Super King Air 300
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 75; Flight Crew Total 8900; Flight Crew Type 500
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types
Aircraft
In cruise for about 1hr at FL310 enroute with 6 passengers and [a] Flight Observer; cabin altitude annunciator and warning tone came on. Verified that cabin alt was now in excess of 10;000 feet and pressurization differential was less than 6psi and declining. Requested an immediate descent to FL250. Despite 1500FPM rate; cabin differential was still declining and cabin altitude increasing. Briefly attempted to troubleshoot by selecting one bleed source OFF. Cabin alt immediately increased and red Master Warning and Cabin Alt illuminated. Returned to both bleeds on to prevent auto deploy of cabin masks. Masks did not drop and passengers were unaware of event. Had First Officer (FO) remove headset and listen for any abnormal cabin leakage noise. None noted. Attempted to adjust pressurization controller up and down but no effect. Advised ATC we were experiencing a pressurization problem and requested descent to 10'000'; but ATC was unable below 14;000 feet due to terrain. Requested and approved for VFR-ON-TOP with all-terrain in site and descended to 9;500 feet. Cabin differential continued to decay to 0 psid; however; cabin altitude did not exceed 11;000 feet and was not in excess of 10;000 feet for more than :30min. Aircraft was now approximately 100nm west of ZZZ. Course deviation was approved to go around terrain ahead and then direct ZZZ. Landing was completed without further incident. After deplaning passengers; Maintenance was contacted by phone and a ground test of the pressurization system conducted. No anomalies were noted. Maintenance control was again consulted and a Maintenance check flight scheduled for the following morning. Maintenance flight was conducted with climb to FL310 and no anomalies noted. Pressurization controller was cycled up and down with no adverse effect or unexpected changes. Maintenance control was again consulted and agreed it was reasonable to continue next revenue flights as planned. A contingency plan to join low airway structure at various points along the route was developed and brief by PIC and FO. All flights [following the incident] were conducted at FL300/FL310/FL300 with no pressurization abnormalities noted.
Captain stated after a maintenance inspection they were unable to find the cause for the loss of pressurization.
B300 Captain reported a loss of pressurization at FL310; a descent to lower altitude and successful landing were made.