DC8-62 COMBI FLT ENRTE TO DOV AFB ENCOUNTERED TURB AT FL310. REQUESTED FL350 AND CLRED. FO ADVISED THEY WERE NOT EQUIPPED TO FLY AT THE RVSP ALTS. LPPO CTL CONTACTED THE FLT BY SELCAL QUESTIONING THE ALT AND THE CAPT IMMEDIATELY REQUESTED FL310.

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DEPART LAGES; AZORES; FOR DOVER AFB AT 295000 TOGW AT XA45Z WITH 30 PAX; 6 CREW MEMBERS (DC8 COMBI) TO FL310. THIS WAS TO BE THE CAPT'S LAST FLT AS AN AIRLINE PLT DUE TO HIS 60TH BIRTHDAY. FL310 WAS CONSIDERED A LOW FLT LEVEL FOR OUR WT. 1 HR INTO THE FLT; WE ENTERED IMC AND MODERATE TURB. CAPT REQUESTED FL350. I STATED THAT WE COULD NOT GO TO FL350. FE AND CAPT ASKED WHY NOT. I STATED THAT A VOICE MAIL 2 WKS PRIOR STATED THAT ATI IS NO LONGER ABLE TO GO ABOVE FL310. WE GOT CLRNC TO FL350 AND CLB TO FL350. 15 MINS LATER; SANTA MARIA CONTACTED US VIA SELCAL; ASKED IF WE WERE RVSP FILED. CAPT REQUESTED FL310 IMMEDIATELY. AFTER WE LEVELED AT FL310; I SHOWED THE CAPT THE RVSP INFO ON ALT CHART 1/2 THAT CLRLY STATED THE RVSP REQUIREMENTS. TOTAL TIME ABOVE FL310 WAS 20-25 MINS. IN HINDSIGHT; I SHOULD HAVE BEEN MORE ASSERTIVE. WE SHOULD HAVE ALSO BRIEFED THE FLT RESTRS BEFORE DEP AND OUR FLT PLAN SHOULD HAVE STATED THE OPERATIONAL RESTR(S). I HAVE SINCE RPTED THE PROB TO THE CHIEF PLT AND D/O. I WON'T LET THIS HAPPEN AGAIN!