Bourdon-tube. This tube gauge is constructed the same as other Bourdon-type gauges except that it
has a small restriction built into the instrument case or into the nipple connection leading to the Bourdon tube. This
restriction prevents the surging action of the oil pump from damaging the gauge or causing the pointer to oscillate too
violently with each pressure pulsation. Operation range markings are placed on the cover glass, or the face of the
gauge, to indicate the safe range of oil pressure for a given installation. Check the applicable maintenance manual.
Dual-type. A dual-type oil pressure gauge is available for use on multi-engine aircraft. The dual
indicator contains two Bourdon tubes, housed in a standard instrument case, one tube being used for each engine. The
connections extend from the back of the case to each engine. There is one common movement assembly, but the
moving parts function independently. In some installations, the line leading from the engine to the pressure gauge is
filled with light oil. Since the viscosity of this oil will not vary much with changes in temperature, the gauge will respond
better to changes in oil pressure. In time, engine oil will mix with some of the light oil in the line to the transmitter, and
during cold weather, the thicker mixture will cause sluggish instrument readings. To correct this condition, the gauge line
must be disconnected, drained, and refilled with light oil. Check the applicable maintenance manual.
Electrical transmitter. In this type of indicating system, the oil pressure being measured is applied to
the inlet port of the electrical transmitter, where it is conducted to a diaphragm assembly by a capillary tube. The motion
produced by the diaphragm's expansion and contraction is amplified through a lever and gear arrangement. The gear
varies the electrical value of the indicating circuit, which, in turn, is reflected on the indicator in the cockpit. This type of
indicating system replaces long fluid-filled tubing lines with an almost weightless piece of wire.
Oil Jets. Oil jets (or nozzles) are located in the pressure lines adjacent to, or within, the bearing compartments
and rotor shaft couplings.
Types. The atomized spray and air-oil mist spray oil jets are the types commonly used. The spray or mist
is produced by tapping high-pressure bleed air from the compressor to the oil nozzle outlet.
Inspection. Inspect oil jets for clogged orifices. The oil jets are easily clogged because of the small size of
the orifice. Oil filters should be checked frequently for contamination and the oil should be free of all foreign particles.
If the last-chance filters in the oil jets should become clogged, bearing failure usually results,
since nozzles are not accessible for cleaning except during engine overhaul.
Maintenance. Refer to the applicable maintenance manual for oil jet maintenance. Ensure that oil
and filters are changed regularly.
Flushing Oil Systems. Flushing of the oil system shall be accomplished when a new engine is installed, when the
oil system is found contaminated, or when oil is changed as specified in the applicable inspection requirements manual.
Flush the oil system as follows:
Drain oil system.
Drycleaning solvent is flammable and solvent vapors are toxic. Use P-D-680, Type II Solvent
in a well-ventilated area. Keep away from open flames. Avoid prolonged solvent contact with
Clean and flush system with a solution of 8 parts kerosene, Federal Specification VV-K-211, 1 part
drycleaning solvent, Federal Specification P-D-680, and 1 part corrosive-preventive, MIL-C-6529. In cold weather, keep
solution warm by using flashproof, thermostatically controlled electrical heating elements. Flushing solution temperature
shall not exceed 80 °F (27 °C).
Lines that cannot be cleaned while installed will be removed for cleaning.
Remove oil screens and filters.
When removing screen of filter, catch oil, which comes from around screen or filter and from screen
or filter itself, in a clean, white cloth.
Dip cloth and its contents into cleaning solution and wash away dirt and carbon.