A RETURN LAND MANDATED AFTER TWR CTLR INFORMS SMA PLTS THAT FUEL IS VENTING FROM WINGS ON TKOF.

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I WAS ACTING AS A CFII ON A LCL TRAINING FLT THAT WAS PLANNED FOR A TOTAL OF 1.5 HRS. BEFORE TKOF STUDENT AND I HAD CGP FILL THE HIGH WINGED SMA FULL OF FUEL. AFTER THE FUEL ATTENDANTS FINISHED FUELING; I GOT OUT AND VISUALLY CHKED THE CAPS. BOTH FUEL CAPS LOOKED FLUSH AND AERODYNAMIC FROM THE FRONT OF THE AIRPLANE. WE THEN STARTED THE ENG AND GOT CLRNC TO TAXI TO RWY 27. THE FUEL VENT WAS VENTING FUEL AS IT SOMETIMES DOES WHEN THE TANKS ARE VERY FULL. THE RUN UP WENT WELL; THEN WE GOT CLRNC TO TKOF FROM TWR. JUST AFTER LIFTOFF THE ARPT MGR CALLED TO THE TWR (ON TWR FREQ) THAT THE AIRPLANE THAT TOOK OFF ON 27 WAS LOSING FUEL. MY STUDENT REMAINED IN THE PATTERN AND THEN LANDED. MY STUDENT AND I NEVER VISUALLY SAW ANY FUEL DISCHARGE EXCEPT FOR THE FUEL VENT. AFTER LNDG I GOT OUT AND THIS TIME STOOD ON THE WING STRUT AND FOUND THE AFT PART OF THE FUEL CAP TO BE LIFTED ABOVE THE LOCKED POS. I THEN CLOSED THE CAP PROPER AND WE COMPLETED THE TRAINING. FROM VISUAL INSPECTION ON THE GND (MY HEIGHT 73 INCHES) BOTH FUEL CAPS LOOKED TO BE SECURE. THE LEADING EDGE OF BOTH CAPS WAS FLUSH WITH THE TOP OF THE WING AND THE HAND GRIP OF THE CAP WAS AERODYNAMIC; THAT IS; IT WAS STREAMLINE WITH THE AIR FLOW. ALWAYS PHYSICALLY CHK THE CAPS -- NOT JUST VISUALLY.