News from ALPA's Committees
Airbus Welcomes New Narrowbody to Fleet
|Aircraft Design and Operations Group|
By Capt. Terry Lutz (Northwest), Director, ALPA’s
Aircraft Development and Evaluation Programs
Air Line Pilot, February 2004, p.22
Modern-technology airplanes are proving to be very adaptable to design changes, as the development of the newest narrowbody entry from Airbus, the A318, demonstrates. From program launch in April 1999, Airbus overcame a number of design challenges to develop an airplane with a shortened fuselage and the same basic operating characteristics as the other members of the A320 family.
The vertical tail and rudder would have to be larger to provide the directional stability and the control necessary for engine-out situations, with a design target to keep the speeds for Vmcg and Vmca in the 108-110 knots range. To achieve this goal, Airbus increased the vertical fin and rudder areas by 7.5 percent, adding 30 inches to the height of the vertical fin.
The horizontal tailplane (the stabilizer/elevator combination, or HTP) remains the same size, but like the vertical-fin/rudder combination, the distance between the HTP and the center of gravity (CG) is shorter. In the angle-of-attack (AOA) protection mode of the flight control system, the engines automatically increase power to takeoff/go-around (TOGA) thrust, producing a significant nose-up pitching moment. Therefore, the trimmable horizontal stabilizer (THS) would need to be active to assist with pitch control. In normal law, the A318 uses an active THS and the elevator to automatically compensate for pitch changes caused by increased thrust.
The worst case, and the determining test for certification, was applying TOGA thrust at maximum AOA and maximum aft cg. Even with the assist from the THS, the aft CG limit of the A318 had to be reduced from 38 to 35 percent mean aerodynamic chord (MAC) to preserve pitch authority. Airbus test pilots also discovered that when flying at high altitude and low airspeed in either alternate or direct law, the airplane did not adequately compensate for the nose-up pitching moment caused by applying thrust, which required further limiting the CG to 33 percent MAC.
One of the major challenges a flight test team must face is determining the speed for "minimum unstick," or Vmu. Vmu determines the remainder of the V-speeds for the airplane. For a very long airplane, the maximum aerodynamic capability cannot be reached on takeoff because the tail will strike the runway first. The A318, however, is not geometry-limited and can be rotated to the AOA for maximum lift. Rather than use significantly lower speeds, Airbus chose to base the A318 V-speeds on the more conservative Vmu determined by flight tests of the A319.
Flight testing identified a few additional areas in which more development was needed. During takeoff testing in which the test pilots used abnormally high rotation rates, interaction between the engine exhaust plume, the horizontal tail, and the close proximity of the runway resulted in a momentary increase in pitch rate at liftoff. Airbus added a "stick smoother" function, which reduces the pitch rate command of the pilot when pitch rate exceeds 6 degrees/second. To prevent overstress on the nose gear caused by reduced HTP authority, the A318’s brake steering control module was modified for lower pressure at initial brake application when in a nose-high attitude.
The airplane flown for this evaluation, the A318 prototype s/n 1599, F-WWIA, was in a flight-test configuration with an engineer’s station located mid-fuselage and a ballast system positioned both forward and aft. Walk-around inspection revealed a few exterior differences. A small strake is above the open nose-gear doors on each side of the airplane. When the landing gear is down, the wake of the open nose-gear doors disturbs the AOA probes when the airplane is at a high AOA with some sideslip present. Some risk exists of entering AOA protections during low speed flight with one engine inoperative during landings in severe crosswinds. The strakes reestablish normal airflow behind the nose landing-gear doors.
The A318 wing incorporates the latest standard in the Airbus Lift Improvement Program, which includes a cover for the telescoping deicing tube and a slat fairing at the fuselage. This produces a slightly higher Clmax and improves performance margins.
On the flight deck, the A318 looks the same as any other narrowbody Airbus aircraft, with a few important differences. All the displays are now LCD screens—brighter with more useable display area on the edges and corners than the original CRTs. The braking system is now "all electric," using an electric backup control unit adapted from the A340-600 program. Nosewheel steering (NWS) is now available with hydraulic pressure being provided from the yellow electric hydraulic pump. This provides NWS for "powered push" operations that will be described later. The A318 flight management system contains the FMS2 software, allowing much greater flexibility for using abeam waypoints and other flight plan modifications.
Ramp weight for the test flight was 121,275 pounds, with 22,000 pounds of fuel and a CG of 25.4 percent MAC. The flight was planned to include approximately 1 hour of air work, which would put us back in the landing pattern at Toulouse just under the maximum landing weight of 113,100 pounds.
The engines installed on the test airplane were CFM56-5B9+ models, rated at 23,300 pounds of thrust. At lower temperatures, the B9+ rated engine is static-thrust-limited, but at higher temperatures, it has the higher thrust characteristics of the B6 engine used on the A319. This gives the A318 excellent takeoff performance in hot, high-altitude conditions. Our takeoff was planned with Flaps 3, flex thrust set for a temperature of 50 degrees C, and trim set at 1.6 degrees nose up. Speeds for the takeoff were V1=116 KIAS, Vr=120 KIAS, and V2=124 KIAS.
Even with flex thrust in use, acceleration was brisk, and V1 and Vr were only about 1 second apart. With the airplane accelerating nicely and a normal 3-degree/second rotation rate, liftoff occurred slightly above V2+10. I followed the flight director commands, cleaned up on schedule, and climbed toward working airspace about 30 nm southeast of Toulouse. Once level at 13,000 feet, I deselected autothrust and set the thrust levers to maintain 250 knots. Flying manually, I visited a few of the protections the flight control system provides in normal law.
Banking the airplane to 33 degrees and slightly beyond felt normal, but even with maximum stick deflection, getting to 66 degrees of bank was difficult. We entered a slow deceleration with the thrust levers at idle and speed brakes extended. Maintaining altitude became increasingly difficult as the airplane passed through the alpha-protection range. At alpha floor, thrust automatically advanced to TOGA, the speed brakes automatically retracted, and the airplane settled at 14 degrees AOA with the stick full aft.
Accelerating back to 250 knots at 13,000 feet, Airbus test pilot Capt. Philippe Pellerin reconfigured the flight control computers to put the airplane into alternate law in the clean configuration. Pitch response to a sharp input and release was immediately damped with no overshoot. To examine the Dutch-roll dynamics, I used a sideslip-release technique. At 250 knots, rudder deflection is limited (as indicated on the lower ECAM F/CTL page). So, with maximum pedal deflection, only 4 degrees of sideslip is produced. When the rudder input was released, the Dutch-roll motions were lightly damped, as expected.
Slowing to 160 knots and selecting Flaps 3, I asked Capt. Pellerin to extend the gear, which put the airplane into direct law. Trimming manually for level flight, I repeated the dynamic investigation in pitch and yaw. Pitch response was slower, but still immediately damped with no overshoot. With more rudder available at this speed, I used the flight-test sideslip indicator to put in just enough rudder to produce 4 degrees of sideslip. When released, the Dutch roll was again lightly damped, with roll response to yaw motion slightly higher than in the clean configuration. From this quick look at the dynamic response of the A318 in degraded control laws in a clean and approach configuration, I couldn’t see any difference from the other members of the A320 family.
Flight test engineer Didier Ronceray then suggested that we return the flight control system to normal law and investigate maximum performance against multiple protections in the landing configuration. I established a normal descent at 160 knots in Flaps 3 and simulated a collision-avoidance maneuver. Pulling hard with full back and full left stick at the same time, the airplane automatically went to TOGA thrust, pitch went to 15 degrees nose high, bank angle was 20 degrees left, and airspeed settled at 101 knots. The pilot can go to those limits immediately, without prior practice, and without fear of stall or upset.
As we began our descent back to Toulouse, Capt. Pellerin demonstrated some new features of the FMS2 software. Radial intercepts have always been a large mental exercise for Airbus pilots; but with the changes in FMS2, the computations are done quickly, and the pilot has full situational awareness of the change. A new page allows the pilot to input either "radial in" or "radial out." Once that is done, a temporary flight plan element appears on the nav display in white. The temporary flight plan is completely separate from the secondary flight plan.
During the descent, I flew four stabilized test points to measure descent rate to find out if the shortened airplane would have a problem getting down in congested terminal areas. Descent rates ranged from 1,100 ft/min in a clean configuration at green dot speed, to 2,500 ft/min at 250 knots clean with the speed brakes extended. These descent rates were enough for us to join the straight-in ILS for Toulouse. In some terminal areas, A318 pilots will have to plan ahead to ensure that there is enough drag on the airplane to meet ATC requirements. On the A318, speed brakes are available with Flaps Full.
The subject of speed brakes is one that ALPA’s Air Safety Structure has discussed with Airbus for several years. A good Airbus pilot will keep a hand on the speed-brake lever as a reminder that the speed brakes are out, because the buffet level is low, and the amber SPEED BRK memo message on the lower ECAM does not command the pilot’s attention. For the A318, Airbus has added an additional caution to the pilot. If below 800 feet radar altitude, or if more than 45 seconds have elapsed and thrust is greater than idle, the master caution light will illuminate with a single chime, and a "SPEED BRAKES STILL OUT" message will appear on the lower ECAM. This is an excellent safety enhancement and one that we hope can be implemented on the other Airbus models.
My pattern work in Toulouse began with a manually flown straight-in approach in normal law, with autothrust off. I had no problem maintaining airspeed using manual thrust. The thrust levers move smoothly with light force and are always perfectly matched. I flew the second approach manually with autothrust on. During rotation for the subsequent touch-and-go, Capt. Pellerin pulled one thrust lever to idle, simulating an engine failure just past V2. Rudder pressure to center the beta target is light, and the pilot can easily center it without overshoot.
For my third and final landing, I specifically wanted to fly the pattern and land in direct law. On the downwind leg, Capt. Pellerin configured the flight controls to put the airplane into alternate law. On base leg, he extended the gear, and we were in direct law. To keep the workload as high as possible, we flew the approach with autothrust off. So in addition to trimming manually, I had to keep a close eye on airspeed. The trim requirement is actually very small, meaning that only small movements are required on the trim wheel. I kept the thrust fairly constant, avoiding any large changes that would require a compensating stick input.
I easily compensated for configuration changes with small stick and trim-wheel movements. From our air work, I knew that the dynamic characteristics in pitch would be excellent for the flare and could see no difference from a landing in normal law. Crosswind landings will require a little extra care because the decrab maneuver may cause the upwind wing to rise. All things considered, including the increased workload with manual thrust and trim, approaches and landings in direct law can be performed with confidence and accuracy.
The engineering team and flight test department at Airbus have done the hard work to make the A318 fit perfectly into the A320 family. They have made changes where necessary and used fly-by-wire technology to control small problems around the flight envelope. The airplane handles and performs like the other family members, and the pilot does not have to remember specific details or compensate for differences that do not exist. The improvements that are incorporated in the A318, including wing changes, FMS2, LCD displays, and the speed brake caution, are all features that should be incorporated in every Airbus airplane we fly.
So where does the A318 fit in the competitive airline world? Airlines most likely to buy this airplane have an existing Airbus fleet and serve either thin markets or markets that ebb and flow on a seasonal basis. Airlines that have Airbus airplanes already equipped with CFM-56B6 engines will see operating economies with the common engine type.
Remember the A318 change in which nosewheel steering is now available on the yellow-system electric hydraulic pump? At the airport terminal, I observed a pushback operation that you have to see to believe. The tug for the pushback is attached to the left main gear and is controlled remotely by an operator who is standing in front of the nose and giving turn instructions to the pilot through the interphone. This is called a "powered push." With nosewheel steering available through an electric pump, an engine does not have to be started until after the pushback is complete. When the airplane stops, the tug is remotely disconnected and driven straight back from the airplane. After the airplane taxis clear, the operator walks to the tug and drives it back to the gate.