This chapter covers the lubrication and sealing systems of turbine engines, which are critical for maintaining engine reliability, performance, and service life. The lubrication system provides cooling and lubrication to bearings and gears, while sealing systems prevent oil leakage into the gas path and protect bearing compartments from hot gases. Understanding the normal operation, failure modes, and troubleshooting of these systems is essential for aircraft maintenance engineers.
Key Concepts Explained
1. Lubrication System Functions and Components
The primary purpose of the turbine engine oil system is to lubricate and cool the bearings and gears. Unlike piston engines, turbine engines do not use oil for hydraulic actuation or combustion chamber sealing.
Major Components and Their Functions
Oil Pumps
Pressure pump: Supplies oil under pressure to bearings and gears
Scavenge pump(s): Removes oil from bearing compartments and returns it to the oil tank. Scavenge pumps typically have higher capacity than pressure pumps to prevent oil accumulation in sumps.
Oil Jets (Squirters)
Direct a precise stream of oil onto main shaft bearings for lubrication and cooling
Also provide lubrication to gear trains in accessory drives
Oil Filters
Pressure filter: Located downstream of the pump, filters oil before it reaches bearings
Scavenge filter: Filters oil returning from bearing compartments to the tank
Filter bypass valve: Opens when the filter becomes clogged to ensure continuous oil flow and prevent oil starvation. A stuck-open bypass valve allows unfiltered oil to circulate, causing accelerated wear.
Oil Cooler and Thermostatic Valve
The oil cooler removes heat from the oil system
The thermostatic valve regulates oil temperature by controlling the proportion of oil flowing through the cooler versus bypassing it
The oil cooler bypass valve (often thermostatically controlled) bypasses the cooler when oil is cold to allow rapid warm-up
Pressure Regulating and Relief Valves
Pressure regulating valve: Maintains constant oil pressure to bearings by metering flow from the pressure pump
Pressure relief valve: Safety device that opens to prevent excessive pressure, protecting components from damage
Check Valves
Allow oil flow in only one direction, preventing backflow and system drain-back when the engine is shut down
Chip Detectors
Magnetic devices that attract and hold ferrous metal particles from the oil
Enable early detection of internal engine wear before catastrophic failure occurs
De-oiler (Oil Separator)
Removes oil mist from air being vented from bearing compartments before it is released overboard or recirculated
Cannot remove 100% of oil mist, so slight oil mist from the breather is normal
De-aerator (Air Separator)
Removes air bubbles from oil before supply to bearings, ensuring proper lubrication and preventing foaming
2. Oil System Ventilation
The oil tank vent system serves three critical functions:
Allows air to enter to prevent vacuum as oil is used
Prevents over-pressurization of the oil tank
Vents vapors overboard or to a separator
The breather allows air to enter and exit the oil tank to accommodate changes in oil level and temperature, preventing vacuum or pressure buildup.
3. Oil Types and Properties
Modern turbine engines primarily use synthetic ester-based oils (e.g., MIL-PRF-23699, MIL-PRF-7808) due to their:
High thermal stability at extreme temperatures
Excellent lubricating properties
Resistance to oxidation and coking
Straight mineral oil is outdated for turbine engines. Ashless dispersant oils are used in piston engines, and multi-viscosity oils are for automotive applications.
4. Sealing Systems
Labyrinth Seals
Non-contacting seals that create a tortuous path for air or oil to flow through
Use a series of fins and pressurized air to minimize leakage between rotating and stationary parts
Prevent oil from migrating into the gas path and prevent hot gases from entering the oil system
Do not lubricate, support the shaft, or dampen vibrations
Carbon Seals
Rotating seals between the shaft and housing
Primary function is to contain oil within bearing compartments
Prevent oil leakage into other engine sections
Subject to wear over time, leading to internal oil consumption
5. Oil System Failure Modes and Troubleshooting
Oil Pressure Abnormalities
Oil Temperature Abnormalities
Oil Consumption and Leaks
Internal leaks (oil entering gas path without external evidence):
Failed carbon or labyrinth seals
Oil consumed in combustion process
Gradual increase in consumption over time indicates progressive seal wear
External leaks:
Visible oil stains on nacelle or engine surfaces
Rapid decrease in oil quantity with normal pressure
Normal conditions:
Slight oil mist from breather is normal (de-oiler cannot remove 100%)
Minor oil seepage past seals may be within manufacturer limits
Oil consumption should be very low and predictable
Oil Contamination Indicators
6. Pre-Oil Procedure
After an oil change or engine installation, the pre-oil procedure is performed to:
Fill the oil system and bearing compartments with oil before engine start
Prevent dry starts and bearing damage
Ensure immediate lubrication upon engine rotation
This is not primarily for leak checking, warming oil, or testing relief valves, though these may be secondary benefits.
7. Oil Cooler Maintenance
Oil coolers are not repairable due to:
Their construction complexity
Risk of internal leakage contaminating the oil system
The correct action for a leaking cooler is replacement, not repair
Important Procedures and Regulations
Oil Level Management
Below minimum: Add oil to bring to specified range. Do not operate without sufficient oil.
Above maximum: Drain excess oil to specified level. Overfilling causes foaming, increased pressure, and oil leaks.
Consistently low without external leaks: Investigate internal seal leakage into gas path.
Leak Troubleshooting Procedure
96.Clean the area to remove evidence of the leak
97.Perform a ground run to visually identify the source
98.Compare leakage rate to maintenance manual allowable limits
99.Do not tighten fittings without knowing the source
Regulatory Requirements (CARs Standard 571)
Standard 571.02 - Maintenance Standards
Where aircraft manufacturer recommendations conflict with engine manufacturer recommendations, aircraft manufacturer recommendations shall be used
Where no manufacturer recommendations exist, standard industry practices are used, including methods published by manufacturers of similar products or generally accepted industry practices
Standard 571.03 - Technical Record Requirements
Must include:
Brief description of work performed
Date of maintenance
Employee identification
Reference to manufacturer's manual used (or alternative standard)
General description of any defect found when disassembly is required (before reassembly)
Standard 571.05 - Part VII Aircraft
Maintenance on Part VII aircraft must be performed under the control of an Approved Maintenance Organization (AMO)
This applies even to elementary work tasks
Elementary Work Recording
Must be recorded in the journey log per CARs Standard 625, Appendix A, and Section 605.94
Maintenance Classification
Minor repair: Restoring to original type design without deviation (e.g., replacing worn carbon seals with identical parts during scheduled maintenance)
Major repair: Deviation from type design with more than negligible effect
