Chapter 11. Safety, Ground Operations, & Servicing
Ground Support Equipment
Electric Ground Power Units
Ground support electrical auxiliary power units vary widely in size and type. However, they can be generally classified by towed, stationary, or self-propelled items of equipment. Some units are mainly for in-hangar use during maintenance. Others are designed for use on the flight line either at a stationary gate area or towed from aircraft to aircraft. The stationary type can be powered from the electrical service of the facility. The movable type ground power unit (GPU) generally has an onboard engine that turns a generator to produce power. Some smaller units use a series of batteries.
The towed power units vary in size and range of available power.
The smallest units are simply high-capacity batteries used to start light aircraft. These units are normally mounted on wheels or skids and are equipped with an extra-long electrical line terminated in a suitable plug-in adapter.
Larger units are equipped with generators. Providing a wider range of output power, these power units are normally designed to supply constant-current, variablevoltage DC electrical power for starting turbine aircraft engines, and constant-voltage DC for starting reciprocating aircraft engines. Normally somewhat top-heavy, large towed power units should be towed at restricted speeds, and sharp turns should be avoided. An example of a large power unit is shown in Figure 11-27.
Self-propelled power units are normally more expensive than the towed units and in most instances supply a wider range of output voltages and frequencies. Another example, the stationary power unit shown in Figure 11-28 is capable of supplying DC power in varying amounts, as well as 115/200-volt, 3-phase, 400-cycle AC power continuously for 5 minutes.
When using ground electrical power units, it is important to position the unit to prevent collision with the aircraft being serviced, or others nearby, in the event the brakes on the unit fail. It should be parked so that the service cable is extended to near its full length away from the aircraft being serviced, but not so far that the cable is stretched or undue stress is placed on the aircraft electrical receptacle.
Observe all electrical safety precautions when servicing an aircraft. Additionally, never move a power unit when service cables are attached to an aircraft or when the generator system is engaged.
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