A C188 AGWAGON PLT TOWING A BANNER IS UNABLE TO MAINTAIN THE REG ALT OVER A STADIUM DUE TO THERMALS.

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OVER RICE STADIUM; WE WERE ENCOUNTERING HVY THERMAL ACTIVITY WITH ASSOCIATED SINK INTERMITTENTLY AROUND OUR FLT PATH. PWR ADJUSTMENTS TO 0% TO 100% WERE OCCURRING TO MAINTAIN PROPER ALT. DENSITY ALT WAS HIGH DUE TO TEMP AND CLB PERFORMANCE WAS DEGRADED; WHICH MADE SLOW CORRECTIVE ACTION UPON ENTERING AN AREA OF SINK; ESPECIALLY WHEN JUST LEAVING A THERMAL WITH HIGH RATE OF CLB AND ASSOCIATED LOW PWR SETTING. THIS SET UP A SIT WHERE I DSNDED BELOW MSA BECAUSE OF SLOW CORRECTIVE ACTION AND POOR PERFORMANCE FROM THE ACFT AND HIGH RATE OF SINK. AN ASSOCIATED FACTOR WHICH SLOWED MY CORRECTIVE ACTION WAS THAT DUE TO EXCESSIVE NOISE CONCERNS I TRIED TO KEEP THE PROP TO A FAIRLY LOW RPM SETTING AND WENT TO HIGHER RPM ONLY WHEN NECESSARY. CORRECTIVE SAFETY ACTION BY PLT: ON JUN/MON/98; I EXPRESSED MY CONCERNS AND PROBS IN A SAFETY MEETING AND IT WAS DECIDED THAT: FULL CLB PWR RPM WILL BE MAINTAINED IN THIS ACFT; WE WOULD AVOID USING THIS ACFT 'IN NOISE SENSITIVE AREAS;' AVOID TOWING HIGH DRAG BANNERS; AND REPLACE THIS ACFT WITH ONE MORE SUITABLE TO OUR REQUIREMENTS AT FIRST OPPORTUNITY.