Bonanza 36
Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 37; Flight Crew Total 6000; Flight Crew Type 20
Distraction; Situational Awareness
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Human Factors; Weather
I was en route to MTN and was cleared to descend from 3;000 to 2;500 FT MSL. I set the autopilot to descend at 500 FPM and checked the GPS to get the distance to the airport. I then realized I was only 6.7 miles away and started searching for the field through the mist at my altitude; forgetting entirely about the level-off at 2;500 MSL. The aircraft continued descending. I was then called and told by Potomac Approach that I was only cleared to 2;500 MSL. I suddenly realized I was at 2;200 MSL. Naturally; I was embarrassed and frightened by the possible consequences my error. Fortunately; the visibility at the time allowed me to see the ground and spot the field. Had it happened in hard IMC; I could have been killed and possibly hurt or killed others in the process. In the future I will be more alert during this phase of flight and of descents in general.
BE36 pilot reported descending below clearance limit toward terrain on approach to MTN.