CL65 CREW HAD LOW CLB RATE DUE TO ACFT GROSS WT; TEMP ABOVE STANDARD; AND ACFT SPD LOWER THAN ACCEPTABLE MACH.

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WE FLY THE CANADAIR CL65 REGIONAL JET WITH 50 PAX SEATS AND 3A1 SERIES ENGS (THERE IS A MORE PWRFUL 'B' SERIES ENG NOW AVAILABLE). IT'S A GREAT AIRPLANE IN ALL RESPECTS EXCEPT FOR AN ANEMIC CLB RATE ABOVE ABOUT FL180. IF YOU'RE HVY AND 'ISA' IS ABOVE STANDARD; THE RATE OF CLB AND AIRSPD DECREASE MARKEDLY IN THE FL20'S. SOMETIMES 500 FPM ABOVE FL270 IS A STRETCH; ESPECIALLY IF YOU'RE RUSHED THE INITIAL CLB AND THE AIRSPD IS AT OR BELOW 250 KIAS/0.70 MACH. WE ARE FORBIDDEN FROM CLBING SLOWER THAN THESE SPDS. WE HAD DEPARTED HOUSTON TO THE NE AND; DUE TO SOME WX/BUMPY-LOOKING CLOUDS; THE CAPT TOLD ME TO CLB STEEPLY (250 KIAS) INTO THE LOWER FLT LEVELS. WE WERE FLT PLANNED AT FL290 AND REACHED IT AT 500 FPM. THE CAPT THEN DECIDED TO CLB TO FL330. WE CONSULTED THE CHARTS (PERFORMANCE) AND; GIVEN OUR WT/TEMP; SHOULD HAVE BEEN ABLE TO MAKE FL330. BUT OUR PLANE JUST COULDN'T MAINTAIN 500 FPM DESPITE THE CAPT SETTING MAX CLB THRUST 'PLUS.' HE THEN SET VERT SPD MODE ON THE AUTOPLT TO 300 FPM TO ADDRESS OUR SLOW SPD IN THE CLB. I ASKED IF WE SHOULD NOTIFY ATC OF OUR INABILITY TO MAINTAIN 500 FPM CLB. HE AT FIRST REFUSED; BUT EVENTUALLY RELENTED. ATC APPROVED 'SLOW CLB.' BUT THE NEXT CTLR WAS UNHAPPY WITH THAT AND WE HAD TO USE OUR REMAINING ENERGY TO CLB THE LAST FEW HUNDRED FT AT A GREAT RATE. HAD WE CONFESSED OUR ANEMIC CLB RATE EARLIER; WHEN I SUGGESTED; THINGS MAY HAVE GONE MORE SMOOTHLY.