737 Bleed Air Test Guide

737-300 / 400 / 500 BLEED AIR TROUBLESHOOTING

When a discrepancy is encountered in this bleed system it is important to note that this system is dividedinto three separate subsystems which interact with each other but are functionally independent.

1. Precooler System----Precooler, precooler valve, precooler sensor, wing thermal anti-ice solenoid (WTAI). Controls the bleed delivery temperature to 390 degrees by varying the amount of air blowing through the precooler from the fan duct.

2. PRSOV System----PRSOV (pressure regulating and shutoff valve), bleed air regulator (remote mount regulator), 450 degree overtemp sensor, and 490 degree overtemp switch. Functions to shut engine bleed air on and off. Also regulates bleed delivery pressure to 40 to 50 PSI through modulation of the PRSOV. Provides bleed air overtemp protection.

3. High Stage System----High stage valve, high stage regulator. Augments bleed pressure from the 9th stage to provide adequate flow at low power settings.

THE FOLLOWING IS A GUIDE ON WHERE TO LOOK FOR THE SOURCE

OF COMMONLY REPORTED FAULTS IN THE 737-classic BLEED SYSTEM.

NOTE: For normal operation the following duct pressures will be observed on a correctly functioning system. TAKEOFF & CLIMB 42-52 PSI

CRUISE 35-40 PSI IDLE & DESCENT 15-20 PSI

NOTE: THE FOLLOWING TROUBLESHOOTING TIPS ASSUME THAT ALL SUPPLY LINES AND B-NUTS ARE NOT LEAKING.

YOUR FIRST STEP SHOULD ALWAYS BE TO CLOSELY EXAMINE THE PLUMBING

TO THE SUSPECTED COMPONENT.

NOTE: When you remove lines from the engine components for test purposes be sure to use anti-seize compound on the treads when re-connecting. These lines are known to gall and lock up tight.

2

JPText BoxFOR INFORMATION ONLY. UNCONTROLLED DATA. ALWAYS USE YOUR OFFICIAL MANUALS.

A. BLEED TRIP AT TAKEOFF POWER bleed parameters otherwise normal

CAUSE excessive pressure sensed in engine ducting upstream of PRSOV causing bleed air regulator over pressure switch to trip bleed off.

PRIMARY SUSPECTS high stage valve, high stage regulator, bleed air regulator.

B. BLEED TRIP AT CLIMB, CRUISE, OR DESCENT bleed pressure low prior to trip or unknown.

CAUSE excessive duct temperature being sensed by 490 degree overtemp switch.

PRIMARY SUSPECTS precooler valve, precooler sensor.

SECONDARY SUSPECTS 450 degree sensor, high stage valve, high stage regulator, PRSOV, bleed air regulator.

C. HIGH DUCT PRESSURE AT HIGH POWER SETTINGS (climb, cruise, & takeoff)

CAUSE PRSOV failing to regulate bleed pressure downstream of valve.

PRIMARY SUSPECTS bleed air regulator, PRSOV.

D. LOW DUCT PRESSURE IN CLIMB OR CRUISE

CAUSE PRSOV restricting bleed air to downstream system because bleed air is to hot. Excessive bleed temp is caused by insufficient fan air through precooler. Low duct pressure can also be caused buy insufficientregulated pressure from bleed air regulator to PRSOV.

PRIMARY SUSPECTS precooler valve, precooler sensor, 450 degree sensor, bleed air regulator, PRSOV.

E. LOW DUCT PRESSURE AT LOW POWER SETTING high power settings duct pressure is normal.

CAUSE high stage valve not open far enough, or PRSOV not regulating properly.

PRIMARY SUSPECTS high stage regulator, high stage valve.

SECONDARY SUSPECTS bleed air regulator, PRSOV.

F. BLEED WILL NOT SHUT OFF OR TURN ON

CAUSE PRSOV is stuck or valve is failing to get a proper control signal from bleed air regulator.


G. BLEED PRESSURE FLUCTUES IN CLIMB OR CRUISE

CAUSE PRSOV hunting due to internal binding or erratic control signal from bleed air regulator.


3

PRECOOLER CONTROL VALVE

INDICATOR

SUPPLY LINE

SUPPLY PRESSURE PORT

SENSOR PORT

SENSE LINE

PRECOOLER SYSTEM TEST

Note: With the engine shut down and the APU bleed valve in the OFF position the precooler control valve should be spring loaded to the OPEN position. If it is not fully open, replace the valve.

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1. Manually wrench the precooler control valve position indicator through full range of motion in both directions. If valve does not operate smoothly, replace the precooler control valve.

2. Disconnect supply line from precooler control valve supply pressure port located on L/H side of engine.

3. Connect test hose between the precooler control valve supply pressure port and regulator Ps of the test box.

4. With 130 PSI supplied to test box, slowly increase pressure at regulator Ps while observing valve position indicator. Precooler valve should be within 30 degrees of full closed when regulator Ps pressure gauge reaches 12 PSI.

a. If valve is not within 30 degrees of full closed, disconnect sense line from R/H side of precooler control valve and plug the precooler sensor port on the valve.

b. Repeat above steps. If the valve still does not close, replace the precooler control valve. If valve closes, there is a leak in the sense line, WTAI solenoid valve, or the precooler sensor.

5. Reduce regulator Ps pressure to zero.

6. Disconnect the sense line from the precooler control valve sensor port located on R/H side of the valve.

7. Connect test hose between the sensor port on the precooler control valve and gauge Pc of the test box.

8. Make sure regulator Pc is fully closed.

9. Slowly increase pressure at regulator Ps to 55 PSI (+/-5 PSI). Pressure on gauge Pc should stabilize between 6 & 13 PSI. Precooler control valve should move to within 30 degrees of full closed.

10. Slowly open regulator (Bleeder Valve) Pc on the test box. Precooler control valve must begin to open when pressure on gauge Pc reaches a minimum of 7 PSI. Precooler control valve must be full open when gauge Pc reaches 3 PSI.

Note: If the valve does not open as outlined it can cause the bleed air temp to be excessively high which will cause the 450 degree sensor to modulate the PRSOV towards the closed position.

11. If the valve does not open as outlined in step #10, replace the precooler control valve.

12. Restore engine to normal configuration.

6

PRSOV SYSTEM TEST

Note: With engine shut down and APU bleed valve in OFF position the PRSOV should be spring loaded to the CLOSED position. If it is not fully closed, replace the valve.

Turn on aircraft power and assure bleed switch for test engine is ON.

450 DegreeSensor

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PRSOV SYSTEM TEST

1. Bleed Air Regulator Test: Note: Leakage from the bleed air regulator honest orifices is not abnormala. Disconnect both supply and control lines from the bleed air regulator pressure ports. b. Connect test hose between the bleed air regulator supply pressure port and regulator Ps of the test box. c. Connect test hose between the bleed air regulator control pressure port and gauge Pc of the test box. d. Make sure regulator (Bleeder Valve) Pc is fully closed. e. With 130 PSI supplied to test box, slowly increase pressure on regulator Ps to 65 PSI (+/-5 PSI). The control pressure on gauge Pc should stabilize at 24 PSI (+/-4 PSI). f. Continue to increase supply pressure on regulator Ps to 120 PSI, control pressure should remain at 24 PSI (+/-4 PSI). If control pressure is significantly higher or lower than 24 PSI, replace the bleed air regulator. g. Reduce regulator Ps pressure to zero and open regulator (BLEEDER VALVE) Pc. h. Disconnect test hose from bleed air regulator control pressure port and reconnect control line to the bleed air regulator control pressure port.

2. PRSOV Minimum Pressure Opening Test: a. Manually wrench the PRSOV position indicator through full range of motion in both directions. If valve does not operate smoothly, replace the PRSOV. b. Slowly increase pressure at regulator Ps while observing PRSOV position indicator. c. The PRSOV must be fully open at 5 PSI maximum supply pressure at regulator Ps. If it takes more than 5 PSI then disconnect the 450 degree sense line from the T fitting located on R/H side of the PRSOV and cap the fitting. Repeat the test. If the test fails again, the PRSOV or the control line is at fault. If the test passes, the 450 degree sense line or the 450 degree sensor is at fault.

3. Bleed Air Regulator Over Pressure Bleed Trip Test: WARNING: Do not use Bleed Air Test Box, regulators do not go high enough to perform this test.a. Using a low pressure nitrogen bottle to regulate pressure, connect test hose from bleed air regulator supply pressure port to low pressure nitrogen bottle. b. Slowly increase pressure while observing the PRSOV position indicator, the PRSOV should open. c. Continue increasing pressure, the PRSOV should slowly go closed and the BLEED TRIP OFF light on the P5-10 overhead (air conditioning module) panel should illuminate when pressure reaches 210 PSI (+/-10 PSI). If the BLEED TRIP OFF light does not illuminate, replace the regulator. d. Reduce pressure to zero and reset BLEED TRIP OFF light.e. Disconnect test hose from bleed air regulator supply pressure port and reconnect supply line to the bleed air regulator supply pressure port.

4. Downstream Sense Line Test: Note: This test should be done if the complaint is high duct pressure and the bleed air regulator control

pressure is verified good per step 1.a. Pressurize pneumatic ducting to the engine using either APU or ground cart. b. Check the downstream sense line for leaks. This line begins on the L/H side of the pneumatic duct just aft and above the precooler, terminating at the PRSOV (L/H side of valve). If this line is leaking it will cause higher than normal duct pressure, replace the downstream sense line.

5. PRSOV Valve Piston Seal Test: Note: Another cause for high duct pressure at high power settings is an internal leak inside the PRSOV

at the valve piston seals. It is normal for a small amount of air to come out of the T fitting.

a. Pressurize pneumatic ducting to the engine using either APU or ground cart. b. Disconnect both bleed air regulator control line and 450 degree sense line from T fitting located on the R/H side of the PRSOV. If large amount of air (jet blast) is coming from the T fitting, replace the PRSOV.

6. Restore engine to normal configuration. 8

INDICATOR

HIGH STAGE SYSTEM TESTNote: This test will verify control function of the high stage system only. It will not test leakage past the butterfly in the valve. This is a common cause for over pressure bleed trips at takeoff power settings.

With the engine shut down the high stage valve is spring loaded to the CLOSED position. If it is not closed, replace the valve.

DOWNSTREAM SENSE LINE

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HIGH STAGE SYSTEM TEST

1. High Stage Regulator Test: a. Disconnect both supply and control lines from high stage regulator pressure ports. b. Connect test hose between the high stage regulator supply pressure port and regulator Ps of the test box. c. Connect test hose between the high stage regulator control pressure port and gauge Pc of the test box.d. Make sure regulator (Bleeder Valve) Pc is fully closed. e. With 130 PSI supplied to test box, slowly increase pressure at regulator Ps to 35 - 40 PSI, gauge Pc pressure should increase to 15 -18 PSI.f. Continue increasing pressure at regulator Ps to 90 PSI. Gauge Pc pressure should remain at 15 18 PSI. If pressure is significantly higher or lower, replace the high stage regulator.

2. High Stage Regulator Pneumatic Shutoff Test: a. Increase pressure at regulator Ps to 110 PSI (+/-10 PSI). Gauge Pc pressure should read 4 PSI or less. If it does not, replace the regulator. b. Reduce regulator Ps pressure to zero and open regulator (Bleeder Valve) Pc. c. Disconnect test hose from high stage regulator control pressure port and reconnect control line to the high stage regulator control pressure port.

3. High Stage Valve Minimum Pressure Opening Test: a. Manually wrench the high stage valve position indicator through full range of motion in both directions. If valve does not operate smoothly, replace the high stage valve.b. Slowly increase pressure at regulator Ps while observing the high stage valve position indicator. c. The high stage valve should be fully open at 10 PSI maximum supply pressure at regulator Ps. If it fails, either the high stage valve or the control line is at fault.

4. High Stage Regulator Downstream Sense Port Leak Test: a. Increase pressure at regulator Ps to 35 PSI. b. Disconnect downstream sense line from high stage regulator downstream sense port. c. There should be no air leakage coming from the high stage regulator downstream sense port. If it leaks, replace the regulator. d. Reconnect the downstream sense line to the high stage regulator downstream sense port.

5. High Stage Valve Internal Leak Test: a. Disconnect the control line from the high stage valve. b. Using a wrench, manually open (full open) the high stage valve. c. While plugging the high stage valve control port with your thumb, remove the wrench and let the high stage valve move to the closed position. If the valve closes in less than 10 seconds, replace the valve.


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AIRCRAFTS WITHOUT REVERSE FLOW PROTECTION

AIRCRAFTS WITH REVERSE FLOW PROTECTION

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AIRCRAFTS WITH REVERSE FLOW PROTECTIONPRE-SB 36-1016AIRCRAFTS WITHOUT REVERSE FLOW PROTECTION

AIRCRAFTS WITH REVERSE FLOW PROECTIONPOST-SB 36-1016

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737-600/700/800/900 BLEED AIR TROUBLESHOOTING

When a discrepancy is encountered in this bleed system it is important to note that this system is dividedinto three separate subsystems which interact with each other but are functionally independent.

1. Precooler System----Precooler, precooler valve, precooler sensor, wing thermal anti-ice solenoid (WTAI). Controls the bleed delivery temperature to 390 degrees by varying the amount of air blowing through the precooler from the fan duct.

2. PRSOV System----PRSOV (pressure regulating and shutoff valve), bleed air regulator (remote mount regulator), 450 degree overtemp sensor, and 490 degree overtemp switch. Functions to shut engine bleed air on and off. Also regulates bleed delivery pressure to 40 to 50 PSI through modulation of the PRSOV. Provides bleed air overtemp protection.

3. High Stage System----High stage valve, high stage regulator. Augments bleed pressure from the 9th stage to provide adequate flow at low power settings.

THE FOLLOWING IS A GUIDE ON WHERE TO LOOK FOR THE SOURCE

OF COMMONLY REPORTED FAULTS IN THE 737NG BLEED SYSTEM.

NOTE: For normal operation the following duct pressures will be observed on a correctly functioning system. TAKEOFF & CLIMB 42-52 PSI

CRUISE 35-40 PSI IDLE & DESCENT 15-20 PSI

NOTE: THE FOLLOWING TROUBLESHOOTING TIPS ASSUME THAT ALL SUPPLY LINES AND B-NUTS ARE NOT LEAKING.

YOUR FIRST STEP SHOULD ALWAYS BE TO CLOSELY EXAMINE THE PLUMBING

TO THE SUSPECTED COMPONENT.

NOTE: When you remove lines from the engine components for test purposes be sure to use anti-seize compound on the treads when re-connecting. These lines are known to gall and lock up tight.

2

JPText BoxFOR INFORMATION ONLY. UNCONTROLED DATA. ALWAYS USE YOUR OFFICIAL MANUALS.

A. BLEED TRIP AT TAKEOFF POWER bleed parameters otherwise normal

CAUSE excessive pressure sensed in engine ducting upstream of PRSOV causing bleed air regulator over pressure switch to trip bleed off.

PRIMARY SUSPECTS high stage valve, high stage regulator, bleed air regulator.

B. BLEED TRIP AT CLIMB, CRUISE, OR DESCENT bleed pressure low prior to trip or unknown.

CAUSE excessive duct temperature being sensed by 490 degree overtemp switch.

PRIMARY SUSPECTS precooler valve, precooler sensor.

SECONDARY SUSPECTS 450 degree sensor, high stage valve, high stage regulator, PRSOV, bleed air regulator.

C. HIGH DUCT PRESSURE AT HIGH POWER SETTINGS (climb, cruise, & takeoff)

CAUSE PRSOV failing to regulate bleed pressure downstream of valve.


D. LOW DUCT PRESSURE IN CLIMB OR CRUISE

CAUSE PRSOV restricting bleed air to downstream system because bleed air is to hot. Excessive bleed temp is caused by insufficient fan air through precooler. Low duct pressure can also be caused buy insufficientregulated pressure from bleed air regulator to PRSOV.

PRIMARY SUSPECTS precooler valve, precooler sensor, 450 degree sensor, bleed air regulator, PRSOV.

E. LOW DUCT PRESSURE AT LOW POWER SETTING high power settings duct pressure is normal.

CAUSE high stage valve not open far enough, or PRSOV not regulating properly.

PRIMARY SUSPECTS high stage regulator, high stage valve.

SECONDARY SUSPECTS bleed air regulator, PRSOV.

F. BLEED WILL NOT SHUT OFF OR TURN ON

CAUSE PRSOV is stuck or valve is failing to get a proper control signal from bleed air regulator.


G. BLEED PRESSURE FLUCTUES IN CLIMB OR CRUISE

CAUSE PRSOV hunting due to internal binding or erratic control signal from bleed air regulator.


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(WTAI)

WTAISolenoid

Valve

Precooler Control Valve

To Precooler ControlValve Sensor

To 450 Degree Thermostat

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Note: With the engine shut down and the APU bleed valve in the OFF position, the precooler control valveshould be spring loaded to the OPEN position. If it is not fully open, replace the valve.

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WTAI SOLENOIDVALVE


1. Manually wrench the precooler control valve position indicator through full range of motion in both directions. If valve does not operate smoothly, replace the precooler control valve.

2. Disconnect supply line from precooler control valve supply pressure port located on L/H side of engine.

3. Connect test hose between the precooler control valve supply pressure port and regulator Ps of the test box.

4. With 130 PSI supplied to test box, slowly increase pressure at regulator Ps while observing valve position indicator. Precooler control valve should be within 30 degrees of full closed when regulator Ps pressure gauge reaches 14 PSI.

a. If valve is not within 30 degrees of full closed, disconnect control line from R/H side of precooler control valve and plug the precooler control pressure port on the valve.

b. Repeat above steps. If the valve still does not close, replace the precooler control valve. If valve closes, there is a leak in the control line, WTAI solenoid valve, or the precooler sensor.

5. Reduce regulator Ps pressure to zero.

6. Disconnect the control line from the precooler control valve control pressure port located on R/H side of the valve.

7. Connect test hose between the control pressure port on the precooler control valve and gauge Pc of the test box.

8. Make sure regulator (Bleeder Valve) Pc is fully closed.

9. Slowly increase pressure at regulator Ps to 55 PSI (+/-5 PSI). Pressure on gauge Pc should stabilize at 9 PSI (+/-3 PSI). Precooler control valve should move to within 30 degrees of full closed.

10. Slowly open regulator (Bleeder Valve) Pc on the test box. Precooler control valve must begin to open when pressure on gauge Pc reaches a minimum of 7 PSI. Precooler control valve must be full open when gauge Pc reaches 3 PSI.

Note: If the valve does not open as outlined it can cause the bleed air temp to be excessively high which will cause the 450 degree sensor to modulate the PRSOV towards the closed position.

11. If the valve does not open as outlined in step #10, replace the precooler control valve.


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PRSOV SYSTEM TEST

Note: With engine shut down and APU bleed valve in OFF position, the PRSOV should be spring loaded to theCLOSED position. If it is not fully closed, replace the valve.

Turn on aircraft power and assure bleed switch for test engine is ON.

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PRSOV SYSTEM TEST

1. Bleed Air Regulator Test: Note: Leakage from the bleed air regulator honest orifices is not abnormal. a. Disconnect both supply and control lines from the bleed air regulator pressure ports.b. Connect test hose between the bleed air regulator supply pressure port and regulator Ps of the test box.c. Connect test hose between the bleed air regulator control pressure port and gauge Pc of the test box.d. Make sure regulator(Bleeder Valve) Pc is fully closed.e. With 130 PSI supplied to test box, slowly increase pressure on regulator Ps to 65 PSI (+/-5 PSI). The control pressure on gauge Pc should stabilize at 24 PSI (+/- 4PSI).f. Continue to increase supply pressure on regulator Ps to 120 PSI, control pressure should remain at 24 PSI (+/-4 PSI). If control pressure is significantly higher or lower than 24 PSI, replace the bleed air regulator.g. Reduce regulator Ps pressure to zero and open regulator (Bleeder Valve) Pc. h. Disconnect test hose from bleed air regulator control pressure port and reconnect control line to the bleed air regulator pressure port.

2. PRSOV Minimum Opening Test: a. Manually wrench the PRSOV position indicator through full range of motion in both directions. If valve does not operate smoothly, replace the PRSOV. b. Slowly increase pressure at regulator Ps while observing PRSOV position indicator. c. The PRSOV must be fully open at 10 PSI maximum supply pressure at regulator Ps. If it takes more than 10 PSI then disconnect the 450 degree sense (flex) line and cap the line. Repeat the test. If the test fails again, the PRSOV or the control line is at fault. If the test passes, the 450 degree sense line or the 450 degree sensor is at fault.

3. Bleed Air Regulator Over Pressure Bleed Trip: WARNING: Do not use Bleed Air Test Box, regulators do not go high enough to perform this test. a. Using a low pressure nitrogen bottle to regulate pressure, connect test hose from bleed air regulator supply pressure port to low pressure nitrogen bottle.b. Slowly increase pressure while observing the PRSOV position indicator , the PRSOV should open.c. Continue increasing pressure, the PRSOV should slowly go closed and the BLEED TRIP OFF light on the P5-10 overhead (air conditioning module) panel should illuminate when pressure reaches 220 PSI (+/-10 PSI). If the BLEED TRIP OFF light does not illuminate, replace the bleed air regulator.d. Reduce pressure to zero and reset BLEED TRIP OFF light.e. Disconnect test hose from bleed air regulator supply pressure port and reconnect supply line to the bleed air regulator supply pressure port.

4. Downstream Sense Line Test: Note: This test should be done if the complaint is high duct pressure and the bleed air regulator control pressure is verified good per step 1. a. Pressurize pneumatic ducting to engine using either APU or ground cart.b. Check the downstream sense line for leaks. This line begins on the top left hand aft side of the precooler, terminating at both the PRSOV and the high stage regulator. If this line is leaking it will cause higher than normal duct pressure, replace the downstream sense line.

5. PRSOV Valve Piston Seal Test: Another cause for high duct pressure at high power settings is an internal leak inside the PRSOV at the valve piston seal.

It is normal for a small amount of air to come out of the PRSOV control pressure fitting.a. Pressurize pneumatic ducting to the engine using either the APU or ground cart.b. Disconnect control pressure sense line from the PRSOV. If large amount of air (jet blast) is coming from the PRSOV, replace the PRSOV.


8

HIGH STAGE SYSTEM TESTNote: This test will verify control function of the high stage system only. It will not test leakage past the butterfly

in the valve. This is common cause for over pressure bleed trips at takeoff power settings.

With the engine shut down the high stage valve is spring loaded to the CLOSED position. If it is notclosed, replace the valve.

9

HIGH STAGE SYSTEM TEST

1. High Stage Regulator Test: a. Disconnect both supply and control lines from the high stage regulator pressure ports.b. Connect test hose between the high stage regulator supply pressure port and regulator Ps of the test box.c. Connect test hose between the high stage regulator control pressure port and gauge Pc of the test box.d. Make sure regulator(Bleeder Valve) Pc are fully closed.e. With 130 PSI supplied to test box, slowly increase pressure at regulator Ps to 35 40 PSI, gauge Pc pressure should increase to 15 18 PSI.f. Continue increasing pressure at regulator Ps to 90 PSI. Gauge Pc pressure should remain at 15 18 PSI. If pressure is significantly higher or lower, replace the high stage regulator.

2. High Stage Regulator Pneumatic Shutoff test: a. Increase pressure at regulator Ps to 110 PSI (+/-10 PSI). Gauge Pc pressure should read 4 PSI or less. If it does not, replace the the high stage regulator.b. Reduce regulator Ps pressure to zero and open regulator (Bleeder Valve) Pc.c. Disconnect test hose from the high stage regulator control pressure port and reconnect the control line to the high stage regulator control pressure port.

3. High Stage Valve Minimum Pressure Opening Test: a. Manually wrench the high stage valve position indicator through full range of motion in both directions. If valve does not operate smoothly, replace the high stage valve.b. Slowly increase pressure at regulator Ps while observing the high stage valve position indicator.c. The high stage valve should be fully open at 10 PSI maximum supply pressure at regulator Ps. If it fails, either the high stage valve or the control line is at fault.

4. High Stage Regulator Downstream Sense Port Leak Test: a. Increase pressure at regulator Ps to 35 PSI.b. Disconnect downstream sense line from the high stage regulator downstream sense port. c. There should be no air leakage coming from the high stage regulator downstream sense port. If it leaks, replace the high stage regulator.d. Reconnect the downstream sense line to the high stage regulator downstream sense port.

5. High Stage Valve Internal Leak Test: a. Disconnect the control line from the high stage valve.b. Using a wrench, manually open (full open) the high stage valve.c. While plugging the high stage valve control pressure port with your thumb, remove the wrench and let the high stage valve move to the closed position. If the valve closes in less than 10 seconds, replace the high stage valve.

6. Downstream Sense Line Test: a. Pressurize pneumatic ducting to engine using either APU or ground cart.b. Check the downstream sense line for leaks. This line begins on the top left hand side of the precooler, terminating at both the high stage regulator and the PRSOV. Replace the downstream sense line if leaks are found.


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737 Bleed Air Test Guide

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