![]() "gyro": the instrument functions by way of gyroscopes."directional": the instrument serves to see the direction of the plane.The name of this instrument describes the 2 essential parts of its characteristics: Heading bug on North, the red knob can rotate the bug across the compass rose. In in the given image the correction is set to 29.92" Hg.ġ" Hg = 1 one inch of mercury = 33.85 millibar = 33.85 hectoPascal The small scale inside the indicator is the correction indicator that can be changed with the knob. The tall arm rotates every 1000 feet, the smaller arm rotates every 10000 feet. The typical altimeter has two arms, like a clock. Depending on the way the altimeter is corrected it can display three different types of altitude. During use the reading can be corrected (with a knob) for the change in air pressure caused by the weather giving a true reading. However the altimeter must be calibrated during installation. This change of shape is measured and expressed in feet. The higher the altitude, the lesser the barometric pressure, the bigger the drum. An airtight drum (the aneroid) changes shape under influence of the surrounding air pressure. The measurement is done with an aneroid barometer. To get exact measurements, make sure that the aircraft does not change its speed.Īn altimeter is an instrument used to measure the altitude of an object above a fixed level. This makes the compass to show more westwards when accelerating and eastwards when decelerating. When changing speed of the aircraft, the compass is subject to acceleration forces. So here is a quick and dirty shell script for retrieving the magnetic variation at a specific coordinate (at sea level and date of invocation) by shamelessly abusing the testmagvar utility from the SimGear source code: Good area charts are not always easy to get (for free). The magnetic variation for a specific spot can be retreieved from the area charts or from other maps. (This is why the instrument settings of the directinal gyro in the menu is "-15" initially, when starting at KSFO. The magnetic variation at FlightGears default airport, KSFO, is about 15° east. In FlightGear, easterly is a positive number, and westerly is negative. ![]() An easterly magnetic variation means that the magnetic north points a bit to the east of the true north. Magnetic variations are expressed in "degrees east" or "degrees west". (The latest version also supports magnetic bearings) The Kelpie flight planner for FlightGear also uses true bearings. And that's why you need to know the magnetic variation: for calculating the wind triangle how much you must hold up against the wind (your heading) when flying to the destination (your course). Wind directions from METAR however, reports winds with true direction. Almost everything in aeronautical navigation is based on magnetic bearings. Even GPS's designed for air navigation uses magnetic bearings. Runway numbering is based on magnetic bearings. Air navigation is generally done using magnetic bearings, and the VOR beacons also use that. When planning a flight it is necessary to know the magnetic variation along the flight path. Not only does the magnetic variation vary with position on the planet, it also slowly varies over time since the magnetic poles are not stationary. The magnetic variation is expressed as an angle east or west of true north. The difference between the magnetic field and true north is called "magnetic variation" or "magnetic declination". Note that The magnetic north pole and the geographic north pole are not located at the same spot. As such, it is subject to various influences that affects the direction in which it points. The compass aligns itself along the magnetic field that surrounds the earth.
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