An optional APU for passenger aircraft
Background: Passenger aircraft are equipped with an Auxiliary Power Unit (APU). Its main purpose is to provide the aircraft on the ground with electric, hydraulic and pneumatic power. These three forms of power are called secondary power. The APU often only provides electric and pneumatic power with hydraulic power converted from electric power by an electric motor driven pump. Any device that produces secondary power could be an APU, but traditionally APUs are always small gas turbines because of their high power-to-mass ratio. A multifunctional fuel cell system is discussed as a possible future APU alternative . However, today most airports offer electric power and pneumatic power (compressed air) thus rendering the APU obsolete. Furthermore, many APUs are “non-essential APUs” with respect to their function in aircraft certification. Still all passenger aircraft are equipped with an APU. APU alternatives: Bio fuel powered gas turbine, hydrogen powered gas turbine, fuel cell, Diesel engine, pressurized air bottles, battery, and “no APU” at all. These are the investigated alternatives for a classical kerosene powered gas turbine APU. Necessity of an APU: Operation on the ground, in normal cruise flight, and on ETOPS flights is investigated. Further investigated are: Aircraft scheduling by the airline, selling an Aircraft with unconventional APU or even without APU. Stakeholder views of the APU: The view of pilots, airlines, aircraft manufacturers, APU manufacturers, airports, and of the residents is considered . Ground operation without APU: Considered are electric supply, air conditioning, engine start, and taxiing without APU and engine operation. Trade-off: APU alternatives including “no APU” are compared with respect to mass (see Figure 1), energy consumption, maintenance requirements, spare holding costs, delay and cancellation costs. These criteria are qualitatively combined in the Scoring-Model (Nutzwertanalyse) and finally also quantitatively combined in a method Direct Operating Costs for aircraft SYStems (DOCsys). Result: The option “no APU” shows strong advantages. An aircraft manufacturer should design a new aircraft type such that it offers the APU only as option. An airline operating from remote airports with little infrastructure will order the type with APU. Most airlines operating (mostly) from well equipped airports will buy the aircraft type (mostly) without APU. The “no APU” option shall be offered in two variants: a) the aircraft is void of all APU interfaces (cables, tubes, ducts), hence an APU retrofit is not possible during later life of the aircraft or b) the aircraft is equipped with all APU interfaces (cables, tubes, ducts) hence an APU retrofit is possible. Option b) has the advantage of a possible retrofit of an APU when the aircraft is sold which avoids the possibility of excessive losses in the aircraft’s resale value. This advantage, however, has to be balanced against the higher purchase price of the first-hand aircraft.