DIVERTING TO ALTERNATE ACCOUNT EXTENDED DELAYS; READBACK FL180; CLEARED TO FL190.

Narrative

AFTER HOLDING IN ICE; ROUGH AIR; ETC.; TO A FUEL POINT NECESSARY TO PROCEED TO OUR ON LINE ALTERNATE OF (MDT) DUE TO TFC AND WEATHER AT OUR DESIRED DEST OF (JFK). THE CAPTAIN MADE THE DECISION TO DIVERT WE WERE ON NY CENTER FREQ 128.57 LEVEL APPROX FL230. I WAS DOING ALL THE COMMUNICATIONS WITH CENTER AS WELL AS COMPANY AND THE CAPTAIN WAS FLYING THE ACFT. SHORTLY AFTER THE REQUEST TO CENTER TO DIVERT THE CTLR GAVE US A HEADING TOWARDS (MDT) AND INSTRUCTED US TO DSND TO WHAT I HEARD TO BE FL180 AND SUBSEQUENTLY READ BACK FL180 TO THE CTLR. UPON REACHING FL180; THE CTLR CAME BACK ON THE FREQUENCY AND ASKED US OUR ALTITUDE. I RESPONDED FL180. THE CTLR THEN INSTRUCTED US TO CLIMB TO FL190 AND INFORMED US THAT WAS OUR ASSIGNED ALTITUDE. WE THEN CLIMBED TO FL190 AND I INFORMED THE CTLR THAT WE HAD BOTH HEARD FL180 IN THE ORIGINAL DSCNT CLRNC AND HAD READ BACK FL180 AFTER HIS INSTRUCTION. SHORTLY AFTER THIS COMMUNICATION; THE CTLR GAVE US A NEW FREQUENCY TO SWITCH OVER TO. PRIOR TO SWITCHING TO THE NEW FREQ THE CAPTAIN GOT ON THE RADIO AND TALKED TO THE CTLR AND ASKED IF THERE HAD BEEN A TFC CONFLICT. THE CTLR QUICKLY RESPONDED 'NO CONFLICT; NO PROBLEM.' WE THEN WISHED THE CTLR A GOOD NIGHT AND CHANGED TO OUR NEW ASSIGNED FREQ AND PROCEEDED TO (MDT). (HUMAN PERFORMANCE CONSIDERATIONS). THE NIGHT OF THE ABOVE MENTIONED INCIDENT WAS EXTREMELY BUSY FOR BOTH THE CTLRS AND FLT CREW DUE TO WEATHER AND DIVERSIONS. THE CAPT AND I WERE NEARING 13 HOURS ON DUTY OF MULTIPLE LEG FLTS. IF INDEED THE MISTAKE WAS OURS (FLT CREW) BY DSNDING TO THE WRONG ALTITUDE I BELIEVE IN THE FUTURE WHEN CONFRONTED WITH LONG DAYS; ADVERSE WEATHER; ETC I WILL DOUBLY VERIFY ALL INSTRUCTION WITH ATC; TO HELP PREVENT THIS KIND OF PROBLEM.