PA28 PLT ALLOWED AUTOMATIC GEAR LOWERING SYS TO PREVENT GEAR RETRACTION; ON TKOF; AT A HIGH DENSITY ALT FIELD.

Narrative

AFTER TKOF FROM CENTENNIAL ARPT RWY 19L; THE PIPER ARROW I WAS FLYING DID NOT REACH A SUFFICIENT CLB RATE TO SAFELY CLR OBSTACLES IN THE VICINITY OF THE ARPT; DUE TO THE FAILURE OF THE LNDG GEAR TO RETRACT. I ATTEMPTED TO RECYCLE THE LNDG GEAR SWITCH; BUT STILL COULD NOT RETRACT THE GEAR. BY THE TIME I REALIZED THE AIRPLANE WAS NOT GOING TO CLB; I WAS NEAR THE DEP END OF THE RWY WITH INSUFFICIENT RWY REMAINING TO SAFELY ABORT THE TKOF. I DECLARED AN EMER TO THE TWR; AND WAS IMMEDIATELY CLRED TO LAND ON ANY RWY. I EXECUTED A SHALLOW-BANKED TURN TO THE L (OVER THE SE CORNER OF THE ARPT); AT APPROX 100 FT ALT; AND LANDED ON RWY 35R. I LATER LEARNED THAT A HELIPAD IS LOCATED ON THAT AREA OF THE ARPT; AND THAT DURING THE L TURN MY AIRPLANE HAD COME CLOSE TO COLLIDING WITH A LIGHT HELI THAT WAS OPERATING THERE. I DID NOT SEE THE HELI DURING MY MANEUVERING. I LANDED SAFELY ON RWY 35R AND TAXIED BACK TO PARKING. DENSITY ALT AT THE TIME OF THE EMER WAS APPROX 7000 FT; AND THE AIRPLANE WAS LOADED APPROX 100 LBS BELOW GROSS (PLT; 2 PAX; BAGGAGE; AND PARTIAL FUEL). ACCORDING TO PERFORMANCE CHARTS THE CLB RATE SHOULD HAVE BEEN APPROX 300-400 FPM. THE POH DOESN'T SPECIFY CLB RATES WITH THE GEAR DOWN; BUT DURING THE EMER THE CLB RATE WAS ON THE ORDER OF 50 FPM. A MECH AT CENTENNIAL ARPT COULD FIND NOTHING WRONG WITH THE GEAR SYS; AND WAS ABLE TO RETRACT THE GEAR. AFTER REVIEWING THE POH; I DISCOVERED THAT THE AUTOMATIC GEAR EXTENSION SYS ON THIS AIRPLANE NORMALLY PREVENTS THE GEAR FROM RETRACTING AT AN AIRSPD BELOW 85 MPH; BUT THAT THIS SPD INCREASES WITH INCREASING ALT. DURING SUBSEQUENT EXPERIMENTATION I FOUND THAT THE REQUIRED AIRSPD FOR GEAR RETRACTION AT THIS DENSITY ALT WAS APPROX 105 MPH; AN AIRSPD THAT I NEVER REACHED DURING THE EMER. SEVERAL HUMAN FACTORS CONTRIBUTED TO THIS EMER. MY PAX WERE IN A HURRY TO GET HOME; AND WE HAD ATTEMPTED TO LEAVE DENVER FROM JEFFERSON COUNTY ARPT EARLIER THAT DAY BUT WERE TURNED BACK BY BUILDING TSTMS AND ASSOCIATED WX TO THE E. HAD I DECIDED TO ABORT THE TKOF EARLIER I COULD HAVE EASILY LANDED ON THE REMAINING RWY; BUT I WAS RELUCTANT TO DO SO BECAUSE I HAD MADE A SUCCESSFUL TKOF UNDER SIMILAR CONDITIONS EARLIER THE SAME DAY. I FELT THAT I WOULD BE ABLE TO GET THE LNDG GEAR RETRACTED; AND THAT THE AIRPLANE WOULD THEN CLB NORMALLY. FINALLY; I DID NOT FULLY UNDERSTAND THE OP OF THE AUTOMATIC GEAR EXTENSION SYS; AND SPECIFICALLY DID NOT REALIZE THAT THE LIMITING RETRACTION SPD INCREASED WITH ALT. HAD I KNOWN THIS I WOULD HAVE ACTIVATED THE SYS OVERRIDE PRIOR TO TKOF.