AOPA Hover Power

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.

The recent media coverage of the damaged military helicopter left at the compound where Osama bin laden was killed has peaked questions about what causes a hard landing. Although, I have witnessed (and performed myself) some landings that were pretty rough, I consider a hard landing as impacting the ground hard enough to cause damage or require an inspection. Due to the sensitive nature of military operations just what caused the hard landing in Pakistan might never be known for sure, but here are a few of the common causes.

In flight training a common cause of hard landings is poor timing at the bottom (the part near the ground) of a practice autorotation. If the autorotation is intended to touch down or power is brought back in too late, the flare height and pitch pull timing are very important. If performed incorrectly, the helicopter can touch down with enough force to spread the skids, cause the main rotor blades to contact the tail boom or roll over. Most of the time these are preventable and no one is hurt (egos maybe). In a real autorotation, options might be very limited and damaging the helicopter while sustaining no injuries would be considered a successful autorotation.

Exceeding the helicopter’s performance capabilities or limitations has caused hard landings. High density altitude, high gross weight or severe winds are the most common situations. Sometimes confused with simply running out of available power is the vortex ring state. The blade tip vortices strengthen when a helicopter descends vertically into a column of air that is already moving downward. The larger vortices consume power and produce a large drop in rotor efficiency resulting in the pilot’s inability to arrest the descent rate. One of the maneuvers prone to encountering this condition is steep high-power approaches. In Pakistan, it was reported that the temperature was higher than expected and I am sure the helicopter was heavy which could have made for critical power management in a very high stress situation.

Harding landings have been caused by striking an object with the tail rotor in a hover. Several EMS accidents have occurred when the pilot hit a sign, light post or other object while hovering at an off airport scene. Night operations increase the risk of not seeing potential threats. One interesting observation regarding the raid in Pakistan was that the tail rotor assembly was outside the compound wall. Was it blown there from the explosion meant to destroy the helicopter or did it hit the wall on approach causing the hard landing? We may never know.

Regardless of how the hard landing in Pakistan occurred, our military performed a highly successful mission for our country. For that I am thankful and very proud.