ATTEMPTED CLB ON THE 'BACKSIDE OF THE PWR CURVE.'

Narrative

OUR ACFT WAS CLBING TO A CRUISE ALT OF FL330. BOTH ENG ANTI-ICE AND AIRFOIL ANTI-ICE WERE IN USE. THE FMS 'VNAV CLB' MODE WAS CTLING THE CLBOUT BUT APCHING FL320; AIRSPD WAS DECREASING AND MACH WAS ALSO ROLLING BACK. BOTH PLTS COULD SEE THAT 'CLB' THRUST WAS ONLY GOING TO MAINTAIN ALT FOR THE MOMENT AND THAT AIRSPD/MACH WAS CONTINUING TO DECAY. AS CLB PERFORMANCE CONTINUED TO DETERIORATE; AND NOT WANTING TO INCREASE OUR ANGLE OF ATTACK; THE CAPT DISENGAGED THE AUTOPLT AND BEGAN OUR DSCNT BACK DOWN TO FL290. AS HE BEGAN THE DSCNT OUT OF FL320; THE FO REQUESTED THE DSCNT WITH ATC AND QUICKLY RECEIVED IT. ACFT WAS DSNDED TO FL290 WHERE PERFORMANCE WAS FINE. LESSON LEARNED: HIGH GROSS WT; HIGH ALT; ANTI-ICE ON AND POOR AIRSPD CTL (IE; BACK SIDE OF THE PWR CURVE) CAN ALL COMBINE TO CREATE A PERFORMANCE BARRIER WHICH CAN ONLY BE RECTIFIED BY DSNDING WELL BELOW THE SVC CEILING AND INCREASING AGAIN TO CRUISE MACH. THE TURB AT FL320 ALSO CONTRIBUTED TO OUR DETERIORATING CLB PERFORMANCE AND WAS A FACTOR IN OUR DECISION TO DSND BACK DOWN TO FL290 WHERE IT WAS SMOOTH.