Pilot reported losing control of aircraft after landing when the aircraft hit a snow berm that hadn't been cleared off the runway correctly.

Narrative

I was assigned to fly a charter flight from ZZZ1 to ZZZ on the morning of DATE. Weather for the trip was variable; as I recall; with good VFR in ZZZ2; and possible lower conditions west of ZZZ2 toward the destination. I chose to fly the last segment of the trip VFR; because the weather looked acceptable; based on conditions at departure time; and I wanted the flexibility to maneuver around and change plans at the destination. I asked for a runway condition report from Flight Service before departure; but they said there was no report in the system. I chose to depart with the intention of getting a runway condition report en-route; and land at ZZZ2 if no report was available.The initial part of the flight was uneventful; but approaching ZZZ2; I asked ATC again for a condition report. They read me a report of 8 inches of wet snow on the runway. Based on this report; I decided to land in ZZZ2; and wait for better conditions. Flying into ZZZ2; I called the company agent in ZZZ2; and asked if he had any information from other pilots about runway conditions in ZZZ. He indicated that the ZZZ maintenance contractor was probably plowing the runway already. Based on this info; I decided to continue; with the intention of calling the plow operator on the radio approaching ZZZ; and getting an up to date report. If I was unable to talk with the plow operator; then we would just turn back and land at ZZZ2. About 20 miles out from ZZZ; the plow operator answered my radio call; and said that he had just started plowing. He said that he would have the runway plowed full length in about 15 minutes. He said he would do 2 passes and that would be plenty wide enough. I decided to circle over the low terrain in the valley east of ZZZ while the plow operator finished the job. Returning to ZZZ2 to wait would have meant about 20 minutes of flying back to ZZZ2; and a further 20 minutes to ZZZ again. I flew a few circles about 10 miles east of ZZZ; and the plow operator called back on the radio; to say that he had just finished the full length of the runway. The weather was variable. Ceiling was indefinite; with reported ceiling at ZZZ between 1;000 and 1;500 ft. Visibility east of ZZZ was variable between about 2 and 5 miles with snow showers. ZZZ was reporting visibility between 1 1/2 and 2 1/2 miles; so I was expecting the visibility to go down as I approached ZZZ. I set up the airplane for landing; and lined up on final approach for Runway XX. About 2 miles out; I got sight of the town and the runway on the far side of the river. The light was dim; with light snow and the terrain was visually featureless. The runway area was a field of uniform white; but I turned on the runway lights; and could make them out clearly enough. I aimed for the center between the runway lights; descended; started the landing flare; and pulled the power back to idle. Just at touchdown; the left main wheel descended into the berm on the left side of the runway; and the airplane started yawing to the left. I applied hard right rudder to keep the airplane under control and steer toward the plowed portion of the runway. The airplane then yawed to the right; onto the plowed portion of the runway. But there was not enough traction or rudder authority to stop the yaw to the right. The airplane slid down the runway while decelerating and yawing approximately 180 degrees to the right. The main wheels probably got enough traction while moving backwards to pull the airplane to the east side of the runway; because the airplane came to rest in the snow berm on the east side of the runway; having yawed a total of about 270 degrees. The event involved smooth; rapid deceleration; but no impacts or abrupt motions. I did not notice any change in engine sound or operation during the event. The engines were still running at idle when the airplane stopped moving. I shut down the engines and secured the airplane normally. We dug out a path in the snow to pull theairplane out; and I taxied it onto the ramp. The ramp had only been partially plowed; and flat light conditions still prevailed; so I walked the path to the ramp before taxi; and noted a route that would get the airplane safely off the runway. I taxied the airplane slowly onto the ramp; but I could not see the berms on the ramp while taxiing; and inadvertently ran the right main wheel slowly into a berm at an angle. I stopped the airplane and shut it down as soon as I felt the wheel start to drag. The right propeller contacted the soft wet snow at about 2-3 knots forward speed; and at idle power (about 1;000 RPM). The propeller displaced some snow; but this did not slow down the engine and put no significant load on the propeller. There were no other anomalies in engine operations or aircraft taxiing. I then called our Director of Operations (DO) on a borrowed satellite phone. I described the event and situation to him; and did a preliminary assessment of the airplane. I did not see any evidence of damage or structural deformation; and I passed that information to him. He told me to do a thorough inspection; and if I found any evidence of structural issues; they would send a maintenance crew out to assess it. He said that if I was satisfied with the condition of the aircraft; then I could fly it back to ZZZ1. Some snow had been thrown up into the wheel wells during the event; so I removed the snow and inspected the condition of the landing gear hardware and attachments. I also did a visual assessment of the straightness and parallelism of the landing gear system. I found no evidence of any structural deformation. I looked for: deformation or bending of the landing gear struts and links; cracks of any kind; bending; buckling or oil-canning of the skins; [and] any evidence of deformation (such as cracked paint) at the interfaces between brackets and primary aircraft structure. I found none of these. The aircraft wing flaps were retracted during initial shutdown; per normal procedure. Some snow was apparently trapped between the left flap and wing; which compressed and re-froze. I removed as much of the snow and ice as possible; with help from one of the passengers; but we were unable to get the flaps to move from their retracted position. I also walked up the runway to see what I could learn from the runway condition. I found the groove in the snow; where the left main wheel first contacted the snow berm. It started a few feet outside of the face of the snow berm; but well within the line of the runway lights and tracked towards the runway centerline. I found no tracks of the wheels on the plowed surface of the runway; and no evidence of the airplane contacting the right side (west side) berm. It is clear that the airplane was on the runway surface during the entire event. I walked up to the runway threshold; and found that the plowed section of the runway was not centered in the space between the runway lights. I paced off the distance from the berm on each side to the innermost runway threshold light. The berm on the west side of the runway was 5 paces (about 15 feet) in from the innermost threshold light; while the berm on the east side was 10 paces (about 30 feet) in from the innermost threshold light. In addition; the berm on the east side appeared to curve toward the runway centerline as it progressed in the landing direction on Runway XX. So while landing on Runway XX; the left side berm was closer to the runway centerline than the right side berm. I was a bit off runway centerline to the left; and the left berm was off center to the right. This combination; along with the flat light conditions caused the left main wheel to contact the berm during touchdown. I returned to the ramp and called back the DO. I reviewed my observations and plans with him. I planned to take the airplane up solo and fly around the pattern; to assess its condition and operation. I then planned to fly to ZZZ2 for more fuel. I planned to not retract the landing gear; for fear of the moisture in the landing gear hardware (wet snow and water) freezing at altitude and rendering some piece of hardware inoperative. Both takeoff and landing with flaps up are normal operations for the aircraft; and the planned runways were adequate for flaps-up operations; so I elected to proceed; even though the wing flaps were inoperative. The DO concurred with my plans; and I consulted with my passengers and got their agreement; so that's what we did. We returned to ZZZ1 with no further difficulties.