ALTDEV. ANSWERED WRONG CALL

Narrative

CLBING OUT OF STL ENRTE TO COU; I WAS OFF OF ATC DEP CTL FREQ TO CALL IN OUR TIMES TO OUR COMPANY. WHEN I RETURNED TO DEP CTL THE FO WAS CLBING THROUGH 4000 FT MSL. I ASKED HIM WHAT ALT HE WAS CLRED TO AND HE SAID 5000 FT. ATC THEN WANTED US TO VERIFY OUR ALT; WE DID AND ATC SAID WE WERE NOT CLRED TO 5000 FT; DSND AND MAINTAIN 4000 FT. ATC CLRED ACR X BACK TO 6000 FT WHICH WAS ORIGINALLY CLRED TO 5000 FT. I ASKED THE FO; AND HE SAID HE READ BACK 5000 FT TO ATC WHICH WE FOUND OUT LATER WAS FOR OTHER ACFT. HUMAN PERFORMANCE CONSIDERATIONS: MYSELF AND THE FO HAD FLOWN 9 LEGS THE DAY BEFORE WITH TSTMS AND DELAYS AND A REDUCED REST. WE WERE INTO THE MIDDLE OF OUR SECOND DAY WHICH CONSISTED OF 10 LEGS THAT DAY WITH TSTMS. COMPANY OPS MANUAL ALLOWS THE CALLS TO BE MADE SOON AFTER TKOF SO THE OPS HAVE OUR TIMES. ANOTHER ACR WAS ON FREQ WHICH COULD BE MISTAKEN FOR OUR FLT. CORRECTIVE ACTIONS: 1) REGS NEED TO BE CHANGED TO NOT LET FLCS FLY SO MANY LEGS IN 1 DAY. REDUCED REST SHOULD BE ELIMINATED. ANY PERSON WITH GOOD WORK ETHICS WILL TRY TO COMPLETE EVERYTHING THAT IS ASSIGNED TO THEM. THIS IS WHERE I FEEL THE DANGER LIES; BECAUSE FLCS ARE FLYING TIRED WITH LITTLE TIME FOR BREAKS AND SLEEP; BUT IT'S LEGAL. 2) DO NOT PERMIT COMPANY CALLS UNTIL CRUISE FLT. 3) FLCS NEED SOME WAY OF NOT READING SIMILAR CALL SIGNS; BACK OF OTHER FLTS IN SAME AREA.