AN EXTRA 300L PLT AND OBSERVER CONDUCTED AN AEROBATIC MANEUVER OVER A RWY AT A NONTOWERED ARPT; NOT IN WAIVED AIRSPACE.

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ON APR/SUN/05; I FLEW MY EXTRA 300L FROM THE REAR SEAT WITH A PLT IN THE FRONT SEAT. OTHER PLT HAD SURGERY RECENTLY AND WANTED SOME DUAL AEROBATIC TIME BEFORE RETURNING TO A SINGLE PLACE EXTRA. AFTER TKOF WITH A CHK OF EVERYTHING BEING OK; I ASKED HER IF SHE WANTED TO TAKE CTL OF THE PLANE. SHE REPLIED 'YES' AND I SAID 'OK; THE PLANE IS YOURS.' SHE ASKED WHERE I PRACTICED AEROBATICS AND I REPLIED; 'AT ZZZ RWY 17.' SHE ASKED; 'WHAT SIDE?' I REPLIED; 'JUST OVER THE RWY.' I HAVE PERMISSION FROM THE OWNER AND MGRS TO DO AEROBATICS THERE AND DO SO REGULARLY. WHEN ARRIVING AT THE ARPT; SHE SAID WE WOULD MAKE A PASS DOWN THE RWY TO CHK FOR TFC AND DURING THIS PASS WE ROLLED AND PUSHED VERT TO A HAMMERHEAD. WHEN I WAS PHYSICALLY ABLE TO REACH THE TALK BUTTON; I INFORMED HER IMMEDIATELY THAT THIS WAS NOT A WAIVED BOX; I PRACTICE HERE ONLY AND MUST STAY AT 1500 FT AGL. SHE WAS NOT AWARE OF THIS; ASSUMING A BOX WAS OPEN; AND IMMEDIATELY GAINED ALT. WE BOTH ASSUMED THE OTHER KNEW THE DISTINCTION OF THE BOX; HER BEING THE PROFESSIONAL AND ME ONLY PRACTICING THERE AT 1500 FT; IT WAS A MISTAKEN COMS ERROR. WE ARE BOTH DISAPPOINTED IN OURSELVES FOR NOT BEING CLR PRIOR TO ANY AEROBATIC MANEUVERS ON THE CONDITION OF THE AIRSPACE IN THE AEROBATICS BOX. WE BOTH WORK HARD TO MAINTAIN A LARGE SAFETY MARGIN; BE PROFESSIONAL AND OBEY THE STANDARDS FOR OUR INDUSTRY. OUR INTENTION WAS TO PRACTICE SAFE MANEUVERS; NOT TO FLY LOW LEVEL AEROBATICS WHERE NOT APPROVED.