Aircraft compass turns

Compass errors display error of the magnetic compass, are caused by the vertical component of the earth's magnetic field in the tilt of the compass. A distinction is made between rotational errors and acceleration errors. The compass error is particularly important for aircraft because the pilot is instructed during instrument flight on the compass.

When unaccelerated climb or descent, however, occurs not display error because the compass rose can be compensated for by their mobile storage, the aircraft pitch and thus continues to be aligned horizontally. The vertical component of the geomagnetic field can therefore exert no torque. The rotation of the magnetic compass error occurs when cornering flights. Depending on the direction and latitude of the rotational error is less pronounced. For curves that start from a northerly course from the magnetic compass shows initially a counter-rotation, which can confuse the inexperienced pilot.

The acceleration errors of the magnetic compass causes during the acceleration phase ( acceleration ) a more northerly at 10-20 ° heading is shown than is actually flown. In the deceleration ( deceleration ) of a flight, a corrupted south course is displayed.

General function of the magnetic compass

Only at the equator run the magnetic lines parallel to the earth's surface - in north-south direction ( the variation is neglected in this analysis ).

At the magnetic poles ( magnetic north pole, south magnetic pole ) run the magnetic lines vertically " in the Ice". In mid-latitudes the lines of force of the magnetic field are more or less inclined to the earth's surface. One can distinguish between a horizontal and a vertical component therefore with the magnetic lines ( geomagnetic field ). At the magnetic equator, the vertical component is zero. Of the magnetic poles of the horizontal component of the earth's magnetic field is zero.

Only the horizontal component of the earth's magnetic field causes the orientation of the compass needle of the magnetic compass to magnetic north. Therefore, the North -setting power of the magnetic compass with approximation to the magnetic north pole ( or magn. Antarctic ) is becoming weaker, as well as the horizontal force component of the geomagnetic field decreases. In the pole then fail completely ordinary magnetic compasses.

The vertical component of the geomagnetic field makes in unaccelerated straight flight not noticeable on the magnetic compass.

To better understand the causes of compass error, you teach as a thought experiment an ordinary marching compass from the horizontal to the vertical. Assuming that the south end and the north end of the magnetic needle are the same weight, the magnetic needle should remain in any desired position. The vertical force component of the magnetic field will drag the north end of the magnetic needle down.

If you look with a marching compass in hand to the northeast ( the northern tip of the needle is beautifully just above the N) and the compass in hand by 30 ° tilts to the right ( the tilt axis runs from 045 ° to 225 ° ), then the northern tip of the needle no longer beautiful show to the north, but she turns around 10-20 ° to the right. If the magnetic needle had only a north point and not even a weight balancing southern tip, the explanation for this would be simple: The weight of the northern tip of the inclined plane allows the needle to the right slide down ( turn away ). The weight ( vertical component - force of gravity ), but also acts on the south end of the needle and is therefore responsible for the rotation during Kompasskippung. The vertical component of the geomagnetic field is the cause. This force acts only on the northern tip of the magnetized compass needle. Strictly speaking, the vertical component of the geomagnetic field affects both North and South Pole on the needle - in opposite direction ( in the southern hemisphere it is the other way around ). The force thus acts as a gravitational force that acts only on the northern tip of the needle. Strictly speaking, however, the gravitational field can not be compared to the magnetic force of gravity, since the magnetic field knows attraction and repulsion, only attraction.

At a slightly tilted magnetic compass needle thus assumes a position which is on the one hand determined by the horizontal component of the earth's magnetic field ( withdraws the needle to the north ) and on the other hand, influenced by the vertical component of the geomagnetic field is ( moves the needle downwards towards the ground, center of the earth ).

Practical significance

For VFR pilots ( VFR), the rotational error and acceleration error do not have much practical significance. The necessary practical knowledge for him reduces itself to the rule that he (if any) may calibrate only during unaccelerated straight flight after his magnetic compass indicator. A badly balanced gyro moves relatively quickly and should therefore be aligned every 20 minutes to the magnetic compass. A well-tuned gyro drifts with the same rate as it corresponds to the Erdrehung, ie Central European latitudes of about 11 degrees per hour. He is like so from the Earth's rotation and then hardly needs to be adjusted in cruise flight.

Also for the instrument flyer (IFR ) of the rotational error is only important if the gyro fails. It is for the flying everyday with key sentences or other thoughts supports memorize the different directions of the rotational error. The IFR pilot must perform under instrument flight conditions (IMC ) its curves exactly. He has on a predetermined course of his curve Dredge ( plus or minus 5 ° tolerance). He must be able to maintain a straight course during the acceleration or deceleration phase.

As a rule, he used to determine the course his gyro, which he aligns regularly to the magnetic compass. In case of failure of the gyro ( usually because of failure of the vacuum system - partial panel) but he has to fly his course as accurate with the magnetic compass. A landing with zero visibility between high mountains with rain and turbulence allows the pilots did not have much time to think about the direction of the rotation error. He can then no longer afford 30 ° auszuleiten a curve too late to correct after stabilization of the magnetic compass course again by 30 °. Also, this correction will succeed in not very good without knowledge of the compass turning error.

Because of the compass rotation error can be small of course, not fly ( to 30 ° ) to the magnetic compass accurate with a stopwatch. When bank in a standard curve (rotation speed 3 ° per second) a 15 ° course change takes 5 seconds.

Rotational errors of the magnetic compass

The rotational error occurs when the compass across inclines to the earth surface and bears a price far north or south of the equator. The larger the bank and the closer the geomagnetic pole, the more does the vertical component of the geomagnetic field at the compass. In the maximum case, at a bank angle of 90 ° directly over a pole, the compass display will rotate by 90 °.

The following information applies only to middle northern latitudes:

The compass rotation error results in a plane that is flying on a northerly course and a curve on an easterly course points out, that the magnetic heading by up to 30 ° initially deflects to the west. Then the magnetic indicator swings back to back North (at this time the aircraft has already one-third of its change of course behind ) and rotates relatively quickly to the east. The magnetic display hurries after the true aircraft heading. On East Course then it has met the aircraft heading again.

After the release of a curve on an easterly course (or West Course ), the pilot must be aware not to rotational error. He can drain out the curve just after the compass display.

In a curve of East Course at North Course ( left curve ) of the rotational error is however tricky. At the beginning of the curve, the magnetic compass display is gradually becoming more behind the true heading back. All the more, the more the airplane will turn toward North Course. Upon reaching the North course, the magnetic needle is left up to 30 °. Thus, it indicates 30 °. What could cause the inexperienced pilot to continue its curve until the magnetic compass indicating exactly 360 °. But then his plane is going to fly 330 ° in reality. So he has the curve turns over and has overshot its course. After the transition to horizontal flight, the magnetic compass corrects itself after a few seconds and the pilot will notice it, and back curves 30 °. That would be an ugly and dangerous flying style.

Because of the rotational error, the pilot must his curve on a north course ( or a similar class - 330 ° - 030 ° ) ends, drain out before. Mnemonic: UNOS - undershoot north.

On a south course the thing is the other way around. In a curve of East Course South Course on the IFR pilot who flies his curve to the magnetic compass must be curve over tighten ( overshoot ) to auzuleiten on the right track: Mnemonic: UNOS - undershoot north, overshoot south.

The nil and deceleration behavior of the magnetic compass depends on rotational speed with which the course change is performed. Therefore ( and for other reasons) course corrections in IFR flight are carried out with the standard rate of rotation (2- min graph, standard turn). So also the None of the magnetic compass for experienced IFR pilots interpretable and in relatively small limits is maintained. The pilot can then balance its rotational error according to experience and feel.

Acceleration errors of the magnetic compass

The acceleration error does not occur on North Course and Südkursen. It has its maximum value at Ostkursen and West courses.

The following applies to the Northern Hemisphere:

When accelerating on a easterly course ( 090 ° ), the lower part of the compass body, so the heavier part moves below the suspension, inertia towards rear of the aircraft, that is, the pilot facing side of the compass body rotates upward, and the front ( the nose, so to speak ) down. The magnetic north needle, which has previously shown exactly to the left ( 360 °), now rotates through the vertical field of the earth right off and the compass dangles a course deviation to the left - so easy Einkurven north.

When speed reduction ( delay ) the magnetic display is distorted to the south.

The mnemonic to is: ANDS - accelerate decelerate north, south.

The acceleration error is noticeable on Ostkursen and West courses in the same way.

Contrary to an often heard and read opinion causes unaccelerated climb or descent does not display error because the compass rosette remains horizontal. It is something else, of course, if accelerated during the ascent or descent, or is delayed.

Near the equator, there are no significant vertical component of the geomagnetic field and therefore no rotational error. Since acceleration errors are due to the structural design of the compass body, they also occur at the equator on a manufactured at the northern mid- latitudes compass. Only with a compass that has no unequal distribution of mass to compensate for the vertical component of the geomagnetic field, no acceleration error would occur more at the equator.

In the southern hemisphere everything is exactly the opposite: ANDS and UNOS help then no more. You would have to be reversed to ASDN and USON.