AN AERO L-29 ON LNDG EXPERIENCED A LOSS OF BRAKING; STRIKING A VASI LNDG BOX AND DEPARTING THE RWY ROLLING OVER A ROCK EMBANKMENT.

Narrative

A STUDENT AND MYSELF DEPARTED ZZZ ON A ROUTINE TRAINING FLT. THE TRAINING MANEUVERS PERFORMED INCLUDED NORMAL STALLS; CYCLING THE LNDG GEAR; TKOF AND LNDGS; AND TAXIING FOR FURTHER DEPS. UPON RETURNING TO ZZZ AND DOING A PRE-LNDG CHK; IT WAS NOTED THAT THE NITROGEN THAT IS USED FOR BRAKING WAS LOW. IT WAS MY OPINION THAT IF I RAISED THE GEAR AGAIN TO GO TO AN ALTERNATE ARPT; THE BRAKE SYS WOULD BE FURTHER DISCHARGED. NOTE: EACH TIME THE GEAR IS CYCLED; THE BRAKES ARE AUTOMATICALLY APPLIED TO STOP THE ROTATION OF THE TIRES IN THE WHEEL WELLS. I DETERMINED THAT I HAVE OPERATED THE PLANE AT THIS LEVEL OF NITROGEN IN THE PAST AND THAT IT SHOULDN'T BE A PROB. DURING THE LNDG ROLL MY STUDENT STATED THAT HE HAD NO BRAKES. I THEN TOOK THE ACFT AND APPLIED THE BRAKES AND ATTEMPTED TO STEER THE ACFT WITH DIFFERENTIAL BRAKING. THERE WAS MINIMAL BRAKE PWR AND I REALIZED THAT WE MAY NOT BE ABLE TO STOP IN THE RWY DISTANCE THAT WE HAD REMAINING. I TURNED THE ACFT TO THE R TOWARD A ROCK EMBANKMENT OR HILL. IT WAS AT THIS POINT THAT I COMMENCED AN EMER SHUTDOWN OF THE ENG; SHUT OFF THE FIRE COCK VALVE AND APPLIED THE EMER BRAKES. THE ACFT; WHILE SLOWING FURTHER; STRUCK A VASI LNDG BOX ON THE N SIDE OF THE RWY CAUSING MINIMAL DAMAGE TO THE AIRPLANE EQUIVALENT TO HANGAR RASH. THE VASI LIGHT SYS WAS COMPLETELY DESTROYED; HOWEVER. AFTER PASSING OVER THE VASI SYS; WE CROSSED THE N TXWY AND ROLLED OVER THE ROCK EMBANKMENT. THE ACFT CAME TO A STOP ON THE EDGE OF THE HILL. WE SECURED THE ACFT AND EXITED. AFTER RECOVERING THE ACFT FROM THE HILL WITH THE HELP FROM A LCL TOWING COMPANY; I DETERMINED THAT THE DAMAGE WAS LIGHT AND LIMITED TO THE LEADING EDGE OF THE WING WHERE IT CAME INTO CONTACT WITH THE VASI BOX. BY APPEARANCES; IT LOOKED AS THOUGH I HAD STRUCK A BIRD. THERE WAS NO DAMAGE RESULTING FROM COMING TO REST ON THE EMBANKMENT AS THE ACFT CAME TO A STOP FROM APPLYING THE EMER BRAKES. THERE WAS NO FURTHER DAMAGE CAUSED DURING THE RECOVERY OF THE ACFT. LATER THAT EVENING; I CYCLED THE LNDG GEAR ON JACKS AND REFILLED THE NITROGEN BRAKE SYS. AFTER TAXIING THE ACFT TO VERIFY THAT WITH THE BRAKE SYS FULL; THE BRAKES OPERATED NORMALLY; I DEPARTED ZZZ TO FLY HOME WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT INCURRED MINOR DAMAGE AND THE CAUSE OF THE LOSS OF BRAKING WAS JUST PLAIN RUNNING OUT OF NITROGEN. EACH TIME THE LNDG GEAR IS RETRACTED; THE NITROGEN PRESSURE STOPS THE ROTATING WHEELS AND SLIGHTLY DEPLETES THE LIMITED NITROGEN SUPPLY. THE NITROGEN BOTTLE PRESSURE WAS CHKED ON THE PREFLT CHK AND APPEARED ADEQUATE FOR THE FLT.