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.
Most helicopters hover within "ground effect". This is defined as a height above ground equivalent to the rotor diameter, that is, if the span of the tip of one rotor to the other is 100 feet, then the helicopter is capable of hovering in ground effect up to 100 feet. The importance of ground effect is that up to that limit, the air is physically compressed beneath the helicopter, between the helicopter and the ground, and a cushion of the air is created. At altitudes above ground in excess of one rotor diameter the ground effect is lost, with the result is that much more power must be provided by the rotors and more fuel burned as the cushion dissipates, forcing the helicopter to hover, by brute force. This also means there is an altitude limit to hovering, even with ground effect.
The highest helicopter hovering capability, under the most unusual circumstances, has been at altitudes of approximately 15,000 to 18,000 feet. Translational flight, or normal forward flight without hovering, is limited to about 30,000 feet, but rarely do helicopters operate at such altitudes.
In addition, during hover flight, at low altitude, a visual obscuration may occur when flying over ground material that can be blown up into the rotors. This causes a "white out" with snow, or a "brown out" with dust and sand. The result is that the helicopter is engulfed in the cloud of a suspended particulate material and the pilot will lose outside reference and horizon cues. Most helicopters are equipped with radar altimeters that permit them to determine their above-ground height, even at one or two feet above ground level, as well as transitional motion. It is important to avoid sideways motion while landing, as a phenomenon known as dynamic roll over may occur: When drifting sideways, sometimes due to a cross wind, a helicopter may touch down with a wheel while moving sideways. This torque will cause the entire helicopter to rotate around its longitudinal axis until the rotor strikes the ground, and the vehicle rolls up in a ball. Maintaining height above ground and ground position is extremely important and crew coordination can help prevent this, particularly when flying over poor reference surfaces, such as water.
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