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.