Narrative:

We were on an IFR flight plan from svn to esnn. There was poor WX at svn and in the area thunderstorms, windshear, rain. One aircraft (L1011) had reported severe to extreme turbulence en route and windshear on approach. A C5A missed the approach and went around. When we taxied out we could see a line of thunderstorms off the end of the runway about 15-20 mi out. There was lightning from cloud to cloud reported. We saw a safe path for our takeoff and departed the airport. At about vr I had an instrument 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 (right hand) fail light came on. I requested to stop climb and stay VMC from ZJX. We stopped at 2500 ft AGL and clear of clouds. I felt that this was safer than to continue a climb 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 thunderstorm (large). ATC advised us to stay VFR and dump south of svn. We dumped fuel following our air carrier's fuel jettisoning procedures, staying clear 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 aircraft in this situation.

Related reports:

Google
 

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.