When properly maintained and operated, hydraulic GSE equipment should provide many years of trouble free service. This article is a supplement to the OEM maintenance manual provided with the equipment. Proper operation should be according to the OEM manual. Longer equipment life, less frequent unscheduled maintenance, and lowest life cycle cost are three good reasons to properly maintain hydraulic GSE equipment. GSE equipment that hydraulically interfaces with the aircraft, if not properly maintained, could contaminate the aircraft hydraulic system and damage sensitive hydraulic aircraft components. All hydraulic systems require regular maintenance; some only call for checking fluid level and seal integrity.
A comprehensive contamination control program should be the foundation of any hydraulic GSE maintenance plan. Development of a contamination program can be broken down into six major steps:
- 1. A company wide agreement to support this program (financial, training, equipment, material, manual labor).
- 2. Training for personnel involved (possibly supplied by the equipment manufacturer, Hydraulic Training Center, or School of Engineering).
- 3. Standards for acceptable level of fluid contamination (airframe manufacturer's recommendation).
- 4. Baseline testing of all GSE equipment (fluid sample sent to an analysis laboratory, or using a contamination monitor).
- 5. Equipment or materials acquired to implement program (contamination monitor, sample analysis bottles, improved filtration, contamination removal equipment).
- 6. Maintenance and testing scheduled (specific intervals for testing and filter replacement, regular evaluation of the program).
Many closed systems (no interface with the aircraft hydraulic systems) only require checking fluid levels, inspection for fluid leaks, and visual or infrequent laboratory fluid analysis.
Contamination problems in hydraulic GSE fluid can be separated into four major categories:
- 1. Particles (solid foreign material in the fluid).
- 2. Water (either in solution or free water).
- 3. Air (either dissolved or entrained).
- 4. Chemical (foreign or fluid deterioration).
Particles: Can cause many types of wear in a system which can lead to component failure. Tight operating clearance components and orifices can stick or plug because of particle contamination - abrasive wear on moving components, edge, or critical surface deterioration.
Water: Moisture can react with almost anything in hydraulic fluids, causing chemical reactions, which can lead to an increase in wear and interference. Water in hydraulic fluids can also promote rust or corrosion through galvanic action. Corrosive wear degrades the surface, bearing fatigue.
Air: Undissolved air can cause premature wear on equipment and pressure changes that compress the air and produce a large amount of heat. Efficiency levels can drop due to the work required to compress the air, and oxidation of metal parts and additives - increased operating temperature, increase in noise level, loss of transmitted power.
Chemical: Incompatible fluids entering the hydraulic system, cleaning solvent residue not removed during component maintenance, or chemical reaction with components (hose material, plated component, elastomer material). Thermal damage, excessive mechanical shear, and additive deterioration are other examples. - viscosity variance, fluid additive breakdown.
The best offense is a good defense - prevent contaminants from entering the system. Make sure the fluid filler cap is in proper operating condition, and only is removed for servicing the reservoir. Change filters often enough to maintain the required contamination level of the system (or annually as a minimum, more frequently depending on usage). Ensure that the air filter/desiccant is not saturated which could either limit air from exchanging with the reservoir, or allow contaminates to enter the reservoir.
Nowhere in an aircraft maintenance manual is there a stated time interval for changing hydraulic fluid.