Recent engine developments have increased altitude performance of light aircraft. Operation at altitudes above 15,000 ft demands changes in the air operated instrument systems and fuel systems in order to assure dependable operation throughout the new altitude ranges.The first section, on pneumatic systems, deals with the requirements and considerations leading to the conversion from vacuum to pressure operated gyro, autopilot, and de-icer boot systems.The second section, on fuel systems, is intended to aid in the solving of the present fuel systems vapor lock problem. A discussion on aviation gasoline includes a tabulation of most of the hydrocarbons, including the classes of hydrocarbons used in this fuel, together with the vapor pressure, boiling point, and heating value of each. A description of the ASTM distillation tests and the Reid vapor pressure equipment is included, since these characteristics affect the volatility and vapor locking tendency. A qualitative description of how vapor lock occurs in the suction piping and its accessories is included. Early Air Force effort is described, tabulating several approaches, and the results. Three twin engine fuel systems are described.