B757 CREW AT FL390 DEVIATING AROUND WX IS INSTRUCTED TO TURN R AND MAINTAIN FL390. ALTHOUGH THE FO WAS FLYING; THE CAPT STARTED THE ACFT DOWN TO FL370 BELIEVING THEY HAD A CLRNC TO DSND.

Narrative

WE WERE DEVIATING AROUND AIRMASS TSTMS AT FL390 (FILED AT FL370) WHEN THE CTLR TOLD US HE NEEDED US TO MAKE A L TURN AND DSND US TO FL370. THIS WOULD HAVE PUT US RIGHT INTO THE WX; SO WE TRIED TO NEGOTIATE AN ALTERNATIVE. BECAUSE THE CTLR AND OURSELVES WERE IN DIFFICULT POS THE PACE AND RATE OF EXCHANGES BECAME HURRIED; AND SLIGHTLY CONFUSED. THE CTLR TOLD US TO TURN R AND REMAIN AT FL390. THE CAPT READ BACK 'TURN R AND DSND TO FL370. THE CTLR ACKNOWLEDGED THIS WITH A 'THANK YOU.' NEITHER HIM NOR MYSELF NOTICING THE CAPT READ BACK FL370. AFTER MAKING THE TURN THE CAPT NOTICED THAT I HAD NOT DSNDED TO FL370 AND INITIATED A DSCNT. I ASSUMED THAT HE HAD HEARD SOMETHING DIFFERENT AND INSTEAD OF VERIFYING THE CORRECT CLRNC I ALLOWED THE ACFT TO DSND. AT ABOUT FL385 I ASKED FOR A CONFIRMATION AND WAS TOLD BY THE CTLR TO CLB BACK UP TO FL390. I SHOULD HAVE INSISTED ON A VERIFICATION AT THE TIME OF DSCNT BUT DID NOT. ASSUMPTIONS SHOULD NEVER BE MADE. SUPPLEMENTAL INFO FROM ACN 443636: WHILE DEVIATING AROUND AIRMASS TSTMS ENRTE OF FL390 EBOUND; X FLT WAS INSTRUCTED THAT NEXT CTLR NEEDED US BACK DOWN TO FL370 AND X FLT NEEDED TO TURN L FOR TFC. THIS WOULD HAVE PUT US INTO THE TOP OF A BUILDUP WE WERE DEVIATING AROUND; SO WE TRIED TO NEGOTIATE EITHER TO REMAIN AT FL390 OR TO REMAIN ON CURRENT HEADING. WHEN WE WERE DSNDING THROUGH FL384; CTLR ASKED US OUR ALT; THEN INSTRUCTED US TO CLB BACK TO FL390 IMMEDIATELY AND CALLED OUT Y TFC AHEAD OF FL370 THAT WE WERE OVERTAKING. I SHOULD HAVE INSISTED THAT THE CTLR CONFIRM OUR READBACK; AND HE SHOULD HAVE CAUGHT OUR ERRONEOUS READBACK.