A DC8-71 CARGOLINER ON TKOF WHEN THE LNDG GEAR WAS SELECTED UP THE ACFT EXPERIENCED A LOUD BANG; WIND NOISE AND VIBRATION UNDER THE FO'S FLOOR. DECLARED EMER; DUMPED FUEL AND RETURNED TO THE FIELD.

Narrative

ON FLT XYZ FROM BFI TO TOL; ON TKOF (FO FLYING) WHEN GEAR WAS SELECTED UP; THERE WAS A LOUD BANG (SOUNDED LIKE AN EXPLOSION) AND VERY LOUD WIND NOISE; ALSO VIBRATION UNDER FO'S SEAT. I THOUGHT THAT WE HAD A TIRE BLOW UP OR A NOSE GEAR DOOR BREAK LOOSE. NOTIFIED ATC AND ADVISED THEM OF THE PROB; DECLARED AN EMER. THEY ASKED US IF WE COULD MAINTAIN VFR AND 1500 FT MSL. WE SAID YES; AND THEY VECTORED US DOWNWIND BACK TO BFI. WE REQUESTED THEM FOR FUEL DUMPING INSTRUCTIONS; AND THEY CLRED US TO GO AHEAD AND DUMP; WHICH WAS DONE MOSTLY OVER WATER BTWN 1500-1800 FT DOWN TO LNDG WT. AFTER ALL CHKLISTS WERE COMPLETED; ATC VECTORED US BACK TO FINAL FOR RWY 13R BFI. THE REASON FOR THE ALT AT 1500-1800 FT WAS DUE TO THE LOW CEILING AND THE IFR ARRS AT SEA ARPT. I TOOK OVER FROM THE FO AND MADE A NORMAL LNDG AT BFI. AFTER LNDG AND INSPECTION OF THE ACFT; FOUND NOTHING WRONG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A DC8-71 CARGOLINER. THE RPTR SAID THE NOISE HEARD WAS A LOUD BANG ACCOMPANIED WITH WIND NOISE AND VIBRATION UNDER THE FO'S FLOOR. THE RPTR STATED IT WAS FIRST THOUGHT THE NOSE TIRES EXPLODED DUE TO THE LOUD BANG OR THE NOSE GEAR DOOR OR DOORS BROKE OFF. THE RPTR SAID THE ACFT WAS INSPECTED AT BFI FOR 3 HRS WITH NOTHING FOUND. THE RPTR STATED THE ACFT WAS FLOWN UNEVENTFULLY TO THE ACR'S MAINT BASE; REINSPECTED AND AGAIN NOTHING WAS DISCOVERED. THE RPTR STATED THE ACFT HAS BEEN OPERATED DAILY WITH NO SUBSEQUENT RPTS. THE RPTR SAID 2 POSSIBLE EXPLANATIONS WERE ICE INGESTION IN THE PACK SYS AND NOSE SHOCK STRUT MALFUNCTION; BUT NEITHER COULD NOT BE PROVED.