particulate and fluid contamination. They may be
further classified according to their type, such as
organic, metallic solids, nonmetallic solids, foreign
fluids, air, and water.
PARTICULATE CONTAMINATION
The type of contamination most often found in
aircraft hydraulic systems consists of solid matter. This
type of contamination is known as particulate
contamination.
The size of particulate matter in hydraulic fluid is
measured in microns (millionths of a meter). The
largest dimensions (points on the outside of the
particle) of the particle are measured when determining
its size. The relative size of particles, measured in
microns, is shown in figure 8-2. Table 8-1 shows the
various classes of particulate contamination levels.
Contamination of hydraulic fluid with particulate
matter is a principal cause of wear in hydraulic pumps,
actuators, valves, and servo valves. Spool-type electro
Figure 8-2.--Graphic comparison of particle sizes.
Table 8-1.--Particle Contamination Level By Class
MICRON
SIZE
PARTICLE CONTAMINATION LEVEL--BY CLASS
RANGE
Acceptable
Unacceptable
0
1
2
3
5
6
4
5-10
2,700
4,600
9,700
24,000
32,000
87,000
128,000
10-25
670
1,340
2,680
5,360
10,700
21,400
42,000
25-50
93
210
380
780
1,510
3,130
6,500
50-100
16
28
56
110
225
430
1,000
Over 100
1
3
5
11
21
41
92
_______
_______
_______
_______
_________
_________
__________
Total
3,480
6,181
12,821
30,261
44,456
112,001
177,592
NOTES
1. The class of contamination is based upon the total number of particles in any size range per 100 ml of
hydraulic fluid. Exceeding the allowable particle count in any one or more size ranges requires that the
next higher class level be assigned.
2. Class 5 is the maximum acceptable contamination level for hydraulic systems in naval aircraft. Fluid
delivered by SE to equipment under test or being serviced must be Class 3, or cleaner.
3. The Class 5 level of acceptability shall be met at the inspection interval specified for the equipment
under test.
8-5