HELICOPTER MECHANIC LEARNS THAT VERBAL AUTHORIZATION TO ALTER CARGO HOOK EMERGENCY RELEASE STC IS NOT VALID.

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OUR HELI HAD BEEN DISPATCHED TO FIGHT FIRES FOR THE UNITED STATES FOREST SVC. UPON ARR; THE ACFT UNDERWENT A FOREST SVC INSPECTION FOR COMPLIANCE TO THEIR CONTRACT SPECS. DURING THIS INSPECTION; IT WAS POINTED OUT TO US THAT WE HAD NOT FULLY COMPLIED WITH A SUPPLEMENTAL TYPE CERT TO ALLOW THE PLT TO FLY FROM THE L-HAND SEAT. THE ITEM IN QUESTION WAS THE CARGO HOOK EMER RELEASE PEDAL. THE SUPPLEMENTAL TYPE CERT CALLS FOR A PUSH ROD TO BE USED TO ACTUATE THE PEDALS FROM EITHER THE L- OR R-HAND POS. WHEN WE INSTALLED THIS PORTION OF THE SUPPLEMENTAL TYPE CERT; WE WERE GIVEN VERBAL AUTH TO USE A CABLE SYS DUE TO THE PARTS NOT BEING AVAILABLE FOR THE REMAINDER OF THE KIT. WE DID NOT RECEIVE ANY AUTH IN WRITING (WE NOW REQUIRE ALL AUTH TO BE WRITTEN AND SIGNED) FOR THIS PORTION OF THE INSTALLATION. WE INSTALLED THE SYS AND CHKED IT FOR PROPER OP AS PER THE SUPPLEMENTAL TYPE CERT AND APPROVED THE ACFT FOR RETURN TO SVC. THE EMER RELEASE PEDAL WAS ALWAYS FULLY OPERATIONAL AND WAS OPERATING NORMALLY AT THE TIME OF OUR DISCOVERY. WE BELIEVED THAT WE WERE IN COMPLIANCE WITH THE SUPPLEMENTAL TYPE CERT DUE TO OUR VERBAL AUTH; AND WE CONTACTED THE MANUFACTURER OF THIS SUPPLEMENTAL TYPE CERT TO ASK THEM FOR SOME GUIDANCE. THEY INFORMED US THAT WE SHOULD HAVE INSTALLED THE PUSH ROD AND THAT THEY WOULD NOT AUTH USE OF THE SYS IN ITS CURRENT CONFIGN. WE HAVE DISPATCHED A MECH TO THE SITE TO INSTALL THE PUSH ROD AND ALL NECESSARY ITEMS TO BRING OUR ACFT INTO COMPLIANCE WITH THE SUPPLEMENTAL TYPE CERT. THIS PROB AROSE THROUGH A VERBAL INSTEAD OF A WRITTEN AUTH WITH SPECIFIC INSTRUCTIONS SPELLED OUT TO ELIMINATE ANY POSSIBLE CONFUSION. WE WILL NO LONGER INSTALL ANYTHING WITHOUT CLRLY WRITTEN AUTH AND CLRLY DEFINED INSTRUCTIONS FOR ALLOWED DEVS FROM THE ORIGINAL SUPPLEMENTAL TYPE CERT. WHILE WE BELIEVED THAT WE WERE OPERATING WITHIN THE SUPPLEMENTAL TYPE CERT; WE NOW UNDERSTAND THAT WE WERE NOT AND HAVE CORRECTED THIS ISSUE. WE REGRET THAT WE DID NOT GET THE ORIGINAL AUTH TO INSTALL THE CABLE SYS IN WRITING. WE HAVE REVIEWED ALL OF OUR SUPPLEMENTAL TYPE CERTS AND ENSURED THAT THERE ARE NOT OTHER ISSUES THAT WE HAVE NOT ADDRESSED. WE WILL CONDUCT TRAINING TO ADDRESS THIS ISSUE WITH ALL OF OUR MAINT PERSONNEL TO INSURE THAT THERE ARE NO FUTURE OCCURRENCES RELATED TO THIS ISSUE. THE FACTORS RELATING TO THIS ISSUE WERE: LACK OF COM: WE DID NOT GET THE AUTH IN WRITING; AND SINCE THE RELEASE PEDALS WERE WORKING; WE 'ASSUMED' EVERYTHING WAS OK. WE WON'T MAKE THAT MISTAKE AGAIN.