A First Officer reported leaving ATC frequency to get the ATIS and upon his return discovered the hand flying Captain leveling at 10;000 FT with Altitude Alerter at 11;000 FT. ATC called and they climbed back to 11;000 FT.

Narrative

Enroute to an air show; Center cleared us to 11;000 FT (descend). We conducted FL180 and below checklist with exception of altimeter setting. I informed Captain I would be off frequency to get arrival ATIS. He handled all communication during this time while hand flying the aircraft (autopilot not used). Very long ATIS due to air show requirements. I returned to frequency and as I looked up; I saw the altitude selector at 11;000 FT (per clearance) but aircraft was leveling 10;000 FT. Center at that point asked our altitude. I heard the Captain say; 'my bad I missed it.' I responded to Center; 'returning to 11;000 FT.' Captain returned to 11;000 FT and flight continued to our destinataion without further incident; but extremely busy due to high volume radio and aircraft traffic. I fly as a contract pilot with this company and am quite familiar with aircraft FMS/EFIS; and NAV systems but note the Captain tends to hand fly the aircraft in the highest traffic; highest work load times. Numerous times I have had to call out 'altitude' when he was flying and going off assigned altitude. I have pointed this out but Captain said nothing and evidently won't change his procedure. I will fly with him in the future but will encourage autopilot use and monitor his climb and descent profiles closely.