• External Hydraulic Leak from the components and engine drain mast.
  • Fuel Leak from Aircraft Tank and engine drain mast.
  • Oil Leak from Engine Drain mast.


  • Leak – Quantity of fluid that comes out of a component that is sufficient to become a drop or drops.
  • Approximately 20 drops = 1cc (1ml)
  • Leakage – Quantity of fluid on the surface of a component that is not sufficient to become a drop.
  • Stain – is an area on the surface of a component that has a different color. It is usually caused when fluid leakage dries on the component surface after high temperature operation.

Leak Analysis

  • Remove all the signs of hydraulic fluid leaks and clean the surface of the components.
  • Operate the components for some cycles before you do an external leak check.
  • If the component has a seal drain connection, disconnect the drain pipe from it.
  • The motors of the Trimmable Horizontal Stabilizer (THS) actuator have seal drain connections, but do not have related drain pipes.
  • Put a piece of cloth or paper below the component, to absorb the fluid. Make sure that it does not touch parts which turn or move.
  • If you find a leak at a hydraulic tube connection, you must tighten the nut to the specified torque.
  • If the leak continues, you must replace the defective components.
  • The crack of the actuator can cause a large leak.


  • Leaks – total leak in a system
  • 50 drops/min – Normal leak (No action required)
  • 50 to 100 drops/min – Dispatchable with conditions
    • Reservoir of the related system is full:
      • Green – 14.5 L // Yellow – 12.5 L // Blue – 6.5 L
  • The drops of hydraulic fluid do not fall on electrical equipment.
  • Particular component leak rate is less than the dispatch limit as given in the AMM table.
  • More than 100 drops/min – NOGO


  • When you find fuel leakage – use STRUCTURAL FUEL LEAK REPORTING FORM (SFLRF).
  • One of the properties of aircraft fuel is that it can flow easily through very small holes. Thus the parts of the aircraft structure that contain fuel tanks are sealed to prevent leaks. If the sealant in these areas is damaged or becomes defective, a fuel leak can occur.
  • Fuel leakage from leaks can be dangerous.

Leakage Analysis

  • Clean & fully dry
  • Wait for 10 minutes
  • Measure the distance across the wet area OR monitor leak rate – 2 ways to identify leak.
  • For measuring – use Steel Rule
  • Leakage can be –
    • Light leakage – upto 4 inch or less than 1 drop/minute.
    • Heavy leakage – b/w 4 inch to 7 inch or less than 2 drops/minute.
    • Continuous leakage – more than 7 inch or more than 2 drops/minute.


Scheduled inspection or repair as per AMM/SRRM

  • Every day inspection 
  • Weekly inspection
  • Temporary repair 
  • Permanent repair 


Hydromechanical Units (HMU) that have fluorocarbon (Viton) external seals can leak fuel for a short time when they are below 0 deg.C. (CFM56 ENGINE).

  • It is recommended to heat the HMU to minimize leakage during starts.
  • 5-minute warm-up at minimum idle.
  • Heat the HMU housing with a suitable hot air blower – until the HMU housing temperature is more than 25 deg.C
  • If the fuel leakage stops in less than five minutes at idle, continue operation of the engine.
  • If the fuel leakage continues after five minutes at idle, shut down the engine and repair the source of the leakage.
  • Fuel leakage rate is less than 60 drops per minute (180 ml/hr or 3 cc/min), you do not have maintenance action.
  • If the fuel leakage rate is more than 60 drops per minute (180 ml/hr or 3 cc/min), but less than 90 drops per minute (270 ml/hr or 4.5 cc/min), do a corrective action within 25 cycles not interfering with revenue service operation.
  • Use a plastic bottle for collecting fuel.
  • Leakage not allowed
    • The fuel manifold shroud


  • Leak < 7 drops/minute – No Action.
  • Leak > 7 drops/minute – Identify leakage source and maintenance action required.
  • Leakage not allowed –
    • The lube unit pad
    • The aft sump area

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