Cooling / Heating
In many climates where an aircraft is on the apron for some time without operation of the APU , auxiliary mobile heating or cooling units are necessary to maintain a suitable internal temperature in the aircraft interior . The airline station engineer is responsible for ensur- ing the availability of such units . With increasing fuel costs and environmental concern , much inter- est has been focused on centralized compressed - air units delivering air to the aircraft gate positions ( usually called fixed air supply or precondi- tioned air , Figure 6.15 ) and to mobile compressors at the gates ( known simply as compressed - air systems , Pneumatic systems can deliver high- pressure air for both heating and cooling and for air - starting the engines .