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.