MD80 CREW CLBED TO A DISPUTED ALT IN ZDV CLASS A AIRSPACE.

Narrative

ON CLBOUT WITH DENVER APCH; DEVIATING THROUGH/AROUND TSTMS; DEN DEP INQUIRED AS TO OUR ALT WHEN ACFT WAS SLOWLY CLBING THROUGH FL264. BOTH CAPT AND FO HAD THOUGHT WE HAD BEEN CLRED TO FL270. ACFT WAS IMMEDIATELY DSNDED BACK TO FL260 WHICH ATC ADVISED WAS OUR CLRNC ALT. UNSURE WHETHER ATC INITIALLY GAVE US WRONG ALT AND DIDN'T CATCH READBACK; OR WHETHER ALT SET KNOB FOR AUTOPLT WAS ACCIDENTALLY BUMPED DURING LONG; SLOW CLBOUT IN WX AND TURB. ATC RPTED NO CONFLICT WITH OTHER ACFT HAD OCCURRED. DUE TO HVY ACFT AND SUBSEQUENT SLOW CLB RATE; SEVERAL MINS HAD ELAPSED BTWN ORIGINAL CLRNC TO CLB AND WHEN ATC INQUIRED AS TO WHAT ALT WE WERE LEVEL AT. BECAUSE OF THIS AND LENGTHY CONVERSATIONS ABOUT ROUTING THROUGH THE WX; NEITHER PLT COULD RECALL THE ORIGINAL ALT CLRNC CONVERSATION. ALTDEV WAS LIMITED TO 400 FT BEFORE CORRECTING. SUPPLEMENTAL INFO FROM ACN 476580: WE BELIEVED WE WERE GIVEN A CLRNC TO FL270. PASSING FL267; CTR CTLR ASKED US OUR ALT AND TOLD US SHE THOUGHT WE HAD ONLY BEEN GIVEN CLRNC TO FL260. SHE TOLD US TO LEVEL OFF AND STAND BY. APPROX 30 SECONDS LATER; SHE ASKED US TO DSND AND MAINTAIN FL260. SHE STATED THAT THERE WERE NO PROBS OR CONFLICTS; THAT SHE JUST NEEDED US AT FL260 UNTIL TFC PASSED; WHICH WOULD STILL BE SEVERAL MINS. SO WE ARE NOT SURE IF WE MISSED THE ALT READOUT OR IF ATC DID. IN ADDITION; DURING THIS TIME; WE WERE DEVIATING DUE TO NUMEROUS LARGE TSTMS IN THE AREA AND WERE ENCOUNTERING TURB.