THRUSH AIRCRAFT, INC – MODEL S2R-R1340
AIRCRAFT MAINTENANCE MANUAL
Effective: 01/01/08
8-7
POWER PLANT INSTRUMENTS
Ref. Figures 8-2 & 8-3 and Table 8-3
This group consists of the oil
temperature, oil pressure, fuel pressure,
fuel quantity and manifold pressure
gauges, as well as the engine
tachometer. See Table 8-3 for
instrument markings. See Section 5 for
more thorough description of fuel
quantity system.
OIL TEMPERATURE SYSTEM
The oil temperature indicating system is
electrically operated by the aircraft
electrical system and therefore only
works when the master switch is on.
The oil temperature transducer is
installed in the “Y” adapter on the
firewall, in the oil stream returning to the
engine from the oil tank. The transducer
varies voltage to the indicator in
proportion to the oil temperature, and
the indicator needle movement is
proportional to the voltage.
Oil temperature is an indicator of the
condition of the oil cooling system and
the internal health of the engine. If the
oil cooling system is operating properly,
a high oil temperature could indicate
such engine problems as broken piston
rings or bearing failure. The excessive
friction of the failed bearing makes the
engine work harder, while broken or
improperly seated rings allow blow-by of
hot gasses into the crankcase, heating
the oil. Oil turning black shortly after an
oil change also indicates piston ring
blow-by.
Before assuming high oil temperature is
an indication of engine problems,
always make sure the oil cooling system
is working properly.
OIL PRESSURE SYSTEM
The oil pressure indicator works on the
Bourdon Tube principle whereby a semi-
circular hollow tube unbends in
proportion to internal pressure. This
unbending motion drives the pressure
indicator needle.
The oil pressure system thus needs no
electricity, but must be connected to a
pressure source; in this case oil piped
from the oil pressure gallery on the left
side of the engine crankcase. This port
is downstream of the engine driven oil
pump. To operate properly, the oil line
between the engine and instrument
must be free of air.
Oil pressure is an indicator of how well
oil is being supplied to the internal
components of the engine. The
desirable pressure is a range because
the pressure is influenced by oil
viscosity, which is in turn influenced by
oil type and the temperature it is
operating at.
Oil pressure higher than this range is
not a “more is better” situation, as it may
be caused by blockages in the oil
distribution system. Normally, however,
high oil pressure is experienced right
after start-up on a cold day, when the oil
is still thick. It is important to allow the
oil to warm up to the operating range
before demanding power from the
engine. Always use the grade and type
of oil recommended by the engine
manufacturer, based on engine usage
and weather conditions.
Low oil pressure is bad news anytime,
as it indicates that the engine is not
getting enough lubrication. Oil viscosity
decreases as oil temperature increases,
so a faulty oil cooling system or internal
engine problems causing high oil
temperature may be at fault. Low oil
pressure when oil temperature is normal
or low could indicate a bad oil pump, the
wrong grade of oil, or an extremely worn
engine.
