ATA 21

In Flight Pressurization Issues:
An operator reported the cabin altitude would climb when the power lever was reduced during routine descents. The aircraft power lever was set to a higher setting and pressurization control was regained, followed by an uneventful landing. Note: The cabin pressure 10,000-foot red warning light did not illuminate.
During the troubleshooting process, it was noted that the left Bleed Switching Valve (BSV) was inoperative and the right High Pressure (HP) side of the right BSV was inoperative. The operator was concerned that they experienced a simultaneous failure of both valves in flight. Past experience has shown that the aircraft most likely did not have a simultaneous failure, but rather the left BSV valve had previously failed and the right BSV valve was the valve that experienced the most recent failure. The ground bypass valve is installed in the right engine bleed system, and due to the increased air flow, it is the most common selection used on the ground. Airplane Flight Manual Section IV, Normal Procedures, Before Take-Off checklist says to set the cabin air selector to both engines; however, it does not require the left bleed source be operationally checked. The aircraft will operate normally on one engine bleed source. If the left and right bleed sources are not both periodically checked, it can lead to pressurization issues in flight. It is recommended that the left bleed source be checked before takeoff to help prevent in-flight pressurization issues. It should also be noted that this condition has been seen only on the Westwind and Astra/Astra SP aircraft.