COMMUTER DEPARTS KNOWING TWR WILL BE CLOSED WITH NO WX PERSONNEL AT DEST. CANCEL IFR AND LAND VFR.

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I WAS FO ON AIRLINES EXPRESS FLT SCHEDULED TO DEPART IAD FOR MHT. THE FLT WAS DEPARTING LATE ENOUGH THAT; BY ARR TIME; THE TWR WOULD BE CLOSED. WE RECEIVED WORD THAT THERE WAS NO LONGER ANY QUALIFIED GND OBSERVER AT MHT FOR A WX AND TA. THEREFORE; ACCORDING TO OUR UNDERSTANDING OF FAR 121 REGS AND OUR OPS SPECS; WE COULD NOT LEGALLY LAND. MY CAPT CONCURRED. THE CAPT THEN CALLED OUR DIRECTOR OF OPS VIA PHONE (WE STILL HAD NOT DEPARTED). THE DIRECTOR OF OPS SAID THAT HE HAD HAD THIS PROB BEFORE; AND THAT ACCORDING TO OUR FAA PRINCIPAL OPERATING INSPECTOR'S INTERP OF THE REGS AND OPS SPECS; WE COULD LEGALLY PROCEED. BASED ON OUR DIRECTOR OF OPS VERBAL ASSURANCE THAT WE WERE LEGAL; OUR CAPT ELECTED TO GO. I DID SO TOO; RELUCTANTLY. THE WX WAS CLR; CALM; AND VISIBILITY UNRESTRICTED. WE WERE ABLE TO CANCEL IFR AND LANDED VFR. I DO NOT BELIEVE SAFETY WAS COMPROMISED AT ANY TIME DURING THIS FLT. IF IT HAD BEEN; I WOULD NOT HAVE DONE IT. HOWEVER; IF A CREW WAS TO PROCEED UNDER THESE CIRCUMSTANCES DURING IMC CONDITIONS; SAFETY WOULD MOST CERTAINLY BE COMPROMISED. UPON FURTHER REVIEW OF FAR 121 REGS AND OUR OWN OPS SPECS; I DO NOT AGREE WITH THE PRINCIPAL OPERATING INSPECTOR'S INTERP. I BELIEVE UNDUE PRESSURE WAS PUT ON US TO CONTINUE THIS FLT. I HAVE LEARNED FROM THIS THAT I CANNOT ALLOW MYSELF TO BE PRESSURED INTO DOING SOMETHING I AM UNCOMFORTABLE WITH IN AN AIRPLANE. IF A SIMILAR INCIDENT OCCURS AGAIN; I WILL REFUSE THE FLT.