Downloaded from https://www.everyspec.com ADS-39A (HDBK) Source Approval and Engineering Test Requirements for Alternate Sources of Helicopter Drive System Assemblies and Components Attachment 2 Test Spectrums 1 Introduction 1.1 This Attachment to Aeronautical Design Standard (ADS) 39A lists helicopter drive system component and assembly endurance test spectrums. 2 Abbreviations GEN Generator SDC Shaft Driven Compressor TTO Tail Take-off 1