The check valve assembly and accumulator are installed to dampen torsional vibrations
encountered in helicopter rotor systems. Because of wind gusts and turbulent air conditions, the rotor
rpm will fluctuate slightly. The power turbine governor senses this rpm change and causes the fuel
control to vary fuel flow. By installing an accumulator and damping out the governing pressure (PG) to
the gas producer fuel control, the design prevents the engine from responding to torsional vibrations.
The fuel nozzle is a single-entry, dual-orifice type nozzle. It threads into the combustor
outer case and extends into the aft end of the combustor liner. The fuel control delivers fuel to the
nozzle which atomizes and injects fuel into the combustion chamber, where it is mixed with air and
The fuel nozzle must properly atomize and inject the fuel in all ranges of fuel flow
from starting to maximum power. This is accomplished by means of a dual-orifice design. The primary
orifice has fuel delivered to it whenever the engine is operating, but the secondary orifice receives fuel
only when the fuel pressure to the fuel nozzle exceeds 150 psi.
AIR BLEED AND ANTI-ICING SYSTEMS
The compressor air bleed system permits rapid engine response by relieving compressor
pressure during engine acceleration. A bleed air control valve is mounted to the bleed air manifold on
the compressor case. The compressor bleed air and anti-icing systems are illustrated in figure 7.15.
Elongated slots between every other vane at the compressor fifth stage bleed compressor air
into a manifold on the compressor case. The air bleed control valve is open during starting and ground-
idle operation, and it remains open until a predetermined pressure ratio is obtained, at which time the
valve begins to move from the open to the closed position.
The engine is equipped with an anti-icing system that conducts hot air to the compressor
front-support struts to prevent ice forming on the struts. The system is entirely separate and
independent of any other bleed air system. The engine anti-icing system must be turned on by the pilot.
As air passes through the compressor, it is compressed. As a result of this compression, the air is heated
and is a source of hot air required by the engine anti-icing system.