SERVICE ENTRY DOOR UNLATCHED JUST AFTER DEP.

Narrative

SHORTLY AFTER TKOF; THE PLT'S AUX HATCH OF THE SMT OPENED WITH THE HANDLE STILL IN THE LOCKED POS. THIS HATCH IS ON THE LEFT SIDE OF THE ACFT ADJACENT TO THE CAPT'S SEAT. AFTER CLBING TO A SAFE ALT; WE ASSESSED THE DAMAGE AND CHKED THE CONDITION OF OUR 6 PAXS. THERE WERE NO INJURIES AND I WAS UNABLE TO CLOSE THE HATCH. AT FIRST I THOUGHT THAT IT LEFT THE ACFT; BUT LATER; AFTER WE LANDED; I NOTICED THAT IT WAS STILL INTACT. IT BROKE AWAY FROM THE RESTRAINING ARM AND MOVED TO A POS OUT OF VIEW WHEN INFLT. CONSIDERING ACFT PERFORMANCE; POOR COMS DUE TO WIND NOISE; LNDG RWY AND TFC IN THE BOSTON AREA; WE DECIDED TO PROCEED STRAIGHT AHEAD TO MANCHESTER; NH. THE AERODYNAMICS WERE AFFECTED AND I WAS RELUCTANT TO TEST ANOTHER MODE OF FLT OTHER THAN STRAIGHT AND LEVEL. WE WERE CLRED FOR AN APCH TO RWY 35 IN MANCHESTER AND MADE A NORMAL LNDG. ALL PAXS WERE UNHURT AND THERE WAS ONLY MINOR DAMAGE TO THE ACFT. MY CONCERN IS THE PLT'S AUX HATCH ON THE SMT. AFTER TALKING TO OTHER CAPTS AT MY AIRLINE; I WAS SURPRISED TO FIND THAT THE SAME THING HAPPENED TO MANY OF THEM. APPARENTLY; EVEN WITH THE HANDLE IN THE LOCKED POS DOES NOT NECESSARILY MEAN THAT THE HATCH IS SECURE. I BELIEVE THAT IT WAS A FAULT IN THE MECHANISM THAT CAUSED THE HATCH TO OPEN INFLT.