The oils ystem (Fig. "Oil system components") 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:
With this, the following problems get specific emphasis:
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.
Fig. "Oil system components" (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.
Aeroengine components which influence problems of the oil/oil system (Fig. "Influences at oil consumption", Fig. "Specifications for oil contaminations", Fig. "Material specific particle content in oil", Fig. "Wear particles point at problems"):
Amount/volume of oil (Fig. "Dangerous overfill")
Spout/suction (Fig. "Problems due to change of the oil type", Fig. "Metallic contaminations in oil show problems", volume 3, Ill. 11.2.5-5.2).
Pressure pumps/fresh oil pumps
Suction pumps/scavenge pumps
Oil filters, sieves, screens
cooled by fuel (Volume 2, Ill. 9.2-4)
Overpressure valves (pressure relief valves, relieve valves)
Bypass valves Control valves (e.g., for oil temperature)
Sensors for display and maintenance
Oil jets/nozzles/injectors for the lubrication of anti friction bearings and gears
Bearings (Fig. "Deteriorating effects by oil coking") Friction bearings/sliding bearings Anti friction bearings (main bearings, bearings in gears and so on)
Squeeze film dampers (Fig. "Damped main bearings")
Gears/gear wheels/gear pinions (Fig. "Deteriorating effects by oil coking")
Fig. "Flight time dictated by oil shortage" (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 (Fig. "Oil system components") demand, comparing with a car motor a frequent re-filling (Fig. "Influences at oil consumption"). 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.
Fig. "Influences at oil consumption" (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 (Fig. "Influences at oil consumption"). 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 (Fig. "Oil coaking endangers main bearings") 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 (Fig. "Importance of resting time at idle").
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, Fig. "Danger also without abrasion and chips") 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:
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 (Fig. "Influences at oil consumption") 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 (Fig. "Temperature limits of fuel and oil systems").
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.
Fig. "Temperature limits of fuel and oil systems" (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 (Fig. "Lubrication oils"). The temperature has essential influence at
the lifetime (Fig. "Influences at oil consumption") 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
As a help for the assessment of the viscosity the value at room temperature for honey, water and oil as an extreme is indicated.
Fig. "Oil viscosity and lubrication by temperature" (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.
Fig. "Oil system heat balance" (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 (Fig. "Oil viscosity and lubrication by temperature"). Service life
and properties are primarily determined by the operation temperatures (Fig. "Influences at oil consumption"). The sketched
oilsystem is a simplified scheme. It shows typical components which contribute markedly to the heat balance
(Fig. "Oil system components").
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.
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
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 (Fig. "Fuel influencing fuel pumps") are more favourable. However its high price led to gear pumps with higher heat production.
Fig. "Lifetime influenced by water in oil" (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. 126.96.36.199-1). The failure mechanisms can olso combine in its effects and so intensify. These are:
Fig. "Metallic contaminations in oil show problems" (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 (Fig. "Specifications for oil contaminations"). With this the chance of an
early identification and clearing of the cause
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).
Fig. "Oil shortage by oil hiding" (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:
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:
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,
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.