EVALUATING SAMPLE RESULTS (cont)

NAVAIR 17-15-50.3

TM 38-301-3

T.O. 33-1-37-3

CGTO 33-1-37-3

c. Determine the wear metal trend between the last sample and the current sample and compare with the

trend limit listed in the Evaluation Criteria Table. Most abnormal increases will usually be readily apparent. The

trend limits in the tables are based on the wear metal concentration increase over a period of 10 equipment

operating hours. Most intervals between samples will not be exactly 10 hours, therefore, a conversion must be

made for approximate comparison purposes. A trend comparison may be made by dividing the wear metal

increase between samples by the operating hours between samples and then multiplying the result by 10. For

example: an increase of 2 ppm in 2 hours is roughly equivalent to a 10 ppm increase in 10 hours; and a 15 ppm

increase over 25 hours is roughly equivalent to. a 6 ppm increase in 10 hours. Trend values for 10 hours are

calculated as follows:

A - B x 10 = trend value for 10 hours

C-D

A = ppm this sample

B = ppm last sample

C = operating hours this sample

D = operating hours last sample

NOTE

The formula shown above for calculating trends is a quick way to determine the

trend values. However, trends calculated using this formula for samples taken very

frequently (less than 5 operating hours between samples) may be much less

accurate and reliable than trends calculated for samples taken less frequently

(more than 5 hours between samples). This possibility of error is due to

spectrometer allowable tolerances and also to the possibility of a variance in the

rate of wear metal production over a period of time. The calculated trends will be

very helpful information in the evaluation process, but if samples taken more

frequently, such as after each flight, once each flying day, etc. are being evaluated,

the calculated trends are not considered accurate for use as equipment

acceptable/not acceptable criteria. If the Abnormal Trend is listed as an asterisk (*),

the value is too low to be determined.

d. Trend values included in the Evaluation Criteria Tables are intended as guidelines for the evaluator.

There are many other factors that must be evaluated to determine actual equipment condition and whether

laboratory recommendations to the customer are required. Generally, trends will fall into one of the following

categories:

(1)

Level (little or no change): Considered normal.

(2) Slightly to moderately increasing or decreasing: usually considered normal because of

spectrometer tolerances, sampling differences, and oil usage/addition factors.

(3) Sharply increasing or decreasing within trend limits: Usually indicative of problems. A sudden

increase may indicate the start of an equipment problem, while a sudden decrease may indicate defective

sampling procedures, oil addition or change without documentation, or sample identification problems.

Recommend verification samples and/or decreased sampling interval for sharp increases, and investigate

sampling procedures or undocumented oil addition for sharp decreases.

(4) Erratic increases and decreases of trend level: Usually indicates a problem in sampling procedure,

(oil addition or change without documentation, sample identification, etc.), and should trigger a request to review

activity sampling procedures and submit a monitored verification sample.

(5) Increases exceeding trend limits: Generally indicative of equipment problems. Consult Decision

Making Guide and review equipment history. This normally results in a resample request and/or maintenance

action recommendation.

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