Length of a nautical mile and compass swirl

[DanishPilot]

(This is also posted on the OATS forum)

. .I just can't figure these two out:

OATS GNAV 061 page 1-20:

The length of a nautical mile is longest at the poles, and shortest at equator, because the Earth being oblate spheroid. I just can't see why it isn't OPPOSITE ! Someone please enligthen me. I figure that the same angle from the center of the Earth to either the pole or the equator must mean that the distance covered should be related to the radius of the earth. Since the equator is further away than the pole, why isn't the nautical mile then LONGER at EQUATOR ?

. .OATS INS 022 page 9-15 and 9-16

I am not happy with the use of "turning through north/south". It is confusing, because how does liquid swirl exactly behave ? Let me give a questions to claify the problem:

On the Northern hemisphere, consider two scenarios:

1) Turning from 345 to 355 degrees

2) Turning from 005 to 015 degrees

Answers:

a) Liquid swirl will increase turning error in both 1) and 2)

b) Liquid swirl will increase turning error in 1) and decrease turning error in 2)

c) Liquid swirl will decrease turning error in 1) and increase turning error in 2)

d) Liquid swirl will decrease turning error in both 1) and 2)

. .I say a) is correct, others say b) is correct. With the phase "turning through north", what is meant ? Because in scenario 2) we are actually "turning south", but we are neither turning through North nor South.

Which one is correct, and why ?

. .LGB

[RVR800]

OATS GNAV 061 page 1-20:

The length of a nautical mile is longest at the poles, and shortest at equator, because the Earth being oblate spheroid. I just can't see why it isn't OPPOSITE ! Someone please enligthen me. I figure that the same angle from the center of the Earth to either the pole or the equator must mean that the distance covered should be related to the radius of the earth. Since the equator is further away than the pole, why isn't the nautical mile then LONGER at EQUATOR ?

A nautical mile is one minute of one degree of a great circle. In an oblate spheroid aren't the poles flattened with the radius distance less . .important?

Polar_stereographic your good on these projections arent you!

[Polar_stereographic]

I actualy chose the name as during the ATPL's, it was one of the most pointless charts you had to know about. Why? Ok, I know, it's a hoop you jump through, I agree, but why not make them relevant.

PS

[RVR800]

It is confusing, because how does liquid swirl exactly behave ?

Liquid Swirl always causes the liquid to rotate. .in the direction you are turning

Turn right liquid swirl right etc..

Liquid swirl is directly related to direction of turn - nothing to do with N or S

Polar - not planning on getting a job with Antarctic Survey then!

[ 01 February 2002: Message edited by: RVR800 ]</p>

[Polar_stereographic]

RVR800

Nope, a job like that's as rare as the proverbial rocking horse *****.

Leave the wife and two kids for that - no way.

PS

[Paul Hickley]

This question has now been answered on the "Ground School" section of the <a href="http://www.oxfordaviation.net/forum" target="_blank">@SK OXFORD FORUMS</a>.

Click on the above link or go to <a href="http://www.oxfordaviation.net/forum." target="_blank">http://www.oxfordaviation.net/forum.</a>

All the best,

Paul

[ 01 February 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]

[ 01 February 2002: Message edited by: Paul Hickley, Gen Nav Spec, Oxford ]</p>

[redbar1]

Just for the record: one nautical mile is one average minute of arc along the earth's meridian (=6.080 feet). .One minute of arc along the equator (=6.087 feet)is called a geographical mile.

We all love these minute details, don't we?

[Aim High]

I have a lovely booklet by W. Myerscough Air Navigation Simply Explained priced at 6d printed in 1942 by Pitman and Sons from which I quote:

Geographical Mile. We can now refer to the effect of the spheroidal shape of the earth on distance. ... an angle of 1' at the earth's centre, formed by two radii of the Equator forms an arc known as the geographical mile, 6087 ft. (The geographical mile is not used in navigation.). .Nautical mile. An angle of 1' at the earth's centre, formed by two radii forms an arc of the meridian known as a nautical mile, and is a variable quantity, being least at the Equator and greatest at the poles. In the vicinity of the English Channel the value of the nautical mile is 6080 ft., and this is the standard adopted in navigation. The error introduced through the use of 6080 ft. for the nautical mile is negligible.

[Gedifroggy]

Hi, This is a very interesting topic!!! What is the official JAR definition of a nautical mile ? Because as far as I know, the French Navy definition of a nautical mile is slightly different from the British Navy one's. The result must be a difference of few centimeters (Pardon, inches). Same kind of question as the Angel sex?. .JAR <img src="smile.gif" border="0"> <img src="wink.gif" border="0"> <img src="smile.gif" border="0">

[Chuck Ellsworth]

Fish:

Gesuus you wouldn't want all these new guys to learn something useful would you? Like oh, lets see,, how to fly?? hmmmm, how to use logic when solving problems?? you know little insignifigant things like that.

I am afraid you just do not understand how important it is to build the bureaucracy so morons can have a place to work.

Don't you understand that the more students that fail the exams the more work it creates.

Unfortunately like you I can't understand it either. Especially when you have to retrain these poor unfortunates in the cockpit so they can actually fly the airplane and make logical decisions. The frightening part is it is getting worse, not better.

................

. . :) The hardest thing about flying is knowing when to say no. :)