scope:

This specification covers the requirements of a gas turbine auxiliary power unit to provide mechanical power for aircraft main engine starting and secondary electrical power.

The paragraphs listed herein are numbered to correspond to those in Specification MIL-P-8686 and either include specific information as required by Specification MIL-P-8686 or indicate revised requirements differing from those of Specification MIL-P-8686. Omission of reference in this specification to a particular requirement of Specification MIL-P-8686 shall be interpreted as compliance therewith.

The gas turbine auxiliary power unit (APU) as defined herein is an airborne Type I APU for a source of mechanical power for main engine starting and for secondary electrical power. The APU consists of two major sections, namely, the gas turbine unit and the accessory gearbox. The gas turbine unit consists of a single stage centrifugal compressor, an annular combustor and a single stage inflow turbine. The compressor and the turbine are mounted back-to-back on a single shaft. The accessory gearbox provides the necessary gearing to provide a speed reduction gear ratios of 7.09 to 1.0 and 6.88 to 1.0 between the turbine shaft and the axial and transverse AND 20002, Type XII-A, power take-off drives respectively. The accessory gearbox also provides support for the lubricating oil sump and drives for other engine driven accessories. The unit incorporates a complete lubricating system including a pressure pump, an oil pressure relief valve, an oil filter and an oil sump permitting at least 30 hours of operation without refilling; a fuel system including a unit driven fuel pump, fuel filter, fuel control for controlling speed and overspeed, and six vaporizing fuel nozzles. A starting fuel nozzle is also provided which is cut-out at 90% unit speed. The electrical system components include an ignition exciter (powered from the airframe installation), a single spark ignitor, tachometer generator, exhaust gas temperature thermocouple, thermal switch, start counter, overspeed switch and speed switch.