CH-47 Hover Limitations

Density Altitude Effects on Military Operations

The capability of helicopters to operate and hover diminishes as the operating altitude increases. This thinning of the air is known as density altitude. At higher altitudes, thin air reduces engine performance known as torque as well as the ability of the rotor blades to grab air and fly or hover.

Density altitude restricts the payload of the helicopter, causing a trade-off between usable payload and fuel. Atmospheric temperature also contributes to the effects of density altitude. Warmer or hot air, especially in the summer, can drastically reduce the payload and capability of helicopters operating at higher altitudes.

Hover-out of ground effect (HOGE) This is the absolute limit of the helicopter's ability to hover. Factors that contribute to this limit are density altitude, atmospheric temperature, available engine torque, and payload.

Hover-in-ground-effect (HIGE) Helicopters are able to hover anywhere from 5-80 feet above high mountain peaks because of the interaction between the ground and the helicopter's rotor blades. This means a helicopter can hover within a few feet of a mountain top but if it were to try to hover in mid air, it would not have sufficient lift.

Density Altitude Effects
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Hypothetical air maneuver corridor based on an operating limit of of 7,000 feet	Example of an air avenue of approach. Note the limitation helicopter vs fighter aircraft
Cover of CH-47D Operators Manual	CH-47D Hover chart showing the effects of density altitude on hover and payload