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Ground Support Equipment

Aircraft and GSE Hydraulic Fluid System Analysis

Recording Action



The procedures in AAP7001.059-TAREG support compliance with AAP7001.053-Technical Airworthiness Regulations, which have been superseded.

Procedures supporting compliance with AAP8000.011-Defence Aviation Safety Regulations are contained in AAP 7001.059-DASR

An organisation’s exposition details which 059 version is applicable


1. Hydraulic fluid systems and equipment are extremely sensitive to the ingress of minute foreign particles and water. These can cause rapid wear, system degradation, serious aeronautical product malfunction and ultimately system failure. The correct specification and quantity of fluid, together with appropriate maintenance and servicing procedures will ensure maximum system performance and serviceability.

2. This chapter prescribes the AMO procedures to be followed when managing the maintenance and operation of hydraulic fluid systems and equipment.


3. When operating, servicing or maintaining aircraft or Ground Support Equipment (GSE) hydraulic fluid systems the following procedures are to be followed:

  1. Only System Program Office (SPO) authorised equipment is to be used for aircraft system replenishment.
  2. Ensure hydraulic fluid cans are wiped clean and dry before piercing. Hermetically sealed containers showing signs of minor surface rust are to have the rust thoroughly removed before opening.

    caution icon

    Piercing type pourers cause contamination and are not to be used.

  3. Pierce the tops of cans with a clean, round marlin type spike.
  4. Do not retain hydraulic fluid in open containers.
  5. Dispose of empty cans to prevent re-use.
  6. Ensure tools used are clean and authorised for use.
  7. Protect the system plumbing during disassembly by using clean, approved blanking caps.
  8. Whenever possible, have the replacement aeronautical product available for immediate installation upon removal of a defective aeronautical product. This applies equally to hydraulic fluid filters.
  9. When replacing filters, if possible, fill the filter bowl with clean hydraulic fluid to reduce the induction of air into the system.
  10. Ensure that the filter element is not loaded or clogged prior to resetting differential pressure indicators.
  11. Filter elements requiring cleaning are to be correctly packaged prior to shipping to an overhaul facility.
  12. Only use approved hydraulic fluids, seals, gaskets or filter elements.
  13. Do not contaminate aircraft systems with fluid from dirty hydraulic dispensing equipment, hydraulic fluid components or dirty test benches.
  14. Reject previously used or unidentified ‘O’ rings.
  15. Store hydraulic fluid system components and elastomers in clean packages in dust free locations, away from direct sunlight and electric motors.
  16. Hydraulic fluid drained or purged from systems is not to be returned to aircraft systems, dispensing equipment, or test benches.
  17. All replenishment points are to be cleaned prior to replenishment being carried out.
  18. Ensure all hydraulic fluid GSE is regularly checked for serviceability and that the hydraulic fluid contained in the GSE is the correct specification and tested for contamination (refer to paragraph 14).
  19. Hydraulic fluid GSE are to be tested for contamination before connection to aircraft.
  20. When using hydraulic fluid GSE, ensure the hose connections of GSE and aircraft are cleaned before coupling together.
  21. Replace dust caps securely after use.
  22. Components are to be disassembled on clean, dry benches or workstands and in a dust free environment.
  23. All cleaning rags used on hydraulic fluid systems are to be clean and lint free.
  24. Care must be maintained to prevent water contamination of aircraft hydraulic fluid systems and rigs. Water in emulsion adversely affects hydraulic fluid system operation and free water supports fungal growth.
  25. Ensure all filler point filters are serviceable prior to use.


4. The replenishment of aircraft hydraulic fluid systems is to be performed by dispensing equipment with 3‑micron (absolute) filtration. A juniper rig, fitted with 5–micron filters, may be used for servicing of aircraft if a 3‑micron filtering unit is unavailable. Dispensing equipment containers are to be marked with the type of hydraulic fluid contained within, and are only to be used for that particular fluid. All dispensing equipment is to be serviced on a periodic basis, including filter servicing in accordance with the relevant technical publication.

5. If approved by the SPO, aircraft hydraulic fluid systems may be replenished directly from a freshly opened hermetically sealed can, provided that all other requirements detailed in this chapter are satisfied.


6. Hydraulic fluid reservoirs are to be replenished to the maximum permissible quantity as specified in the relevant aircraft publication.

7. Only the fluid specified in the relevant aircraft publication (or an approved alternative) is to be used in replenishing an aircraft hydraulic fluid system. If required record the quantity of hydraulic fluid added to the aircraft in the aircraft’s maintenance documentation.



8. Aircraft hydraulic fluid systems are to be sampled and analysed on the following occasions:

  1. when called for in the aircraft/aeronautical product servicing schedule
  2. after hydraulic pump failure
  3. whenever contamination is suspected
  4. after filter by-pass indication
  5. as part of preparation for service after storage
  6. post system flushing
  7. at the direction of a higher authority.

9. Samples from the aircrafts hydraulic fluid system are to be taken as soon as possible after aircraft shutdown, prior to system replenishment. The period between shut down and sampling is not to exceed 20 minutes unless otherwise stated in the relevant maintenance publication. If this cannot be achieved, the system is to be cycled in accordance with relevant maintenance publications, to ensure the hydraulic fluid has cycled through the system prior to sampling.

Ground Support Equipment

10. GSE (hydraulic test equipment) used in the servicing, testing or replenishment of aircraft or aeronautical product hydraulic fluid systems, are to be cycled and tested before connection to such systems and are to also have the following:

  1. GSE without Contamination Monitoring Systems (CMS). Samples are to be taken monthly, after cycling in accordance with paragraph 12 through 19, these are visually inspected for discolouration and the presence of sediment or water. Irrespective of the results of the visual check a sample from the GSE must be sent for laboratory analysis every three months; and
  2. GSE with CMS. Checks are to be carried out 6 monthly by cycling in accordance with paragraph 12 and observing the contamination levels indicated by the CMS.

11. If any discolouration has occurred, water or sediment is evident, or excessive contamination is identified by the CMS, the GSE is to be drained, flushed, refilled and cycled in accordance with paragraph 12 and the fluid retested.

12. Cycling. Cycling is carried out to bring the hydraulic fluid to temperature and also allow CMS, where fitted, to settle and provide accurate readings. Prior to taking samples or carrying out tests, GSE is to be cycled in accordance with AAP 7605.014‑2M -Contamination Control Hydraulic Test Rigs and Benches or other applicable technical publications; however, in the absence of technical instructions, GSE is to be cycled as follows:

  1. Aircraft Hydraulic Test Rigs. Operate the test rig at maximum flow rate and minimum pressure for sufficient time to provide at least five changes of hydraulic fluid in the reservoir for GSE without CMS or a period of 15 minutes for GSE with CMS, flushing should be continued until the NAS 1638 reading is Class 4 or below.
  2. Workshop Hydraulic Test Benches. Operate the test bench at 50% flow rate and pressure capacity for sufficient time to provide at least five changes of hydraulic fluid in the reservoir for GSE without CMS or a period of 15 minutes for GSE with CMS, flushing should be continued until the NAS 1638 reading is Class 4 or below.

13. To minimise the introduction of contaminants into the sample, sampling points, equipment and bottles are to be cleaned immediately before taking a sample by:

  1. Flushing the sampling bottle with filtered Isopropyl Alcohol and inverting and allowing to drain and dry.
  2. Cleaning the sampling point and equipment with a lint free cloth and Isopropyl Alcohol and then rinsing with Isopropyl Alcohol and allowing to dry.

14. When the sampling bottle, point and equipment are dry, samples are to be taken using the following steps:

  1. Initiate a flow of fluid from the designated sampling point and allow 500 millilitre (ml) to flow into a suitable waste container.
  2. Without interrupting the flow, obtain a 500 ml sample by placing the clean sample bottle into the stream of the flowing fluid.
  3. Remove the sample bottle when the 500 ml sample has been obtained.
  4. Terminate the flow of the fluid.

15. Each sample is to be labelled with:

  1. the serial number of the aircraft, airframe hours and system from which the sample was taken
  2. in the case of GSE, the equipment number
  3. the section to which the equipment/aircraft belongs
  4. the date the sample was taken
  5. the reason the sample was taken
  6. the name and contact number of the person taking the sample.

Aircraft and GSE Hydraulic Fluid System Analysis

16. The routine method of analysis is the colour patch method, keeping a full record of each analysis carried out. If a more comprehensive analysis is required, the responsible SPO will issue further instructions.

17. Where a sample fails the colour patch test, the relevant system is to have the filters changed or cleaned, the system cycled in accordance with paragraphs 8 and 9, as applicable, and the test repeated. If this sample fails a second time, the system is to be drained, flushed, refilled and cycled in accordance with paragraphs 9 and12, as applicable. The sample is to be forwarded to a laboratory as designated by the responsible SPO. Dependent on the results of this test, the responsible SPO will advise further action to be taken.


Hydraulic fluid samples are to be adequately packaged to ensure no leakage occurs during transit. This includes the taping of sample bottle lids and the packaging of samples in a container filled with absorbent material.

18. Colour patch testing is performed utilising a Contamination Analysis Kit, Part No XX65 047 00, which enables effective monitoring of the quality aircraft hydraulic fluid systems and associated GSE. The test kit is used for rapid monitoring of the degree of particulate contamination by observing the degree of discolouration of a disc filter through which a standard volume of test liquid has been drawn by vacuum filtration. The colouration of the filter is then compared to recognised standard levels of contamination, in accordance with the instructions supplied with the kit.

Recording Action

19. The date and the result of the aircraft hydraulic fluid sample inspections are to be entered in the applicable recording and certification system. Sampling of GSE must be recorded in the respective GSE servicing record.

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