Commercial Fixed Wing
Communication Breakdown; Distraction; Human-Machine Interface; Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Human Factors; Environment - Non Weather Related
At 5;000 FT MSL roughly 15 nm SE Runway 31 at AMA we accepted a visual via direct IMOBE then Runway 31. We don't have a RNAV 31 chart but it's in the database. We had previously been given direct from FUZ to AMA and that put us just a little east of the final for 31. We flew to IMOBE in LNAV and I selected 4;200 FT in the MCP and started to descend via V/S. I was tired and just assumed we were in Class B airspace. Wrong; it's Class C. The Airport and Runway were in sight but everything felt low and I realized I was going through 1;400 FT AGL so I leveled; slowed and configured and continued to IMOBE using the PDI in VNAV. (All we ever heard from ATC was cleared to land and afterward cleared to the hangar.)
An Air Carrier First Officer reported feeling low on approach about 15 NM from AMA and realized he was 1;400 FT AGL so he leveled; slowed and configured for landing. ATC was silent.