ACR MLG'S LOSS OF IRU'S AFTER TKOF IN IMC CONDITIONS CREATES A HDG TRACK DEV IN DEP PROCS. NON ADHERENCE TO ATC CLRNC INSTRUCTION PROC.

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APPROX 20 SECONDS AFTER TKOF; WE LOST OUR INERTIAL REF UNIT (IRU) SYS; WHICH RESULTED IN LOSS OF OUR PRIMARY FLT DIRECTOR (PFD) INDICATIONS AND LOSS OF NAV MAP (ND) DISPLAY. CREW COMPLETED STRAIGHT AHEAD DEP IN COMPLIANCE WITH ABNORMAL CHKLIST WITH WINGS LEVEL CLBOUT WHILE PNF INITIATED ABNORMAL CHKLIST PROCS. THIS RESULTED IN A DELAYED TURN FROM RWY HDG (272 DEGS MAGNETIC TO ASSIGNED HDG OF 250 DEGS AT MIDDLE MARKER). UPON CONTACTING DEP CTL; CREW ADVISED DEP OF OUR DEGRADED HDG SYS AND POSITIONING INDICATIONS. DEP CTL ADVISED/DIRECTED A FURTHER TURN TO THE L TO DIRECT US TO A 250 DEG HDG FOR THEIR DEP CORRIDOR. AFTER TURNING TO A HDG OF 250 DEGS (WITH ATC ASSISTANCE) DUE TO CONTINUED DEGRADED FLT DIRECTION AND ATTITUDE INFO IN IMC CONDITIONS; THE CREW COMPLETED THE ABNORMAL CHKLIST (ABNORMAL SECTION 'NAV' PAGE 4; 'ATTITUDE HDG SELECTION'). IN THE 'ATTITUDE' MODE OF THE IRS; WE CONTINUED TO CLB INTO VMC CONDITIONS. PRIMARY ATTITUDE REFS WERE RESTORED AND RELIABLE. WE WERE ABLE TO RESTORE IRS HDG INFO; BUT HDG INFO CONTINUED TO DRIFT; REQUIRING FREQUENT UPDATES. VOR AND ILS RAW DATA INFO WAS AVAILABLE. ATC VECTORED CREW FOR ILS TO RWY 17 AT PENSACOLA MUNICIPAL (PNS). WX WAS GREATER THAN 5000 FT AND 5 MI. A VISUAL APCH WAS MADE TO RWY 17 WITH AN ILS BACKUP. APCH AND LNDG WERE UNEVENTFUL.