Flying the Longest-Range Airplane in Production
The Airbus A340-500
By Capt. Terry L. Lutz (Northwest)
Air Line Pilot, February 2003, p. 22
On Aug. 2, 2002, a Virgin Atlantic A340-600 flew nonstop from Singapore to Las Vegas, a distance of 7,820 nautical miles. The newest versions of the A340, the -500 and -600, are not just lengthened and strengthened airplanes; they have been redesigned in critical areas to provide a significant increase in range. And they are targeted for operations from the Pacific Rim to the United States. In June 2002, Capt. John Cox (US Airways), ALPA’s Executive Air Safety Chairman, First Officer Dave Hayes (Northwest), director of ALPA’s Certification Programs, and I, as director of ALPA’s Aircraft Development and Evaluation Programs, met in Toulouse, France, to learn the engineering details and see the results from the pilot perspective.
Engineers have no magic formula for designing long-range airplanes. The basic considerations are lift-to-drag ratio (L/D), weight fraction (the weight at takeoff compared to the weight at landing), and specific fuel consumption.
|The use of carbon fiber in critical areas may be surprising, but this proven technology was expanded in the A340 family and will be applied extensively in developing the A380.|
Airbus began developing plans for this airplane by adding a wing box insert to previous A340 designs, which increases the chord of the wing by three full frames, or 5.6 feet, at the fuselage, tapering to the wingtip. This accomplished several things simultaneously. Overall wing area increased by 28 percent to 4,704 square feet, and more significantly, internal wing fuel volume increased 38 percent to 49,353 gallons (330,700 lbs at 6.7 lbs/gal).
The wing box insert changed the airplane’s aerodynamic characteristics. The wing span increased by 10.8 feet with a wingtip extension, but aspect ratio actually decreased from 9.26 to 8.57. The wing became more swept, moving from 29.7 to 31.1 degrees. Increasing the chord at constant thickness reduces the thickness ratio of the airfoil by 2 percent.
The increased sweep and decreased thickness ratio combined to make the airfoil more supercritical, pushing the divergence drag mach number higher, which moved the design cruise speed up to Mach .825. Airbus uses Mach × L/D as a measure of cruise efficiency, and has been able to increase efficiency in the A340-500 by 2 percent over the A340-300.
Selecting the engine was an interesting challenge. The original A340-200 and –300 are powered by four CFM56-5C4s, rated at 31,200 to 34,000 lbs of thrust. Airbus asked Rolls Royce to develop a new engine, the RR Trent 500, rated at 56,000 lbs thrust for the A340-600, and 53,000 lbs of thrust for the A340-500. To accomplish this, Rolls Royce took the Trent 700 fan, fan case, and accessories and mated them to the core of the Trent 800. The Trent 800 has a number of internal improvements, including a 3-D compressor blade design, and higher turbine blade speeds to reduce aerodynamic loading while improving efficiency. The resultant higher bypass ratio and internal efficiency of the Trent 500 design provides 7 percent lower specific fuel consumption than the Trent 700.
With the design nearly set for significantly increased range, Airbus engineers, seeking to keep the airframe as light as possible to further improve the weight fraction, used advanced materials throughout the airframe—carbon fiber for the complete horizontal tail, aft pressure bulkhead, and keel beams; titanium in the engine pylons; and lighter weight, higher strength aluminum alloys for numerous skin panels. The use of carbon fiber in critical areas may be surprising, but this proven technology was expanded in the A340 family and will be applied extensively in developing the A380.
Before we flew the A340-500, we wanted to know about any flight control system changes in the new design. The cable system that controls the rudder in the event of electrical failure is changed in the A340-500/600. The rudder is now fully electric and has a damper in the control system to provide normal rudder feel. In normal law, the rudder is powered by the blue, green, and yellow hydraulic systems, and controlled by the primary flight computers (PRIMs). If the PRIMs lose electrical power, a back-up control module, which gets electrical power from two permanent magnet generators driven by the blue and yellow hydraulic systems, control the rudder. From the pilot perspective, rudder feel and response is just like previous Airbus models.
As you might expect from a very long airplane (the A340-600, at 246 feet 11 inches long, is 7 feet longer than the A380 currently in development), designers must take into consideration the possibility of a tailstrike on both takeoff and landing. On the PFD (primary flight display), a pitch limit chevron, shown in red, provides the pilots with a visual pitch reference for the tailstrike pitch attitude. For the A340-500, this limit is shown at 10 degrees for takeoff and 8.7 degrees for landing. More importantly, the flight control laws now contain a "soft protection" to make the pilot’s pulling to the tailstrike pitch attitude more difficult. The soft protection is a function of pitch rate, pitch attitude, and stick position, but will still allow the pilot to pull through the protection, if necessary.
Pitch axis changes
The designers have made two more important changes in the pitch axis. The first is the high AOA protection phase-out logic. With this change, if the high AOA protections are triggered and actual AOA is less than AOA protection and the sidestick is neutral, the normal control laws are automatically recovered. The aim of this new logic is to avoid being stuck at high AOA while in AOA protection in reaction to a gust, should the pilot not push forward on the stick. The second change is noted after engine start, or during a touch-and-go, when the pitch trim automatically repositions corresponding to the actual cg in the FMGC (flight management and guidance computer).
|The electric rudder feel system provides positive control, and small corrections can be made during taxi without the steering feeling crisp or heavy.|
The test flights
The airplane we flew was A340-541, ship number 0394, the first A340-500 that Airbus manufactured. It was in a test configuration, with a water ballast system installed, and an engineer’s station in the forward part of the cabin. Our test pilot for the flight was Capt. Lucien Benard (see biography, page 25), and we were most fortunate to have Gilles Robert (see biography, page 25) on board as flight test engineer. Our ramp weight was 528,000 lbs, and we dispatched with 110,000 lbs of fuel. That’s not very close to the maximum takeoff weight of 804,000 lbs, but our pattern work would put us near the maximum landing weight of 520,000 lbs.
We pushed back from the docking bay of the Airbus flight test facility in Toulouse with Capt. Cox at the controls. A taxi-aid camera system (TACS) provides two views of the airplane—one from the vertical stabilizer and one from behind the nosewheel.
Switches on the overhead panel bring the split screen TACS display either up in front of the captain and first officer or on the lower ECAM. The upper portion shows a clear view from behind the nosewheel, with marks to show where the main gear will travel. The lower display depicts the overhead view from the tail, with brackets indicating the position of the main gear. In the nosewheel view, we watched as the tug was disconnected and moved clear of the airplane. All of us commented that this enhanced our awareness of activity around the airplane.
Most of the A340-500/600’s controls and displays are similar to those of the other Airbus product line. We easily started the engines two at a time after selecting ignition on and moving the console-mounted start switches forward. Airbus learned during high-altitude testing that the engines are best started individually when on APU bleed. At engine start, a box is drawn in the upper ECAM around the engine being started. When the start is complete, the boxes disappear, and grey shading appears on the EPR display, indicating the normal EPR operating range.
As we taxied for takeoff, Capt. Cox evaluated some of the new features of the A340-500/600. The electric rudder feel system provides positive control, and small corrections can be made during taxi without the steering feeling crisp or heavy. Normal ground steering through the tiller is improved with installation of a damper, making ground handling very positive for an airplane of this size and weight. The TACS allowed precise turns with a simple technique: when approaching the turn, allow the curved taxi line to track down the fuselage until it meets the wing line, then start the turn and keep the line on the fuselage/wing intersection.
Takeoff trim is set automatically at engine start, depending on what information the pilot enters in the FMGC. If the trim setting inserted on the MCDU Perf page does not agree with the position of the trim wheel, a PITCH TRIM MCDU/CG DISAGREE caution message will appear. For this Flaps 3 takeoff, the cg was 30.0 percent MAC. The pilots are responsible for checking that the proper information is inserted. We would prefer that the normal method of setting pitch trim refer to percentage cg rather than to index units. The use of percentage cg would eliminate errors of + and – ambiguity, and make the pilots more aware of actual cg location at takeoff.
Lining up for Runway 33, Capt. Cox advanced the thrust levers for a FLEX thrust takeoff. TOGA thrust for the A340-500 is 53,000 lbs per engine, which is derated for Vmcg considerations because the -500 has a shorter fuselage than the -600. When the thrust levers are in the MCT/CL (max continuous thrust/climb) detent, thrust is the same as on the longer A340-600.
Takeoff speeds for our Flaps 3 departure were V1=136, Vr=138, and V2=151. With the thrust levers in the FLEX detent, the red pitch-limit chevron appeared on the PFD and remained in view until the airplane was above 400 feet. At Vr, Capt. Cox smoothly rotated, and the airplane lifted off at 7 degrees pitch, well below the displayed pitch limit of 10 degrees, and continued rotation to the flight director target of 15 degrees. Cleanup was on schedule, and climb continued to FL250, southwest of Toulouse. We noticed a good amount of noise from the nose gear mechanism during retraction, but otherwise noise levels in the cockpit during climb at high power setting were ideal for normal conversation. During cruise above FL250, actual CG is calculated in the FMGC based on initial CG, ZFW, and burn schedule. To achieve the optimum CG for cruise, fuel begins to transfer to the trim tank in the horizontal tail, up to a total of 14,300 lbs.
Our main purpose for this flight was to explore some specific flight control features in the A340-500/600 and not to revisit the well-documented "hard protections" in the Airbus control laws. Many large airplanes exhibit structural response modes in turbulence or during sharp control inputs. These modes can cause, in the aft cabin, motion that differs from what the cockpit is experiencing. For the A340-500/600, Airbus has developed control laws for flexible airplanes. In addition to six gyrometers located near the CG, Airbus has added three vertical and two lateral accelerometers near the cockpit, and three rear lateral accelerometers in the aft fuselage. Inputs from these additional sensors provide aircraft motion data to the PRIM computers to significantly reduce aircraft motion caused by structural response.
All three ALPA pilots had the chance to investigate aircraft response with rapid, and at times cyclic, inputs in pitch and roll, and noted the absence of aircraft motion from structural modes. Aircraft response to aggressive inputs was smooth and predictable, within existing flight control system limitations, and without exciting an uncomfortable airframe response on the flight deck, or in the cabin.
To investigate changes to the high AOA protection logic, F/O Hayes slowed to V2 in the landing configuration, stabilized, and reduced thrust to idle. Holding altitude as speed decreased, the airplane eventually went to alpha floor, thrust automatically went to TOGA, and the airplane began to climb at very slow speed as F/O Hayes released the stick to neutral. As the airplane accelerated past the speed for AOA protection, it returned to nearly level flight and began autotrimming. From the pilot’s perspective, this small change made the airplane feel and appear much more predictable in the pitch axis. With the original version of the high AOA protection logic, the airplane would maintain the AOA computed as the airplane decelerated through the speed for alpha protection (also the speed at which autotrimming stopped), and continue the slow speed climb until the pilot pushed forward on the stick to regain autotrimming.
F/O Hayes also investigated some of the other control logics, including the "dual input" audible and visual warnings that come on if both pilots are providing control inputs (either additive or subtractive), and auto spoiler retraction. If the spoilers are extended and the thrust levers are moved forward of the climb detent, the spoilers will automatically retract, followed by a chime and a Spoiler Disagree message on the lower ECAM. However, using spoilers in a descent and leveling off with spoilers extended is still possible. With no significant buffet at lower speeds, with no audible alert, and with pilots not checking the ECAM, the spoilers could remain out until Flaps 3 are selected for landing. At that point, they will automatically retract.
In preparing for engine-out events on landing, I assumed the left seat and flew some V1 cuts at altitude. If the airplane senses a difference in thrust, the beta target on the bank-angle pointer turns blue. When centered, the beta target provides the correct sideslip angle to keep spoiler deflection at zero and achieve minimum drag. Because we were flying a test airplane with sideslip indicator, we could note that in landing configuration with Flaps 3, the actual sideslip angle and bank angle were both about 2 degrees.
We descended into Toulouse for landing on Runway 33. The fully configured final approach speed was 134 knots at a weight of 512,000 lbs. To allow the airplane to fly more slowly in the landing configuration, Airbus added, to each engine nacelle, strakes that provide a 5-knot reduction in approach speed at the same pitch attitude (see photograph of landing at La Paz, Bolivia, page 23).
Because we would be vectored for a straight-in ILS, the first approach and landing was planned to be an autoland, with winds from 280 degrees at 10 knots, well under the autoland crosswind limit of 15 knots. We followed standard Category III procedures, using both autopilots, and monitored the final portion of the approach on the upper flight-mode annunciator line, as the autopilot mode changed from approach to land to flare. Touchdown was normal and precisely on centerline. Derotation to nosewheel touchdown was also done on the autopilot, at which point we disconnected the autopilots and reconfigured for the touch-and-go.
We turned onto the visual downwind at 1,500 feet AGL, keeping the pattern as tight as possible. The next approach was a normal four-engine approach with flaps full to a touch-and-go. Touchdowns in this airplane are smooth and predictable. The A340-500/600 has a braked center landing gear with four wheels, and on landing, the 12 main tires touch down at nearly the same time. We had no feeling of a double touchdown during derotation—a well-known characteristic of the A330, which has no center landing gear.
After reconfiguring for the go-around, Capt. Benard brought the No. 4 thrust lever to idle, simulating engine failure at V1. I centered the beta target, which had again turned blue to indicate that sideslip was not optimum. We cleaned up at 1,000 feet AGL as we turned to downwind. The next approach was a three-engine approach with Flaps 3. Even though we were just below max landing weight, thrust on three engines is about the same as thrust would be at flaps full with four engines. Touchdown was normal at 5 degrees nose-up pitch, well below the pitch limit of 8.7 degrees.
After a normal touchdown, all four thrust levers were brought up for the go-around. During the subsequent turn to downwind, we swapped seats so that F/O Hayes and Capt. Cox could repeat the pattern work. Sometimes the jumpseat is a good place from which to observe and learn. While the next patterns were flown, I noticed on the flight test instrumentation that the airplane does almost everything at the same angle of attack, 4 degrees. I also observed something that I have always felt is quite remarkable about the Airbus control system design. Even though the sidestick controller has limited displacement and is rather well damped, the pilot still has the ability to "sample" for the runway in the flare. F/O Hayes used this technique, and I observed him make three or four smooth stick corrections to constantly improve his sink rate at touchdown. And he was rewarded in those efforts with the smoothest touchdown of the day, at 6 degrees pitch attitude.
After Capt. Cox completed our full stop landing, I swapped back into the seat to evaluate the taxi-aid camera system. Sure enough, during our pattern work some unfortunate insect had splashed itself on the lens cover of the tail-mounted camera. But it did not degrade our ability to use the TACS for precise taxiing. Using both cameras is a good idea, because at times the taxiway centerline is worn, or not continuous at an intersection; at those times, the lower view will ensure that the main gear stay on the paved surface. Pulling back into the dock at the Airbus flight test facility, I noted that as you get closer to the gate, the most important reference is still the marshaler. The TACS should only be used as a brief crosscheck when close to the gate.
In the tightly contested Pacific market, range and payload are the name of the game. Changing the wing, increasing fuel capacity, using lighter weight materials, and lowering specific fuel consumption are the significant engineering challenges that Airbus has successfully met with the design of the A340-500/600. They have used the advantages of the fly-by-wire control system by adapting it to changes in the airframe, without creating hardware changes in the flight control system itself. A good example is the soft protection designed to prevent tailstrikes on takeoff and landing. The impression that we had when manually flying the airplane was that we were flying a large airplane with precise control response similar to that of the A320. Long range and precise control are good things for airlines at the end of a financial year, and for flight crews when landing after a 14-hour flight. We expect that the A340-500 and 600 will be highly successful additions to the Airbus product line.
Capt. Terry Lutz, director of ALPA’s Aircraft Development and Evaluation Programs, also wrote "Flight Evaluation: Boeing 767-400ER," February 2001.
|Meet the Test
Pilot and Flight Test Engineer
Capt. Lucien Benard, the Airbus test pilot who flew with us during our evaluation of the A340-500, graduated from the French Air Force Academy in 1967 and flew reconnaissance missions in the Mirage IIIR from Strasbourg, France, for the next 7 years. In 1980, he attended the Empire Test Pilot School at RAF Boscombe Down in the United Kingdom. From there, Capt. Benard was assigned to the French Flight Test Center at Istres, France, where he flew the Mirage F1CR and the Mirage 2000. He also served as an instructor pilot at Écôle Personnel Navigant Essais Reception. In 1985, Capt. Benard began a civilian test pilot career at Reims Aviation, where he was assigned to the Caravan 2 test program. He moved to SOGERMA Bordeaux in 1989, where he flew the Jaguar, Mirage F1, Nord 262, C-130, and Falcon 20. He moved to Toulouse in 1994 and became a test pilot for Aerospatiale, flying the ATR 42 and 72. He was project test pilot for the first flight and development test program of the Beluga transports, which are A300 aircraft extensively modified to bring outsize airframe components to Toulouse from manufacturing facilities around Europe. Benard joined the Airbus team in 1996 and is currently project test pilot on the A340-500/600 program.
After graduating as an engineer from ENSA (Écôle Nationale Supérieure de l’Aeronautique) in 1964, Gilles Robert qualified as a flight test engineer (FTE) at EPNER (Écôle Personnel Navigant Essais Reception), the French test pilot school, in 1966 and was assigned to the simulator center in Istres for 4 years with particular responsibilities for the Mirage IIIB variable stability aircraft developed for handling qualities of new projects, such as the Concorde. Robert was then designated as an FTE for the certification of civil aircraft and was responsible for developing a Falcon 20 variable stability aircraft and for the coordination, as a flight test specialist, of activities linked to evolutions of the regulations. In 1984, Robert joined Aerospatiale Toulouse as an FTE in charge of handling qualities and performance of the ATR 42. In 1986, he joined Airbus Industrie Flight Division, also at Toulouse, as the FTE in charge of the development and certification of flight control laws and handling qualities of Airbus Industrie new products, such as the A320, A340, and their derivatives. Since 1994, Robert has been the director of the Test and Development Department within Airbus. He has logged more than 5,000 flight test hours on some 150 aircraft types.