On September 29, 2016, about 0600 central daylight time, N361SF, a Bell 407 helicopter, impacted terrain following a loss of control while attempting to land at the Comanche County Memorial Hospital Heliport (18OK), Lawton, Oklahoma. The pilot and 2 crew members had minor injuries. One crew member was seriously injured, and the helicopter was substantially damaged. The helicopter was owned and operated by Survival Flight under the provisions of 14 Code of Federal Regulations Part 135 as a positioning flight. Night visual meteorological conditions prevailed for the flight which operated on a company flight plan.The pilot reported that he approached the helipad from the southwest. It was his first landing to this helipad but had departed from the helipad on the day prior. Due to trees and transmission lines within 40-50 ft of the elevated helipad, the pilot flew a slight right-turning, steep approach. When the helicopter was approximately 125 ft above the pad and 150 ft to the southwest, the pilot commanded left cyclic to stop the right turn. He estimated the helicopter was below 40 knots, but above effective transitional life, with wind off the nose of the helicopter or slightly left, and a stable 500-ft per minute descent. The helicopter did not respond to his control input and the pilot announced his intension to the crew to go-around. He increased left cyclic until it was against his left leg and the helicopter still did not respond. The pilot lost control of the helicopter and it landed hard colliding with a wall. An inspector from the Federal Aviation Administration (FAA) examined the airframe with the assistance of a technical representative from Bell Helicopter. No preimpact anomalies were discovered with the wreckage. The engine control unit (ECU) was removed from the helicopter and sent to Triumph in West Hartford, Connecticut. With oversight from an FAA inspector, data from the unit was downloaded. The data extracted was consistent with the engine producing the required power and responding to collective control inputs. Exceedance information captured by the ECU recorded an exceedance of main rotor speed (Nr) and torque (Q). The unit recorded 10 lines of data with this exceedance which contained information consistent with the accident sequence. Prior to the accident there were 2 spikes in engine parameters. Without changes in collective inputs, demands of flight control inputs could impact a spike on engine demand. On the NTSB Form 6120, the pilot stated that the helicopter was loaded with 3 crew members on the right side of the helicopter, and a near full fuel load. Up to the accident landing, the helicopter had flown for 6 hours including 6 approaches and night landings at other hospitals without incident.