MD88 FLT CREW ATTEMPTS TO CLB TO A HIGHER ALT FOR WX AVOIDANCE AND EXCEEDS THE ACFT'S PERFORMANCE ENVELOPE.

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DURING CLBOUT OF RICHMOND; REQUESTED CLB FROM PLANNED CRUISING ALT OF FL310 TO FL350 TO AVOID WX. ZDC APPROVED THE CLB WITH A STATED RESTR TO LEVEL AT FL350 FOR CONFLICTING TFC. THE CLB WAS STARTED AT MACH .76; SLOWING TO APPROX MACH .64 AT FL350. FMS GENERATED MAX ALT WAS FL364. AT FL350; THE ACFT WOULD NOT ACCELERATE TO CRUISE MACH OF .76. THE FLT ENTERED LIGHT TURB WHICH RESULTED IN ADDITIONAL AIRSPD DECAY. I REQUESTED IMMEDIATE DSCNT CLRNC TWICE AND IT WAS DENIED DUE TO TFC. I DECLARED AN EMER AND THE CAPT DSNDED THE ACFT TO FL310. CONTRIBUTING FACTORS: 1) HIGHLY QUALIFIED CAPT WHOSE JUDGEMENT AND FLYING SKILLS WERE EXEMPLARY WHICH MADE ME LESS LIKELY TO QUESTION HIS ZOOM MANEUVER. 2) LAST LEG OF A LONG 4 DAY TRIP WHICH MAY HAVE RESULTED IN DECREASED SITUATIONAL AWARENESS. 3) WHAT MUST HAVE BEEN A WARMER THAN STANDARD DAY BECAUSE TYPICALLY THE ACFT; AT THAT GROSS WT; ACCELERATES JUST FINE. 4) PERHAPS LACK OF EXPERIENCE WITH THE ANSWER TO THE QUESTION: 'HOW SLOW IS TOO SLOW?' 5) DESIRE TO PLEASE BOTH PAX AND ATC BY GETTING TO FL350 QUICKLY.