Date: 201604
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Dash 8-100
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Cargo Door
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We climbed to a cruise altitude of FL220; in VMC and smooth air above icing conditions. Our filed altitude of FL180 was unavailable and icing conditions existed up to FL200. After approximately 7 minutes of cruise flight the CABIN PRESS warning light illuminated. After verifying the cabin altitude was above 10;000 ft and climbing and the cabin differential pressure was dropping; the FO and I suited-up and communicated and began our immediate action items and checklist for a rapid depressurization. The seatbelt sign was ON but we also made a PA for everyone to take their seats immediately. We informed ATC and began an emergency descent using the memory items and checklist to 10;000 ft. Shortly after beginning the emergency descent and at approximately FL200 the BAG DOOR warning light illuminated. I slowed the airspeed in our descent fearing a structural problem with our bag door. We then completed the BAG DOOR WARNING checklist which also directed us to the UNPRESSURIZED FLIGHT checklist. I told ATC I would divert to ZZZ; but learned the weather was low IMC there. ZZZ1 weather was significantly better so I chose to divert there. After stabilizing the aircraft at 10;000ft; both the CABIN PRESS and BAG DOOR warning lights extinguished and were off for the remainder of the flight. I exchanged flight controls with the FO while I established communications with the FA to ensure the cabin; FA; and passengers were safe and secured. I then called dispatch but kept getting a busy signal which I later learned was because our dispatcher was calling us at the same time. I called a different dispatch desk and was transferred to our dispatcher to review our situation and convey our return plan to ZZZ1. The passengers were briefed for a normal landing and all non-normal and normal checklists were completed. I resumed flight controls and had a normal landing in ZZZ1. Fire rescue inspected the aircraft to ensure no structural damage; specifically in the bag door area; and we were given the all clear. We made a normal taxi to the gate and I directed the FO to tell operations to only allow maintenance to open the bag door. We had a normal deplaning at the gate and I was not advised of any passenger or crew member injuries.There was some failure of the pressurization system. When the BAG DOOR warning light illuminated I suspected our bag door was compromised and was the cause of the loss of pressurization. Even though the aircraft is certified to fly up to FL250; be extra cautious when operating at or near these altitudes. Always be prepared for the unexpected and have a plan for your emergency descent and diversion options at all times.


Second narrative: [Report narrative contained no additional information.]


Synopsis: DHC-8-100 Flight Crew reported receiving a cabin pressure warning at FL220 seven minutes after leveling off. ATC was advised and a descent was initiated with the bag door warning light illuminated passing FL200. The crew elected to divert to the nearest suitable airport.