ATA 21

Pressurization Tips
Due to the number of calls we recieve, we thought we would provide a few tips for flight crews and maintenance people concerning the pressurization system.
The first thing to remember, especially for new crews, is anytime the aircraft is started, the DV window MUST be opened. This is because both bleed switching valves are failed open, and they require electrical power AND bleed air pressure to close. Therefore, air is being pumped inot the cabn as soon as the engine rotates, and no vacuum is being generated to open the outflow valve.
Keep the outflow valve CLEAN. use mild soap of isopropyl alchohol to wipe the knife-edge seal. Spray a little LPS-1 into the vent hole and on the valve shaft.
Use the Ground Pressurization Control (GROUND PRESS) switch. If your doesn't work, get it fixed!

Before trying to pressurize the aircraft, check the following items first:
1. Normal system - outflow valve opens during/after engine start (could be either valve)
2. After starting, close doors and DV window. GROUND PRESS switch engaged - outflow valve closes 3/4 of the way, safety valve starts to open, outflow valve closes almost fully.
3. Trip LH ground switch to flight - safety valve closes fully, outflow valve opens and meters (put controller below S/L and max rate).

To Pressurize
1. Trip isobaric valve to close.
2. Open manual dump valve fully.
3. Start both engines.
4. Select "Both" on bleed selector.
5. Close DV window slowly.
6. Pull L/H pressurization circuit breaker.
7. Close manual dump valve at whatever speed your ears can handle.
8. You may have to increase engine speed or toggle temp to HOT to reach max differential, but it should get there and operate on the outflow valve limiters. Now you can do all the door and window leakage checks. Don't forget the forward pressure bulkhead bungs and flight control cable bungs.

To De-Pressurization:
1. Slowly open the manual dump valve.
2. Reset all circuit breakers and open and safety the isobaric valve.
3. CLOSE the manual dump valve.

If you want to check the cabin leakage rate, just leave the aircraft pressurized to max differential and simply shut down the engines. One word of cation - If avionics technicians have been working in the instrument panel, make sure NO vacuum connections are open or loose because that would ruin your whole day.
One last thing - Do not replace the controller! In over 25 years of maintaining Westwinds, we have replaced controllers approximately 5 times. It is as trouble free as you can get. Look for vacuum leaks, Ty-wraps or rags in the outflow valves, dirty ejector screens, weak dump solenoids, etc.

ATA 21

When you are pressurizing the aircraft on the ground, remove the inlet duct from the ram-air valve before starting the engines. When you get the aircraft pressurized, see how much air is blowing out of the valve inlet. If it is a lot, the butterfly seal in the valve is gone or deteriorated and the valve must be replaced.
This is also a good time to check the regulating pressure on the Fluid Pressure Regulating Valve. If it is low, the air cycle machine will not see enough inlet air pressure to come up to the correct speed.

ATA 21

Bleed Line Over-pressurization
Your flight crew has returned from a routine flight and reported the illumination of the RED BLEEDLINE OVERPRESS warning light during the landing roll out. You perform an operational check of the bleed air system per the Aircraft Maintenance Manual. You are unable to duplicate the discrepancy. All functions appear normal and no abnormal indications are observed during the checks.

Questions:

1. What pneumatic system device(s) activate the BLEED LINE OVERPRESS warning light?
2. What bleed air control configuration may cause this condition (over-pressure/temperature in the bleed manifold) to occur during landing operations?
3. Why did the warning appear, and was it a true or false indication?

Answers:

1. The BLEED LINE OVERPRESS warning light is activated by either the over-temperature or over-pressure switch located on the bleed air common manifold upstream of the refrigeration unit.
2. If the cabin air selector is set to “RIGHT”, the ground bypass air valve will open upon landing because all conditions are met. When the crew deploys reversers and increases engine RPM with the piggyback lever(throttles are at idle), pressure and temperature in the bleed air common duct may rise above over-pressure switch or over-temperature sensor limits. As the right engine spools up, the open ground bypass air valve routes unregulated HP bleed air directly to the common manifold.
3. The BLEED LINE OVERPRESS warning will appear and it will be a “true” indication.