LOW TIME B767-300 CAPT RELATES APCH TO IAD WITH WINDSHEAR WARNINGS; TURB AND ENGS APPEARING TO BE MALFUNCTIONING. GAR FOLLOWS FIRST APCH; SECOND APCH TO PARALLEL RWY IS SUCCESSFUL.

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IT WAS THE END OF 2 LEGS FOR THE DAY; DEN TURN OUT OF IAD. HAD A JUMPSEATER B737 CAPT (HAD BEEN A STANDARDS CAPT IN HIS PAST). NO BAD WX IN IAD AREA NOR FORECAST; NO PIREPS OF WINDSHEAR NOR BAD RIDES DURING DSCNT; ONLY WIND GUSTS 15-22 KTS AND 'WINDSHEAR ADVISORIES' ON ATIS. GAVE AN EARLY 'PREPARE FOR LNDG' TO FLT ATTENDANTS; EVERYONE WAS SEATED WHEN WE ENCOUNTERED ROUGH AIR AT 3000 FT ON R DOWNWIND TO RWY 1R. FO WAS HAND FLYING AT THIS POINT. AFTER FREQ CHANGE TO TWR; HEARD WINDSHEAR RPTS OF 15 KT GAINS/LOSSES. STARTED HEARING QUIET 'BOOM-BOOMS' AND A FEW SECONDS LATER FO ASKED IF THERE WERE ENG PROBS; WERE WE HAVING COMPRESSOR STALLS? ALL ENG INSTS WERE NORMAL; BUT AUTOTHROTTLE WAS VERY BUSY SO HE DISCONNECTED THEM. WE OPTED TO CONTINUE THE APCH. NO CALLS CAME FROM THE CABIN; WE PRESSED ON. ON FINAL APCH; CONFIGURED EARLY; EVERYTHING WAS STANDARD. AT 200 FT; I NOTICED THE FMC GENERATED WIND VECTOR AT 42 KTS; L XWIND. SAID '40 KT XWIND; IF YOU FEEL UNCOMFORTABLE ABOUT ANYTHING DON'T HESITATE TO GO AROUND; BUT EVERYTHING LOOKS OK FROM THIS SEAT.' GUST OF WIND IN THE FLARE CAUSED FO TO INITIATE A GAR AT ABOUT 10 FT. ACCOMPLISHED EVERYTHING AS SOP; ON DOWNWIND FINALLY HAD MOMENT TO TALK. DISCUSSION ENSUED AMONG ALL 3 AS TO OPTIONS: TRY AGAIN? GO SOMEWHERE ELSE? FUEL? ENGS? ME FLY? ASKED FOR APCH TO RWY 1L OR RWY 30. ATC QUESTIONED OUR CHOICE; BUT SET US UP FOR RWY 1L (RWY 30 WOULD BE A LONG DELAY). FO GAVE ME AIRPLANE ON AUTOPLT SO HE COULD GET OUT APCH PLATES; AND THIS IS THE MOMENT I REALLY FELT HOW BAD THE AIRPLANE AND ENGS WERE BATTLING THE AIR. WE OPTED TO HAVE ME FLY THE APCH/LNDG; AND IT WAS AT THIS POINT I STARTED EARNESTLY WORRYING ABOUT THE ENGS. TO MYSELF; 'WHAT IS THAT NOISE COMING FROM? THE GAUGES ARE NORMAL. KICK OFF THE AUTOTHROTTLES. EVERY THROTTLE MOVEMENT IS COINCIDING WITH THAT BOOM-BOOM NOISE. DO WE HAVE LOOSE CARGO? ARE THE NOSE GEAR DOORS MAKING THAT NOISE? ALMOST EVERY THROTTLE MOVEMENT IS COINCIDING WITH THAT BOOM-BOOM NOISE. IS IT ONE ENG? BOTH ENGS? WILL IT GO AWAY IF WE GET OUT OF THIS ROUGH AIR? SHOULD WE GO SOMEWHERE ELSE; BAG THIS WINDSHEAR? WILL MY ENGS CONTINUE TO OPERATE DURING A DIVERT? THE BOOM-BOOMS ARE BECOMING MORE FREQUENT.' MY CONCERN OVER THE ENGS GREW EACH MOMENT ALONG THE QUICK APCH TO RWY 1L. I LEFT THE AUTOPLT ON FOR THE APCH FOR THE WINDSHEAR; TOOK ALL OTHER WINDSHEAR PRECAUTIONS; BUT KEPT THE AUTOTHROTTLES DISCONNECTED. AROUND 1500 FT TWR RPTED A 20 KT SHEAR. THIS DEFINITELY EXCEEDED OUR PARAMETERS; AND WE HAD A LOT OF DISCUSSION AT THIS POINT; INCLUDING THE FO DIRECTING OUR JUMPSEATER TO START TYPING A MESSAGE TO DISPATCH AS TO THE CLOSEST PLACE TO GO IF WE GO AROUND. I COULD HEAR THE CONCERN OOZING OUT OF THE FO'S VOICE; BUT I ELECTED TO CONTINUE THE APCH AS WE HAD NOT ENCOUNTERED THAT LEVEL OF SHEAR YET OURSELVES. AT 200 FT WE HAD A MOMENTARY 20 KT INCREASE; BUT ALL OTHER PARAMETERS WERE SPOT ON JUST LIKE THE FIRST APCH. ON GS; VERT SPD STEADY AT 700 DOWN; NO DRASTIC THROTTLE CHANGES. I HAD TO DECIDE: CONTINUE TO LNDG; WRESTLING THE WINDSHEAR; OR GO AROUND AND WRESTLE WITH ' A DUAL ENG FLAMEOUT.' I OPTED TO CONTINUE; DISCONNECTED THE AUTOPLT AT 100 FT; LANDED SOFTLY WITHIN THRESHOLD; FELT STRONG GUSTS DURING THE ROLLOUT; NEVER LEFT CTRLINE. DURING TAXI IN IT WAS VERY DIFFICULT TO HOLD COMMENTS; BUT WE DID UNTIL REACHING THE GATE. AT WHICH TIME THE FOOD GATES LET LOOSE. THE JUMPSEATER SUGGESTED I SHOULD HAVE DECLARED AN EMER AND DEMANDED RWY 30; BUT IN RETROSPECT; THIS WOULD ONLY HAVE GIVEN US MORE OF A HEADWIND AS THE APCH END WAS IN THE SAME SPOT AS RWY 1; AND WE WOULD HAVE HAD TO INCREASE OUR TIME IN THE AIR WITH QUESTIONABLE ENGS TO GET REALIGNED WITH THAT APCH. HE ALSO SAID HE SAW A FLASH AT SOME POINT OUT OUR R SIDE DURING THE SECOND APCH; THINKING IT WAS REFLECTION OF FLAME. THE FO STEPPED OUT TO SAY GOODBYE TO THE PAX WITH THE HOPES OF HEARING SOME INPUT OR QUESTIONS FROM THEM. NO ONE MENTIONED ANYTHING ABOUT SEEING ANYTHING UNUSUAL WITH OUR ENGS. NONE OF THE FLT ATTENDANTS HAD NOTICED ANY UNUSUAL SIGHTS OR SOUNDS BEYOND THE BOUNCY RIDE FROM THE WINDS. I HAVE ONLY BEEN ON THIS FLEET FOR 3 MONTHS; I WAS BEGINNING TO WONDER IF WHAT WE HEARD WAS NORMAL. BUT THE FO HAS BEEN ON IT FOR 8 YRS AND HAS 'NEVER' HEARD THAT NOISE BEFORE. SHOULD WE WRITE IT UP? IT WAS CONCERNING ENOUGH TO MAKE ME CONTINUE AN APCH AFTER A SEVERE WINDSHEAR ENCOUNTER; BUT WHAT CAN I SAY ABOUT IT IN A WRITE-UP? EVERYTHING WAS ABSOLUTELY NORMAL OTHERWISE; WE HAVE NO IDEA WHAT WAS REALLY TRULY CAUSING THAT NOISE. IT 'SEEMED' TO COINCIDE WITH THROTTLE MOVEMENT; BUT NOT 100% OF THE TIME (95%). THERE'S NO LOG HISTORY ABOUT THIS. I LEFT THE AIRPLANE WITHOUT WRITING ANYTHING UP AND I AM NOW REGRETTING THAT. BUT IT WASN'T AN INTENTIONAL DECISION TO DO SO; WE WERE SO WRAPPED UP IN CONVERSATION; I SIMPLY FORGOT. I JUST LOOKED AT THE LOGS ON THE ACFT SINCE WE HAD IT; IT WENT TO ZZZ AND BACK AND HAS NO SIMILAR WRITE-UPS. MUST SEND THIS RPT NOW. ABRUPT ENDING.