Caravan 208B
Flight Crew Commercial
Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
Flight Crew Returned To Departure Airport
Weather
While climbing enroute; we encountered icing at 6900 feet msl.  We could no longer climb and maintain 120 knots.  We advised Center that we were returning to [the departure] airport and requested direct to the runway initial approach fix.  ATC advised that the minimum vectoring altitude in that sector was 7000 feet msl.  We then advised Center that we would not be able to maintain 7000 feet msl.  The ice continued to accumulate on the aircraft while we were enroute to the initial approach fix.  In order to maintain 120 knots we maintained a descent rate of between 100 and 300 feet per minute.  Despite being below the minimum vectoring altitude; we were able to stay well above the terrain during our slow descent and verified our terrain and obstruction clearance using Capstone and our Garmin GPS.  At 4000 feet msl we were below the clouds and at 3500 feet msl the outside air temperature was above freezing and the ice began to break free of the aircraft. At approximately 3000 feet msl; we were able to cancel our IFR clearance and turn directly to the airport and landed without any further incident.  Filing pilot reports on inflight icing conditions can prevent other aircraft from flying into those same conditions.
C208B Captain reported icing in flight that made it impossible to maintain minimum vectoring altitude in an area of high terrain. They descended to warmer air and landed safely at departure airport.