HARD LNDG. FIREWALL DAMAGE.

Narrative

I WAS DEMONSTRATING A SHORT FIELD LNDG TO A STUDENT IN HIS SMA WHICH RESULTED IN A HARD LNDG WHICH DAMAGED HIS FIREWALL. I USED 70 MPH; WHICH IS WHAT THE PLT MANUAL SUGGESTED; FULL FLAPS; PWR WAS CUT TO IDLE OVER AN IMAGINARY 50 FT OBSTACLE. AFTER PWR WAS CUT; I PITCHED FORWARD TO MAINTAIN PROPER AIRSPD; WHICH RESULTED IN THE INCREASED RATE OF SINK DESIRED AFTER PASSING THE OBSTACLE. IN THIS CASE; DUE TO THE DENSITY ALT AND POSSIBLE GUSTY WINDS (10 KTS MAX) THE RATE OF SINK WAS MUCH GREATER THAN DESIRED. AT THIS POINT; I BEGAN THE FLARE AND ADDED FULL PWR ONLY TO IMPACT VERY HARD WITH THE RWY. I BELIEVE SOME OF THE FACTORS IN THIS EXTREMELY POOR LNDG WAS THE WARM CONDITIONS AND TOO SLOW OF AN AIRSPD. THE PLT MANUAL FOR THE SMA SUGGESTS 70 MPH PWR OFF APCH FOR A SHORT FIELD LNDG. MY EXPERIENCE IN THE PAST DOING MANY SHORT FIELD LNDGS IN SMA TYPES HAS ALWAYS BEEN THAT THE MIN AIRSPD IN THE PLT MANUALS GIVES PLENTY OF EXTRA ENERGY AT THE FLARE. THE AIRSPD IN THE MANUAL AND MY PAST EXPERIENCE IN OTHER SMAS LEAD ME TO INCORRECTLY BELIEVE THAT EVEN WITH THE WARM TEMP; THE LNDG AT WORST WOULD BE FIRM. THE WINDS THAT AFTERNOON WERE SLIGHTLY GUSTING. I BELIEVE THIS COULD; MAYBE; HAVE PLAYED A SMALL PART IN NOT ENOUGH AIRSPD FOR A PROPER FLARE. AS FOR THE WT OF THE ACFT; 1 HR OF FUEL WAS BURNED; 3 ADULTS WEIGHING 185 POUNDS; 180 POUNDS; 125 POUNDS AND A CHILD; NO BAGGAGE. I IMMEDIATELY AFTER LNDG REALIZED 70 MPH WAS MUCH TOO SLOW FOR AN 'OVER THE OBSTACLE' SHORT FIELD LNDG AND WILL; IN THE FUTURE FOR ANY PLANE; USE A GREATER BUFFER OF AIRSPD FOR THESE LNDGS. IN ADDITION; IT MAY BE HELPFUL TO KNOW I SAW THE OWNER MAKE A SHORT FIELD LNDG AT 80 MPH JUST PRIOR TO MY ATTEMPT AT 70 MPH; THE SHORT FIELD SPD IN THE SMA'S MANUAL.