The high temperatures of the last few days altered the performance of aircraft during the take-off and climb phases, as a consequence of which the perimeter of the areas in which the local residents hear the noise shifted.
The heatwave recorded in the last few days in Geneva was felt particularly by those airport staff who work on the apron concrete, where temperatures were over 40°C at times. Out of concern for the welfare of staff working outside, the airport authorities distributed water at regular intervals to help them keep cool and combat dehydration.
The heat also affected the geographical perimeter within which the local residents around the airport hear aircraft noise. This is explained by some physical principles.
When the temperature is high, the performance of jet engines is reduced. In simple terms, planes fly more slowly and therefore climb more slowly, so that they fly lower on take-off.
After taking off, aircraft have to fit their flight path into a specific circuit that varies according to their destination and traffic conditions. Each circuit traces a “roadway” in the sky, punctuated by points that planes have to fly over at minimum altitudes.
Since planes have a lower climb rate during a heatwave, those altitudes are reached later, which means that turns may be made later relative to the ground, and that the perimeter shifts within which noise pollution generated by planes can be heard.
People living in some communes might therefore have thought that the take-off flight paths had been changed, which is not the case. The return to normal temperatures will allow the aircraft noise to be confined once again to the allocated perimeter and envelope.
By way of information, the airport authorities work in close collaboration with the air navigation services (Skyguide) and the airlines to ensure that aircraft movements (landings and take-offs) always generate as little noise pollution as possible within the smallest possible perimeter.
In normal conditions, as part of the techniques employed, pilots try to take off as fast as possible on the long runway (high thrust) and use the highest possible climb rate. By making planes reach a high altitude very fast, the loudest noise is contained within the perimeter of the airport. Once the planes have left that perimeter and are flying over populated areas, pilots reduce the climb rate (by cutting thrust), so that they generate less noise for as long as it can be heard from the ground. Once a high altitude has been reached, they can reapply thrust without it being heard from the ground.