BE36 pilot reported descending below clearance limit toward terrain on approach to mtn.

Attributes
ACN	1008521
Time
Date	201205
Local Time Of Day	1201-1800
Place
Locale Reference	MTN.Airport
State Reference	MD
Aircraft 1
Make Model Name	Bonanza 36
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Direct
Flight Plan	IFR
Person 1
Function	Single Pilot Pilot Flying
Qualification	Flight Crew Flight Instructor Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 37 Flight Crew Total 6000 Flight Crew Type 20
Events
Anomaly	Deviation - Altitude Overshoot Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Narrative:

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.

Original NASA ASRS Text

Title: BE36 pilot reported descending below clearance limit toward terrain on approach to MTN.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.