Air start units provide a supply of compressed air to operate pneumatic starters on turboprop and gas turbine engines. Air start units may be mounted on trailer units to be towed to the aircraft, or they may be selfpropelled units similar to that shown in figure 11-5.

A typical air start unit consists of the following components: A GTC, a high capacity storage battery, and the necessary fuel, oil and electrical systems, controls, and compressed air lines.

The typical GTC is basically a two stage centrifugal compressor assembly directly coupled to a radial inward flow turbine. In addition to compressed bleed air, the compressor supplies compressed air for combustion to drive the turbine wheel. The combustion gas is passed through the combustion chamber to the turbine nozzle assembly. The power extracted by the turbine wheel is transmitted to the compressor, accessory section, and control system components.