B737-700 FLT CREW HAVE A TRACK DEV ON DEP.

Narrative

SHORTLY AFTER DEP FROM ISP WE WERE GIVEN DIRECT BEADS (APPROX 080 DEG HDG) THEN AS FILED. AT 15000 FT IN THE CLB; AUTOPLT ENGAGED; LNAV ENGAGED AS WE APCHED BEADS (APPROX 7 NM) AND I STARTED A LEAD TURN (AUTOPLT/LNAV) TO INTERCEPT THE COURSE TO THE NEXT FIX OF EMJAY. THE NEW COURSE WAS ROUGHLY A 150 DEG COURSE REVERSAL FROM OUR PREVIOUS COURSE INTO BEADS AND THE EARLY LEAD TURN DIDN'T SEEM UNUSUAL WITH THE STRONG TAILWIND. 1/2 WAY THROUGH THE TURN; THE ATC CTLR DIRECTED US TO ROLL OUT ON OUR CURRENT HDG (150 DEGS) AND WANTED TO KNOW WHY WE HADN'T FLOWN TO BEADS PRIOR TO STARTING OUR TURN. SHORTLY AFTER; ATC CLRED US BACK ON COURSE. AFTER LNDG; THE CAPT PLACED A CALL TO OUR ATC SPECIALIST AND INFORMED HIM OF THE SIT. SUPPLEMENTAL INFO FROM ACN 689997: AFTER WE STOPPED THE TURN; THE CTLR ASKED IF WE WERE GIVEN A HDG BY DEP. I REPLIED IN THE NEGATIVE AND HE TOLD US WE WERE 10 MI FROM BEADS. BY OUR DISPLAY AND INSTRUMENTATION; IT DID NOT APPEAR TO BE THE CASE BUT WE DID RECEIVE AN 'UNABLE TO COMPLY WITH RNP NAV' MESSAGE EARLIER IN THE FLT. I DID NOT CONSIDER THIS UNTIL WRITING THIS RPT. FROM MY PERSPECTIVE; NOTHING SEEMED TO BE OUT OF THE ORDINARY.