About flying-talking about altitude

? When you see the eagle flying high on the plain, you will feel that it is amazing that the eagle flies so high; When you see an eagle hovering on the plateau, your first feeling is how the eagle flies so low!

? There is a height problem here. An eagle flying 500 meters on the plain at an altitude of 10 m and an eagle flying 3000 meters on the plateau at an altitude of 100 m, which eagle do you think flies higher?

? The so-called height in our mind is nothing more than the vertical distance from the ground to a certain position. It's not that simple. Strictly speaking, this height is called relative height. For example, a high-rise building is 2 15 meters high, which is the relative height of 2 15 meters. If your ground is15m above sea level, then the height of this building is 215m+15m = 230m above sea level, which is an absolute height.

? So what's the altitude? Simply put, altitude is sea level. The altitude15m is the vertical distance from the sea level to the ground where you are. But the problem is coming again. How to calculate the high tide and low tide? Is the altitude constantly changing with the rise and fall of sea level? In fact, in order to unify the standards, it is necessary to have a reference point to determine the altitude. The datum point for determining altitude in China is Qingdao. All heights are calculated based on this base point. So, if the altitude of your home is15m, the vertical distance from this datum point is15m.

? In our daily life, because altitude has little connection with our life, no one will pay attention to it, but it is closely related to the flight safety of aircraft. If you don't understand the height properly, the instrument is wrong and the height is wrong, you will make a big mistake. In many plane crashes, some of them are caused by flying in mountainous areas and not controlling the height of the plane well, which leads to crashing into mountains.

? In aviation, when it comes to altitude, it must be linked to atmospheric pressure, and an important instrument installed on the plane-barometric altimeter.

? There is pressure in the atmosphere, and the air pressure decreases linearly with the increase of height. The altitude value can be converted by measuring the pressure at this altitude, which is the principle of aircraft barometric altimeter.

? The flying height of an aircraft refers to the height difference between the position of the aircraft in the air and the selected datum. Because the datum planes selected are different, the definition of height is different. For example, a plane flew over the plateau at an altitude of 3000 meters, and now it is 500 meters above the ground. For people on the ground, this plane flies quite low, but the pointer of the barometric altimeter on the plane shows 3500 meters instead of 500 meters.

? Relative height: refers to the height between the plane and the ground of the designated airport (such as the landing airport). The barometric scale of altimeter is based on the airport, which is called the scene barometric altitude. Pilots must know this altitude when taking off and landing.

? True altitude: refers to the vertical distance between the plane and the ground directly below it.

? Absolute altitude: refers to the altitude between the aircraft and the sea level, also known as the barometric altitude at sea level.

? Standard atmospheric pressure height: refers to the height between the aircraft and the standard atmospheric pressure plane. The standard pressure surface is artificially set. This surface is based on sea level, and the atmospheric pressure is1013.25pa (15℃). Because this height does not change with the influence of temperature and humidity, it is not completely consistent with the real sea level height, so the standard atmospheric pressure height is different from the absolute height. According to the regulations of ICAO, when an aircraft enters the route, it will use the standard atmospheric altitude.

? For some airports that are difficult to directly measure the altitude, the relative altitude is obtained by subtracting the altitude of the airport from the standard atmospheric pressure altitude. There are two kinds of altimeters on the plane: barometric altimeter and radio altimeter. Radio altimeter is used to measure the real height of an airplane from a certain point on the ground, especially for measuring the low height of an airplane when it takes off and lands.

? The barometric altimeter is actually a barometer, whose height is converted from the measured barometric pressure. Its key part is the vacuum diaphragm box in the instrument. The inner cavity of the vacuum membrane box is vacuum sealed, and the surface has certain elasticity. When the pressure in the table is high, the vacuum capsule is compressed, and when the pressure is low, it expands. The compression and expansion displacements are transmitted to the indicator through mechanical devices, indicating the external atmospheric pressure. If the dial is engraved with the corresponding height, the pressure gauge becomes an altimeter. The altimeter is equipped with an adjusting knob, which is used to adjust the set pressure on the vacuum diaphragm box to set the datum plane, so that the altimeter can display the heights of different reference objects.

? When the plane takes off or lands, the pilot is most concerned about the relative altitude to the airport, so the air pressure in the reading window is set as the airport air pressure (also called the field pressure, which is reported by the airport weather station). Set the airport height to zero, and the altimeter indicates the height of the plane relative to the airport when it takes off (or lands), that is, the air pressure height at the scene.

? When the flight altitude exceeds 400 meters, the plane enters the route. In order to keep the distance between planes during flight, all planes must set the altimeter at standard atmospheric pressure. At this time, the altimeters on the aircraft uniformly represent the standard atmospheric pressure altitude, so that there will be no difference in altitude indications between aircraft, thus avoiding the occurrence of collision accidents.

? When the plane flies over obstacles, it is necessary to know the absolute height of the plane. At this time, adjust the setting knob of the altimeter to the local air pressure at sea level, and the absolute height will appear on the altimeter. The absolute height minus the terrain height (shown by the navigation chart) is the true height of the plane and the terrain below.

? The barometric altimeter is simple in structure and low in price. After long-term practical application, it has become the main instrument of aircraft, but its measurement accuracy is lower than that of newly emerging air data computer and satellite navigation systems, so it has become a backup system in new aircraft. Because the barometric altimeter is directly measured, it becomes a reliable backup system when other systems fail.