2015-09-15 Yesterday afternoon, Chongqing General Aviation Co., Ltd. B-7702 Bell 407GX helicopter crashed during the Wanzhou-Liangping aerial mission, and all four crew members were killed. In recent years, there have been many helicopter safety accidents in China. In the face of blood lessons, we should pay more attention to safety. In this small series, I also talked about the relevant precautions when helicopters are flying in the mountains.
Main factors affecting helicopter mountain flight
The factors affecting the flight of helicopters in the mountains are, in general, flight altitude, mountainous terrain, mountain airflow and meteorological conditions.
The impact of flight altitude on helicopter mountain flight is mainly reflected in the following aspects:
1) As the flying height increases, the air density gradually decreases, and the atmospheric pressure also decreases, resulting in poor performance and maneuverability of the helicopter.
2) The maximum flight speed of a helicopter will also decrease as the flight altitude increases, and its surface speed will vary with changes in air density and temperature.
3) The wind speed will increase as the altitude increases. Below the friction layer, the wind direction changes greatly; above the friction layer, the wind changes more regularly with height. Attention should be paid to the change of airflow during flight. Part C of this section focuses on mountain airflow.
4) As the altitude of the flight increases, the atmospheric temperature gradually decreases, and after a certain height, the risk of helicopter freezing increases, especially when flying near the cloud and the humidity.
1) When the mountain area is flying, the external reference objects are similar and difficult to identify, and the landing site is more complicated. To ensure flight safety, external observations should be strengthened.
2) It is difficult to select a landing site in a mountainous area because the landing site is usually on a lateral or longitudinal hillside, which is neither flat nor irregular compared to the heliport. Dynamic tumbling and ground resonance are likely to occur during the take-off and landing of such a site.
Research data shows that the influence of airflow is easy to put the helicopter into a dangerous state. At this time, the pilot mistakenly thinks that it is the cause of manipulation, and there is often a reverse correction, which increases the danger. Therefore, understanding the characteristics of mountain airflow is extremely beneficial to ensure safe flight in mountainous areas.
1) The airflow layer near the surface is highly susceptible to changes in wind and terrain. When the airflow passes through the obstacle, some of it is disturbed to produce moving air vortices and turbulence.
2) A Venus effect occurs in the vicinity of a shrinking valley or mountain pass. Its performance is to accelerate the gas flow, so that the static pressure of the air is reduced, resulting in a large index of the altimeter.
3) The windward side of the obstacle is in a relatively calm updraft zone, the leeward side is in the downdraft zone, and there may be turbulent and drum-like airflow. The stronger the wind, the stronger the turbulence and the wider the spread.
4) The airflow passes through a single mountain range. When the slope is regular and gentle, the upward airflow is relatively gentle; when the airflow passes through the mountaintop, it becomes a downward airflow; when the airflow passes through a mountain with steep mountains and mountains, the flow of the airflow is basically unchanged. But in the concave land will produce a strong turbulence.
5) The airflow moves in the terrain of the mountains. When the angle between the wind and the ridge is near right angle, the steeper the mountain and the higher the wind speed, the turbulent flow is not only strong, but also the thickness and width are correspondingly increased, which often produces a series of rollers. Airflow.
6) The contrast of airflow changes between the areas with sunshine and the areas without sunshine in the mountainous areas is very obvious. The greater the difference in surface properties on adjacent areas, the greater the disturbance airflow generated; the intensity of the low-level air disturbance airflow is obvious. The daily change, the updraft in the afternoon reaches the maximum intensity.
7) The airflow situation in mountainous areas is relatively complicated, with many influencing factors, rapid changes and difficult to predict. The turbulence, strong downdrafts and wind shear caused by changes in mountain airflow have become the main factors that threaten flight safety.
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July 19, 2024