SA226 ENCOUNTERS ICE AND IS UNABLE TO MAINTAIN REQUESTED SPD AND ALT SE OF BURLEY; ID.

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COPLT FLYING. ACFT AT 17000 FT; 180 KTS; FULL PWR; AND LIGHT RIME ICE ON WINGS. CTR REQUESTS 150 KTS. COPLT REDUCES PWR AND AT 150 KTS COPLT INCREASES PWR. AIRSPD SLOWS TO 140 KTS AND BEGINS TO DSND. FULL PWR APPLIED; NO INCREASE IN AIRSPD ACFT CONTINUES DSCNT. AT 200 FT BELOW ALT CAPT REQUESTS BLOCK ALT BUT DOES NOT SPECIFY ALT. 300 FT BELOW ALT CAPT INFORMS CTR UNABLE 150 KTS AND 17000 FT. AT 16600 FT AIRPLANE ACCELERATES TO 160 KTS AND CLBS BACK TO 17000 FT. PROB: 1) CAPT DID NOT KNOW ACFT PERFORMANCE CAPABILITIES IN THIS ENVIRONMENT. 2) CAPT UNABLE TO COMMUNICATE CLRLY WITH CTR AHEAD OF TIME TO KEEP AIRPLANE WITHIN IFR PARAMETERS. SOLUTION: LEARNED MORE ABOUT AIRPLANE PERFORMANCE CAPABILITIES. IN THE FUTURE; I KNOW THERE WILL BE SITS THAT WILL CATCH ME BY SURPRISE; AND AS ALWAYS I WILL AVIATE; NAV; AND COMMUNICATE. BUT THE NEXT TIME I COMMUNICATE IN SUCH A SIT I WILL STATE EXACTLY WHAT I NEED. 'CTR 150 KTS IS TOO SLOW; I NEED LOWER IMMEDIATELY TO INCREASE AIRSPD AND RETURN TO ALT.'