Sampling and In-Service Limits

During aircraft operations, hydraulic fluid is subjected to an environment which, in time, alters certain properties of the fluid. No two types of aircraft hydraulic systems operate under the same conditions. Heat, moisture, and overall cleanliness are factors that affect the operating life limit of hydraulic fluid. Therefore, the operating life of Skydrol fluids is determined on the basis of both physical and chemical properties rather than by a fixed number of operating hours. We suggest that you have your hydraulic fluid analyzed periodically to assure it meets airframe manufacturer in-service limits.

Fluid In-Service Quality Limits

The airframe manufacturers have established in-service limits for aviation hydraulic fluids. These limits are listed in the following table. Continued use of a fluid that does not meet one or more of the limits can adversely affect the life of system components.

During aircraft operations, hydraulic fluid is subjected to an environment which, in time, alters certain properties of the fluid. No two types of aircraft hydraulic systems operate under the same conditions. The heat that is produced during system operations, the moisture from the environment to which the fluid is subjected, and the cleanliness of the system (i.e., freedom from solids and liquid contaminants), will affect the operating life limit of the Type IV hydraulic fluid. Therefore, the operating life of Skydrol fluids is determined on the basis of both physical and chemical properties rather than by a fixed number of operating hours.

Eastman's Technical Service fluid sampling program offers comprehensive analysis and evaluation of the fluid, with recommendations for corrective action when required.

Fluid Sampling

Eastman suggests that hydraulic fluid be analyzed periodically to assure that it meets airframe manufacturer in-service limits. Eastman’s Technical Service fluid sampling program offers comprehensive analysis and evaluation of the fluid, with recommendations for corrective action when required.

  • Routine Sampling - Each system should be sampled about once a year and/or whenever the airframe manufacturer suggests.
  • Unscheduled Maintenance - When malfunctions may have a fluid relationship, samples should be taken.
  • Suspicion of Contamination - Usually fluids should be drained and replaced, with samples taken before and after the maintenance procedure.

Fluid Contamination

Contamination in the hydraulic system can affect the performance of both the hydraulic fluid and the system components. Common liquid contaminants are chlorinated cleaning solvents, water, petroleum-based or synthetic oils and engine lubricants. Liquid contaminants can alter the fluid's fire resistant properties, affect seal performance, cause gel formation and lead to acid development Common solid contaminants include component wear particles as well as contamination external to the hydraulic system. Solids can cause physical damage to components as well as line and filter pluggage. Certain solids accelerate fluid degradation. Proper in-line filtration is required to maintain a low solids contamination level. Filters should be checked and replaced as required. If a high contamination level persists, procedures outlined by the aircraft manufacturer and component manufacturer should be followed.

Airframe Manufacturers' Service Limits for Type IV Fluid

Analysis Boeing-Seattle Boeing-Long Beach (Douglas) Airbus Bombardier Gulfstream Lockheed BAE Fokker Embraer
Appearance No cloudiness, phase separation or
precipitation, any color is acceptable
No cloudiness or precipitation
Color: Blue/purple to gray
-- -- --
Specific Gravity @ 77°F/25°C 0.970-1.066 0.900-1.066 .970-1.066 0.995-1.066 0.995-1.066 0.989-1.065 0.995-1.066 0.999-1.057 0.995-1.066
Moisture % (max) 0.8 0.7 0.8 0.8 0.8 1.0 0.6 0.6 0.8
Neutralization No.
mg KOH/gm (max)
1.52 1.5 1.5 1.5 1.5 1.5 0.75 1.0 1.5
Kinematic Visc. @ 100°F/38°C, cSt 6.0-12.5 7.0-12.0 6.0-12.5 6.0-12.5 6.0-12.5 7.0 (min) 6.0 (min) 7.0 (min)6.0-12.5
Elemental Contamination1

Calcium

50 ppm max -- -- -- 50 ppm max -- -- -- --

Potassium

50 ppm max -- -- -- 50 ppm max -- -- -- --

Sodium

50 ppm max -- -- -- 50 ppm max -- -- -- --

Chlorine

200 ppm max 200 200 200 200 ppm max 200 200 200 200

Sulfur

500 ppm max -- -- -- 500 ppm max -- -- -- --
Particle Contamination #/100ml (max)

Class

9 9 9 9 9 8 9 9 9

5-15 microns

128,000 128,000 128,000 128,000 128,000 64,000 128,000 128,000 128,000

15-25 microns

22,800 22,800 22,800 22,800 22,800 11,400 22,800 22,800 22,800

25-50 microns

4,050 4,050 4,050 4,050 4,050 2,025 4,050 4,050 4,050

50-100 microns

720 720 720 720 720 360 720 720 720

>100 microns

128 128 128 128 128 64 128 128 128

Footnotes:

1. Contamination means quantities in excess of those introduced as a part of the basestock or the additives package.
2. Neutralization number for B-757 Center System is .5 mg KOH/gm (max).



Fluid Color

Phosphate ester-based aviation hydraulic fluids are purple fresh out of the can and may change color during service. Color is not a reliable indicator of fluid quality, and there is no in-service specification for fluid color. The fluid should be clear with no cloudiness or suspended solids.