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In aviation, aircraft compass turns are turns made in an aircraft using only a magnetic compass for guidance.
| | This article needs additional citations for verification .(March 2009) |
In aviation, aircraft compass turns are turns made in an aircraft using only a magnetic compass for guidance.
A magnetic compass aboard an aircraft displays the current magnetic heading of the aircraft, i.e., the aircraft's directional orientation relative to the Earth's geomagnetic field, which has a roughly north-south orientation. The compass can be used in turns to verify the aircraft is travelling in the desired direction at the conclusion of a turn. The nature of the instrument and the alignment of the magnetic pole of the earth cause the magnetic compass to have several significant limitations when used for navigation. A pilot aware of those limitations can use the compass effectively for navigation. The compass continues to operate despite failures in the electrical, vacuum or pitot static systems.
Compass turns (turns using the magnetic compass as the primary reference instrument) are not standard practice in modern aircraft. Compass turns are typically performed in simulated or actual failures of the directional gyro or other navigational instruments. A magnetic compass is a simple instrument when the compass is not moving and is on the earth. A magnetic compass installed in an aircraft is subject to compass turning errors during flight. Pilots must compensate for such errors when using the magnetic compass.
Most of the errors inherent in the heading indications of a magnetic compass are related to the compass' construction. An aircraft compass consists of an inverted bowl with a magnetized bar attached. The bowl is balanced on a low friction pin. The bowl and pin assembly is enclosed in a case filled with non-acidic kerosene. The magnetized bar tends to orient the assembly with the local geomagnetic field. The bar turns the visible bowl of the compass. The outside surface of the bowl includes markings to indicate a magnetic heading. As the aircraft (and the compass housing) turns, the bowl remains somewhat stationary with respect to the Earth due to the magnetic attraction. In summary, the aircraft is free to turn around the stationary bowl.
The standard practice when flying with a gyro-stabilized compass (or heading indicator) is to read the magnetic compass only while in straight and level unaccelerated flight. This reading is then used to set the gyro-stabilized compass. The gyro compass will read correctly in a turn, whereas the magnetic compass can't be read properly while turning. Thus the pilot will always ignore the magnetic compass while turning, but periodically check it in straight and level unaccelerated flight.
Several types of error will affect the heading indication provided by a magnetic compass if the aircraft is not in steady straight and level unaccelerated flight.
A limitation imposed by a compass' construction is that the balancing bowl's pin, which is connected to a pivot point, only allows, in most compasses, the bowl to tilt by approximately 18 degrees before it will touch the side of the casing. When this happens its freedom to rotate is lost and the compass becomes unreliable.
A second limitation is magnetic dip. The compass dial will tend to align itself with the geomagnetic field and dip toward the northern magnetic pole when in the northern hemisphere, or toward the southern magnetic pole when in the southern hemisphere. At the equator this error is negligible. As an aircraft flies closer to either pole the dipping error becomes more prevalent to the point that the compass can become unreliable because its pivot point has surpassed its 18 degrees of tilt. Magnetic dip is caused by the downward pull of the magnetic poles and is greatest near the poles themselves. To help negate the effect of this downwards force, the center of gravity of the compass bowl hangs below the pivot. [1] Compass navigation near the polar regions, however, is nearly impossible due to the errors caused by this effect.
When in steady straight and level flight the effect of magnetic dip is of no concern. However, when the aircraft is accelerated or turned to a new heading the following two rules apply:
First, when on an easterly or westerly heading and the aircraft accelerates, the center of gravity of the bowl lags behind the pivot, making it tilt forwards. [1] Because of magnetic dip the compass will show a false turn towards the north if in the northern hemisphere or vice versa a false turn towards the south if in the southern hemisphere. Also if the aircraft is decelerated the compass will show a false turn towards the south in the northern hemisphere and false turn towards the north in the southern hemisphere. The error is neutralized when the aircraft has reached its velocity and the magnetic compass will then read the proper heading. Pilots in the northern hemisphere remember this by the mnemonic ANDS: accelerate north, decelerate south. The opposite occurs when flying in the southern hemisphere. This error is eliminated while accelerating or decelerating on a heading of exactly North or exactly South.
Second, when on a northerly heading and a turn towards the east or west is made, the magnet causes the compass to lag behind the actual heading the aircraft is flying through. This lag will slowly diminish as the aircraft approaches either east or west and will be approximately correct when on an east or west heading. When the aircraft turns further towards South, the magnetic compass needle will tend to lead the actual heading of the aircraft. When a turn is made from south to an east or west heading the compass will lead the actual heading the aircraft is flying through, it will diminish as the aircraft approaches either east or west, and it will lag as the aircraft turns further towards North. This happens in a coordinated turn because of the bank of the aircraft and resulting bank of the compass. The North-seeking pole of the magnet is pulled towards the Earth's magnetic field in the turn. This results in an angular displacement of the compass. The magnitude of the lead/lag will be approximately equal to the aircraft's latitude. (An aircraft at 30° north latitude will need to undershoot 30° while turning directly north, and overshoot 30° while turning directly south). This guideline is based not on a standard-rate turn, but on a bank angle of 15°-18°, which would equal a standard rate turn at the airspeeds typical of light aircraft. The pilot community uses the mnemonic UNOS (undershoot North overshoot South) to memorize this rule in the Northern hemisphere. In the Southern hemisphere the mnemonic ONUS is used. Other mnemonics used in the northern hemisphere are NOSE (North Opposite, South Exaggerates), OSUN (Overshoot South, Undershoot North), and South Leads, North Lags. These are reversed in the Southern hemisphere.
Standard rate turn is a standardized rate at which the aircraft will make a 360 degree turn in two minutes (120 seconds). Standard rate turn is indicated on turn coordinator or turn-slip indicator.
All turns during flights under instrument rules shall be made at standard turn rate, but no more than 30 degrees of bank. In case of vacuum-driven instruments failure (i.e. directional gyro, attitude indicator) the rollout to new heading is timed: let's say the aircraft is flying 060 degrees heading and it needs to fly new heading 360. The turn will be 60 degrees. Since the standard rate turn is 360 degrees in 120 seconds, the plane will need 20-second standard rate turn to the left.
In case of electrical instrument failure, which include turn coordinator or turn-slip indicator, the following formula will help to determine turn bank at which the turn will be made at standard rate: In order to calculate bank angle for a standard rate turn knowledge of airspeed must be known. The rule of thumb using airspeed requires that the last digit of the airspeed be dropped then add five. For example, if the airspeed is 90 knots, the bank angle would be (9+5=) 14 degrees. For 122 knots, it would be (12+5=) 17 degrees. The line of latitude is the maximum lead or lag a compass will have.
The following explanations are for the northern hemisphere.
For example, an aircraft flying at 45°N latitude making a turn to north from east or west maintaining a standard rate turn a pilot would need to roll out of the turn when the compass was 45 degrees plus one half of the bank angle before north. (From east to north at 90 knots 0+45+7=52) A pilot would begin to roll out to straight flight and on a heading of north when 52 degrees was read from the compass. (From west to north at 90 knots (360-45-7=308). A pilot would begin to roll the aircraft out of the bank at 308 degrees read from the compass to fly on a north heading. Making a turn towards south from west the pilot would have to roll the aircraft out of the turn when the compass was 45 degrees minus half the bank angle (from west to south at 90 knots 180-45+7=142, from east to south 180+45-7=218).
From the examples we see that when turning to north from east or west the bank angle used to calculate the time to roll the plane out of the turn must begin at the greatest number of degrees or further away from north. Conversely for turns to south from east or west the bank angle is calculated to decrease the number of degrees to lead the roll out or closer to south.
Generally pilots will practice making these turns using half standard rate turns. This will decrease the bank angle so that it is half of the calculated bank angle. When turns are made at half standard rate the line of latitude will only cause the compass to have an error of half as much. So our new calculation using a half standard rate turn is as follows: (From east to north at 90 knots 0+22.5+3.5=26) the lead roll out heading read from the compass would be 26 degrees to fly on a north heading. (From west to north 360-22.5-3.5=334) The lead roll out heading read off the compass would be 334 degrees.
Turns made for other directions should be interpolated. For example, a left turn made from a heading of west to south east (SE). The compass would initially show a heading that is correct as the turn gets closer to south the compass would indicate a lead heading of the greatest error, as the aircraft passes through south the error would decrease and show less of a lead. As the aircraft approaches south east the error would only lead half as much as it did when the aircraft was rolling through south. So if the turn was made using a half standard rate at 90 knots and the SE heading required to fly was 135 degrees the roll out heading would be 135-11.25+3.5=127 degrees. Hence a roll out heading read from the compass of 127 degrees would be used to actually fly the heading of 135 degrees.
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with magnetic north. Other methods may be used, including gyroscopes, magnetometers, and GPS receivers.
Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information in flight. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon. Visual flight rules (VFR) require an airspeed indicator, an altimeter, and a compass or other suitable magnetic direction indicator. Instrument flight rules (IFR) additionally require a gyroscopic pitch-bank, direction and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, and a clock. Flight into instrument meteorological conditions (IMC) require radio navigation instruments for precise takeoffs and landings.
Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north and true north. This angle varies depending on position on the Earth's surface and changes over time.
The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometres per hour (km/h), knots (kn), miles per hour (MPH) and/or metres per second (m/s). The recommendation by ICAO is to use km/h, however knots is currently the most used unit. The ASI measures the pressure differential between static pressure from the static port, and total pressure from the pitot tube. This difference in pressure is registered with the ASI pointer on the face of the instrument.
The heading indicator (HI), also known as a directional gyro (DG) or direction indicator (DI), is a flight instrument used in an aircraft to inform the pilot of the aircraft's heading.
The attitude indicator (AI), formerly known as the gyro horizon or artificial horizon, is a flight instrument that informs the pilot of the aircraft orientation relative to Earth's horizon, and gives an immediate indication of the smallest orientation change. The miniature aircraft and horizon bar mimic the relationship of the aircraft relative to the actual horizon. It is a primary instrument for flight in instrument meteorological conditions.
The basic principles of air navigation are identical to general navigation, which includes the process of planning, recording, and controlling the movement of a craft from one place to another.
A non-directional beacon (NDB) or non-directional radio beacon is a radio beacon which does not include inherent directional information. Radio beacons are radio transmitters at a known location, used as an aviation or marine navigational aid. NDB are in contrast to directional radio beacons and other navigational aids, such as low-frequency radio range, VHF omnidirectional range (VOR) and tactical air navigation system (TACAN).
An automatic direction finder (ADF) is a marine or aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the ship or aircraft to a suitable radio station. ADF receivers are normally tuned to aviation or marine NDBs operating in the LW band between 190 – 535 kHz. Like RDF units, most ADF receivers can also receive medium wave (AM) broadcast stations, though these are less reliable for navigational purposes.
Visual flight or "Visual Attitude Flying" is a method of controlling an aircraft where the aircraft attitude is determined by observing outside visual references.
In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are:
An instrument landing system localizer, or simply localizer, is a system of horizontal guidance in the instrument landing system, which is used to guide aircraft along the axis of the runway.
Aircraft maneuvering is referenced to a standard rate turn, also known as a rate one turn (ROT).
A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments. Other instruments that might be connected are air data computers, flight data recorders, altitude encoders, cabin pressurization controllers, and various airspeed switches. Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot static system, such as altitude, is potentially safety-critical. Several commercial airline disasters have been traced to a failure of the pitot-static system.
A primary flight display or PFD is a modern aircraft instrument dedicated to flight information. Much like multi-function displays, primary flight displays are built around a Liquid-crystal display or CRT display device. Representations of older six pack or "steam gauge" instruments are combined on one compact display, simplifying pilot workflow and streamlining cockpit layouts.
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Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth's magnetic field lines. This angle varies at different points on the Earth's surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement, and negative values indicate that it is pointing upward. The dip angle is in principle the angle made by the needle of a vertically held compass, though in practice ordinary compass needles may be weighted against dip or may be unable to move freely in the correct plane. The value can be measured more reliably with a special instrument typically known as a dip circle.
The chandelle is an aircraft control maneuver where the pilot combines a 180° turn with a climb.
The aileron roll is an aerobatic maneuver in which an aircraft does a full 360° revolution about its longitudinal axis. When executed properly, there is no appreciable change in altitude and the aircraft exits the maneuver on the same heading as it entered. This is commonly one of the first maneuvers taught in basic aerobatics courses. The aileron roll is commonly confused with a barrel roll.
In aviation, the turn and slip indicator and the turn coordinator (TC) variant are essentially two aircraft flight instruments in one device. One indicates the rate of turn, or the rate of change in the aircraft's heading; the other part indicates whether the aircraft is in coordinated flight, showing the slip or skid of the turn. The slip indicator is actually an inclinometer that at rest displays the angle of the aircraft's transverse axis with respect to horizontal, and in motion displays this angle as modified by the acceleration of the aircraft. The most commonly used units are degrees per second (deg/s) or minutes per turn (min/tr).
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