HDG TRACK DEV OCCURS WHEN THE FLT CREW OF AN MD88 MISREADS THEIR ACARS SCREEN AND FLIES THE FILED PLAN VERSUS THE ATC 'CLRED VIA' FLT PLAN 15 NM E OF PXR; AZ.

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RECEIVED ATC CLRNC VIA ACARS. HAD FILED A 'DRYHEAT 2' SID. ACARS SCREEN ON BACK OF THROTTLE AREA HAS 3 OR 4 PAGES FOR ATC CLRNC. IN SCROLLING THROUGH PAGES; SAW THE FILED DEP AND MISTOOK IT FOR THE ATC CLRED DEP (SJN2 SID). ON DEP AT 4 DME; TURNED TO 100 DEP HDG; IN ACCORDANCE WITH DRYHEAT SID VERSUS 45 DEGS; IN ACCORDANCE WITH SJN SID. AFTER ABOUT 8 MI; CTLR ASKED WHAT DEP WE WERE FLYING; THEN TURNED US TO 45 DEGS TO COMPLY WITH SJN SID. CONTRIBUTING FACTORS: DESIGN -- ACARS SCREEN ON MD88 IS TO SIDE OF PLTS AND IS MORE DIFFICULT TO READ THAN SCREENS MORE FORWARD; SUCH AS THE B737-800. DEP PROCS: PHX HAS A LARGE NUMBER OF SIDS AND STARS. DEP SIDS CHANGE FREQUENTLY DEPENDING ON RWY ASSIGNED. BECAUSE OF DENSITY ALT; DEP RWY IS FREQUENTLY DIFFERENT FROM RWY ATC EXPECTS AS ACFT OPERATE AT MAX WTS IN HOT WX. PROCEDURAL: SINCE SIDS CHANGE FREQUENTLY; WOULD HELP IF SIDS WERE NOT FILLED; LET THEM ALWAYS BE ATC ASSIGNED. PLT PROC: WE CAREFULLY BRIEFED THE FILED SID. I WILL ALWAYS PRINT OUT ATC CLRNCS AFTER THIS TO REDUCE POSSIBILITY OF CONFUSING 'CLRED' VERSUS 'FILED' RTES!