A B777 ON CLB AT FL240 ENCOUNTERED A SUDDEN AND MODERATELY STRONG JOLT WITH NOISE. ACFT RETURNED TO ZZZ FOR INSPECTION.

Narrative

ON CLBOUT FROM ZZZ; DEST LONDON'S HEATHROW ARPT; A SUDDEN AND MODERATELY STRONG JOLT WAS FELT AT APPROX FL240 AT A NORMAL CLBOUT AIRSPD OF 300+ KTS. THE JOLT FELT AS IF AN ENG HAD SUFFERED A SIGNIFICANT COMPRESSOR STALL; AND THERE SEEMED TO HAVE BEEN AN ACFT YAW WITH THE EVENT. IN ADDITION; A NOISE OR MUFFLED SOUND WAS ALSO HEARD IN THE COCKPIT. THE EVENT WAS FELT AND HEARD THROUGH THE ENTIRE AIRPLANE. THE 2 FLT ATTENDANTS BY DOORS 4R AND 4L LATER DESCRIBED THE NOISE AS SOUNDING LIKE A MUFFLED FIRECRACKER. THE RELIEF PLT WAS SENT BACK TO GATHER INFO. DISPATCH AND SYS ACFT MAINT CTLR WERE BROUGHT INTO THE ANALYSIS VIA COMMERCIAL RADIO; ALTHOUGH THE COM WITH SYS ACFT MAINT CTLR WAS NEVER VERY GOOD AND EVEN THE CONNECTION WITH DISPATCH WAS NOT WHAT IT SHOULD HAVE BEEN. AFTER A WHILE; A SATCOM CONNECTION WAS ESTABLISHED WHICH GREATLY IMPROVED THE COM LINK. AS THERE WAS NO APPARENT REASON THAT COULD BE DETERMINED TO HAVE CAUSED THIS JOLT; A DECISION WAS MADE TO NOT CONTINUE THE FLT OUT OVER THE NORTH ATLANTIC; BUT INSTEAD MAKE A PRECAUTIONARY RETURN LNDG AT ZZZ. EVENTS CONSIDERED AS POSSIBLY CAUSING THE JOLT RANGED FROM AN UNSEEN AND UNVERIFIED ENG COMPRESSOR STALL; SIGNIFICANT CARGO SHIFT; BLOWN TIRE IN THE WHEEL WELL; AND EVEN A FOREIGN DEVICE EXPLODING SOMEWHERE ON THE AIRPLANE. THIS WAS OF SOME CONCERN DUE TO A VERY RECENT FAA WARNING ON TERRORISM WITH THE EMPHASIS ON EAST COAST TARGETS. AS WE WERE ALMOST 50000 LBS OVERWT WE DUMPED FUEL TO REDUCE THE LNDG WT. A 2+ HR INSPECTION OF THE ACFT AT ZZZ; INCLUDING REMOVAL AND INSPECTION OF ALL CARGO CONTAINERS AND BULK CARGO; REVEALED NOTHING. IN ADDITION; AN INSPECTION OF THE AUTOPLT SYS WAS PERFORMED AS THERE WAS SOME MINOR ANOMALIES WITH THE SYS DURING THE RETURN. AFTER A FULL DISCUSSION WITH DISPATCH AND MAINT; WE THEN DEPARTED FOR LONDON AND THE FLT WAS UNEVENTFUL.