Compass error
Compass errors are display errors in the magnetic compass that are caused by the vertical component of the earth's magnetic field when the compass is tilted . A distinction is made between rotation errors and acceleration errors. The compass error is particularly important for aircraft, as the pilot relies on the compass for instrument flight.
On the other hand, there is no display error during unaccelerated climb or descent, since the compass rose can compensate for the aircraft inclination due to its movable mounting and is therefore still aligned horizontally. The vertical component of the earth's magnetic field cannot exert any torque. The rotation error of the magnetic compass occurs when turning. Depending on the direction of the compass and the latitude, the turning error is differently pronounced. In the case of turns starting from a north course, the magnetic compass will initially show a counter-rotation which may confuse the inexperienced pilot.
The acceleration error of the magnetic compass has the effect that during the acceleration phase a course 10–20 ° more northerly than is actually flown is displayed. In the deceleration phase of a flight, a wrong course to the south is displayed.
General function of the magnetic compass
Only at the equator do the magnetic lines run parallel to the earth's surface - in a north-south direction (the variation is neglected in this consideration).
At the magnetic poles (magnetic north pole, magnetic south pole) the magnetic lines run vertically "into the ice". In middle latitudes the lines of force of the magnetic field run more or less inclined to the earth's surface. One can thus differentiate between a horizontal and a vertical component of the magnetic lines (earth's magnetic field). At the magnetic equator, the vertical component is zero. At the magnetic poles, the horizontal component of the earth's magnetic field is zero.
Only the horizontal component of the earth's magnetic field causes the alignment of the compass needle of the magnetic compass to the magnetic north pole. Therefore, the north-pointing force of the magnetic compass becomes weaker and weaker as the magnetic north pole (or magnetic south pole) approaches, as the horizontal force component of the earth's magnetic field also decreases. Ordinary magnetic compasses then fail completely near the pole.
The vertical component of the earth's magnetic field is not noticeable on the magnetic compass in unaccelerated straight flight.
In order to better understand the causes of the compass errors, use a thought experiment to point an ordinary marching compass from the horizontal to the vertical. Assuming that the south end and the north end of the magnetic needle are equally heavy, the magnetic needle would have to remain in any position. However, the vertical force component of the magnetic field will pull the north end of the magnetic needle down.
If you look to the northeast with a marching compass in hand (the northern tip of the magnetic needle is nicely above the N) and tilt the compass in your hand by 30 ° to the right (the tilting axis runs from 045 ° to 225 °), then the The northern tip of the magnetic needle no longer points nicely to the north, but rotates 10–20 ° to the right. If the magnetic needle only had a north point and not also a south point to compensate for the weight, the explanation for this would be simple: The weight of the north point on the inclined plane causes the needle to slide down to the right (turn away). The weight (vertical component - gravitational force) also acts on the south end of the needle and is therefore not responsible for the rotation during the tilting of the compass. The vertical component of the earth's magnetic field is the cause. This force only acts on the magnetized north point of the compass needle. Strictly speaking, the vertical component of the earth's magnetic field acts on both the north and south poles of the needle - in opposite directions (in the southern hemisphere it is the other way around). The force thus acts like a gravitational force that only acts on the northern tip of the needle. Strictly speaking, one cannot compare the magnetic force with the gravitational force, since the magnetic field knows attraction and repulsion, but the gravitational field only attraction.
With a slightly tilted magnetic compass, the needle assumes a position that is determined on the one hand by the horizontal component of the earth's magnetic field (pulls the needle to the north) and on the other hand is influenced by the vertical component of the earth's magnetic field (pulls the needle down, towards the ground, to the center of the earth).
Practical meaning
For the visual pilot ( VFR ), the turning errors and acceleration errors are of little practical importance. For him, the necessary practical knowledge is reduced to the rule that he may only calibrate his course gyro (if available) according to the magnetic compass during unaccelerated straight flight. A badly balanced course top moves relatively quickly and should therefore be aligned with the magnetic compass every 20 minutes. A well-adjusted course top drifts at the same rate as it corresponds to the rotation of the earth, i.e. at around 11 degrees per hour in Central European latitudes. It compensates for the rotation of the earth and then hardly needs to be readjusted when cruising.
For the instrument pilot ( IFR ), too , the turning error only becomes important when the heading top fails. He will memorize the different directions of the turning errors for everyday flying with mnemonics or other aids to thinking. The IFR pilot must perform his turns precisely under instrument flight conditions ( IMC ). He has to take his turn on a predetermined course (plus minus 5 ° tolerance). He must be able to keep a straight course even during the acceleration or deceleration phase.
As a rule, he uses his gyro to determine the course, which he regularly aligns with the magnetic compass. If the course gyro fails (mostly due to failure of the vacuum system - partial panel), however, it must be able to fly its course just as precisely with the magnetic compass. A landing approach with zero visibility between high mountains with rain and turbulence does not leave the pilot much time to think about the direction of the turning error. He can then no longer afford to take a curve 30 ° too late and correct the course by 30 ° again after the magnetic compass has stabilized. Even this correction will not be particularly successful without knowledge of the compass rotation error.
Because of the compass rotation error, small course corrections (up to 30 °) can be made more precisely with the stopwatch instead of the magnetic compass. If you bank into a standard curve (turning speed 3 ° per second), a 15 ° course change takes 5 seconds.
Rotation error of the magnetic compass
The rotation error occurs when the compass tilts across the surface of the earth and a north or south course is applied far away from the equator. The greater the bank and the closer the earth's magnetic pole, the stronger the vertical component of the earth's magnetic field acts on the compass. In the maximum case, with a bank angle of 90 ° directly above a pole, the compass display rotates by 90 °.
The following statements only apply to mid-northern latitudes:
The compass rotation error in an aircraft that is heading north and making a turn on an east course causes the magnetic course to initially deflect up to 30 ° to the west. Then the magnetic display swings back to north (at this point the aircraft has already made a third of its course change behind it) and turns relatively quickly to the east. The magnetic display hurries after the real aircraft course. On an easterly course, it then adhered to the aircraft course again.
When exiting a curve on an east course (or west course) the pilot does not have to pay attention to the turning error. He can derive the curve exactly according to the compass display.
With a curve from east to north (left turn), however, the turning error becomes insidious. At the beginning of the curve, the magnetic compass display gradually lags behind the true course. The more the aircraft turns northbound. When reaching the north course, the magnetic needle remains up to 30 ° behind. So it shows 30 °. Which could cause the inexperienced pilot to continue his turn until the magnetic compass reads exactly 360 °. But then his plane is actually already flying 330 °. So he over-turned the curve and overshot his course. After the transition to level flight, the magnetic compass corrects itself after a few seconds and the pilot will notice his mistake and turn back 30 °. That would be an ugly and dangerous flying style.
Because of the turning error, the pilot has to take his turn, which ends on a north course (or a similar course - 330 ° - 030 °), beforehand (undershoot).
On a south course, things are exactly the other way around. In the case of a curve from east to south, the IFR pilot, who flies according to the magnetic compass, has to overshoot his curve in order to be on the correct course: Mention: UNOS - undershoot north, overshoot south.
The false display and deceleration behavior of the magnetic compass depends on the speed at which the course change is carried out. Therefore (and for other reasons) course corrections in IFR flight are carried out at the standard rate of turn (2-minute curve, standard turn). In this way, even the false reading of the magnetic compass can be interpreted by the experienced IFR pilot and is kept within relatively small limits. The pilot can then compensate for his turning error based on experience and feeling.
Acceleration error of the magnetic compass
The acceleration error does not occur on north and south courses. It has its maximum value on east courses and west courses.
The following applies to the northern hemisphere :
When accelerating on an easterly course (090 °), the lower part of the compass body, i.e. the heavier part below the suspension, moves towards the rear of the aircraft due to inertia, i.e. the side of the compass body facing the pilot rotates upwards, and the front (the Nose, so to speak) down. The magnetic north needle, which previously pointed exactly to the left (360 °), now rotates to the right through the vertical earth field and the compass display pretends a course deviation to the left - i.e. a slight curve to the north.
When the speed is reduced (deceleration), the magnetic display is falsified to the south.
The motto is: ANDS - accelerate north, decelerate south.
The acceleration error is noticeable in the same way on east courses and west courses.
Contrary to an opinion often heard and read, an unaccelerated climb or descent does not cause a display error, as the compass rosette remains horizontal. It is of course something else when accelerating or decelerating during the climb or descent.
In the vicinity of the equator there is no significant vertical component of the earth's magnetic field and therefore no rotation errors. Since acceleration errors are caused by the construction of the compass body, they also occur at the equator in a compass manufactured for the mid-northern latitudes. Only with a compass that does not have an uneven mass distribution to compensate for the vertical component of the earth's magnetic field would no acceleration error occur at the equator either.
In the southern hemisphere everything is exactly the opposite: ANDS and UNOS will no longer help. They would have to be reversed to ASDN and USON.