Narrative:

We departed ZZZ1. The sky was clear at the time. All annunciators were indicating normal when we did the final flaps; annunciators; trims; speed check before takeoff. Approach control gave us a vector; clearance to climb to 13000 ft; and handed us off to center. Climbing through approximately 5000 ft the door unlocked light came on. We left the fasten seatbelt light on and the sic tried to visually check the door from the right seat. He was unable to verify anything from his seat. I put the airplane on autoplt; passed control to the sic; and quickly left my seat to check the door visually. I saw right away that the white lines on the latching mechanism were misaligned; indicating that the door was not properly latched. I advised the passenger that we would be landing at ZZZ2 and to keep their seatbelts fastened; returned to my seat; took aircraft control and directed the sic to run the abnormal checklist for the door unlocked light. I leveled the plane at 11000 ft and informed center that we needed to land at ZZZ2 to check out the door. We were right over ZZZ2 at the time and I am familiar with the airport. Center cleared us direct to ZZZ2 and handed us back to approach. We made a normal landing at ZZZ2; pulled onto the taxiway and set the brake. I then went back to the door; latched it properly; saw nothing abnormal about the door; and the sic checked that the door unlocked light was out. We departed ZZZ2 and completed the trip uneventfully. With the citation 500 series door; it is critical to not go over center when putting the door handle in the lock position. I have always demonstrated to new sic's what can happen in that case. The door pins are not latched! I was planning this for the layover; but should have done it sooner.

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Original NASA ASRS Text

Title: C500 DIVERTS WHEN DOOR UNLOCKED LIGHT ILLUMINATES. DOOR NOT PROPERLY LATCHED.

Narrative: WE DEPARTED ZZZ1. THE SKY WAS CLR AT THE TIME. ALL ANNUNCIATORS WERE INDICATING NORMAL WHEN WE DID THE FINAL FLAPS; ANNUNCIATORS; TRIMS; SPD CHK BEFORE TKOF. APCH CTL GAVE US A VECTOR; CLRNC TO CLB TO 13000 FT; AND HANDED US OFF TO CENTER. CLBING THROUGH APPROX 5000 FT THE DOOR UNLOCKED LIGHT CAME ON. WE LEFT THE FASTEN SEATBELT LIGHT ON AND THE SIC TRIED TO VISUALLY CHK THE DOOR FROM THE R SEAT. HE WAS UNABLE TO VERIFY ANYTHING FROM HIS SEAT. I PUT THE AIRPLANE ON AUTOPLT; PASSED CTL TO THE SIC; AND QUICKLY LEFT MY SEAT TO CHK THE DOOR VISUALLY. I SAW RIGHT AWAY THAT THE WHITE LINES ON THE LATCHING MECHANISM WERE MISALIGNED; INDICATING THAT THE DOOR WAS NOT PROPERLY LATCHED. I ADVISED THE PAX THAT WE WOULD BE LNDG AT ZZZ2 AND TO KEEP THEIR SEATBELTS FASTENED; RETURNED TO MY SEAT; TOOK ACFT CTL AND DIRECTED THE SIC TO RUN THE ABNORMAL CHKLIST FOR THE DOOR UNLOCKED LIGHT. I LEVELED THE PLANE AT 11000 FT AND INFORMED CENTER THAT WE NEEDED TO LAND AT ZZZ2 TO CHK OUT THE DOOR. WE WERE RIGHT OVER ZZZ2 AT THE TIME AND I AM FAMILIAR WITH THE ARPT. CENTER CLRED US DIRECT TO ZZZ2 AND HANDED US BACK TO APCH. WE MADE A NORMAL LNDG AT ZZZ2; PULLED ONTO THE TXWY AND SET THE BRAKE. I THEN WENT BACK TO THE DOOR; LATCHED IT PROPERLY; SAW NOTHING ABNORMAL ABOUT THE DOOR; AND THE SIC CHKED THAT THE DOOR UNLOCKED LIGHT WAS OUT. WE DEPARTED ZZZ2 AND COMPLETED THE TRIP UNEVENTFULLY. WITH THE CITATION 500 SERIES DOOR; IT IS CRITICAL TO NOT GO OVER CTR WHEN PUTTING THE DOOR HANDLE IN THE LOCK POS. I HAVE ALWAYS DEMONSTRATED TO NEW SIC'S WHAT CAN HAPPEN IN THAT CASE. THE DOOR PINS ARE NOT LATCHED! I WAS PLANNING THIS FOR THE LAYOVER; BUT SHOULD HAVE DONE IT SOONER.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.