PA18 PLT LOSES DIRECTIONAL CTL ON TKOF FROM TOUCH AND GO LNDG; STRIKES A TXWY SIGN; AND LANDS ON A PARALLEL TXWY. PROX OF HELI'S HOVERING AT THE ENDS OF A XING RWY NOTED AS A POSSIBLE CONTRIBUTING FACTOR.

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THE PLT FLYING THE SUPER CUB WAS PRACTICING TOUCH AND GOES FROM THE BACK SEAT POS. BOTH PLT/CFI'S WERE PROFICIENT AND CURRENT IN THE ACFT. 7 TOUCH AND GOES WERE ACCOMPLISHED WITHOUT INCIDENT; EVEN WITH A HELI PRACTICING HOVERING ON THE WEST X OF THE CLOSED; CROSS RWY. WHEN LOSS OF CTL HAPPENED AFTER THE BEST LNDG OF THE SERIES; THERE WERE 2 HELI'S ON EACH END OF THE CLOSED; CROSS RWY; PRACTICING HOVERING ON EACH OF THE X'S. CTL WAS LOST ON ROLLOUT AS THE CUB APCHED THE CLOSED RWY INTXN WITH THE ACFT EXITING THE RWY ON THE R SIDE; HITTING A TXWY SIGN JUST AFTER GETTING AIRBORNE; AND COMING TO REST ON THE E SIDE OF THE RWY. IT IS ASSUMED THE TURB OF THE 2 HELI'S AFFECTED DIRECTIONAL CTL OF THE CUB AS IT WAS ROLLING OUT AFTER TOUCHDOWN. SUPPLEMENTAL INFO FROM ACN 628472: WHILE MAKING TKOFS AND LNDGS FROM THE BACK SEAT OF A PIPER L21 SUPER CUB; PWR WAS SLOWLY APPLIED FOR TKOF. THE ACFT TURNED TOWARD THE R SIDE OF THE RWY. FULL PWR AND FULL L RUDDER WERE APPLIED IN AN ATTEMPT TO REMAIN ON THE RWY. AS THE ACFT LIFTED OFF THE L MAIN LNDG GEAR AND HORIZ STABILIZER STRUCK A RWY SIGN. A LNDG WAS MADE ON THE PARALLEL TXWY. DUE TO THE DAMAGED LNDG GEAR THE PLANE GND LOOPED TO THE L. SINCE I WAS FLYING FROM THE REAR SEAT I WAS UNABLE TO SEE THE SIGN. THE CFI IN THE FRONT SEAT ALSO WAS UNABLE TO CORRECT THE PATH OF THE ACFT. TO AVOID THIS TYPE OF INCIDENT BE SURE THE ACFT IS ON THE CTRLINE AND MOVING STRAIGHT AHEAD DURING PWR APPLICATIONS.