HELICOPTER PILOT PRACTICING AN AUTO ROTATION AT 4B8 HAS A SMALL ACFT PASS BENEATH HIM AS HE IS ENTERING THE FLARE AT 15 FEET.

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I DEPARTED WITH A STUDENT AND I WENT TO ROBERTSON ARPT (4B8) TO DEMONSTRATE AN AUTOROTATION. 5 MI OUT I STARTED MAKING MY CALLS TO 4B8 CTAF; BUT WHEN REQUESTING TFC ADVISORY I GOT NO RESPONSE SO SINCE THE WIND WAS COMING FROM THE N I DECIDED TO USE RWY 2. BECAUSE I WAS APCHING THE PATTERN FROM THE SE I JOINED A R BASE FOR RWY 2 AT 800 FT AGL. THE WHOLE TIME I WAS MAKING MY RADIO CALLS STATING MY POS AND INTENTIONS. WHEN I WAS ON MY R BASE I COULD SEE A SMALL AIRPLANE ON WHAT WOULD BE A L DOWNWIND FOR RWY 2. THE SMALL AIRPLANE WAS NOT MAKING ANY RADIO CALLS; BUT AT THIS POINT ACFT SEPARATION WAS NOT AN ISSUE AND I CONTINUED TO FINAL. ON FINAL I ENTERED A NORMAL STRAIGHT-IN AUTOROTATION WHICH WAS TO BE TERMINATED IN 3-5 FT HOVER ON THE RWY. AS I WAS COMING TO THE END OF THE MANEUVER; I STARTED TO INITIATE MY FLARE AND JUST AS I DO I CAN SEE THE SMALL AIRPLANE ROLL OUT FROM R UNDERNEATH ME. THE ACFT CLRNC WAS UNCOMFORTABLY SMALL AND I RIGHT AWAY HAD TO MAKE AN EARLY PWR RECOVERY AND SIDESTEP TO AVOID THE AIRPLANE.