PIPER ARROW ACFT. STUDENT PLT PRACTICING SLIP APCH TO LNDG LOST CTL OF ACFT. AIRSPD DECAYED AND INSTRUCTOR LOST SIGHT OF THE LNDG THRESHOLD DUE TO SLIP MANEUVER. SINK RATE HAD DEVELOPED BY THE TIME INSTRUCTOR TOOK CTL. TOO LATE AND ACFT HIT ARPT 4000 FT RWY MARKER.

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DURING AN INSTRUCTIONAL ACFT RENTAL CHKOUT FLT; A PIPER ARROW CONTACTED A 4000 FT MARKER AT THE OXR RWY 25 LNDG AREA. THIS WAS THE 8TH TOUCH-AND-GO BEFORE A RETURN TO CAMARILLO ARPT WAS PLANNED. THE TYPE APCH REQUESTED BY ME; THE CFII; WAS A NO FLAPS FORWARD SLIP APCH TO A TOUCH-AND-GO. THE RENTER WAS NOT PROFICIENT IN THE MANEUVER REQUESTED. THE TURN WAS A SLIPPING L TURN WHICH CAUSED A LACK OF VIEW OF THE LNDG THRESHOLD. THE AIRSPD DECAYED AS THE ACFT PROGRESSED DOWN SHORT FINAL. I WAS CAUGHT OFF GUARD BY THE STUDENT'S INABILITY TO SIDE-SLIP BACK TO THE RWY. BY THE TIME I TOOK THE CTLS; THE SINK RATE HAD DEVELOPED TO A POINT WHERE FULL PWR AND FULL ELEVATOR DEFLECTION WERE NOT ENOUGH TO GLIDE OVER THE MARKER. THE THUMP THAT WAS HEARD WAS ASSUMED TO BE CONTACT WITH THE GND. THE OXR CTL TWR THEN INFORMED US OF THE DAMAGED 4000 FT MARKER. SINCE THE ACFT WAS ALREADY AIRBORNE; A RETURN TO CAMARILLO ARPT WAS INITIATED AND A LNDG THERE WITHOUT INCIDENT. AFTER DISCUSSING THIS INCIDENT WITH SEVERAL STAFF INSTRUCTORS; I HAVE COME UP WITH SOME IMPROVEMENTS THAT I AM MAKING IN MY EVERYDAY INSTRUCTIONAL SVCS GIVEN TO STUDENTS. I WILL PERSONALLY RAISE MY STUDENT STANDARDS AS FAR AS THE TOLERANCES ALLOWED IN DIFFERENT PHASES OF FLT BEFORE I TAKE OVER THE CTLS. I HAVE INTENSIFIED MY PREFLT BRIEFINGS TO A POINT WHERE I BRIEF WHAT I FLY AND I FLY WHAT I BRIEFED. THIS WAY; THERE WILL BE NO UNEXPECTED SURPRISES FOR MYSELF OR ANY STUDENT. I PLAN TO RESTRICT ANY FORWARD SLIP TO FINAL APCHS ONLY. THIS WILL ELIMINATE THE LOSS OF FORWARD VISIBILITY BY THE PLT OR FLT INSTRUCTOR. IF LESS THAN 150 FT AGL AND THE ACFT IS NOT ESTABLISHED TO LAND ON THE RWY; A GAR WILL BE INITIATED BY THE FLT INSTRUCTOR. IF AIRSPD FALLS BELOW A SPECIFIED AIRSPD; A GAR WILL BE INITIATED BY THE FLT INSTRUCTOR. I RECALL WHEN I FIRST ACHIEVED CFI STATUS; MY EXAMINER SAID THAT I WILL TEND TO TAKE THE CTLS MORE THAN NECESSARY; THEN THERE WILL BE A TREND WHERE I WON'T TAKE THE CTLS AS OFTEN AS NECESSARY. I HOPE OTHER CFI'S WILL RECOGNIZE THIS TREND BEFORE IT CAUSES ANOTHER INCIDENT.