The very nature of helicopters, especially in EMS operations, requires maneuvering in close proximity to objects. Large solid objects like walls and fences can create very turbulent air by interfering with a normal rotor downwash pattern. In a hover near the ground, the air being accelerated down hits the ground and travels away from the helicopter. When this air encounters a large solid object it slows down and gets redirected disrupting the air flow through the rotor system Typically, the side close to the object sees more disturbed air creating a lift imbalance felt by the pilot as pitch and roll instability. Depending on the location of the object relative to the wind (which can hurt or help the situation) the instability can be mild to severe. Turbulent air that gets near the tail rotor can cause yaw instability as well. The constantly changing air flow creates fluctuating lift from the rotor requiring power changes to maintain a hover. For a helicopter that is heavy and operating at a high density altitude, insufficient power available could cause a hard landing.
However, a bigger danger is actually striking the object with the rotor system. According to the NTSB, on Aug. 31, 2002, the pilot in command of a Sikorsky S-76 taking off from a hospital helipad became distracted by a large section of torn awning that was flapping in the wind due to rotor wash. The main rotors struck the top corner of the building and the helicopter began to settle. The pilot lowered the collective and navigated between the building and the parking garage to the street below, applying full collective pitch to help cushion the hard landing. The pilot and two medical crewmembers received minor injuries.
That same year the pilot and flight nurse of a BK117 were not so lucky when their helicopter struck the hospital building at night during gusty wind conditions. The paramedic was the only survivor and said that when the helicopter was about 20 ft. above the helipad, he was programming the GPS receiver and felt a sudden gust of wind push the helicopter from behind. He was not alerted to anything unusual until he looked up and noticed the helicopter’s close proximity to a 16-floor brick building that extended above the height of the helipad by four floors. The paramedic yelled, “Building, building, building!” to alert the pilot, who then made a rapid right cyclic input to avoid hitting the building. But the helicopter struck the building and fell about 13 floors to ground level.
Operating in a confined area during gusty wind conditions requires enhanced concentration especially during operations at high density altitudes.