Passenger aviation without noise, soot particles and carbon dioxide - in short "emission-free flying" - has evolved from vision to reality. The HY4 - the world's first four-seat passenger aircraft with hydrogen fuel cell technology and electric drive - marks a milestone on this journey. The electric flying with the HY4 is quiet, protects the environment and increases the range and safety compared to conventional combustion drives or full battery applications. Long-standing aerospace and energy research activities in the fields of batteries, fuel cells and hydrogen technology make the Hy4 team experts in the fields of More Electric Aircraft (MEA) and All Electric Aircraft (AEA). Fuel cells can be used in aviation in many areas. Common to all applications is operation at high altitudes, at low pressure and low outside temperatures. The safety requirements and reliability must meet very high standards.