LOSS OF ACFT CTL IN TAXIING TFC.

Narrative

AFTER START-UP I CHKED MY BRAKES; AND DURING TAXI A BRAKE CHK WAS PERFORMED BY BOTH PLTS. THE STUDENT NOTICED THE R BRAKE DIDN'T FEEL NORMAL; I APPLIED MY BRAKES; THEY WORKED AS THEY SHOULD. THE STUDENT AGAIN APPLIED HIS BRAKES; AT THIS TIME THEY APPLIED NORMALLY. TAXI WAS CONTINUED. STUDENT WAS TAXIING ACFT ON TXWY BRAVO. UPON APPLYING BRAKES THE STUDENT COMMUNICATED 'WE HAVE NO BRAKES.' AT THE SAME TIME THE ACFT SLOWLY BEGAN TO VEER L OF THE CTRLINE. I IMMEDIATELY APPLIED FULL R RUDDER AND FULL R BRAKE. THE ACFT DID NOT RESPOND TO THESE INPUTS IN ANY WAY. I THEN ATTEMPTED TO STOP THE ACFT BY APPLYING BOTH L AND R BRAKES; ALSO WITHOUT RESULT. NEITHER THE L OR THE R SIDE BRAKES OPERATED NORMALLY WHEN APPLIED AT THIS TIME. I THEN BROUGHT BOTH ENG MIXTURE CTLS TO THE IDLE CUTOFF POS. SOON AFTER; THE ACFT DEPARTED TXWY BRAVO L MAIN WHEEL FIRST. THE ACFT SEEMED TO PIVOT AROUND L MAIN WHEEL CAUSING THE NOSEWHEEL TO SHEAR OFF. THE NOSE CAME TO REST ON THE GND; AT WHICH TIME EACH ENG HAD ONE BLADE OF THE PROP IN THE GND. THE R ENG PROP SUSTAINED A BEND WHILE THE L ENG PROP DID NOT SUSTAIN ANY DAMAGE. ALL CORRECTIVE ACTION MADE BY ME HAD FAILED. THE FAA INVESTIGATOR I SPOKE WITH CHALLENGED MY DECISION OF BRINGING THE MIXTURES TO THE IDLE CUTOFF POS. HE SAID I SHOULD HAVE USED THE ASYMMETRIC THRUST. I TOLD HIM THAT AT THE TIME THAT WOULD HAVE BEEN INEFFECTIVE; AND CONSIDERING I HAD NO BRAKES IT WOULD HAVE INCREASED MY SPD. TXWY WIDTH WAS AN IMPORTANT FACTOR IN MY DECISION PROCESS. BRAVO TXWY IS 40 FT WIDE; WHICH GAVE ME 20 FT TO USE. IF I HAD USED ASYMMETRIC THRUST AND STILL GONE OFF THE TXWY; THE L ENG WOULD HAVE SUSTAINED MUCH DAMAGE; AS IT IS NOW IT DOES NOT. ALSO; IF I HAD USED ASSYMETRIC THRUST AND IT KEPT ME FROM DEPARTING THE TXWY; I STILL HAD NO BRAKES TO STOP. THE END OF TXWY BRAVO WAS APPROACHING; I WOULD HAVE GONE OFF THE END. I KNOW MY JUDGEMENT IN THIS INCIDENT WAS CORRECT AND ANY OTHER REACTION THAN THE ONE I HAD; WOULD HAVE BEEN INAPPROPRIATE AND CAUSED MUCH MORE DAMAGE. I HAVE NO IDEA WHY THE ACFT STARTED TO STRAY L AND WHY CTL INPUTS WERE NOT EFFECTIVE. NOR DO I HAVE ANY IDEA AS TO WHY THE BRAKES FAILED.