FLT CREW OF WDB HAVE A TRACK DEV WHEN A MECHANICAL PROB DISTRACTS THEM FROM THEIR CLRED DEP ROUTING.

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I WAS THE RELIEF FO SEATED IN THE OBSERVER SEAT. WHILE THE CREW BRIEFED THE DEP; I WAS HANDLING PAPERWORK DUTIES; BUT HEARD THE CLRNC AND THE FO BRIEF THE LOGAN 2 DEP DIRECT PEASE AS FILED. I THINK THE DELAY WE EXPERIENCED AFTER THE BRIEF CONTRIBUTED TO CONFUSION. WE HAD OVER 1 HR DELAY AFTER THE BRIEF. I ALSO REMEMBER SOME CONFUSION READING BACK THE CLRNC WHEN THE FO READ BACK DIRECT (PEASE) AFTER DEP. THE CLRNC CTLR SAID 'NEGATIVE; LOGAN 2; DIRECT (PEASE).' SHORTLY AFTER TKOF; CLBING THROUGH 1500 FT; I NOTICED A RED MANIFOLD FAIL LIGHT ON THE #3 ENG. THE CAPT WAS ON THE RADIO WITH ATC AND THE FO WAS HAND FLYING THE ACFT. I WAS VERY CONCERNED WITH THE MANIFOLD FAIL LIGHT AN STARTED A QUICK MENTAL DIAGNOSTIC OF THE PROB. I FAILED TO NOTICE THE FO PASSING THE 4 DME FIX FROM BOS WHICH REQUIRED A R TURN TO 090 DEGS (WE WERE DEPARTING FROM RWY 4R). WHEN THE CAPT FINISHED HIS RADIO XMISSION; I STATED WE HAD A MANIFOLD FAILURE AND THAT THE AUTOMATIC AIR SYS CONTROLLER CLOSED THE #3 BLEED; WHICH ALSO MEANT WE HAD NO ANTI-ICE ON THE R WING. DURING THE SHORT TIME I BRIEFED THE CAPT ON OUR PROB; ATC QUESTIONED WHERE WE WERE GOING. THE CAPT RESPONDED DIRECT (PEASE). THE CTLR SAID 'WHO GAVE YOU THAT?' HE THEN SAID WE WERE TO FLY THE LOGAN 2 DIRECT. I COULD HAVE PREVENTED THIS MISTAKE IF I DIVIDED MY ATTN BTWN THE MAINT PROB AND OBSERVING THE FLT CREW ACTIONS. I ALSO THINK A QUICK REVIEW JUST BEFORE TKOF OF THE DEP PAGE IS A GOOD SOLUTION. 'NEVER ASSUME THE PF WILL DO WHAT HE BRIEFS' IS AN IMPORTANT LESSON.