AIR - DESCRIPTION AND OPERATION
1. General
A. Engine compressor air is tapped for operating various engine accessories and airplane
systems. Some bleed systems are basic with the engine as received from the engine
manufacturer. Other engine bleed systems are installed by the airframe manufacturer. In
addition to engine bleeds for accessory and system use, a compressor bleed system is
incorporated to permit engine operational flexibility by allowing high compressor discharge air
to bleed into the fan discharge duct.
B. Those systems using engine bleed air and containing coverage within this chapter are as
follows:
SYSTEM SECTION
Compressor Bleed 75-31-0
Engine Anti-icing 75-11-0
Engine Gravel Protection *[1] 75-09-100
Engine Nose Cowl Anti-icing 75-11-0
Generator Cooling 75-21-0
C. Those systems using engine bleed air and containing coverage in other chapters are as
follows:
SYSTEM CHAPTER
Pneumatic Starting System 80
Engine Fuel Deicing 73
Pneumatic System 36
*[1] GJ ALL EXCEPT B-2509 AND B-2510
PW 732, 742, 752, 761, 762 AND 772
ENGINE GRAVEL PROTECTION SYSTEM - DESCRIPTION AND OPERATION
EFFECTIVITY
GJ ALL EXCEPT B-2509 and B-2510
1. General
A. The engine gravel protection system is provided for the two engines to prevent dirt, gravel and
other debris from getting into the engines. The system uses 13th stage engine bleed air ducted
through the nose cowl and discharged at a point below the inlet of the engine through air
nozzles (Fig. 1).
B. The system uses 28-volt dc power from the P6 load control center.
C. The system consists of a gravel protection boom, gravel protection valve, GRAVEL PROTECT
or ENG ANTI-FOD light, and a pressure switch for each engine, and a GRAVEL PROTECT or
ANTI-FOD switch for the two engines.
D. The GRAVEL PROTECT or ANTI-FOD switch at the P5 overhead panel controls the operation
of the system and permits testing valve operation during flight. System operation is indicated at
the control cabin by the GRAVEL PROTECT or ENG ANTI-FOD light at the center instrument
panel. A press-to-test feature checks the circuit continuity of the GRAVEL PROTECT or ENG
ANTI-FOD light (Fig. 2).
E. The landing gear module (M338) through the air-ground switch allows operation of the system
while the airplane is on the ground. When the airplane lifts off the ground the system is
automatically de-energized even though the GRAVEL PROTECT or ANTI-FOD switch is ON.
2. Engine Gravel Protection Boom
A. The engine gravel protection boom conveys bleed air from the engine and discharges it to the
ground through nozzles located at its end. The boom is located below the nose cowl and
extends from a point slightly forward of the aft end to a few inches beyond the inlet end. The
boom has a forward strut support which is provided with internal air lines for anti-icing the strut
(Fig. 1).
B. The nozzles in the boom direct the air at an angle towards the rear of the engine. The high
velocity air prevents dirt, gravel and other debris on the area below the engine inlet from being
sucked into the engine.
C. For removal/installation of the gravel protection boom, refer to Engine Nose Cowl, 71-11-31.
3. Engine Gravel Protection Valve
A. The engine gravel protection valve controls the flow of air that goes through the air nozzles in
the gravel protection boom. It is a solenoid- piloted, pressure-actuated regulating valve.
B. The valve is located below the engine just aft of the nose cowl. It is connected to the gravel
protection air ducts by means of couplings.
C. The valve is opened by positioning the GRAVEL PROTECT or ANTI-FOD switch at the P5
overhead panel to ON. This allows 13th stage engine bleed air to pass through the valve and
then to the air nozzles in the boom. Air pressure downstream of the valve is monitored by
means of an air sensing line tapped on the duct a few inches away from the outlet end of the
valve and connected to the valve body. Air pressure beyond the minimum or maximum
pressure limits at which the valve has been set will increase or decrease the opening of the
valve thereby regulating the flow of air.
D. The valve has an indicator which can be used in checking valve position during maintenance. A
regulating screw on the valve permits adjustment of the downstream air pressure to the
recommended value (Ref Adjustment/Test).
4. Engine Gravel Protection Pressure Switch
A. The gravel protection pressure switch in conjunction with the GRAVEL PROTECT or ENG
ANTI-FOD light at the P2 center instrument panel indicates that the gravel protection system is
operating (Fig. 2). The pressure switch is located downstream of the gravel protection valve.
When the valve is opened, pressure developed downstream of the valve actuates the pressure
switch which in turn closes the GRAVEL PROTECT or ENG ANTI-FOD light circuit and
illuminates the GRAVEL PROTECT or ENG ANTI-FOD light. The pressure switch closes at a
pressure of approximately 10 psi and opens when the air pressure goes below 5 psi.
5. Operation
A. The engine gravel protection system is operated on the ground by placing the GRAVEL
PROTECT or ANTI-FOD switch at the P5 overhead panel to ON. This energizes the solenoid
on the gravel protection valve of each engine which opens the valve. Opening the valve allows
13th stage bleed air to go through the gravel protection air duct in the nose cowl and to the
gravel protection boom below. The air goes through the gravel protection boom and is
discharged through the air nozzles.
B. Air pressure developed downstream of the valve actuates the pressure switch which causes the
GRAVEL PROTECT or ENG ANTI-FOD light at the P2 center instrument panel to illuminate.
Illumination of the GRAVEL PROTECT or ENG ANTI-FOD light in the control cabin indicates
that the engine gravel protection system is operating.
C. During flight when icing condition occurs, anti-icing of the gravel protection boom is done by
placing the GRAVEL PROTECT or ANTI-FOD switch to ANTI- ICE/TEST. This operates the
engine gravel protection system, allowing warm air to pass through the boom thereby
preventing the formation of ice. Aside from anti-icing, this position of the GRAVEL PROTECT or
ANTI-FOD switch may also be used to test system operation during flight or on the ground
during maintenance.
ENGINE GRAVEL PROTECTION SYSTEM - ADJUSTMENT/TEST
EFFECTIVITY
GJ ALL EXCEPT B-2509 and B-2510
1. Engine Gravel Protection Valve Test
A. Equipment and Materials
(1) Pressure gage, 0-100 psig, 8-inch dia., with 4-foot long high pressure hose extension and
provided with a hook for attaching gage to engine
B. Prepare to Test Engine Gravel Protection Valve
(1) Check that GRAVEL PROTECT or ANTI-FOD switch is off and the following circuit
breakers are open:
(a) Applicable ENG GRAVEL PROTECT or ENG ANTI-FOD circuit breaker.
(b) MASTER DIM BUS circuit breakers
(c) DIM and TEST circuit breakers
(2) Open left and right side engine cowl panels.
(3) Check that engine gravel protection valve is closed.
NOTE: An indicator on the valve will show valve position.
(4) Remove tube cap from tee on air sensing line and attach end of pressure gage hose
extension on tee.
(5) Secure the pressure gage at a suitable place at the side of engine where it can be read at
a safe distance.
(6) Provide electrical power.
C. Test Engine Gravel Protection Valve
(1) Position GRAVEL PROTECT or ANTI-FOD switch to ON and close all circuit breakers in
par. 1.B.(1).
(2) Start the engine according to Chapter 71, Power Plant.
(3) With the engine running at the EPR setting listed in the following table, record pressure
gage readings as regulated by the valve (Parker #F61D0236M Series).
(4) If the recorded pressures are not within limits, run engine at idle and adjust valve to bring
regulated pressure within tolerance. If pressure cannot be adjusted by moving upper
adjustment screw 1/2 turn, replace valve assembly.
NOTE: There are two adjustment screws on the valve, the lower and upper adjustment
screws. Adjustment on this test should be made only to the lower adjustment
screw.
(5) Shutdown engine according to Chapter 71, Power Plant.
(6) Position GRAVEL PROTECT or ANTI-FOD switch to OFF.
D. Restore Airplane to Normal Configuration
(1) Open MASTER DIM BUS, DIM and TEST circuit breakers.
(2) Remove pressure gage connection at tee on air sensing line and install tube cap.
(3) Close left and right side engine cowl panels.
(4) Close MASTER DIM BUS, DIM and TEST circuit breakers.
(5) If no longer required, remove electrical power from airplane.
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