Eight months ago, the Airbus A380 programme suffered a potentially disastrous setback when the International Civil Aviation Organisation (ICAO) said the giant aircraft’s wake vortex characteristics demanded special treatment and implemented a provisional set of separation requirements for the aircraft during approach, take-off and cruise. These requirements threatened to wipe out any capacity gain the A380 could offer busy airports like London Heathrow, by reducing the number of landings per hour that the airport could handle and, not surprisingly, Airbus has protested strongly about them.
Until that ICAO bulletin, Airbus was hopeful that the A380 would be included in the “heavy” aircraft category for separation, alongside other large aircraft such as the 747. This would have required separation distances on approach of 4nm (7.4km), 5nm and 6nm from other heavy-, medium- or light- category aircraft respectively. In cruise, separation from other aircraft would have been 5nm.
However ICAO’s interim recommendations announced last November pushed the A380’s approach separation from any category of aircraft (including another A380) to 10nm, and 15nm in the cruise. Airbus strongly contested the proposals and set about compiling more detailed comparative data. “We leased a 747 and a 777 and flew circuits with an A380, A340-600 and A318 at the Istres flight-test airfield to compile comparative data on the same day in the same weather conditions,” says Airbus senior vice-president flight operations Claude Lelaie. He says the data shows the model used for vortex computation was incorrect and Airbus has since flown around 100h of tests to establish a new model.
Meanwhile, the ICAO A380 working group which met in Montreal on 9 June to discuss relaxation of the interim recommendations broke up without agreeing new limits. “It had been planned to reach a conclusion at this meeting, but it became apparent very quickly that there was some information missing in some areas, so we will perform additional tests,” says Lelaie.
“It was agreed that the existing limits were too conservative and that a new set of interim requirements would be established, pending additional information from Airbus,” says Airbus senior vice-president flight test Fernando Alonso.
Airbus immediately set about more back-to-back testing, but in the meantime “targets” have been set for new interim recommendations, which “will still be conservative. We have further meetings planned and aim to have these new interim recommendations issued before entry into service,” says Lelaie.
The target for the new separation distances behind an A380 are likely to be: heavy - 6nm; medium - 7nm; light - 9nm (see graphic).
“We believe that these should be the targets for the interim situation and that they are conservative,” says Lelaie, adding: “It’s urgent to come to a conclusion before entering into service. But we can continue to work on the subject.”
Although it was Airbus’s aim to have the A380 classed as “heavy” and retain the existing three-category system, if it proves necessary to create a new classification for the A380 then “we have to use this benefit in both directions”, says Lelaie. “If we say that the vortex of an A380 is stronger than that of a 747, then it means that the A380 will resist better the vortex of a 747 than another 747. Maybe you can put an A380 at 3nm behind a 747 [instead of 4nm],” he says. Lelaie is also hopeful that ICAO will recommend that the distance between two A380s on approach be 4nm.
Lelaie says the amount of hold-time required for an aircraft departing behind an A380 is still under discussion. Last year’s recommendation was 3min, compared with 1-2min for heavy-category aircraft, depending on routings, and Lelaie says he believes the recommendation will be “plus 30s” for aircraft departing behind an A380. “That is our feeling, but we need to perform more back-to-back tests,” he says. More studies will be undertaken to establish the separation requirements in the cruise and for the take-off hold-time, he adds.
Lelaie does not rule out tweaks being made to the A380’s aerodynamics, in the longer term, to mitigate the impact of its vortices on other aircraft: “We have tried several things and there are quite of lot of ideas, but we have no time to make serious aerodynamic configuration changes before service entry,” he says.
Source: Flight International
