Narrative:

We were deviating around airmass thunderstorms at FL390 (filed at FL370) when the controller told us he needed us to make a left turn and descend us to FL370. This would have put us right into the WX, so we tried to negotiate an alternative. Because the controller and ourselves were in difficult position the pace and rate of exchanges became hurried, and slightly confused. The controller told us to turn right and remain at FL390. The captain read back 'turn right and descend to FL370. The controller acknowledged this with a 'thank you.' neither him nor myself noticing the captain read back FL370. After making the turn the captain noticed that I had not descended to FL370 and initiated a descent. I assumed that he had heard something different and instead of verifying the correct clearance I allowed the aircraft to descend. At about FL385 I asked for a confirmation and was told by the controller to climb back up to FL390. I should have insisted on a verification at the time of descent but did not. Assumptions should never be made. Supplemental information from acn 443636: while deviating around airmass thunderstorms en route of FL390 eastbound, X flight was instructed that next controller needed us back down to FL370 and X flight needed to turn left for traffic. 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 descending through FL384, controller asked us our altitude, then instructed us to climb back to FL390 immediately and called out Y traffic ahead of FL370 that we were overtaking. I should have insisted that the controller confirm our readback, and he should have caught our erroneous readback.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 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.