 As a result of these studies and recommendations from the U.S. National Transportation Safety Board (NTSB), in 1974 the FAA required all (Part 121) certificate holders (that is, those operating large turbine-powered airplanes) and some (Part 135) certificate holders (that is, those operating large turbojet airplanes) to install TSO-approved GPWS equipment.In 1978, the FAA extended the GPWS requirement to Part 135 certificate holders operating smaller airplanes