PLT OF PIPER CHEROKEE NOTICED SHORTLY AFTER TKOF FROM COS THAT ACFT PERFORMANCE WAS POOR TO NIL REQUIRING AN EXPEDITIOUS RETURN TO FIELD FOR LNDG.

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ON NOV/XA/98 AT APPROX XX00; I DEPARTED COS ARPT FOR A PLANNED TRIP TO GREELEY; CO. AS I LIFTED OFF FROM RWY 17R; I NOTICED A DISTINCT LACK OF ENG AND ACFT PERFORMANCE. MY CLB RATE WAS LESS THAN 200 FPM AND ANY ATTEMPT TO INCREASE THE RATE OF CLB RESULTED IN ILLUMINATION OF THE STALL WARNING INDICATOR. COS TWR ADVISED ME TO 'TURN R WHEN ABLE...' WHICH I INTERPED TO MEAN THAT THEY THOUGHT THAT I MAY BE LOWER THAN NORMAL. ONCE ESTABLISHED ON A NBOUND HDG; I WAS DIRECTED BY TWR TO 'RECYCLE XPONDER CODE XXXX.' AT THIS TIME I STILL WAS UNABLE TO CLB MORE THAN 200 FT ABOVE THE ARPT ELEVATION. I ASSUMED THAT THE RECYCLE XPONDER CALL INDICATED THAT I WAS TOO LOW FOR COS DEP TO PICK ME UP ON RADAR. EVENTUALLY; WHILE NBOUND ABEAM THE RWY; I WAS PASSED TO COS DEP CTL. I RPTED MY HDG AND ALT (STILL NO MORE THAN 200 FT AGL) AND ATTEMPTED TO CLB ONCE AGAIN. AT APPROX 1 MI NW OF THE ARPT; AND STILL UNABLE TO CLB; I REQUESTED A TURN BACK TO COS. DEP CTL INQUIRED AS TO WHETHER I HAD A PROB; AND I RESPONDED THAT I WAS NOT SATISFIED WITH MY ACFT PERFORMANCE. HE THEN HANDED ME OFF TO COS TWR FOR AN UNEVENTFUL APCH AND LNDG. ALTHOUGH MY ACFT WAS LOADED WITHIN APPROVED LIMITS -- 3 PEOPLE (185; 200; AND 125 LBS); LITTLE BAGGAGE; AND ONLY 40 GALLONS OF FUEL ONBOARD -- THIS WT PROVED TO BE TOO MUCH FOR THE EXISTING CONDITIONS. MOREOVER; PRIOR TO TKOF; I PROPERLY LEANED THE ENG FOR HIGH ALT CONDITIONS AND REQUESTED 'BACK TAXI' ON THE ACTIVE RWY TO INCREASE THE 4500 FT AVAILABLE FOR TKOF FROM THE NEAREST AVAILABLE OPEN TXWY (A3) TO OVER 5500 FT. COS ATIS HAD RPTED A TEMP OF 22 DEGS C AND AN ALTIMETER SETTING OF 30.07. USING A PRESSURE ALT OF 6184 FT (COS FIELD ELEVATION); UNDER THESE CONDITIONS; I CALCULATE THE DENSITY ALT WOULD HAVE BEEN 8185 FT. WHILE THIS FIGURE IS NOT EXCESSIVELY HIGH; I BELIEVE THAT THE COMBINATION OF A NEARLY MAX GROSS WT ACFT AND A HIGHER THAN STANDARD DENSITY ALT RESULTED IN MY PERFORMANCE PROBS. IN MY OPINION; THE MOST SIGNIFICANT CAUSE OF THIS INCIDENT WAS MY ERROR IN ASSUMING THAT I HAD SUFFICIENT PERFORMANCE TO SAFELY ACCOMPLISH THE FLT. I FAILED TO DO COMPLETE PREFLT PLANNING; INCLUDING: 1) AN ACCURATE WT AND BAL CALCULATION; 2) DENSITY ALT COMPUTATION; AND 3) A CHK OF MY TKOF AND CLB PERFORMANCE UNDER THE PLANNED CONDITIONS. HAD I ACCOMPLISHED THESE PROPER PREFLT ACTIONS; THE SIT I FOUND MYSELF IN (100-200 FT ABOVE RESIDENTIAL AREAS OF COLORADO SPRINGS) WOULD NOT HAVE HAPPENED. IN THIS CASE; I WOULD HAVE OPTED TO DRIVE INSTEAD OF FLY TO GREELEY; AS I ULTIMATELY ENDED UP DOING ANYWAY.