This Aerospace Recommended Practice (ARP)identifies and defines a method of measuring those factors affecting installed power available for helicopter power plants. These factors are installation losses, accessory power extraction, and operational effects. Accurate determination of these factors is vital in the calculation of helicopter performance as described in the flight manual. It is intended that the methods herein prescribe and define each factor as well as an approach to measuring said factor. Only standard installations of turboshaft engines in helicopters are considered. Special arrangements leading to high installation losses, such as the fitting of an infrared suppressor may require individual techniques for the determination and definition of engine installation losses.