JET AIRCRAFT ENGINE LUBRICATION SYSTEMS

The increased complexity of aircraft engines has added to the requirements for proper
lubrication. Jet engines require lubrication to prevent friction from reducing the engines’
efficiency. Oil is the lifeblood of the aircraft engine. If the oil supply to the bearings stops,
the lubricating films break down and cause seizing and burning between moving parts. Like
the heart and circulatory system of the human body, they quietly perform their function so
well we forget their importance.

LUBRICANTS
Lubricating oils provide a film that permits surfaces to glide over one another with less
friction. Therefore, lubrication is essential to prevent wear in mechanical devices where
surfaces rub together.
The Navy uses many types of lubricants. The selection of the proper lubricant depends on
the design of the equipment and the operating conditions. Maintenance instruction
manuals (MIMs) or maintenance requirements cards (MRCs) list the type of lubricant
required for specific aircraft maintenance tasks. Using the wrong type of lubricant, mixing
different types, or lubricating improperly can cause extra maintenance man-hours, part
failures, and accidents.

Types of Lubricants

Lubricants are classified according to their source—animal, vegetable, petroleum, mineral,

or synthetic. Animal oils are not suitable lubricants for internal-combustion engines. They
form fatty acids, which cause corrosion when exposed to high temperatures. Vegetable oils
have good lubricating qualities, but break down (they change in chemical structure) after
long periods of operation in internal-combustion engines. Mineral-base lubricants are
usually divided into three groups—solids, semisolids, and liquids. Petroleum-based oils (for
example, MIL-PRF-6081 grade) were used in early jet engines. The types of lubricants used
in the engines of today are different from the lubricants used years ago. As the power
output of jet engines increased, aircraft were able to fly higher. The operation of jet engines
at these higher, colder altitudes and higher engine temperatures created greater demands
on lubricating oils. This, in turn, required the development of synthetic lubricants that could
withstand these higher bearing temperatures.

LUBRICATION SYSTEMS

Oil systems used in jet engines are relatively simple in design and operation, but their
function is important. The principal purposes of the oil system are the same as those
covered under lubricating oils—to provide an adequate supply of clean oil to bearings
and gears at the right pressure and temperature, to remove heat from the engine, and to
remove contaminants from the system and deposit them in the filters.
The ability of the oil to lubricate correctly depends upon its temperature and pressure. If
the oil is too hot, it will not have enough viscosity. If it is too cold, the oil will resist
movement between the parts and flow too slowly for proper lubrication. If the oil pressure
is too low, not enough oil will be supplied to the bearing for proper cooling. If the
pressure is too high, it may cause high-speed antifriction bearings to skid and not roll
properly.

Types of Lubrication Systems

Engines use a wet-sump, dry-sump, or a combination of both as lubricating

systems. Wet-sump engines store the lubricating oil in the engine or gearbox. Dry-
sump engines use an external tank mounted on the engine or somewhere in the
aircraft structure near the engine.

Wet-Sump System

Engines needing a limited supply of oil and cooling can use a wet-sump type. The
reservoir for the wet-sump system is either the accessory gear case or a sump
mounted to the bottom of the accessory gear case. This system is similar to your
car’s engine. In the wet-sump oil system, the gearbox provides space for foaming
and heat expansion because the oil level only partly fills the casing. Deaerators, in
the gearbox, remove oil from the air and vent the air outside.
The main disadvantages of a wet-sump system are as follows:
 The oil supply is limited by the sump capacity.

 It is hard to cool the oil. Oil temperatures are higher because the oil is
continuously subjected to the engine temperature.

 The system is not adaptable to unusual flight altitudes, since the oil supply
would flood the engine.

Dry-Sump System
The dry-sump system is the most common. In the dry-sump lubrication system, a tank
located in the airframe or mounted on the engine holds the oil supply. This type of
system carries a larger oil capacity, and an oil cooler is usually included to control
temperature. The lubrication design of the engine may use either an air-oil or a fuel-oil
cooler.

Oil System Components

There are two primary types of oil systems. Some of these parts are unique to one type
of system, while other parts are used in both systems. The main parts of a typical oil
system include an oil tank, oil pumps, valves, filters, and chip detectors. Other parts
are oil coolers, oil jets, gauge connectors, vents, and oil system seals.
Oil Tanks
The oil tank stores the system oil supply. An oil tank may be a simple sealed
container (similar to a car’s fuel tank) where oil is gravity-fed to the engine. Older
low-performance aircraft engines could use this simple tank design. Today’s high
performance aircraft require a more complicated pressurized type of oil tank; this
assures positive lubrication during all flight conditions.

The dry-sump oil system uses an oil tank located either in the airframe or mounted
on the engine. Oil tanks mounted on the airframe are normally located within or near
the engine compartment. Additionally, designers place it high to gain as much
advantage as possible from gravity flow to the oil pump inlet.

The tank is designed to furnish a constant supply of oil to the engine in any attitude,
and during negative g loading or forces. This is done by a swivel outlet assembly
mounted inside the tank, a horizontal baffle mounted in the center of the tank, two
flapper check valves mounted on the baffle, and a positive vent system.

All oil tanks provide an expansion space. This allows for oil expansion from heat and
oil foaming. Some tanks also have a deaerator tray for separating air from the oil.

Another feature common in oil tanks is a sump with drain and shutoff valves in the
bottom of the tank. The drain valve permits oil to be drained for oil changes. An oil
shutoff valve is a motor operated, gate-type valve attached to the oil sump. This
valve can be operated electrically or manually to shut off the oil supply to the engine
in emergency conditions.

Oil Pumps
The oil pump supplies oil under pressure to engine points that require lubrication.
Most lubrication pumps have both a pressure supply element and a scavenge
element. However, some oil pumps serve a single function; they either supply or
scavenge the oil. The number of pumping elements, both pressure and scavenge,
depends largely on the type and model of the engine. For instance, the axial-flow
engines have a long rotor shaft and use more bearings than a centrifugal-flow
engine. Therefore, there must be more oil pump elements for both supply and
scavenging, or they must be of larger capacity.
It is common to use small individual scavenge pumps in the remote sections of an
engine. This assures proper scavenging of the lubricating oil. In all types of pumps,
the scavenge elements have a greater pumping capacity than the pressure element.
This is to prevent oil from collecting in the bearing sumps.
The pumps may be one of several types, each type having certain advantages and
limitations.
The three most common oil pumps are gear, gerotor, and piston types, the first being
the most used and the last the least used. Each of these pump types have several
different designs.
The gear-type oil pump has only two elements (one for pressure oil and one for
scavenge). However, this type of pump could have several elements. This valve limits
the output pressure of the pump by bypassing oil back to the pump inlet. Also notice the
location of the shaft shear section, which will allow the shaft to shear if the gears should
seize.

Gerotor Oil Pump

The gerotor pump usually has a single element for oil feed and several elements for
scavenging oil. Each of the elements, pressure and scavenge, are almost identical in
shape. However, the capacity of the elements is controlled by varying the size of the
gerotor elements. The elements are all driven by a common shaft. Engine revolutions
per minute (rpm) determine oil pressure, with a minimum pressure at idling speed and
maximum pressure at maximum engine speed. Each set of gerotor is separated by a
steel plate, making each set an individual pumping unit. Each set consists of an inner
element and an outer element. The small star-shaped inner element has external lobes
fitting within and matching with the outer element, which has internal lobes.

Piston Oil Pump

The piston lubrication pump is always a multi-plunger type. Output of each piston
supplies a separate jet. Oil drained from the points of lubrication is scavenged by a
separate pump element.

Valves
Valves control the pressure and flow of oil in the lubrication system. There are three
types of valves common to oil systems that are discussed in this text. They are relief
valves, check valves, and bypass valves.

Oil Pressure Relief Valve

An oil pressure relief valve limits the maximum pressure within the system. The relief
valve is preset to relieve pressure and return the oil to the inlet side of the lube pump.
This valve is important if the system has an oil cooler, because the cooler’s thin walls
rupture easily.

Check Valves
Check valves installed in the oil supply lines or filter housings prevent oil in the reservoir
from seeping (by gravity) into the engine after shutdown. Check valves prevent
accumulations of undue amounts of oil in the accessory gearbox, rear of the compressor
housing, and combustion chamber. Such accumulations could cause excessive loading
of accessory drive gears during starts, contamination of the cockpit pressurization air,
internal oil fires, and hot starts.

Thermostatic Bypass Valves

Thermostatic bypass valves are included in oil systems using an oil cooler. Their
purpose is to maintain proper oil temperature by varying the proportion of the total oil
flow passing through the oil cooler. This valve consists of a valve body (having two inlet
ports and one outlet port) and a spring-loaded thermostatic element valve.