Deviation (navigation)

Eastern deviation

Western deviation

The deviation or deflection ( English deviation , DEV ) is an addition to the magnetic declination to be observed deflection of a magnetic compass , with the interference fields in the vicinity of the measuring point connected. It is also called vehicle magnetism after its most important cause .

definition

The deviation (Latin for deviation ) corresponds to the angle difference between the direction in which a faultless magnetic needle points, which is only influenced by the magnetic field of the earth (MN), and the direction in which a magnetic needle rotates, which is both from the earth's magnetic field and is also influenced by the magnetic field of the aircraft or ship.

causes

The proximity of objects containing cobalt, nickel and iron (more precisely: ferromagnetic ) objects ( bunch of keys, metal glasses, bicycles, cars, steel ships, trains) changes the direction of the magnetic field lines on which the magnetic compass needle aligns. Therefore, compasses that are used on vehicles with ferromagnetic materials are inevitably subject to their disturbing influences. If these are not distributed evenly around the compass by chance, a systematic error in the direction measurement is the result. Also circuits on aircraft and ships generate magnetic fields, especially when direct current flows through the wires.

In addition, there are cyclical, course- dependent effects whose influence on the compass changes with the direction in which the vehicle “looks”.

A vehicle, be it an airplane, a vehicle or a ship, can take on the most varied of directions with regard to the earth's magnetic force, whereby the direction and strength of the vehicle's own magnetic field changes. Therefore the size and direction of the deviation of the compass on different courses assumes different values, which are mainly repeated with the periods 360 °, 180 ° and 90 °. They are therefore called full, semicircular and quarter-circle and can be filtered out of careful, overdetermined measurements.

Discovery of the deviation

The changes in the magnetic field due to deviation follow certain laws that were first discovered by Matthew Flinders . Soon after, Siméon Denis Poisson presented the deviation in mathematical form.

Decrease in deviation

Deviation beacons for compass compensation

One possible measure against the deviation is to mount a master compass in a location that is better suited than the bridge or the cockpit . Otherwise or beyond that, the deviation can be reduced by additionally attached and usually adjustable, small magnets or iron masses (magnetic compensation ). The remaining deviation is recorded in a so-called deviation table , from which the necessary correction can be found for each course direction.

Deviation in practice

If a device is installed or dismantled in the cockpit of an aircraft, this can have an influence on the deviation of the compass needle. Therefore, the deviation must be redetermined after significant changes to the aircraft. This happens in steps of 30 °. The results of the deviation determination are recorded in a deviation table, which is hung up near the compass so that the pilot can easily see it.

Special situation with ships

There is a special situation with ships made of steel. The entire (huge) iron hull of the ship becomes a magnet due to the earth's magnetic force, the axis of which is in the direction of the magnetic meridian . Part of this total magnetism is permanently absorbed by the shock of the hammering of the ship lying on top of the stack. This is known as the ship's permanent magnetism . The axis of the permanent magnetic part of the total magnetism also remains unchanged later if the ship adopts a different position to the magnetic meridian.

Another part, in particular the magnetism present in the softer iron masses, always changes its direction of induction with the change in the meridian direction . This part of the total magnetism is called volatile magnetism . Here, however, the change in direction does not always take place instantaneously, but only gradually. For this, Robert Koldewey introduced the term remanent magnetism .

Navigational treatment

The deviation ( DEV ) is referred to as the location misappropriation ( VAR ) with EAST (+) or WEST (-).

If compass north ( English compass north , CN ) lies east of magnetic north ( English magnetic north , MN ), then it is an eastern deviation ( English east deviation ).

An easterly deviation must always be deducted from the magnetic heading ( mwK , English magnetic heading , MH ) in order to obtain the compass heading ( English compass heading , CH ). Exactly the opposite is the case with western deviation. This must be added to the MH. The result is the compass course ( CH ) that can also be found on the (sea) map .

Similar to the variation, the following applies to the deviation:

" West is best (+!), East is least (-!) "

As already mentioned, an easterly deviation is positive, since a clockwise angle is defined as positive. A western deviation is correspondingly negative. Above saying can be formulated mathematically: . If the deviation east it is deducted from the ill-looking heading (magnetic heading) . ${\ displaystyle - (DEV)}$ ${\ displaystyle - (+ 5) = - 5}$

Is it west they must be added: . ${\ displaystyle - (- 5) = + 5}$

The following donkey bridge has been used in navigation for this purpose:

“ From the wrong to the right course with the right sign,
from the right to the wrong course with the wrong sign! "

Sample calculation

${\ displaystyle {\ begin {aligned} MH-DEV & = CH \\ 10 - (+ 1) & = 9 ^ {\ circ} \ end {aligned}}}$

literature

Peter Dogan: The Instrument Flight Training Manual 3rd, revised edition. Aviation Book, Seattle, WA 2004, ISBN 0-916413-26-8 . (English)
Ernst Götsch: Aircraft technology. Introduction, basics, aircraft science. 4th edition. Motorbuch, Stuttgart 2008, ISBN 978-3-613-02006-1 .
Wolfgang Kühr: The private pilot. Volume 3, technology 2 . Revised edition. Luftfahrtverlag Schiffmann, Bergisch Gladbach 1981, ISBN 3-921270-09-X .
Jürgen Mies: Radio navigation . In: private pilot library . Volume 3. Motorbuch, Stuttgart 1999, ISBN 3-613-01648-6 .
George H. Reid: The Quick and Easy Guide to Compass Correction. Sheridan House, Dobbs Ferry, NY 1997, ISBN 1-57409-023-2 . (English)
Jeppesen Sanderson: Private Pilot FAA Practical Test Study Guide. Englewood, CO 2000, ISBN 0-88487-265-3 . (English)
Jeppesen Sanderson: Private Pilot Manual. Englewood, CO 2001, ISBN 0-88487-238-6 . (English)
Walter-Air: CVFR textbook. Mariensiel 2001.