4
3
7
6
4
8
5
3
1
2
5
1
2
9
10
10
14
11
AIR FLOW
13
11
13
12
15
ASf12018
12
16
1.
Solenoid receptacle
9. Rate head
2.
10. Servo assembly
Solenoid valve
3.
Modulating valve spring tension adjusting cap
11. Butterfly valve assembly
4.
Exhaust thermostat boss
12. Filter assembly
5.
Drain boss
13. Constant pressure regulating assembly
6.
14. Thermostat assembly
Oil out boss
7.
15. Constant pressure regulator adjustment screw
Oil in boss
8.
Capillary assembly
16. Thermostat removal and test pressure plug
Figure 12-18.--Modulating and shutoff valve.
The s u rg e c o n t r o l c o m p o n e n t s f u n c t i o n t o
harsh surge of bleed air, which could damage aircraft
modulate the engine acceleration schedule and
starter shafts.
reduced speed schedule. Modulation of the engine
When the output air switch is placed in the "OFF"
acceleration schedule (rate) is accomplished by the
position, the modulating and shutoff valve closes,
action of the pneumatic shutoff valve when it is
eliminating bleed air (and a pressure differential acting
actuated by the centrifugal switch. This valve operates
on the cutout switch). Hence, the cutout switch
between 35 percent and 95 percent of engine rated
contacts close and re-actuates (opens) the pneumatic
speed and bleeds off turbine inlet air as required for
shutoff valve until the engine speed, again, increases
smooth acceleration.
above 95 percent (no-load condition).
M o d u l a t i o n o f t h e e n g i n e 's r e d u c e d s p e e d
MODULATING AND SHUTOFF VALVE.--
schedule is performed by the action of the pneumatic
The modulating and shutoff valve is the principal
shutoff valve when it is actuated by the cutout switch.
component of the bleed-air system. Bleed loading of
The circuit to the cutout switch energizes when the
output air switch is selected for lower engine speed
the engine is accomplished by bleeding compressed air
operation. The cutout switch actuates the pneumatic
from the engine. This valve allows for air to be bled
shutoff valve to bleed off excess compressed air until it
from the engine while maintaining governed speed for
is signaled by the pressure differential sensing probe.
engine operation.
The probe, located in the bleed-air flow, senses a
DUCT ASSEMBLIES.--Ducting is used to
pressure differential as the modulating and shutoff
transport the air bled from the engine to the unit being
valve starts to open. This differential acts on the cutout
serviced, normally an aircraft engine requiring air for
switch diaphragm and opens the contacts, which, in
starting.
turn, closes the pneumatic shutoff valve. A smooth
INTERNAL DUCTING.--Ducting within the
transition occurs between the controlled opening
enclosure allows air discharged from the modulating
speed of the modulating and shutoff valve and the
closing of the pneumatic shutoff valve to prevent a
and shutoff valve to reach the outer shell of the
12-21
