To inform flight crews of a Cabin Pressurization Control System anomaly.
An anomaly has been discovered with the Cabin Pressurization Control System (CPCS). A recent software change in the CPCS may result in the outflow valve closing and the airplane pressurizing on the ground. In some cases, this has resulted in the inability to open the passenger entry doors.
Boeing has discovered this anomaly was introduced in a recent software change in the Cabin Pressurization Controllers (CPC). Specifically, airplane pressurization can occur after an electrical power transfer is accomplished. This includes, but is not limited to, electrical power transfers as a result of normal engine start and shutdown operations.
The affected Honeywell CPC software is:
2117388-11,
2117388-12, and,
2117388-13.
The following highlighted procedural steps should be executed as shown below immediately after the "Fuel Control switches" step as published in the Normal Procedures section of The Boeing Company Operations Manual, Volume I:
FUEL CONTROL switches ............................................ CUT OFF C
Verify ENG VALVE and SPAR VALVE lights extinguished.
CABIN ALTITUDE MODE SELECTOR ............................ MAN F/O
If outflow valve not fully open:
CABIN ALTITUDE MANUAL CONTROL ....................... CLIMB F/O
Position outflow valve fully open.
The above highlighted procedural steps should be accomplished during all shutdown operations. This will preclude the CPC anomaly from inadvertently pressurizing the cabin. Proper execution of the "Preflight Procedure - First Officer" during the subsequent flight will ensure the pressurization system is properly configured for flight.
This anomaly is corrected by Boeing Service Bulletin 767-21-0166 or 767-21-0168. This Operations Manual Bulletin will be canceled after Boeing is notified that all affected airplanes in the operator’s fleet have been modified.
Boeing Maintenance Tip 21-017 is related to this Operations Manual Bulletin.