320 family is the A318, which is 2.39m (4.5 frames) shorter than the A319. A standard two-class layout can accommodate 107 passengers at a pitch of 36in (91cm) in first class and 32in in economy.
As well as rounding out the lower end of the product line, the A318 is being marketed as an entry-level Airbus. Although airliner pricing is far from transparent, based on published list prices for a mid-weight aircraft, a 107-seat A318 can be bought for $45 million, while the A319 costs $10 million more, making each seat on the A318 $23,000 cheaper than a seat on the A319.
Clearly, the A318 is aggressively priced relative to its larger sibling, but it is by no means a stripped model. The A318's level of standard equipment is equal to, or better than, that of the A319. The only significant feature of other A320 derivatives that is not available on the A318 is a containerised cargo capability.
The A318's certification programme kicked off in January 2002 with the maiden flight of the first of two test aircraft (F-WWIA). This aircraft was powered by the planned lead engine, Pratt & Whitney's PW6000, which the company had developed specifically for short-haul, high-frequency operations in which maintenance costs rather than relative fuel consumption were the dominant design factor.
The PW6000 first featured a five-stage high-pressure compressor (HPC) that helped give the engine a maintenance-friendly low parts count. The engine proved reliable and capable during the initial stages of the test programme. However, actual fuel consumption was exceeding target levels and in an effort to reduce it, P&W abandoned the five-stage HPC in favour of an MTU-designed six-stage unit.
After 180h of flight, the PW6000 engines on the first aircraft were replaced with CFM International CFM56-5Bs. Handling qualities for the vast majority of the flight envelope were not adversely affected by the new engines, although the additional nose-up pitching moment generated at maximum thrust required the aft centre of gravity (CG) to be limited to a line 3% forward of the original limit to ensure adequate elevator authority at low speeds. The first prototype has been the primary workhorse in the certification effort and has amassed 670h (180h/P&W and 490h/CFM).
Certification schedule
Airbus obtained European Joint Aviation Authorities certification for the CFM-powered variant on 23 May. US Federal Aviation Administration certification is scheduled for 23 June, ahead of delivery to the first customer, US carrier Frontier Airlines next month. Certification of the P&W-powered variant has been delayed until the end of 2005, with first deliveries scheduled for early in 2006 to America West Airlines.
On the ramp at Airbus's Toulouse flight test facility, the A318's relatively short 31.44m length is eclipsed by its wing span - common to the whole A320 family - of 34.09m. Further contributing to its squat appearance is the large-diameter fuselage and long undercarriage, the former having contributed directly to the success of the A320 family, in which the passenger cabin is 178mm wider than that of a Boeing 737, allowing each economy-class seat to be 1in wider. Airbus also offers a wide-aisle option for the A320 family to allow faster passenger loading and unloading, shortening turnaround times. Short-haul, high-frequency carriers such as EasyJet are interested in this option.
On the pre-flight inspection of F-WWAI, Airbus test pilot Jacques Rosay pointed out the minor external differences between the A318 and the larger A319. The 2.39m fuselage shrink was accomplished by removing 0.8m (1.5 frames) forward of the wing and 1.59m (three frames) aft. The shorter forward fuselage section required modification of the wing belly fairing. During flight test with the landing gear down at high angles of attack (AoA), vortices from the nose gear doors were impinging on the fuselage-mounted AoA probes. This had no direct impact on flying qualities, but it did corrupt AoA inputs to the flight-control system. Two strakes, aft and outboard of the nose gear doors, have been fitted to redirect the vortices and allow the AoA probes to read clean air. The shorter rear fuselage and resultant lower moment arm for the vertical stabiliser/rudder combination required the stabiliser fin to be heightened by 750mm and the rudder itself by 100mm.
Design strengths
Once in the cockpit, I was reminded of one of the strengths of Airbus's FBW aircraft design philosophy - the common cockpit layout. The forward instrument panel has six 185 x 185mm colour liquid crystal displays, a primary flight display (PFD) and navigation display (ND) for each pilot, plus two displays for the electronic centralised aircraft monitoring (ECAM) system. These displays have the same external dimensions as the CRTs in other members of the A320 family, but a slightly narrower border gives a larger viewable area and a sharper display.
Airbus estimates the LCD display suite is $10/h cheaper to maintain and operate than comparable CRTs. The three centre panel standby flight instruments on the A320 have been replaced by a single Thales LCD integrated standby instrument system (ISIS), like the one in the A340-600. The rest of the cockpit layout is essentially identical to other A320 series aircraft and only slightly different from that of the A330/A340 widebodies. The overhead panel is arranged logically and uses a dark cockpit philosophy, in which no lights illuminate if systems are functioning properly.
Design eye reference balls on the centre windscreen pillar helped me to obtain a good seating position and the adjustable left armrest enabled me to grip the sidestick comfortably.
After Rosay had programmed the flight management and guidance system (FMGS) through one of the two centre console-mounted multifunction control and display units (MCDUs), the pre-start checklist was completed. The engines were started by APU bleed air one at a time using the full authority digital engine control (FADEC) automatic start sequence. Both started normally, with idle N1s of 20.5% and fuel flows of 154kg/h (340lb/h). Had the FADEC detected a hot, hung or no-start condition, it would have aborted the start and auto-cranked the engine to blow out residual fuel vapours.
After engine start, a white sidestick order "cross" was displayed on each PFD. The "cross" shows commanded sidestick displacement, and allowed me to practise the aft stick pull required for rotation. Commanded control positions were crosschecked on the ECAM flight-control synoptic page. Idle power was sufficient to start the aircraft rolling once the parking brake was released.
Rosay set the flaps to position "3" - the third of four positions available - in preparation for take-off from Toulouse's runway 32L. A reduced-power take-off would be performed, relating to an assumed temperature setting of 50°C (120°F), while the actual temperature was 15°C. On the runway, the thrust levers were advanced and engines allowed to stabilise at 50% N1. Once we were cleared for take-off, the brakes were released and the thrust levers set to the "FLEX" autothrust detent.
The CFM56-5B9 engines stabilised at 84.3% N1 for the take-off roll. Rosay called V1 at 116kt (215km/h), followed shortly by a half aft stick rotation at 121kt. The flight director (FD) provided pitch guidance to maintain V2+10kt, giving 134kt for the 56,800kg (47,000kg zero fuel weight) aircraft. Acceleration and clean-up were uneventful. As we passed 1,500ft above ground level (AGL), the "LVR CLB" advisory message flashed on the PFD, reminding us to move the thrust levers to the "CL" (climb power) detent. We turned towards the working area south of Toulouse and established a 250kt climb.
Climb restrictions
Local air traffic conditions prohibited us from climbing to a realistic cruise altitude, so we stopped the climb at FL170 (17,000ft), at which level Rosay showed me several of the areas where Airbus's active FBW technology has been used to make flying the A318 - or any other FBW Airbus - easy.
A very effective yaw damper limits side slip induced by rolling, so the A318 can be flown feet on the floor for most of the flight envelope. For bank angles up to 30°, the pilot does not need to apply aft stick to keep the nose level on the horizon. The flight controls compensate and automatically apply the correct nose-up input to maintain level flight.
For banks of more than 30°, however, the pilot must apply aft stick pressure as required in a conventional aircraft. In the roll axis, the sidestick commands roll rate based on stick displacement. About 0.5kg of force is required to break the stick out of the neutral position and command a roll. The force required to displace the stick is linear, but not symmetrical, to take account of the typically greater arm strength when twisting the wrist towards the body, rather than away from it. The maximum attainable bank angle in the normal control laws is 67°. The stick must be held to the lateral stop to reach the 67° angle of bank limit. Releasing the stick from there will cause the flight controls to return the aircraft to a 33° bank angle in the same direction.
In the pitch axis, the normal flight controls are a g command system. In wings level flight, a neutral stick position will command 1g flight. Stick displacement fore and aft commands an associated g either less than or greater than 1g. The flight controls prevent the pilot from overstressing the aircraft by limiting aircraft loading to published maximums - +2.5g to -1g flaps up, and +2g to 0g flaps down. Pilot control authority in pitch is further limited by the flight controls, because they will, at most, allow an aircraft nose-up pitch attitude of 30° and a nose-down one of 15°.
The g command scheme has a number of benefits. The pilot does not need to continually retrim the aircraft as it speeds up or slows down. Configuration changes, such as flap extension and retraction, do not require pilot compensation to maintain current flightpath. Even though the engine thrust line is below the CG, thrust increases or decreases do not change the pitch attitude, as they would on a conventional aircraft.
Major difference
Although a g command scheme has a number of benefits, there is one major difference with conventional aircraft, in which adding or reducing power while at a trim speed condition will cause it to climb or descend to maintain the trim speed. In level flight, pulling the power to idle will cause the A318 to slow. As it slows, there is no tactile feedback to the pilot because the stick remains in the neutral position and the flight controls maintain a constant altitude. The aircraft's pitch attitude will increase automatically as it approaches a stalling angle of attack which, left unchecked, could result in a full stall.
Airbus has implemented a number of features to prevent the A318 from entering a stalled flight condition. In the normal flight control laws, three dynamic speeds, or AoAs, are computed. The highest speed, AlphaPROT, is shown on the PFD at the top of yellow hash marks to the right of the speed tape. If AlphaPROT is reached, the autopilot will disconnect and the speedbrakes will retract if extended. Also, the stick will stop commanding g and command AoA directly, like a conventional aircraft in which continued aft stick pressure is required to slow the aircraft further.
The next slower speed is AlphaFLOOR, which is not displayed to the pilot. Before reaching AlphaFLOOR, a low-energy warning is triggered, causing an audible "SPEED, SPEED, SPEED" to be sounded every 5s. At AlphaFLOOR, the autothrust system will automatically engage and advance both engines to the take-off/go around (TOGA) power setting regardless of the actual thrust lever position. In no case will the aircraft slow below AlphaMAX, a speed below AlphaFLOOR and above the actual stalling AoA, indicated by the top of a red band to the right of the speed tape.
With the flaps set to "3", gear down and 8,120kg of fuel, I retarded the thrust levers to idle to experience this sequence of events. Everything progressed as outlined above, except Rosay disconnected the autothrust shortly after they engaged at AlphaFLOOR. This allowed me to slow the aircraft to its AlphaMAX speed of 98kt for these conditions. I put the stick full right while continuing to hold it at its aft stop. The aircraft rolled gently to the right, the flight controls now limiting the maximum bank angle to 45°. The A318 was docile and controlled at this slow speed, displaying truly carefree handling qualities at the extreme.
After completing the slow-speed manoeuvres, I accelerated the aircraft with the flaps still set to "3" and gear down. At 150kt, I extended the speedbrakes to full and again retarded the thrust levers to idle. As the aircraft slowed, Rosay called "pull up" to simulate a ground proximity system warning. I rapidly advanced the thrust levers to the TOGA position while simultaneously pulling full aft on the stick. The nose pitched up to a 30° attitude and the speedbrakes automatically retracted. Airspeed bled off to about 100kt and the A318 maintained a 3,500ft/min (17.78m/s) climb away from the simulated hazardous terrain.
Acceleration
Once safely above our simulated terrain, I pulled the thrust levers out of the TOGA detent to a mid-range position and lowered the nose to accelerate. The engines, however, stayed at the TOGA setting of 92.7% N1. Since the aircraft had been at AlphaFLOOR, the autothrust had locked the engines at the TOGA power setting. I used the thrust lever-mounted switch to disconnect the autothrust from the locked setting and regain control of the engines.
The A318's FBW flight-control system allows pilots to effortlessly attain the maximum level of allowed performance even in the most critical situations. This capability should enhance flight safety in almost all cases. One possible exception would be terrain closure at higher speeds, where more than the placarded g limit is aerodynamically available. Only in the direst circumstances would a pilot knowingly exceed the structural limit of his craft. Given the choice between certain ground impact or possibly bending or breaking the aircraft, some pilots may prefer to have the option of the latter.
Return to Toulouse was via radar vectors to an instrument landing system approach to runway 32L. Rosay loaded and activated the approach in the FMGS. He also entered the surface winds into the MCDU. The first approach was flown with the flaps set to "4". Glideslope intercept was at 3,000ft MSL, about 2,500ft AGL.
The PFD's split-cue flight detector allowed me to track precisely both the localiser and glideslope during the approach. The thrust levers were placed in the CL detent, and stayed there throughout the approach. The approach was flown in managed speed, the autothrust maintaining a minimum airspeed above 1.23VSTALL minus the ground headwind component.
Because the winds were stronger at altitude, the practical effect was that as the aircraft tracked the glideslope, the target airspeed decreased from 128kt at 2,000ft AGL to 123kt at 1,000ft AGL. At 50ft radar altitude (RA), the A318's pitch axis flight control laws changed to a pitch attitude command system, with neutral stick commanding the actual pitch attitude at 50ft RA, about 6° nose-up for this approach.
At 30ft RA, the flight controls lowered the neutral stick pitch attitude to 2° nose-up over 8s, forcing the pilot to pull aft to maintain the approach attitude. I started the flare manoeuvre at about 30ft, retarding the thrust levers from the CL detent at 15ft. The flare manoeuvre itself felt like that of a conventional aircraft. The A318 touched down smoothly on the runway centreline about 400m from the approach end.
Touch-and-go manoeuvre
Once on the runway, Rosay set the flaps to "3" and pitch trim to 1 unit nose-up for a touch-and-go manoeuvre. I advanced both thrust levers to the TOGA detent, the engines stabilising at 87.3% N1. Rosay called "rotate" at 120kt.
After lifting off the runway in a 10° nose-high attitude, he pulled the right engine to idle. Only about 60% of the available rudder displacement, as shown on the ECAM flight control display, was required to maintain runway heading.
Centring the sideslip index on the PFD required about 15° of rudder trim to zero out rudder forces. It should be noted that in the event of an engine failure, the sideslip index becomes a beta target, commanding optimum performance for the asymmetric thrust condition and allowing a small amount of sideslip. Flap setting for an engine out approach, position "3" or the full position "4" setting, is a function of required go-around performance.
With only 4,760kg of fuel, flaps "4" would have provided adequate go-around performance, but I elected to use flaps "3" to experience another landing condition. The engine out approach was flown in managed speed like the previous one. I centred the rudder trim after intercepting the glideslope.
Only a slight amount of left rudder was required to keep the beta target centred at the approach speed of 130kt. Again I started the flare manoeuvre at about 30ft AGL. I only had to relax pressure slightly on the left rudder pedal as I retarded the thrust lever out of the CL detent at 15ft AGL. Again the aircraft touched down softly on the runway centreline, within 500m of the approach end of the runway. After clearing the runway, taxi back to the ramp and shutdown procedures were straightforward.
During my flights the A318 showed itself to be a commendable addition to the single-aisle A320 family. It offers the same level of passenger comfort as its larger siblings, yet in a smaller package more suited to lightly travelled routes.
For airlines that already operate other members of the A320 family, those seeking a 100-seat aircraft may find the A318 the ideal choice because it has the advantage of common pilot type rating and spares requirements, generating savings.
As a standalone product for the 100-seat niche, the decision becomes less clear. Both Boeing and Embraer offer 100-seat aircraft at a substantially lower price, roughly $35.5 million for the Boeing 717 and $31.3 million for the Embraer 195. Although Airbus has orders for 84 of its smallest airliner, it has not won any orders from airlines that do not already operate A320 family aircraft. As a low-risk derivative of the A320, the A318 will doubtless be a commercial success for Airbus while offering airlines a highly capable and technologically advanced 100-seat airliner.
Source: Flight International
