What was found
NR 1 INERTIAL REF UNIT RACK FAN INVERTER SUFFERED A CATASTROPHIC FAILURE DURING GND OPERATION WITH GND POWER APPLIED. THE ENCLOSED UNIT APPEARS TO HAVE EJECTED ITS COVER, INTERNAL PARTS & POTTING, INCLUDING A TOROIDAL TRANSFORMER. THERE ARE SIGNS OF HEAT DAMAGE & MELTED VARNISH & POTTING COMPOUND INTERNAL & EXTERNAL ON THE CASE & EJECTED PARTS. THE CREW DID DETECT AN ELEC BURNING ODOR MOMENTARILY, EMITTED FROM THE LT AVIONICS RACK AREA. NO CB POPPED & ONLY FURTHER INDICATION TO THE CREW OF A FAILURE, WAS A LONG DELAYED & INTERMITTENT NR 1 IRU FAN FAIL, CAS MSG AT 1 POINT DURING NR 1 IRU OPERATION. AFFECTS TO THE AREA SURROUNDING & EXTERNAL TO THE UNIT INCLUDE CONTAMINATION FROM EJECTED DEBRIS & MELTED VARNISH. ONLY A THOROUGH CLEANUP OF THE AREA REQUIRED TO CORRECT. REPLACING THE OTHER 2 REMAINING LIKE UNITS THAT ARE INSTALLED IN THE ACFT. THE MFG HAS A NEW & IMPROVED PART TO REPLACE THIS.