This incident occurred in early 2007. On the day the Boeing 747 flight had already been delayed for 3 hours. When the crew had reported for briefing, the aircraft still had six malfunctions, one of them being Left Outflow Valve AUTO INOP. On board the airplane, the flight crew found that the left outflow valve was indicating fully open. The maintenance was questioned by the PIC as to why the left outflow valve had not been deactivated closed as described in the MEL. At the time, the maintenance engineer was busy trouble shooting a thrust reverser problem. The reply was that the aircraft had been dispatched with the same malfunction items for quite some time and since there were no relevant EICAS messages displayed, Therefore, it should be OK for dispatch with no need for any further verification. With the maintenance engineer being busy with his job at hand, the flight crew decided not to bother him further and started their ground preparation procedures.
After 2 malfunctions were cleared, the maintenance log was signed and the PIC endorsed it without verifying the outflow valve’s position. The aircraft took off and after passing 6000ft, unusual cabin altitude indication appeared. The crew attempted to level off the aircraft at about 9000ft. However, the jet overshot 10000ft and the CABIN ALTITUDE warning horn went off. A descent was made to 9000ft and the problem was investigated. The crew found that the outflow valve was still in the fully open position, when it should have been closed after takeoff. The crew checked the operations manual, however they misunderstood the note written on the manual for this malfunction item to be- ‘the switch (would allow the outflow valve to be close manually) should not be operated’. They then made an attempt to reach the maintenance department, however, this failed at low altitudes. At the same time, they were under pressure by the ATC that they would soon be out of his control airspace and that they have to decide whether to climb or to turn back. After a brief discussion, and with all the contingency fuel used, the crew followed the PIC’s decision to return to the departure aerodrome. They dumped the fuel down to maximum landing weight. During the fuel dumping, they were able to contact the maintenance again, and they were advised to manually close the left outflow valve. The valve was closed and they landed back at the departure aerodrome.
The threat in this incident is that this aircraft was dispatched with multiple malfunctions. The flight had already been delayed for a couple of hours, and time pressure was put on the maintenance department to complete fixing the two malfunctions as soon as possible. Improper handling of the required maintenance actions by the maintenance department was the primary factor that caused this incident. The resolution of the crew after the discovery of the malfunction was safe, but was somehow unwise. The crew showed insufficient Threat Management skills. The outflow valve was not positioned according to the operations manual, and the crew did not take any action to manually correct its position. The Quick Reference Manual (QRH) checks were not initiated. This incident also shows lack of communication with the maintenance department.