MICHAEL GERZANICS / TOULOUSE
Airbus' lastest creation, the stretched A340-600, may be the world's longest aircraft yet - but its handling belies its size.
Airbus' A340-600 first took off on 23 April - its mission to give airlines an alternative to the Boeing 747-100/200/300 series. Since the -600's maiden flight, Airbus has pursued an aggressive flight-test programme designed to obtain Joint Aviation Authorities certification by April. So far, the three test aircraft have flown over 550h. They have clearedthe normal operating envelope to a maximum ceiling of 41,100ft (12,500m)and maximum dive speed of 365kts (675km/h)/Mach 0.93 (Vmo/Mmo being 330kts/Mach 0.86). In addition, they have demonstrated a maximum take-off weight of 365t and full range of centre-of-gravity position. Test pilot Claude Lelaie, head of flight tests, says flight control development is progressing well.
When Flight International flew Airbus' newest and largest aircraft recently, the test example was the first prototype F-WWCA. At a length of 75.3m, it is the world's largest airliner; being over 4m longer than the 747 and its wingspan is larger than all but the Boeing 747-400. The large 2.47m-diameter fans of the four Trent 500 engines presented a huge contrast to the smaller and less powerful CFM56. Unusual on a four-engine aircraft was the ram air turbine (RAT), which was housed in a canoe pod between engines three and four. In an emergency, the RAT provides hydraulic power to the primary control surfaces and/or electrical power to a number of essential busses.
The all-composite moving horizontal tailplane is 38% larger than the -300's, while the vertical tail and rudder are based of those of the A330-200s. The centre gear, unlike the -300's, is a four-wheel twin tandem boogie arrangement. All main gear and centre-gear wheels are fitted with carbon multi-disc brakes and electric cooling fans. There are two cameras comprising the taxi aid and camera system (TACS). One is located in the vertical fin, and provides a forward view of the top of the aircraft out to the outboard engine nacelles. The other, located on the lower centreline of the fuselage, provides a view of the nose-gear and taxiway.
The flight-deck layout reflects one of Airbus' strongest selling points: a cockpit in common with its other fly-by-wire (FBW) aircraft. The six primary displays are all liquid crystal, and the stand-by instrument cluster has been replaced by a single Smiths Industries integrated stand-by indicator system display.
Programming the flight-management system was similar to that of other modern aircraft equipped with "glass" flightdecks. Before disconnecting external power, Lelaie pushed the auxiliary power unit (APU) start pushbutton. The APU page was automatically displayed on the lower electronic centralised aircraft monitor (ECAM) display, allowing us to monitor its start. The before-engine-start and push-back checklists were easily completed. After a short push-back, the engines were started via automatic start. After placing the ENG START selector to the IGN/START position, I moved the engine master switches for engines numbers 1 and 2 to the ON position. The full authority digital electronic control automatically controlled APU bleed air, engine ignition and fuel application to ensure an orderly start. Once core RPM, N3, reached 50%, on the first two engines, the start valves closed, completing the start cycle. Engines 3 and 4 were started in the same manner.
Once cleared to taxi to Runway 15L, idle power got the 259.2t aircraft rolling. Lelaie selected the TACS display on the lower ECAM. The lower part of the display showed the view from the tailfin camera, looking down on the aircraft. The top of the display provided a view of the taxiway via the nose-wheel camera. The nose-wheel steering was precise, allowing me to track the taxiway centreline with the side-mounted tiller and rudder pedals. While I found the TACS good for taxiing in confined areas, most jumbo pilots would not find it necessary for routine operations. But it would be required, say, for a 180¼ turn on a narrow runway. Unlike the Boeing 747 or 777, the A340-600 lacks steerable body gear and could require up to 59m ofpavement to negotiate a 180¼ turn.
While taxiing to the take-off point, the flaps lever was put to position 3: fullleading-edge slats, flaps to 29¼ and ailerons drooped 10¼. Pitch trim for our 25.2% MAC (mean aerodynamic chord) condition was set to 3.2¼ nose up. Once in position and cleared for take-off, I advanced the throttles to 1.05 engine pressure ratio (EPR) and released the brakes. Gross weight at brake release was 258.4t, well under the maximum 365t. The empty weight of 223tincluded around 55t of flight-test equipment and ballast.
Once all four engines had stabilised I advanced the thrust levers to the take-off go-around (TOGA) detent. Acceleration was brisk as the engines stabilised at the maximum EPR setting of 1.361 on aday on which ambient temperature was 14¼C. Lelaie called "V1" at 140kt and "rotate" at 147kt.
Two-thirds aft stick travel was required to obtain the desired take-off pitch attitude. To help prevent a tail-strike during rotation, a large red pitch limit "V" was displayed on the primary flight display (PFD). Should the visual warning be ignored, the digital flight controls limit stick input to avoid a hard tail-strike. With a 15kt headwind the aircraft lifted off the runway approximately 30s after brake release and a ground run of 2,250m.
Lelaie raised the landing gear and I followed flight director (FD) guidance for our initial climb at 167kt (V2 plus 10kts). At 2,000ft above ground level, I retarded the thrust-levers to the climb (CLB) detent, selecting climb power with the autothrust system. At the acceleration altitude of 3,000ft AGL, I reduced pitch attitude, and selected flaps 1 passing 180kt. At 200kt the flaps were set to 0. Once in a clean configuration the FD commanded a 250kt climb. flight control system (FCS) automatic compensation was impressive.
The aircraft maintained the desired pitch attitude without additional pilot inputs. After levelling off at flight level (FL) 70 we turned towards a working area about 50km (30nm) south of Toulouse and north of the Pyrenees. I engaged the number 1 auto-pilot (AP) by a pushbutton on the glareshield-mounted flight control unit (FCU) and used its speed selector knob to set a 280kt climb to FL150. The AP was fairly responsive as I varied the aircraft's heading via an FCU selector knob.
I clicked the AP off with the stick-mounted thumb button. The aircraft' attitude remained constant. I did a half-stick roll to the left and released the stick just before 30¼ of bank. The aircraft smoothly stopped its roll. No back stick-pressure was required to maintain altitude as the FCS automatically does it at bank angles of 33¼ or less. Then I did a full stick roll to the right. As the aircraft rolled through 33¼ of right bank, I had to blend in aft stick-pressure to maintain a level flight. At 67¼ angle of bank the aircraft stopped rolling despite my continued stick deflection. In the FCS normal law, the envelope protection feature of the FCS will not allow bank angle to exceed 67¼. Steady-state roll rate was around 15¼/s. Once I released the stick, the aircraft rolled to the left and maintained a level 33¼ bank turn to the right. All turns were completed feet-on-the-floor as the FCS provides yaw-damping and turn co-ordination at all bank angles. Like earlier A340 models, the-600's turn co-ordinator and yaw damper are active even in direct law, the most degraded FCS mode available.
In normal law fore/aft stick movement commands normal acceleration or g. Neutral stick commands 1g un-accelerated flight along the flight path (note that un-accelerated in this case does not refer to airspeed). From a stable condition pushing the thrust-levers up causes the aircraft to speed up: the FCS automatically commands level flight, even as the angle of attack (AoA ) and resulting pitch attitude decrease. In a conventional aircraft, the pilot would need to trim nose down as speed increased. This phenomenon is called speed stability. Most aircraft display positive speed stability: from a trim condition the pilot must trim nose-down as speed increases or trim nose-up as speed decreases. Airbus' FBW aircraft, like the-600, demonstrate neutral speed stability. Changing stick forces as an aircraft speeds or slows from a trim condition are valuable tactile cues as to what an aircraft is doing. A g command FCS eases pilot workload for some tasks but removes some valuable tactile feedback in the process. Approaching both the high and low speed extremes of the flight envelope, however, some feedback is provided through the stick.
With the aircraft level at FL140, I advanced the thrust-levers to the CLB detent and accelerated the aircraft from 280kt to 330kt. During the acceleration no stick inputs were required to maintain level flight. When stable at Vmo , I did a number of sharp control inputs in all three axes. In each case the resulting aircraft movement was small and all residual vibrations damped out immediately. Even at high speed the cockpit noise level was low, allowing clear communication at normal voice levels. In addition to aural and ECAM warnings, the FCS willcommand a climb to reduce airspeed, should the pilot inadvertently exceed Vmo/Mmo. Extension of the speed brakes to the half and full positions caused a slight burble, but their extension and retraction caused no change in pitch attitude.
Slowing to 250kt, Lelaie selected direct law. Pitch control was good in this degraded mode, but speed changes required manual re-trimming via the console-mounted pitch-trim wheel. Roll control was more sensitive than in normal law, but 20¼ bank angle turns were easily accomplished. Overall in direct law the aircraft felt much like a first generation 747, a marked contrast to the crisp handling qualities it displayed in normal law.
Initial flight control laws, derived from earlier A340 models, are being refined during the test effort. The 11.6m fuselage stretch has led to the flight controls exciting the fuselage's 1.8Hz vertical and lateral bending modes. Finding a solution has required a few trade-offs. A single notch-filter was installed to extract the part of the control input causing the effect, but this did not cover minor frequency variations caused by many passenger and cargo distributions. A second notch-filter was installed to cover a wider range of excitation frequencies. This arrangement worked in cruise, but not for take-off and landing. Using in-flight configurable flight control software, at FL140 and 250kt, I saw the aircraft's response to sharp stick inputs for all three cases.
The basic aircraft/unfiltered response was very noticeable with residual vibrations only moderately damping out. The single notch response was more damped than the unfiltered, and the double notch response was essentially deadbeat, with no residual vibrations. Airbus has chosen a hybrid solution to cope with the 1.8 Hz bending modes. In cruise flight, where the aircraft will spend most of its time and passenger comfort is critical, the flight controls will use the double notch filter. For take-off and landing, where faster pitch response is needed, the single notch-filter will be used.
After re-engaging the normal-law flight controls, I slowed the aircraft to get a feel for its low-speed flying qualities. With a weight of 253t, landing gear down and flaps full, the lowest selectable airspeed (Vls) was 150kt, and roughly equates to 1.23 Vs1g (stall speed). At 5kt above this speed I was able to manoeuvre the aircraft precisely. As mentioned before, theA340-600 generally displays neutral speed stability. To improve flying qualities and protect from slow speed stalls, the flight controls do a number of things. The aircraft will not slow below Vaprot , a speed below Vls but above Vs1g , without an aft stick-input. When at Vaprot , a stall protection scheme is activated if continued aft stick-pressure is maintained.
If the aircraft continues to slow to Vafloor(a speed below Vaprot), the autothrottle will engage in TOGA mode to power the aircraft out of an impending stall. Additionally, maximum roll-rate is limited to half its normal value when the stall protection scheme is active. Disarming the autothrottle and throttling back allowed the aircraft to slow to its maximum AoA. With full-aft stick the aircraft reached 15.8¼ AoA at a speed of 120kt, slightly faster than Vs1g. The wings were rock steady as the aircraft entered a nose-high wings-level descent.
In most airliners with wing-mounted engines immediate pilot action (application of rudder) is required to handle the loss of an engine. On the 777, Boeing has installed a stand-alone thrust asymmetry compensator (TAC). The TAC operates in addition to the normal flight-control laws and does an excellent job of automatically countering yaw induced by an engine failure while keeping the wings level. In normal law the -600's flight controls automatically limit the effects of the loss of an engine. I simulated an engine failure on a go-around manoeuvre, gear down and flaps set to 4, by rapidly advancing the thrust-levers to TOGA at 170kt.
Once stabilised in a climb, Lelaie brought engine number 4 to idle. With feet and hands off of the controls, the right wing dropped slightly, less than 10¼ of bank, and aircraft entered a slow right-hand turn. On the PFD a blue "beta target" symbol appeared to aid the proper application of rudder. Much like a slip indicator, it guided my application of half-left rudder to zero the side slip. I then used lateral stick to level the wings. With the exception of the TAC-aided 777, the -600 has the most benign response to an engine failure of any wing-mounted engine-configured aircraft I have flown.
With the area work complete, we accepted vectors and a descent to Toulouse for an instrument landing system (ILS) approach to runway15L. I engaged AP1 for our return. AP interface through the FCU was quite intuitive as we set up on an intercept leg for final approach. Lelaie selected the ILS approach and runway in the multifunction controller/display unit, which automatically tuned the ILS. Once level at the glide slope intercept altitude of 3,000ft above mean sea-level, the aircraft slowed to green dot, a speed of 212kt (clean manoeuvring speed). The thrust-levers were now in the CLB detent, and remained there until I retarded them for landing. The autothrottle system does a fine job of maintaining speed, but does not move the thrust-levers. The pilot gets a sense of what the engines are doing via the ECAM engine display, but there is no tactile feedback for level of, or changes in, thrust output. At 200kt the gear was extended. Flaps were set to 1, slats only, and the aircraft slowed to S speed, 194kt for its 240t weight (maximum landing weight is 254t). Just before localiser capture, I disconnected the AP and extended the flaps to full, the landing position.
At glide slope capture, I lowered the nose 3í to track the glide path. The autothrottle now slowed the aircraft to its approach speed of 150kt, thrust-levers still in the CLB detent. As with all other phases of flight, the aircraft was always in trim and helped reduce pilot workload. At 400ft AGL, the red pitch-limit "V" was displayed on the PFD, a visual cue to the maximum flare attitude before a tail-strike. Normal pitch law is a g command scheme and would be unsuitable for the landing phase, as most pilots expect to pull aft on the stick to flare the aircraft. At 100ft AGL the flight-control pitch-axis went in to flare mode, direct law with some damping.
Entering ground effect caused a nose-down pitching motion which the pilot countered by pulling aft on the stick and flaring the aircraft. For my landing I pulled the thrust levers out of the CLB detent to idle at 50ft AGL and started the flare manoeuvre. Touchdown was on centreline but at the far end of the desired touchdown zone. Main landing gear touchdown caused the spoilers, armed on final, to extend. After lowering the nose to the runway, I selected mid-range reverse thrust and used manual braking to slow to a safe taxi speed. Taxi back to parking and post-flight procedures, similar to those of other large civil aircraft, were easily accomplished.
Overall, I found the A340-600 a delight to fly, with handling qualities that belie its large size. The spacious flight-deck was quiet, a feature sure to reduce fatigue on long flights. Airbus' common FBW cockpit philosophy can lead to significant savings in crew training costs, but its implementation does reduce tactile feedback cues.
The larger wing and Trent 500 engines give the A340-600 a 0.83 Mach cruise speed, an 0.01 Mach improvement over earlier A340s, but still slower than the 747s. With a range of 13,900km (7,500nm) and, typically, three-class seating of 380 passengers, the A340-600 is an ideal candidate for airlines looking to replace older 747 Classics, while significantly reducing operating costs.
Source: Flight International