A3XX Technology

Airbus Industrie's A3XX is not just the biggest airliner ever designed. Airbus engineers intend the new jet to set a new standard in technology, with the aim of delivering significantly better efficiency than any straightforward stretch of the 747.

The basic design is heavily influenced by one driving requirement: airport compatibility, which set an 80-meter (262-foot) limit on length and wingspan. This not only drove Airbus to a double-deck fuselage, but has profoundly affected the design of the wing.

The A3XX wing is huge, with an area of 9,000 square feet-two-thirds bigger than the 747 wing. Several factors drove the wing size, says A3XX engineering VP Robert Lafontan. "We considered high-speed performance and drag. We considered low-speed performance, and we needed a wing that would carry enough fuel for all versions."

Operationally, Airbus found its customers wanted a high initial cruise altitude, so that the aircraft could climb above weather and puddle-jumper traffic even with a full load. The goal for the A3XX is 35,000 ft-4,000 ft higher than some of today's aircraft. Together with the need for fuel capacity growth, this set the wing's area, and the 80-meter span constraint results in an aspect ratio of 7.6-lower than the A330/340.

Regularly zinged by its rivals on the cruise speed issue, Airbus has designed the A3XX for Mach 0.85. The result is rather more wing sweep than the A340.

The big wing is lightly loaded-even the heaviest A3XX has a wing loading around 140 lbs/sq ft, versus 160 lbs/sq ft and more on many long-range jets. Consequently, the A3XX gets by with a simple one-segment slotted flap.

Another key issue in wing design is the vortex wake: the A3XX's extra capacity will be little use in reducing congestion if landing separation distances have to be increased. According to Lafontan, Airbus research-including test flights and the use of free-flight models -has shown that weight and wingspan are only two of several factors in the vortex equation. In the A3XX the flap design, engine location and pylon design all play a part in ensuring that the vortex pattern is weak and unstable, decaying quickly behind the aircraft.

Aerodynamic design plays a part in reducing noise: on landing, with today's quiet engines, half the noise comes from the airframe. The A3XX's new engines, with 110-inch fans and higher bypass ratios than today's turbofans, will also play a part in meeting current and future noise rules. (The 747X will use similar engines, but with smaller fans.)

A big wing can entail a weight penalty, so Airbus is applying new technology to reduce weight wherever possible. For the first time, the central wing box will be made from carbon fiber composite material, saving some 1,800 pounds. Airbus is mulling the choice between composites and advanced bonded aluminum for the outer wings. (The mid-section will be metal because of complex loads and design requirements.) The horizontal tail, as big as an A310's wing, will be the largest composite structure outside the B-2.

The upper fuselage skins will be made from a new laminated material, comprising very thin alternating skins of S-2 fiberglass and aluminum. GLARE (glass fiber reinforced) panels are 25% lighter than aluminum, are highly resistant to fatigue and cracking, and cost less than carbon composites. In the lower lobe of the fuselage, Airbus will use continuous laser welding to join the stringers to the skins, eliminating fasteners and providing an inherently corrosion-resistant structure.

The A3XX should be the first "more electric" production aircraft. It will have two hydraulic systems-running at 5,000 psi to reduce size and weight-backed up by two electro-hydraulic actuators, each run by a separate electrical system, at each control surface.

Throughout the aircraft, according to Lafontan, an important goal is to make the A3XX "fault-tolerant"-it will be able to be dispatched safely with failures that would ground today's aircraft, returning to a main base where the problem can be fixed. Prognostic maintenance and remote monitoring will be used on all systems.

Given Airbus' established approach to technology, the next Airbus-whatever it may be-will build on A3XX technology. For instance, Lafontan says that the hybrid electric/hydraulic system is an interim step: if it lives up to its promise, the next Airbus will be all-electric. Airbus, which has been bold in its use of technology in the past, does not seem to be getting conservative in its old age.

By Bill Sweetman