B737-800 CREW COMPLAINED THE AUTOFLT SYS EXCEED THE MAX ACFT SPD 'BARBER POLE' WHEN DSNDING IN VNAV.

Narrative

ON THE MOSEY 5 ARR INTO CVG; ATC GAVE US CLRNC TO CROSS THE NABB VOR AT 13000 FT MSL. WE WERE AT FL220 WHEN THE CLRNC WAS RECEIVED. THE DSCNT CLRNC WAS GIVEN A LITTLE LATE AND VNAV WITH AUTOPLT USED FOR THE DSCNT. THE AUTOPLT IN THE DSCNT TRIED TO MAINTAIN VNAV PATH TO MAKE THE XING AT NABB AND DUE TO A DESIGN FLAW; WHEN TOO HIGH; ADJUSTS THE NOSE UNTIL THE ACFT SPD EXCEEDS VNAV SET SPD BY 10 KTS. IN THIS CASE; THE SPD INCREASED AND AS DESIGNED THE VNAV KICKED OFF AT +10 KTS ABOVE VNAV SPD SENDING US SLIGHTLY INTO THE BARBER POLE BY 2-3 KTS (WITH CLACKER WARNING). THE CAPT (PF) PUT OUT THE SPOILERS AND ACFT SLOWED BELOW BARBER POLE IN LESS THAN 5 SECONDS. THIS LOGIC FLAW IN VNAV DSCNT PERFORMANCE NEEDS TO BE CORRECTED TO PREVENT THIS OCCURRENCE. DESPITE CAREFUL MONITORING; IT IS NOT TOO DIFFICULT FOR VNAV; IF TRYING TO DSND BACK ONTO THE PATH FROM ABOVE; TO RIDE CLOSE TO; AND IF KICKS OFF; BUMP THROUGH THE BARBER POLE. A SLIGHTLY SLOWER SPD (MORE CUSHION FROM BARBER POLE) AND CONSERVATIVE TOP OF DSCNT POINT WOULD CORRECT THIS.