Chapter 11. Safety, Ground Operations, & Servicing

Hydraulic Ground Power Units

Portable hydraulic test stands are manufactured in many sizes and cost ranges. [Figure 11-29] Some have a limited range of operation, while others can be used to perform all the system tests that fixed shop test stands are designed to perform. Hydraulic power units, sometimes called a hydraulic mule, provide hydraulic pressure to operate the aircraft systems during maintenance. They can be used to:

  • Drain the aircraft hydraulic systems.
  • Filter the aircraft system hydraulic fluid.
  • Refill the aircraft system with clean fluid.
  • Check the aircraft hydraulic systems for operation and leaks.

This type of portable hydraulic test unit is usually an electrically powered unit. It uses a hydraulic system capable of delivering a variable volume of fluid from zero to approximately 24 gallons per minute at variable pressures up to 3,000 psi.

Operating at pressures of 3,000 psi or more, extreme caution must be used when operating hydraulic power units. At 3,000 psi, a small stream from a leak can cut like a sharp knife. Therefore, inspect lines used with the system for cuts, frays, or any other damage, and keep them free of kinks and twists. When not in use, hydraulic power unit lines should be stored (preferably wound on a reel) and kept clean, dry, and free of contaminants.

Ground Support Air Units

Air carts are used to provide low pressure (up to 50 psi high volume flow) air which can be used for starting the engines, and heating and cooling the aircraft on the ground (using the onboard aircraft systems). It generally consists of an APU built into the cart that provides bleed air from the APUís compressor for operating aircraft systems or starting engines. [Figure 11-30]

Ground Air Heating and Air Conditioning

Most airport gates have facilities that can provide heated or cooled air. The units that cool or heat the air are permanent installations, which connect to the aircraft by a large hose that connects to the aircraftís ventilation system. Portable heating and air conditioning units can also be moved close to the aircraft and connected by a duct, which provides air to keep the cabin temperature comfortable.

Oxygen Servicing Equipment

Before servicing any aircraft, consult the specific aircraft maintenance manual to determine the proper type of servicing equipment to be used.

Two personnel are required to service an aircraft with gaseous oxygen. One person should be stationed at the control valves of the servicing equipment and one person stationed where he or she can observe the pressure in the aircraft system. Communication between the two people is required in case of an emergency.

Aircraft should not be serviced with oxygen during fueling, defueling, or other maintenance work, which could provide a source of ignition. Oxygen servicing of aircraft should be accomplished outside hangars.

Oxygen used on aircraft is available in two types: gaseous and liquid. The type to use on any specific aircraft depends on the type of equipment in the aircraft. Gaseous oxygen is stored in large steel cylinders, while liquid oxygen (commonly referred to as LOX) is stored and converted into a usable gas in a liquid oxygen converter.

Oxygen is commercially available in three general types: aviatorís breathing, industrial, and medical. Only oxygen marked ďAviatorís Breathing Oxygen" which meets Federal Specification BB-0-925A, Grade A, or its equivalent should be used in aircraft breathing oxygen systems. Industrial oxygen may contain impurities, which could cause the pilot, crew, and/or passengers to become sick. Medical oxygen, although pure, contains water, which can freeze in the cold temperatures found at the altitudes where oxygen is necessary.

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