Most aircraft use an auxiliary power unit (APU), a small engine which produces power for the aircraft when it is on the ground and the main engines are turned off. This allows cleaners to plug in their equipment and air conditioning to run when passengers are boarding.
The APU is located in the tail of the aircraft and runs off normal jet fuel. Whilst APUs are useful at airports where there is little ground support, most airports have now started installing fixed electrical ground power and pre-conditioned air, usually at the gate.
These fixed supplies allow the aircraft to get electricity straight from the local grid (or even solar power – see page 52) and use the airport’s air conditioning to control the temperature on board. Because the auxiliary power unit is needed to help start the main engines, it can be switched on just before the plane is due to depart.
Nice Côte d'Azur Airport installed such a system for its general aviation departing area in 2014, and the use of the system has cut annual CO2 emissions by 416 tonnes. It has also reduced noise on the apron. The system at Nice is on remote stands, away from the terminal. The connections are therefore provided in an innovative ‘pop-out’ system which comes up from the tarmac. Training was carried out with ground handlers to encourage the use of the system.
A number of airports have made it mandatory to switch off the APU when aircraft are parked at the gate. Barcelona El Prat Airport mandates use of its fixed ground power and pre-conditioned air from two minutes after the aircraft arrives at a terminal gate until five minutes before it leaves, saving 58,000 tonnes of CO2 per year. Other Spanish airports with mandatory use include: A Coruña, Madrid-Barajas, Alicante-Elche, Fuerteventura, Gran Canaria, La Palma, Lanzarote, Palma de Mallorca, Tenerife Norte, Tenerife Sur, and Vigo.
In December 2014, Hong Kong Airport banned the use of auxiliary power units by aircraft. To facilitate the ban, the airport invested over $7.8 million to upgrade all its fixed ground power and pre-conditioned air systems. In addition, the airport engaged airline and line maintenance operators in the development of the new policy through preparatory meetings and trials.