FLC OF ACR LTT ACFT INADVERTENTLY DSNDED BELOW CLRNC AND MEA ALT LIMIT DURING DSCNT FOR AN EMER RETURN TO THE ARPT.

Narrative

DURING DSCNT FOR APCH INTO MTJ THE ACFT DSNDED 400 FT BELOW THE MEA OF 12400 TO 12000. THE ERROR WAS POINTED OUT BY THE CTLR. THE FLT HAD BEEN CLBING OUT FROM MTJ ENRTE DEN WHEN A MALFUNCTION IN THE PRESSURE CTLR CAUSED THE CABIN TO START TO CLB ABOVE 10000. SINCE MEA'S FURTHER ALONG THE RTE PRECLUDED UNPRESSURIZED FLT BELOW 10000 FT IT WAS DECIDED TO RETURN TO MTJ. CLRNC WAS RECEIVED TO RETURN AND THE CREW DONNED OXYGEN MASKS. THE CAPT TOOK CTL OF THE ACFT WHILE THE FO DONNED HIS MASK. THE ACFT WAS CLRED FOR AN APCH INTO MTJ UPON BEING ESTABLISHED ON V26 AND TO RPT DSNDING THROUGH 13000. THE FO HAD SOME TROUBLE WITH HIS MASK STRAPS AND THE CAPT MISSED THE 13000 CALL. THE CAPT THOUGHT THE MEA WAS 12000 AND DSNDED TO THAT ALT. UPON BEING INFORMED OF THE ERROR BY CTR; AN IMMEDIATE CLB WAS PERFORMED TO 12400. THE CAPT THOUGHT THE MEA WAS 12000 AS HE HAD SHOT THE APCH INTO MTJ BUT FROM A DIFFERENT AIRWAY WHERE THE MEA WAS 12000; AND MISREAD THE CHART DURING THE APCH BRIEFING. CONTRIBUTING FACTORS WERE THE DISTRS OF COMPANY CALLS; PAX BRIEFING; AND THE DIFFICULTIES ENCOUNTERED BY THE FO WITH HIS FACE MASK STRAPS. ALSO; THE CREW HAD FLOWN 5 LEGS ALREADY FOR A TOTAL OF 5 HRS 18 MINS AND HAD BEEN ON DUTY SINCE AM31. ALTHOUGH FATIGUE HAD NOT REACHED A HIGH LEVEL; IT WAS AT LEAST A FACTOR TO CONSIDER. IN RETROSPECT; THIS WAS A CHILLING REMINDER OF THE NEED TO CONSTANTLY MONITOR THE MSA AND NOT ALLOW EXTRANEOUS FACTORS TO DISTRACT THE CREW.