This document defines the technical guidelines for the safe integration of Proton Exchange Membrane (PEM) Fuel Cell Systems (FCS)@ fuel (considered to be liquid and compressed hydrogen storage types only)@ fuel storage@ fuel distribution and appropriate electrical systems into the aircraft. Editorial Note: Today PEM systems and fuel storage represent the most mature FCS technology and currently forms the basis for this standard. Other types of fuel cell systems and fuels (including reforming technologies and electrolyzers)@ may be covered by a further update to this document. Purpose The purpose of this document is to provide introductory mechanical and electrical system safety guidelines that should be considered when designing fuel cell power systems for use on FAR/CS@ Part 25 aircraft applications. Field of Application This document covers primarily fuel cell systems in the various intended applications for FAR/CS@ Part 25 large aircraft and the installations should comply with all appropriate regulatory requirements.