AN ACR FREIGHTER FLC LOST 2 OF THEIR 3 INERTIAL NAV SYS AFTER TKOF INTO AN AREA OF TSTMS. THE FLC REMAINED CLR OF THE CLOUDS AND SUCCESSFULLY RETURNED TO THE ARPT.

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WE WERE ON AN IFR FLT PLAN FROM SVN TO ESNN. THERE WAS POOR WX AT SVN AND IN THE AREA TSTMS; WINDSHEAR; RAIN. ONE ACFT (L1011) HAD RPTED SEVERE TO EXTREME TURB ENRTE AND WINDSHEAR ON APCH. A C5A MISSED THE APCH AND WENT AROUND. WHEN WE TAXIED OUT WE COULD SEE A LINE OF TSTMS OFF THE END OF THE RWY ABOUT 15-20 MI OUT. THERE WAS LIGHTNING FROM CLOUD TO CLOUD RPTED. WE SAW A SAFE PATH FOR OUR TKOF AND DEPARTED THE ARPT. AT ABOUT VR I HAD AN INST FAILURE WARNING. THE #1 INS HAD FAILED AND I LOST MY GYROSCOPE HEAD AND COMPASS. I SWITCHED TO #3 INS AND ROTATED FOR LIFT-OFF. AT ABOUT 1500 FT THE #2 INS WARNING (R HAND) FAIL LIGHT CAME ON. I REQUESTED TO STOP CLB AND STAY VMC FROM ZJX. WE STOPPED AT 2500 FT AGL AND CLR OF CLOUDS. I FELT THAT THIS WAS SAFER THAN TO CONTINUE A CLB INTO VERY POOR WX NOT KNOWING WHAT WAS WRONG WITH MY INS'S. I CONSIDERED MY OPTIONS TO EITHER CONTINUE TO JFK OR RETURN TO SVN. I FELT IT WAS SAFER TO RETURN. WE INFORMED ATC AND REQUESTED A VECTOR TO DUMP FUEL. THE NORMAL DUMP AREA WAS COVERED BY A TSTM (LARGE). ATC ADVISED US TO STAY VFR AND DUMP S OF SVN. WE DUMPED FUEL FOLLOWING OUR ACR'S FUEL JETTISONING PROCS; STAYING CLR OF CLOUD AND LIGHTNING. AT THE END OF THE DUMP WE RETURNED TO SVN AND LANDED. BOTH #1 AND #2 INS'S HAD TO BE REPLACED AND WE LEFT THE NEXT EVENING FOR SNN IN IRELAND. I FEEL TAKING EVERYTHING INTO CONSIDERATION; THIS WAS THE SAFEST WAY TO OPERATE THE ACFT IN THIS SIT.