Narrative:

On climb out from ZZZ, destination london's heathrow airport, a sudden and moderately strong jolt was felt at approximately FL240 at a normal climb out airspeed of 300+ KTS. The jolt felt as if an engine had suffered a significant compressor stall, and there seemed to have been an aircraft 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 flight attendants by doors 4R and 4L later described the noise as sounding like a muffled firecracker. The relief pilot was sent back to gather information. Dispatch and system aircraft maintenance controller were brought into the analysis via commercial radio, although the communication with system aircraft maintenance controller 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 communication link. As there was no apparent reason that could be determined to have caused this jolt, a decision was made to not continue the flight out over the north atlantic, but instead make a precautionary return landing at ZZZ. Events considered as possibly causing the jolt ranged from an unseen and unverified engine 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 pounds overweight we dumped fuel to reduce the landing weight. A 2+ hour inspection of the aircraft at ZZZ, including removal and inspection of all cargo containers and bulk cargo, revealed nothing. In addition, an inspection of the autoplt system was performed as there was some minor anomalies with the system during the return. After a full discussion with dispatch and maintenance, we then departed for london and the flight was uneventful.

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Original NASA ASRS Text

Title: 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.

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.