The oils ystem (Ill. 22.3-1) is compared with a fuel system something special. It is of essential meaning for the longtime function of important components like main bearings and (sliding) seals, gears and as transfer of actuation forces and power (hydraulic). Above this oil has to transport the heat from bearings, gears, damping films (squeeze film dampers) and sliding surfaces. This heat is delivered to the air or fuel by heat exchangers like oil cooler or fuel preheater.

During its function, oil is loaded in different ways:

  • Thermal between non friction surfaces and sliding surfaces, at hot pipeline/tube walls.
  • Under shear load between sliding surfaces (sliding ring seals, friction bearings) and non friction surfaces(gears, non friction bearings).
  • Spraying, like at a spray nozzle for the lubrication of anti friction bearings and gears.
  • Pressure, respectively pressure changes.
  • Pressure fluctuations/oscillations in damping films of bearings.
  • Oxydation during intense contact with air. Formation of foam and fine droplets.Oil as working fluid has a relative long lifetime and circulates.

With this, the following problems get specific emphasis:

  • Aging under operation conditions.
  • Developing of contaminations in the system.
  • Influencing the components of the oilsystem by longtime mechanismy like material fatigue, fretting/frictionwear, cavitation and corrosion.
  • Continuous cleaning from contaminations.
  • Problems in connection with the oil change and refilling/top up.
  • Absorption of air/foaming.
  • Monitoring the system for contaminations.

Above this, special effects like oil fire, vapour bubbles/pads, amounts of oil which are detracted from the function (oil hiding) and coke/coal formation play a role.

Ill. 22.3-1 (Lit. 22.3-1 and Lit. 22.3-6): An oil system consits of several sub systems. The fresh oil system/pressure oil system and the scavenge oil system as well as the monitoring system. To the components of the oil system don't count components like bearings, bearing chambers or gears, whose function and operation safety is a task of the oil system. Problems and failures of the typical components from an oil system are discussed at different positions of this book and the other volumes. This applies also to the oil properties.
In the following will be pointed at the associated illustrations.

Oil properties

  • Types (Ill. 22.3.1-1)
  • Density
  • Viscosity
  • Acidity/effective acid
  • Compatibility with other materials of the system (Ill. 22.3.1-2, Ill. 22.3.1-3, Ill. 22.3.1-4, Ill. 22.3.1-5)
  • Discolourations (Ill. 22.3.1-6)
  • Thermal stability:
    Thermal limits
    Structure (Ill. 22.3.2-2.1, Ill. 22.3.2-3.1)
    deteriorating effect (Ill. 22.3.2-4.1), Ill. 22.3.2-4.1, Ill. 22.3.2-4.2)
    Operation influence (Ill. 22.3.2-6.1, Ill. 22.3.2-6.2)
    Deposits (Ill. 22.3.2-2.1, Ill. 22.3.2-3.1)
  • Contaminations (Ill., Ill.
    Particles (Ill., Ill., Ill., Ill.
    Water (Ill., Ill., Ill.
    Proof (Ill., Ill.
  • Proneness for cavitation (Ill.
  • Oil tests/investigations of properties (Ill. 22.3.4-1, Ill. 22.3.4-1, Ill. 22.3.4-2.1, Ill. 22.3.4-5)

Aeroengine components which influence problems of the oil/oil system (Ill. 22.3-3, Ill. 22.3.4-4, Ill. 22.3.4-6, Ill. 22.3.4-9):

Oil tank
Amount/volume of oil (Ill. 19.2-13)
Oil consumption
Oil level
Spout/suction (Ill. 22.3.1-4, Ill. 22.3-8, volume 3, Ill. 11.2.5-5.2).

Oil pumps
Pressure pumps/fresh oil pumps
Suction pumps/scavenge pumps

  • Damage/deterioration, malfunction caused by oil contaminations (Ill. 23.2.1-18)
  • Cavitation (Ill. 23.2.1-18, volume 1, Ill. 5.3.1-11.2 and Ill. 5.3.1-11.3)

Oil filters, sieves, screens

  • Blocking/clogging (Ill. 22.4.2-1)
  • Identification of deposits (Ill., Ill.

Oil cooler
Air cooled
cooled by fuel (Volume 2, Ill. 9.2-4)

  • Damage (Ill.

Overpressure valves (pressure relief valves, relieve valves)
Bypass valves Control valves (e.g., for oil temperature)

  • Stuck/jamming
  • Clogging


  • Clogging

Sensors for display and maintenance

  • Pressure
  • Temperature
  • Contaminations
    Magnetic chip detector, Ill. 22.3.4-7, Ill. 22.3.4-8, Ill. 22.3.4-10)
  • Function (Ill. 22.3.4-7, Ill. 22.3.4-11)
  • Malfunction (Ill. 22.3.4-12, Ill. 22.3.4-13, Ill. 22.3.4-14).

Oil jets/nozzles/injectors for the lubrication of anti friction bearings and gears

  • Clogging/blocking by
    Foreign objects
    Coking (Ill. 22.3.2-7)
  • Fracture/cracks

Seals (Ill. 22.3.2-4.1)

  • Static seals like O-rings (Ill. 23.4.1-4)
    Damage/deterioration (Ill. 23.4.1-12)
  • Moving seals/dynamic seals (Ill. 22.3-3)
  • Contact seals/sliding seals/sliding ring seals (Ill.

Bearings (Ill. 22.3.2-4.1) Friction bearings/sliding bearings Anti friction bearings (main bearings, bearings in gears and so on)

  • Failures (Ill. 23.1.2-8.1)
  • Contaminations from outside (Ill. 22.4.2-1)
    Forming/developing (Ill. 23.1.1-5.1)
  • Contaminations from the bearing itselfForming/developing
  • Coking (Ill. 19.1.3-6, Ill. 22.3.2-6.2)

Squeeze film dampers (Ill. 23.1.1-2)

Bearing chambers (Ill. 22.3.2-5, Ill. 22.3.2-7)

  • Oil fire (volume 2, chapter 9.2)
  • Contaminations (Ill. 23.1.2-4)

Gears/gear wheels/gear pinions (Ill. 22.3.2-4.1)

  • Contaminations from outside
  • Contaminations from the gear itself

Ill. 22.3-2 (Lit. 22.3-1): The amount of oil in an aeroengine is comparatively small and about that of a delivery-van. This is especially astonishing if we keep in mind the power, respectively the converted energy. At fighters the small amount of oil that fits into the tank volume of about 5 liters is not at last forced by the small usuable room in the fueslage. Also the small number of lubricated moving parts, compared with a piston engine, need less oil. For this reason for military operations the amount/volume of oil and not the carried fuel can limit the flight time.
The oil volume of big fan engines can be significant above 50 liters. In each case to the tank volume the oil volume in the system must be added.
Aeroengine typical oil losses at the seals (e.g., labyrinthseals) and breathers (Ill. 22.3-1) demand, comparing with a car motor a frequent re-filling (Ill. 22.3-3). The consumption of oil from a fighter aeroengine is typically about 1 liter per flight hour. This demands the check of the oil level before every flight.

Ill. 22.3-3 (Lit. 22.3-3): The oil consumption of an aeroengine is determined by many influences. Leakage losses, e.g., at labyrinth seals and in the breather. Of especially importance is the replacement volume which is necessary because the property changes of the oil. Those can be assigned two groups: On one hand design and operation as well on the other oil quality/oil properties, safeguarded by tha monitoring system (chapter 22.3.3).

Influence of operation and design:

Utilisation period/change
of an oil is essential determined by the operation temperature (Ill. 22.3-3). A rise of the temperature about 8°C doubles the reaction speed. During surpassing a temperature limit noticeable degradation products develop. Therefore the service time of an oil at high temperatures is determining. This is especially true for the maximum operation temperature. The requirements for the oil increase also in modern aeroengines with the tendency to longer operation periods and higher temperature levels. It is especially influenced by the pressure ratio of the compressor and the gas temperature.
The decomposition can be accelerated during contact with hot, catalytic acting metal surfaces. The type of these reaction products determines about the effectivity of an oil change respectively the dumping of the oil at lasting consequences in the oil system.
With this the design of the oil system plays an essential role for changes of the oil. Of especial influence is the heat shielding of oil-conveying components in a hot environment. To this belong the bearing chambers (volume 2, Ill. 9.2-1) and the connected oil-conveying lines. Pipe lines, guided through hot struts (Ill. 22.3.2-7) in the casings, are intensively heated and demand sepcial attention.
At most hot lekage air deteriorates the oil system. It enters the region of bearing chambers at labyrinth seals (volume 2, Ill. 9.2-2). Deteriorates during the operation time the sealing effect of labyrinths (abrasion, srosion, volume 2, Ill. 7.2.1-2) increases the oil aging and with this corresponding the oil consumption.
Natturalls the operation profile of an aeroengine (temperature-time-trend) and with this the lifetime of the oil plays a special role. So a civil use at long distances the oil consumption can differ markedly from short haul with many starts.
Leakage rates and oil temperatures are specific for particular operation conditions like aircraft speed (e.g., during supersonics, volume 2, Ill. 9.2-4 and Ill. 9.2-8) and maneuvers (military mission, volume 2, Ill. 9.2-9). In an extreme case an inner, from the outside not visible, oilfire can be ignited (volume 2, Ill. 9.2-9). With this an unexperienced high oil loss can be expected. This can also apply as indication of an oil fire.
If cracks develop in oil bearing structures like pipelines or bearing chambers during operation, e.g., by thermal fatigue or high frequency vibrations (volume 2, Ill. 7.1.2-10), a break-in of hot gas/hot air into the system can occur. With this an oil fire will be quite probable (volume 2, Ill. 9.2-10 and Ill. 9.2-11).
Not only during the operation of the aeroengine, oil will be deteriorated by temperature. A markedly influence has also the time after shut down of the engine. Heats the temperaturee of the hot parts by radiation and conduction the oil locally during stand still (heat soaking), it can decomposite (Ill. 22.3.2-6.1) and cause dangerous failures (Ill. 22.3.2-6.2).
With rising service temperature and time, metals like copper/brass (brazings, bearing cages), magnesium (gear casings) or lead (thin running-in coatings, „flash“) can be attacked intensified. Reaction products, absorbed by the oil deteriorate the thermal stability and thus promote the formation of oil coke. So a self-energising effect develops.
For the thermal oil stability, wear products (by fretting, Ill. 23.2.2-7) have proved as especially damaging. Typical is a mixture of abrasion from the base material (steel) and sliding coatings/platings. A typical source are spline toothings (volume 2, Ill. 6.2-18 and Ill. 6.2-19), as they are used manifold at shafts of pumps and gears. Requirement is, that these have a direct connection to the inside of the oil system. The rate of this effect depends from the design. Especially deteriorating act coatings like dry lubrications with fine distributed copper or zinc placers/soaps.
As an susceptible design for blocked oil, flown through openings, the oil supply through the main shaft has shown. Therby the centrifugal force is used to supply the bearings with oil through small bores. Through these, decomposition products (coke) and contaminations from the outside (dust) can be inserted.
The susceptibility of an oil system for deposits depends from the position and mesh size of the sieves/screens. Here an early blocking must be prevented.

A special danger comes from dust particles in the intake air of the aeroengine. Those can get with the cooling air through labyrinths of the bearing chambers into the oil. Often these particles carry adsorbed sea salt. Then they act particularly as 'condensation nuclei' for a coke formation in the oil.

Monitoring system for oil quality/properties:

Primarily three mechanisms act deteriorating:

  • Thermal decomposition produces acids and reactive hydrocarbons.
  • Oxidation originates acids and water.
  • Hydrolysis by entrance of water develops acids and alcohols.

These deterioration mechanisms change the properties of the oil. Additionally deposits form. Is an aeroengine specific value specified by the OEM surpassed, it must be proceeded according to the manual. Understandably exists here a connection with the intervals of the oil dumping (drain periods).

Sludge: Normally the content lays in civil aeroengines below 0,001 % (10 ppm). Higher amounts can block small oil-bearing openings. The percentage of sludge an aeroengine can tolerate will be specified by the OEM.

The viscosity (Ill. 22.3-3) rises with the decomposition of the oil. With this the function of the oil system and so of the lubricated components is influenced.
A too high viscosity can decrease the flow and so increase the heating of the oil and accelerate its aging.
Viscous oil can trigger so called skidding (slip of the cage) with damages in the main bearings (chapter 23.1).
The viscosity is also important for the function of the oil cushion from damped bearings (Ill. 23.1.1-2). Only the OEM can evaluate a possible consequence. He has the necessary background knowledge.
With the viscosity rises the loading capacity of the oil . This is an advantage e.g., for anti friction bearings and gears (Ill. 22.3-4).
Formation of acids (acidity) during the decomposition of the oil. This deterioration is characterised by the increase of the acid number and less by the acute value. This behaviour depends from the product. It is connected with acetous additives for a higher loading capacity of the oil.
The formation of acids increases the danger of corrosion damages in the oil system. There are standardised laboratory analysis/tests for the corrosion proneness of the materials, respectively the aggressivity. These are extensive and therefore not suitable for an oil monitoring on site.
From experience the actual danger of corrosion comes rather from introduced oil contaminations. Primarily concerned are washing media/cleaning media, hydraulic fluid and water. If necessary it is due to an oil change.
Cadmium is used in elder aeroengine types as corrosion protection for steel parts. At high temperatures it dissolves in synthetic oils. With this the corrosion sensitivity of the concerned surfaces rises. Lead and all lead alloys are attacked from synthetic oils. Therefore they should be avoided in oil systems. Lead appears as a solder component in filters or as plating/coating ('Flash') at the contact surfaces.
Silver is also in braze materials and because of its good sliding properties used as plating/coating. It reacts with certain additives which increase the pressure strength of synthetic ester oils. This can lead to the remove of the silver from thin platings. Magnesium is only attacked when there is water (hydrolysis).
Zinc serves as galvanic corrosion protection also as cadmium alternative. It develops with synthetic oils zinc placers (zinc soaps). Therefore it should not be used in the oil system.

Coke and lacquer like deposits: Aged oil tends to the formation of deposits. Already small amounts are sufficient, that valves jam or for the clogging of the breather. The dumping of the „used oil” does not remove contaminations which accumulate at already existing deposits and so can concentrate. This further accelerates their buildup.
So together with unfavourable local operation conditions, a heavy buldup of coke in aeroengine specific zones can be explained.
Already adherent deposit can not be removed with the draining of the oil. Also fresh oil lacks under operation conditions the necessary solubility. In contrast sieves/screens and filters already contain loose deposits.
At lacquer like (varnish) deposits the oil change interval has from experience hardly no influence. Therefore it was supposed, that fresh oil does not age as fast, if traces of instable compounds did not yet settle. Anyway the change of the viscosity and acid number can demand the change of the oil.

Ill. 22.3-4 (Lit. 22.3-2): The indicated temperatures accord approximately the maximum acceptable values for oil and fuel of civil aeroengines. Because of long time intervals till the change of oil these indications are rather conservative. For fighter airplanes, higher oil temperatures must be expected. This is considered with elevated limits (Ill. 22.3.1-1). The temperature has essential influence at the lifetime (Ill. 22.3-3) and the operation behaviour of an aeroengine oil. If necessary a fuel cooled heat exchanger (preheating of the fuel) plays an important role. Its effectivity depends from the fuel temperatures. On one hand a sufficient heat dissipation from the oil must guaranteed. At the other hand the icing of the fuel must be prevented.
As a help for the assessment of the viscosity the value at room temperature for honey, water and oil as an extreme is indicated.

Ill. 22.3-5 (Lit. 22.3-3): The viscosity, of an aeroengine oil influences essential its load capacity. This is the resistance with which an oil film prevents the touch of two surfaces. Such a situation especially occurs at sliding bearings and anti friction bearings as well as at gear tooth flanks. The lower the viscosity, i.e. thin fluided the oil, the lower its load capacity.The effect of the viscosity for the load capacity is based on two effects. First because the oil must be extruded out of the gap during normal movement of the surfaces against each other. Secondary, that at another sliding surfaces, the oil is pulled by the adhesion into the gap. So a separative pressure can build up dynamically. It occurs not only in sliding bearings/sleeve bearings but also between roll off surfaces. Examples are races of anti friction bearings and gear tooth flanks.
Is the dividing lubrication film thinner than the designed fineness of the filter, possible contaminations of the oil are squeezed between the load transferring surfaces. The consequences are mechanical damages, indented particles and grooves/scratches. Those shorten the lifetime by wear/abrasion and fatigue fractures (fatigue pittings, chapter 23.1 and chapter 23.2).
Acute danger of deterioration/damage exists by wear and seizing/galling.

Ill. 22.3-6 (Lit. 22.3-2): The heat balance of the (lubrication) oil system is essential for its operation behaviour. With this also the parts which must be supplied are concerned (Ill. 22.3-5). Service life and properties are primarily determined by the operation temperatures (Ill. 22.3-3). The sketched oilsystem is a simplified scheme. It shows typical components which contribute markedly to the heat balance (Ill. 22.3-1).
Heat sources (see chart below):

Main bearings add by splashing, churning and friction (mechanical, gas) the main part of the oil heating. Additionally hot gas entrance as leakage of the labyrinth seals must be expected. In the labyrinth seals the leakage stream can be further heated by air friction and windage.
Bearings, damped (damped bearings) by an oil film, chapter 23.1) produce in the film heat by churning. Additionally this heat is transferred from the outer ring/race into the oil film.

Accessory gears and main bearing chambers possess several heat producing components. From these the air friction must not be neglected.

  • From the hot surrounding much heat can get even in spite of heat insulation through the walls of pipe lines and bearing chambers into the inside and with this into the oil.
  • Gears produce especially much heat by friction and churning in the load transferring tooth systems and by splashing.
  • Anti friction bearings.
  • Seals also produce heat. At face seals heat develops in the oilfilm between the sliding surfaces.
  • Couplings/clutches with relative movements between the contact surfaces.

Oil pumps: The amount of produced heat is mostly influenced by the function principle. In grear pumps it is produced by friction (meshing of the teeth, seals, friction bearings), shearing of the oil film and churning.

Oil cooler: For the heat balance of the oil system the heat removal from the oil is essential. For this, heat exchangers are used. These transfer the heat into the air or fuel. The effectiveness depends not at least from the temperature of thes transportation media.
For example in oil/fuel heat exchangers the fuel temperature is codetermined from the aerodynamic heating. Thereby especially the wing tanks are heated during high aircraft speed and the icing of the wings is decreased (volume 1, Ill. 5.1.4-7 and volume 2, Ill. 9.2-4). This gets obvious especially during supersonic flight and rises the oil temperature. Was this not sufficient considered, the oil temperature can be no more lowered at the aspired value (volume 2, Ill. 9.2-8). With this even oil fires will be promoted (volume 2, Ill. 9.2-3 and Ill. 9.2-4).
Also the type of fuel pomp determines the heat production and influences with this the fuel temperature. Because of its adjustability, axial piston pumps (Ill. 22.2-10) are more favourable. However its high price led to gear pumps with higher heat production.

Ill. 22.3-7 (Lit. 22.3-7): The water content of an oil can be connected with the expected lifetime of the lubricated parts. Thereby changes caused by water play a role (Ill. The failure mechanisms can olso combine in its effects and so intensify. These are:

  • Corrosion: Especially steels rust, i.e. they develop iron oxides. Did acid form (Ill. 22.3-3) also austenitic steels are attacked. Rust paricles act abrasive and produce fresh metallic surfaces. These corrode faster and so accelerate the deterioration.
  • Erosion can occur as mere drop impact (volume 1, Ill. 5.3.1-11.1). This deterioration is intensified by the surface temperature (water etching). Thereby vapour development and thermal fatigue may accelerate the formation of pittings.
  • Cavitation (volume 1, Ill. 5.3.1-11.2) is the result from the implosion of vapour bubbles in the oil. Water tends to evaporate sooner during a pressure drop. That is true for the suction side of pumps or at sliding surfaces (e.g., face seals). This effect is intensified by an air absorption of the oil.
  • Hydrogen embrittlement (volume 1, chapter 5.4.4) develops when the water in the oil intrudes into micro cracks and gets there under high pressure during opening and closing of the crack surfaces. Thereby it decomposes and releases atomic hydrogen which diffuses ito the steel. This promotes the crack propagation. For example this effect occurs at fatigue cracks in roll off surfaces and so accelerates fatigue breakouts (pittings, Ill. 23.1.1-4).
  • Oxidation of sliding coatings/patings on sliding surfaces by the formation of acids and decomposing of water with the development of oxygen. Also seemingly corrosion-resistant coatings/platings from tin and lead ('babbitt'/white metal) can be affected. Such coatings can be found in friction bearings/sleeve bearings of some propeller gears.
  • Wear by metallic contact of sliding surfaces, caused by a too low load rating of the water contaminated oil. Also hard deposits from the water can cause wear (abrasion).

Ill. 22.3-8 (Lit. 22.3-5): Failures of the components from an oil system can show as particles. These can be also very fine distributed in the oil. The chemical composition of those particles allows conclusions at the concerned components (Ill. 22.3.4-4). With this the chance of an early identification and clearing of the cause exists.
In the chart below the alloying elements of typical materials from components in the oil system are assigned to the analysis of the particles (see chapter 23.3). Frequently to this, detailed aeroengine specific informations/specifications, are given from the OEM or military operators (Lit. 23.3.4-16).

Ill. 22.3-9 (Lit. 22.3-2): Does oil gather locally in the system, it can no more used for its duty. The low circulating amount of oil is faster heated by the constant heat which must be dissipated. There are two main causes which detract oil from the circular flow:

  • Oil hiding: There are sufficient possibilities for an oil accumulation in an oil system. So this oil can no more be sucked. This is especially true for fighter aeroengines during special maneuvers with high negative g-loads (e.g., upside-down flight). Indeed oil accumulations are minimised in the tank but can not ruled out in spite of snorkel systems (volume 3, Ill. 11.2.5-5.2). The relatively complex form af the accessory gears (sketches above, volume 3, Ill. 11.2.5-3) or in regions of the scavenge system, promote oil accumulations.

  • Failing or not sufficient drain of the oil: Foam formation and bubble formation can prevent sufficient oil in the circulation. This is the case if foam from the suction is not sufficient captured. Bubbles developed by air or vapour (e.g., water in the fuel). Those act as plug (vapour locking) in the scavenge system.

During oil deficiency and lubrication problems, overheating and overload of anti friction bearings and sliding surfaces (bearing races, gear tooth flank) must be expected. The consequences are dangerous failures like at main bearings or of accessory devices.
Also oil accumulations (frame below) itself can come into contact with rotating parts and act as a failure cause. Thereby are concerned:

  • Splashing which heats the not flowing off oil.
  • Increased lekage rates in the labyrinths of the bearing chambers. Consequence is the exit of oil with contaminations/fouling in the aeroengine. In an extreme case the danger of an oil fire exists (volume 2, Ill. 9.2-13).


22.3-1 I.E.Traeger, „Aircraft Gas Turbine Engine Technology, Second Edition“, Verlag : Glencoe/McGraw-Hill 1994, ISBN 0-07-065158-2., page 267-293.

22.3-2 H.Streifinger, „Fuel/Oil System Thermal Management in Aircraft Turbine Engines”, Paper RTO MP-8, des RTO AVT Symposium on „Design Principles and Methods for Aircraft Gas Turbine Engines“, Toulouse, France, 11-15 May 1998 page 12-1 up to 12-10.

22.3-3 The AeroShell Book, Edition 18, 2003, „Aeroshell Turbine Oils”, www.shell.com/aviation, page 1-90.

22.3-4 D.W.Bendell, „Factors Influencing Aircraft Turbine Engine Oil Drain Practices“ Paper des SAE Automotive Engineering Congress, Detroit, Michigan, January 10-14, 1966, page 148-152.

22.3-5 „Tracking Footprints from Engine Oil”, Zeitschrift: „Aviation Engineering & Maintenance Magazin“, 1979, Sept, page 30-33.

22.3-6 M.J.Kroes, T.W.Wild, „Aircraft Powerplants, Seventh Edition”, Verlag : Glencoe/McGraw-Hill 1990, ISBN 0-02-801874-5, page 343-356, 470.

22.3-7 M.Duncanson, „Detecting and Controlling Water in Oil“, Exxon Mobil, www.practicingoilanalysis.com, 17.02.2006, page 1-9.

© 2021 ITTM & Axel Rossmann
22/223/223.txt · Last modified: 2021/03/16 22:06 (external edit)

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