Astronomic precession

[manucordier]

Hello,

I found a question on the JAA ATPL(A) Operation Procedure about Astronomic precession. I suppose they are talking about Earth axis precession (with a period of about 26000 years).

The question is :
Astronomic precession:

Answer: causes an apparent spin of heading gyro to the right in the southern hemisphere.

Well I've tried to think it and to see it many different ways and I just don't get it.
Does someone has a schema or an explanation for that ?

Thank you very much,
Emmanul Cordier.

[Dick Whittingham]

Earth rotation

Dick

[Keith.Williams.]

As Dick has stated, this question concerns Earth Rate drift. If you are not familiar with it, a brief summary is provided below.

Because of its property of rigidity, a spinning gyroscope remains aligned with a fixed point in space. If a gyroscope is placed upon the earth it will initially be aligned with both a point in space and a point on the earth. This might for example be the North Pole. As the earth rotates it carries the gyroscope with it. The gyroscope will remain aligned with the same point in space but this will no longer be aligned with the same point on the earth. This phenomenon is called earth rate error or apparent drift.

The magnitude of earth rate drift is proportional to the latitude

In the northern hemispheres this drift is to the right. In the southern hemisphere this drift is to the left.

To visualise this, imagine that you are looking down on a gyro that is lying horizontally at the North Pole. The Earth spins anti-clockwise to the East while the gyro spin axis remains unchanged. So relative to the Earth, the gyro spin axis appears to be rotating clockwise to the right.

The JAR CQB has two very similar questions on this subject. For one the correct anser is "It causes the gyro to spin to the left in the southern hemisphere". In the second the correct answer is "It causes the gyro to spin to the right in the northern hemisphere".

[manucordier]

Ok thanks.

However it seems to me (but of course I am not en expert) that it is badly named because the term "astronomic precession" recalls much more the earth axis precession ( Astronomy: precession of earth (http://astro.wsu.edu/worthey/astro/html/lec-precession.html) ) than the "apparent wander" (or "earth rate drift", I believe both terms are synonyms, right ?).

One more question, do you agree Kieth with that one:

If we consider an aircraft flying from a point A to a point B:

Travel precession is synonym for transport precession = (LongitudeB - LongitudeA) * sin(mean lattitude)

and

Earth rate drift is synonym for apparent wander = (15°/h)*sin(mean lattitude)

Is that right ?

If what I've said is right, then the following is false :
(from FlyAround JAR Bank)

Posit:

g, the longitude difference
Lm, the average latitude
Lo, the latitude of the tangent

The correct formula expressing the travel precession applied during a transoceanic and polar navigation, is equal to:

Right answer (according to FlyAround): 15°/h sin(Lm)

Do you agree Kieth ?

Thanks again.

Emmanuel.

[Keith.Williams.]

I would certainly agree that the question is badly worded. This probably stems from langauge differences between the various JAR states. A question originally drafted in Portugese, then translated into english by a frenchman, is likely to exhibit a few problems?

Your use of the following expression;

Travel precession is synonym for transport precession = (LongitudeB - LongitudeA) * sin(mean lattitude)

is a good example of langauge differences. The equation is OK but I have never heard the term "Travel pecession" before. Although the use of the term is quite logical. Most UK Texts would use the term "Transport Wander".

Your use of the term:

Earth rate drift is synonym for apparent wander = (15°/h)*sin(mean lattitude)

is also unusual. Again the equation is OK, but most UK texts would group Transport Wander and Earth Rate Wander as two parts of Apparent Wander.

If we consider there to be two types of wander:

REAL WANDER
Which as its name indicates is real. This is caused by manufacturing imperfections in the gyro.

APPARENT WANDER
This is not real, but appears to take place because of rotation of the Earth and movement of the gyro across the surface of the Earth.

The only argument that I can offer in support of the quoted answer to the final question is that if we take "transpolar" to mean up one meridian and over the pole to the anti-meridian without any gradual change of longitude, then only Earth rate will occur as a gradual process throughout the trip. There will be no Transport wander while the longitude isn't changing, but there will of course be an instantaneous change as we go over the pole.

[manucordier]

Ok thanks Kieth.

Have a good weekend.

Emmanuel.