On December 31, 2019, about 1415 central standard time, an Enstrom F28A helicopter, N57SJ, sustained substantial damage when it nosed over during a forced landing following a partial loss of engine power. During the nose-over, the tailboom was separated and the main rotor system and fuselage were damaged. The student pilot and flight instructor were not injured. The helicopter was registered to and operated by a private individual under the provisions of Title 14 Code of federal Regulations Part 91 as an instructional flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local flight originated from the Salaika Aviation Airport (07TA), Danbury, Texas, at an unconfirmed time. The certificated flight instructor stated that he and his student had departed for a second pattern during a training mission. After the helicopter made the turn onto the crosswind leg of the traffic pattern the engine suddenly began to run rough and had lost a lot of power. He took the controls and executed an autorotation into a clearing, flared and touched down. The skids of the helicopter sunk into the soft mud, the rotor blades contacted the ground, and the helicopter rolled onto its side. The operator reported that examination of the engine after the accident revealed that a portion of the intake valve for the No. 3 cylinder had broken off and had migrated into the No. 1 cylinder where it lodged between the No. 1 intake valve and seat, holding that valve in the open position.