Date: 201911
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: IMC
Light: Dawn
Aircraft: Beech 1900
Operator: Air Carrier
Operating under: Part 135
Flight plan: IFR
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Door Warning System
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: As is standard [company] B1900 block out procedure; both the forward and aft doors were verified closed by checking the visual latch indications within the viewing portals on the door. The forward door latching mechanism was lit and the latching mechanism was verified locked in place and the door correctly secured. During the before start checklist; all caution and warning annunciators appeared normal; and both door micro switch clusters produced a 'lights out' condition on the annunciator panel. All start; taxi; takeoff and initial climb indications were normal. The aircraft blocked at XA45Z and was wheels up at XA59Z. During the cruise climb phase at approximately XB05Z (estimated;) through roughly 8;000 feet MSL and roughly 15 miles downrange; I noted momentary; quick single flashing of the master warning annunciator. I scanned all warning lights; but noted none illuminated. I cautiously continued enroute; considering the light as a possible momentary glitch. The activity continued sporadically throughout the climb until well after level flight was established at 16;000 feet MSL; some 40 miles downrange. Sometime after level flight was established (and cruise checklist verifying cabin differential around 4.6 PSI;) both the master warning and the forward cabin door annunciator lit in a steady red. The Emergency Checklist was consulted; and the cabin differential was reduced to zero while a descent to 10;000 feet MSL was requested for oxygen regulatory compliance. With the differential at zero and the cabin altitude at 10;000 ft MSL; the Forward Door Warning Annunciator remained on; (the master warning had been cancelled/reset for trigger.) The remainder of the checklist; then; called for landing at the nearest suitable airport; that airport being ZZZ. I determined that ZZZ1 was some ten minutes further flying; and that to fly an approach into ZZZ (an unfamiliar airport); off of a familiar route without the benefit of a weather and NOTAM briefing in IMC was less of a safe option than continuing to my scheduled destination. I therefore elected to continue to ZZZ1 IN THE INTEREST OF SAFETY.  In order to deviate from an approved checklist procedure (landing at the nearest suitable airport;) I elected to request priority handling; thus granting me authority to deviate as necessary to the extent necessary to meet the emergency.  An uneventful descent; ILS approach; landing and block in at ZZZ followed at XB56Z.I became aware through the triggering of the aircrafts master warning and warning annunciator system.Forward door micro switch cluster out of correct alignment.Followed the BE1900 [company] approved emergency checklist. Deviated from 'land at the nearest suitable airport' in the interest of safety.




Synopsis: BE-1900 Captain reported diversion caused by Master Warning Door Open light indication.