A PT17 STEARMAN WAS UNABLE TO CLB OR MAINTAIN ALT AFTER TKOF FROM A GRASS STRIP DUE TO AN APPARENT TAILWIND OR DOWNDRAFT RESULTING IN AN OFF ARPT LNDG.

Narrative

TKOF IN THE STEARMAN WAS WITH PLT; PAX AND A FULL LOAD OF FUEL FROM AN UNCTLED 2500 FT GRASS STRIP. WINDSOCK INDICATED CALM WINDS; HOWEVER; THE FIELD IS SURROUNDED BY TREES WHICH CAN MASK OR CAUSE SWIRLING WINDS. RUNUP AND TKOF WERE NORMAL WITH ALL ENG PARAMETERS NORMAL. AIRBORNE AND INDICATING 60 MPH; STARTED A GENTLE CLB AND A SLIGHT L TURN TO CLR TREES AT THE SE END OF THE FIELD. AS WE CLRED THE TOP OF THE TREES; NOTICED THAT AIRSPD WAS BLEEDING OFF. RECHKED MIXTURE AND PWR LEVERS FULL FORWARD AND WITH CONTINUED DECREASE IN AIRSPD LOWERED NOSE SLIGHTLY TO REGAIN SPD. AS SPD CONTINUED TO DECREASE (I SUSPECT FROM A SLIGHT TAILWIND OR DOWNDRAFT); I LOWERED THE NOSE TO MAINTAIN ABOVE STALL SPD. WITH RISING TERRAIN AHEAD AND NO INCREASE IN PERFORMANCE; I ELECTED TO MAKE A PRECAUTIONARY LNDG IN AN UNPOPULATED OPEN FIELD LOCATED JUST NE (1/2 - 3/4 OF A MI) OF THE DEP STRIP. THE NEGATIVE PERFORMANCE (TAILWIND AND/OR DOWNDRAFT) ENCOUNTERED AFTER TKOF; COMBINED WITH THE MARGINAL PERFORMANCE OF THE ACFT (DUE IN PART TO A WOODEN PROP) LEFT ME WITH NO MARGIN FOR ERROR AND RESULTED IN THE PRECAUTIONARY LNDG. WILL DEFINITELY RECONSIDER TAKING OFF WITH A FULL FUEL LOAD WHEN IT IS NOT REQUIRED AND ONLY CONSIDER TKOF WITH A KNOWN HEADWIND. ALSO A MORE EFFICIENT PROP WOULD IMPROVE TKOF PERFORMANCE.