On April 24, 2018, about 1000 mountain daylight time, a Cessna T303 airplane, N9746C, impacted terrain during an emergency off-airport landing after encountering severe icing conditions near Pine Ridge, South Dakota. The commercial pilot and two passengers sustained minor injuries, and one passenger was not injured. The airplane sustained substantial damage to the wings and fuselage. The airplane was registered to and operated by Aberdeen Flying Service, Aberdeen, South Dakota, as a Title 14 Code of Federal regulations Part 135 ondemand air taxi flight. Day instrument meteorological conditions (IMC) prevailed, and an instrument flight rules flight plan was filed for the flight. The flight departed Aberdeen Regional Airport (ABR), Aberdeen, about 0930 central daylight time, and was destined for the Pine Ridge Airport (IEN), Pine Ridge, South Dakota. Prior to the flight, the pilot obtained a weather briefing via the company computer system and reviewed the weather information with the company chief pilot. The pilot stated that based on the computer briefing, which did not include icing conditions, he was aware of the forecasted weather conditions along the route of flight and at the intended destination. After takeoff and during the climb to 12,000 ft mean sea level (msl), the airplane encountered light rime ice, and the pilot activated the de-ice equipment with no issues noted with the equipment. After crossing over the Pierre Regional Airport (PIR), Pierre, South Dakota, the pilot heard via the radio reports of better weather at a lower altitude, and the pilot requested a descent to between 5,000 and 6,000 ft. During a descent to 6,000 ft msl, the airplane encountered light to moderate icing conditions. Prior to the descent, the airplane was clear of ice accumulation. The pilot thought about turning back to PIR but could not get clearance until the airplane was closer to IEN due to poor radio coverage. Shortly thereafter, the pilot stated it felt "like a sheet of ice fell on us" as the airplane encountered severe icing conditions. The pilot applied full engine power in an attempt to maintain altitude. The airplane exited the overcast cloud layer about 500 ft above ground level (agl). The pilot decided to execute an off-airport emergency landing because the airplane could not maintain altitude. The airplane touched down in a field about 25 miles from IEN. During the emergency landing, the landing gear separated, and the airplane came to rest upright.The pilot reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation, and the airplane was below its maximum gross weight. A review of photograph images obtained by the operator confirmed the airplane retained structural icing after the landing. At 0852, the IEN Automated Surface Observation System (ASOS), located 19 miles westsouthwest of the accident site, reported wind from 350° at 15 knots gusting to 22 knots, 5 miles visibility in unknown precipitation and mist, ceiling overcast at 1,200 ft agl, temperature 2° C, dew point 1° C, altimeter 30.43 inches of mercury (Hg). At 0952, IEN ASOS reported wind from 360° at 11 knots gusting to 20 knots, 6 miles visibility in unknown precipitation and mist, ceiling overcast at 1,300 ft agl, temperature 2° C, dew point 1° C, altimeter 30.45 inches of Hg. The National Weather Service Aviation Weather Center Current Icing Products, which were available online for the preflight briefing, reported the probability of icing at 5,000 ft, 7,000 ft, and 9,000 ft, as follows: the probability indicated a greater than 75% probability of icing over South Dakota from below 5,000 ft through 9,000 ft. In addition, the icing intensity was depicted as light to moderate intensity, with a high threat of Supercooled Large Droplets at 5,000 ft and 7,000 ft over the region. The preflight weather briefing did not include any inflight weather advisories, which would have alerted the pilot of moderate icing conditions expected over the flight route in the form of airmen's meteorological information (AIRMET) Zulu that was issued at 0700 and valid or 0900. AIRMET Zulu included moderate ice between 5,000 ft and flight level 180 with conditions continuing beyond 0900. The preflight weather briefing did include a pilot report which indicated light rime icing conditions encountered by a commercial airplane climbing through IMC conditions between 3,500 ft and 10,000 ft. There was no current significant meteorological information (SIGMET) to prohibit the flight from operating at the time of the accident. According to the Federal Aviation Administration's Instrument Flying Handbook FAA-H8083-15B; Chapter 10 on page 10-24: "The very nature of flight in IMC means operating in visible moisture such as clouds. At the right temperatures, this moisture can freeze on the aircraft, causing increased weight, degraded performance, and unpredictable aerodynamic characteristics. Understanding avoidance and early recognition followed by prompt action are the keys to avoiding this potentially hazardous situation … Structural icing is a condition that can only get worse."