ACR MLG REQUIRED RWY 8R ACCOUNT AT MAX TKOF GROSS WT; RWY IN USE RWY 26. CLRED RWY 8R MAINTAIN RWY HEADING CLRED FOR TKOF. CAPT; PF; RUSHING DEP FORGOT RWY HEADING RESTR AND BEGAN A TURN AT 4 DME PER SID. DEP CTLR QUESTIONED THE TURN AND THE CAPT IMMEDIATELY TURNED BACK TO RWY HEADING.

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WE WERE AT OUR MAX GROSS TKOF WT AND NEEDED RWY 8R AT PHX. RWYS HAD JUST CHANGED TO RWY 26. MUCH COORD WAS NEEDED TO WORK OUT A RWY 8R DEP. WE WERE ASKED TO EXPEDITE OUR TAXI TO RWY 8R. TKOF CLRNC WAS RECEIVED AND WE WERE TOLD TO MAINTAIN RWY HEADING. I MISSED THE 'MAINTAIN RWY HEADING.' WE WERE CLRED ON THE ST JOHNS 2 DEP. TFC WAS INBOUND ON FINAL FOR RWY 26R AND WE RPTED THEM IN SIGHT. WHEN TFC WAS RPTED IN SIGHT TO DEP CTL; I THOUGHT WE WERE CLRED NORMAL NAV ON THE DEP AND I CALLED FOR LNAV. I STARTED A L TURN AT THE PXR 075 DEG/4 DME TOWARDS 020 DEG HDG. PASSING ABOUT 040 DEGS DEP CTL ASKED IF WE WERE STILL HDG 080 DEGS. WE IMMEDIATELY TURNED R TO 080 DEGS. THERE APPARENTLY WAS AN ARR ACFT THAT COULD HAVE BEEN A POTENTIAL CONFLICT. CONTRIBUTING FACTORS: RWY CHANGE DURING TAXI OUT. CONCERN WITH FLAPS 5 DEGS IMPROVED MAX TKOF WT. EXPEDITING TO MAKE A SMALL WINDOW OF OPPORTUNITY -- DEPARTING OPPOSITE DIRECTION OF TFC. BRIEFING AN LNAV DEP. MISSING 'MAINTAIN RWY HEADING.' DISTR WITH LOOKING FOR TFC. SLOW IT DOWN AND LISTEN MORE CAREFULLY.