TM 55-4920-432-13&P
SECTION III - TECHNICAL PRINCIPLES OF OPERATION
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PRINCIPLES OF OPERATION
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a.
A small highspeed pump serves dual functions by producing system vacuums up to 29 inches of
mercury and system pressures up to 25 psi to simulate aircraft atmospheric and airspeed
conditions. These vacuums and pressures are delivered to the respective aircraft system and
tester instruments through oil reservoir, oil sumps, and a network of hoses, rigid pressure lines,
fittings, adapters, valves and ports on the tester along with external hoses and adapters supplied
with the tester.
b.
The pump serves the dual function of developing pressure as well as vacuum for operation of the
Tester. In the pressure section, a mixture of air and oil is pumped into the reservoir where the oil
and air are separated. The separated oil drops to the bottom of the reservoir where it is bled to the
pump for lubrication purposes, the air being forced out at the top of the reservoir under pressure.
The pressurized air, after going through the check valve whose function is to prevent oil from
entering instrument lines during operation, enters an oil sump and passes through a filter which is
an integral part of the sump. Air from the sump blows through two pressure control valves. The
pressure INCREASE valve controls the amount of air permitted in the system and the pressure
DECREASE valve opens the line to the ambient air, allowing system pressure to bleed off. The
available pressure level, as fixed by the control valves is sent to the pressure relief valve and
PRESSURE selector valve. Setting the PRESSURE selector valve at the desired test position
completes the circuit to the instrument under test and the master instrument to one of the pressure
relief valves. The pressure relief valve is in the line at all times and protects the instruments from
pressure overload when the pump is producing vacuum which passes through the check valve, and
the sump which prevents oil from entering the instrument lines. The vacuum INCREASE valve
controls the amount of vacuum in the system and the vacuum DECREASE valve opens the line to
the ambient air allowing the system vacuum to bleed off.
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