THE PILOT OBSERVED THAT THE TOW PLANE WAS DIRTY SO HE WASHED IT. THE PILOT FAILED TO SECURE THE TWO ENGINE COWL DOORS. DURING INITIAL CLIMB, WITH A GLIDER IN TOW, THE DOORS BEGAN FLOPPING UP AND DOWN. THE GLIDER PILOT OBSERVED THE MISHAP, RELEASED THE TOW LINE AND SAFELY LANDED. THE TOW PLANE PILOT REVERSED COURSE TO RETURN FOR LANDING. THE WITNESSES REPORTED THAT WHILE REVERSING COURSE THE AIRPLANE'S PITCH ATTITUDE STEEPENED. THE AIRPLANE STALLED AND SPUN UNTIL CRASHING ON DESERT TERRAIN ADJACENT TO THE AIRSTRIP.