Commercial aircraft are usually equipped with auxiliary engines, which are used in particular while on the ground for supplying onboard systems and starting up engines. Auxiliary engines are often designed as gas turbines coupled with a generator, which provide a high electrical power while requiring little installation space, and also a high pneumatic power in conjunction with a bleed air connection. As a consequence, the supply of electrical components of the airplane on the ground and operation of an air conditioning system can be ensured through the supply of bleed air. However, the gas turbine design may be expected to result in a significant level of noise, and gas turbines further consume a large quantity of supplied fuel. Depending on the used propellant, emissions of unburned hydrocarbons, nitrogen oxides and other pollutants into the environment of the airplane may be expected.
The operation of auxiliary engines is significantly restricted at many airports, especially in urban areas.
EP 1 928 745 B1 and EP 2 222 562 B1 show fuel cell systems that are integrated into an airplane.