Date: 201708
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Super King Air 350
Operator: Air Carrier
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization Control System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: My First Officer (FO) and I experienced a sudden depressurization of the cabin while at FL260. We first experienced the pressures difference in our ears followed by seeing the cabin altitude VSI go to a sudden climb greater than 3;000 feet per minute and the cabin altimeter increasing. We then got both the red 'CABIN ALT HI' annunciation and white 'CABIN ALT' annunciation.FO was the pilot flying; I took controls and operated the radios and we proceeded by donning our oxygen masks; [advising ATC] and starting a descent down to 14;000 feet with vectors for the terrain. Air Traffic Control informed me of the altimeter setting for [our location] and I put it in my altimeter. In the descent FO got out the QRH to run the 'loss of pressurization' checklist. I asked him to check the dump switch. He noticed the switch was in the 'DUMP' position. He informed me and then cycled the switch through the 3 positions and back to the normal operation position. At the start of the flight we had 1700 pounds of oxygen; in the descent we noted we still had 1000 pounds of oxygen remaining which was a sufficient amount to make it to [destination]. Upon reaching 14;000 feet we leveled off and the cabin stabilized back to a normal cabin altitude. We notified the passengers of the situation and followed all or descent checklists. We [advised ATC] and requested to climb back to 16;000 feet in order to proceed back direct [destination]. From this point on the flight proceeded as normal with no further issues.Upon reaching [our destination] I spoke with the passengers about the situation. Everyone seemed a bit noticeably shaken but in good spirits everything was okay and we were on the ground. I informed dispatch of the situation and contacted maintenance.In debriefing after the flight neither FO nor I know how the 'Dump Switch' could have been moved to the 'DUMP' position. Noting; at the time of depressurization neither I nor FO's hands were [anywhere] near the switch. The cause for depressurization is not known to us; but considering we had no 'BLEED AIR FAIL' annunciation and the cabin re-pressurized when leveling at 14;000 feet the switch is the only possible determination.




Synopsis: A King Air 350 Captain reported that while in cruise they experienced a sudden loss of pressurization in the cabin.