Hydraulic Leak, Contamination and Dispatch
A regional turboprop has a slow hydraulic leak from a landing-gear actuator. The reservoir level has dropped from FULL to the LOW band over about 40 flight hours. A sample taken from the reservoir shows visible particulate and a slight milky appearance. The maintenance manual specifies a filter element change and a fluid sample check after any reservoir top-up exceeding a defined volume, and requires fluid cleanliness to a stated ISO class. The operator wants to defer the leak under the minimum equipment list (MEL) for three days to keep the aircraft on schedule. The MEL entry for this system permits operation with a leak only if the leak rate is within a stated limit and the reservoir can be maintained above the minimum level. The mechanic has not measured the leak rate.
1. What does the milky appearance and visible particulate tell you, and how should it change the plan?
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Milky appearance indicates water or air entrainment in the fluid; visible particulate indicates solid contamination. Both are abnormal and both can damage pumps, valves and seals.
Effect on the plan:
- The fluid no longer meets the stated ISO cleanliness class, so the aircraft does not meet the manual's fluid condition requirement.
- A filter element change and fluid sample check are required per the manual after the top-up volume threshold is reached; the contamination finding makes this mandatory rather than optional.
- Water contamination must be addressed (drain/sample, identify source) before further flight, because it can cause corrosion and, at altitude, ice formation in small passages.
- Simply topping up and deferring would mask the problem and propagate contamination through the system.
The judgement is that a contamination finding overrides a schedule-driven deferral decision.
2. Can the leak be deferred under the MEL as proposed? Justify your answer.
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Not on the evidence available. The MEL entry is conditional: it permits operation only if the leak rate is within a stated limit and the reservoir can be kept above minimum level. The mechanic has not measured the leak rate, so the condition cannot be shown to be met.
Required before any deferral:
- Measure and record the leak rate against the MEL limit.
- Confirm the reservoir can be maintained above the minimum level for the planned flights.
- Confirm all other MEL conditions (e.g. any performance or operational restrictions) are satisfied.
- Record the deferral correctly with the specified time limit and any crew notification.
Additionally, the contamination finding is a separate defect that the MEL entry does not cover; it must be rectified regardless. Deferral cannot be used to avoid a mandatory fluid condition requirement.
3. The operator pressures you to sign the deferral 'as is'. How should you respond, in terms of both technical and human-factors considerations?
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Technically: signing a deferral without the required measurements and without addressing contamination would be a false certification. The correct response is to decline, state the specific unmet conditions, and offer the compliant path (measure leak rate, rectify contamination, then reassess deferral).
Human factors:
- Schedule pressure and authority gradient are recognised error precursors; the mechanic should not let commercial pressure override airworthiness judgement.
- Use the operator's own procedures: raise the discrepancy, document the refusal and the reasons, and escalate to a supervisor or the continuing airworthiness management organisation.
- Keep the communication factual and non-confrontational: 'The MEL condition requires a measured leak rate; I cannot certify it without that measurement.'
- Record the event so the pressure is visible and the decision is traceable.
The exam point is that a mechanic's signature is a personal certification; it cannot be delegated to, or coerced by, the operator's schedule.