Escape From Frame

CONTENTS

INCLINATION:

Earth behaves like a big magnet. A magnetic needle, freely suspended in its center of gravity, points to the magnetic north pole with his north.

The force that affects the magnetic needle is called the directing force or intensity. On the Northern Hemisphere the magnetic needle will be closer to the magnetic north pole than to the magnetic south pole. Consequently, the gravitation force of the magnetic north pole will be larger than the one of the magnetic south pole. A freely suspended magnetic needle will therefore be more or less aimed downwards, see figure1.

The angle made by the magnetic needle with the horizontal surface is called INCLINATION.

The directing force can be disintegrated into a horizontal component H and a vertical component V.

By suspending the magnetic needle above its center of gravity, it will be nearly positioned horizontally, as if the south pole of the magnetic needle has become heavier, see figure 2.

Consequently, the needle's center of gravity will move a little to the south of the turning point.

THE MAGNETIC COMPASS:

In private airplanes the magnetic compass is usually a spherical compass, which takes its name from the somewhat spherical glass. The compass consists of a compass binnacle, in which the magnetic needles are attached to a floater, which floats in the compass liquid. A circle graduated with the courses surrounds this floater. Everything is elastically suspended in the compass binnacle.

The circle is called the compass rose, which can be seen through a window. Behind this window, a sail strip has been fixed. 

As the pilot is sitting behind the compass, the courses on the compass rose are put oppositely vis-à-vis the directions  of the compass rose, see figure 3.

As such, the pilot can see the right course indication. As the magnetic needle always points to the north, the plane will actually turn around the compass rose when it changes course.

However, the magnetic compass is liable to some mistakes. One of these mistakes is the so-called deviation, caused by the plane's magnetic mush influence. In order to limit these disturbances as much as possible, a micro-adjuster is put above the compass. This micro-adjuster should reduce the deviation on all courses to a minimum.

This adjustment should be considered as a deliberate inclusion of magnetic mush forces to remove the plane's mush forces. As such, it does not adjust the compass, but the plane itself.

The micro-adjuster is based on the principle that two magnets put on top of each other - the north pole on top of the south pole - neutralizes one another, see figure 4.

Once the magnets are removed from each other, a magnetic field is created, on which the N-S direction is at right angles to the original axis. By putting the two pairs of magnets across each other, a magnetic field of any direction can be created and the deviation can be reduced to zero for every course.

However, the stand of the micro-adjuster before the zero deviation differs from course to course, so that an average deviation must be accepted. These deviations are listed in the deviation table. 

THE COMPASS MUST BE RE-ADJUSTED: 

• when the plane has undergone thorough reparations;

• when the compass has been changed;

• after a stroke of lightning or electric discharges near the plane;

• e.g. when the plane has flown through a thunderstorm;

• when there is doubt about the correctness of the given deviations.

THE ACCELERATION MISTAKE:

We already noticed that the center of gravity of the magnetic needles is always a little to the south of the center of rotation.

When we fly an eastern or western course and the pace is raised, the center of gravity will lag behind vis-à-vis the point of suspension, as a result of the slowness.

In case of a eastern course, the compass rose will turn a little to the right during an acceleration and show a course reduction, see figure 5.

 In case of a western course, the compass rose will turn a little to the left during an acceleration and show a course increase during the acceleration.

In case of a speed reduction, the center of gravity will try to maintain its speed, which will result in the opposite effect. (Z= Center of gravity)

NORTHERN ROTATION FAULT:

The northern rotation fault: this means that the compass rose, influenced by the vertical component of the terrestrial magnetism, will not continue to point to the north, but turn in the direction of the curve in case of a northern course.

Imagine that the plane flies a magnetic course to the north and starts to turn to the right. Then the compass rose will also turn to the right, see figure 6.

The vertical intensity can now be resolved into a component at right angles to the plane of the magnetic needle (Vv) and a component in the plane of the magnetic needle (Vh). The latter will make a turn of 90° with the horizontal intensity H, once the airplane has started to turn, so that the needle will turn to the direction of the resultant of H and Vh, i.e. in the direction of the turn.

After the turn has been taken, the compass will lag. Then the magnetic needle will turn to the magnetic north again.

In case of a southern course, the opposite effect will take place when taking a turn and this turn will be over-clearly indicated.

On the Southern Hemisphere, the vertical intensity will work in the opposite direction, which will result in the opposite effect.

RATE ONE-TURN:

In case of a turning speed of 3° per second (2 minutes bend or rate One-turn), the following rules are to be observed:

• turning to a course north, the turn will be ended when the compass indicates 30° less than north ("UNDERSHOOT");

• turning to a course south, the turn will be ended when the compass indicates 30° past south ("OVERSHOOT");

• turning to a course east or west, the turn will be ended when the compass indicates the desired course.

• turning to a course between east or west and north or south, an "undershoot", or "overshoot" between 30° and 0° is used, depending on whether the desired course is closer or less close to the north or south.

General rule of thumb: >>>"NEVER SEE THE NORTH".

Remark:

When making steep turn, the magnetic needle system may cant so much that the rose touches the compass binnacle, so that the compass blocks.