Operation Procedure JAA ATPL(A) Exam

[manucordier]

Hi,

Could someone confirm the interpretation I did of this exercise:

(see annexes 071-2084A below)
The inertial navigation system (INS) indicates:

Position: 71° 55.1'N 094°55.3'W
Drift: 6°L (left)

The route followed in order to return to the VOR station is the meridian of the VOR station used:
RESOLUTE BAY (74°43.6'N 094°55.3W), this VOR has North set on the local geographic meridian.

The gyro compass has just been reset to Grid North (chart used: polar stereographic chart, grid parallel to zero meridian, Grid North in the direction of geographic North Pole).

The correct representation of the RMI VOR/DME is at this moment:
(the distance DME is in the box) http://www.cours-de-math.eu/Drift-left.jpg


---> my interpretation is that I am flying on a True North Track with wind coming from the right. I apply a drift correction angle to the right of 6°. So the arrow of my VOR RMI is pointing towards the VOR station with an angle of 6° to the left of the nose of my aircraft. My True heading is 006° and my Grid heading is 90,27 + 6 = 96,27°

Is that right ? Do you agree ?

The Answer given by "Fly Around" is picture 2 (well,... the pictures don't have numbers) (http://www.cours-de-math.eu/Drift-left.jpg)

Thank you very much for your help and explanation.
Emmanuel Cordier.

[Keith.Williams.]

I’m not sure where you got the “90,27” from.


You are due south of the VOR so to fly to it you must fly 000 True.

You are on the 094°55.3W meridian so adding this convergence to your True course will give a course of 094°55.3 Grid.

But you need to compensate for 6 degrees left drift so adding 6 degrees gives grid heading of 094°55.3W + 6 = 100°55.3.

Rounding these up to the nearest degree give Heading 101 Grid and the VOR is 6 degrees to your left which is 095 Grid.

[downwind24]

The answer the CAA want is the diagram on the top right. It is however totally beyond me how or why this question and polar stereographics turn up in Operational Procedures :hmm:

[Keith.Williams.]

You are right in saying that the correct answer is the top right diagram.

The heading required to fly to the VOR is 101 Grid (which is 006 True).

And the VOR is 6 degrees to the left of the nose of the aircraft (on heading 095 Grid which is 000 True).

But the VOR is aligned with the local meridian (True North), so the VOR needle shows 000 True (instead of 095 Grid which it would show had it been aligned to Grid North).

The trick in this question is to notice that the gyro and compass card are aligned to Grid North, but the VOR is aligned to True North. This will tempt some candidates to select the bottom left diagram.

[manucordier]

Thanks to you both Kieth and Dowind24, this helps a lot !

A few more questions to make sure I get it right...

* 1.
Keith: to make it 100% correct, the meridian convergence shouldn' t be computed this way :
C = 94°55,3' * sin(mean latitude)
= (94+55,3/60) * sin([74,73°+71,92°]/2) = 90,2°

So the grid course to follow to reach the VOR should be 90,2° Grid.
Adding 6° drift correction angle to the right and my grid heading would be 96,2°.
The VOR RMI would still mark 000° with its arrow positioned with a relative bearing of 96,2° to the left.


*2.
If the instrument used was an ADF, it would have been quite different, right ?
I still would have taken a heading 6° to the right of the true course to correct for cross wind and the arrow of the ADF would have marked a 6° relative bearing to the left. Thus the bottom left picture in the annexe (http://www.cours-de-math.eu/Drift-left.jpg) would have been the correct one. Is that correct ?

Again thanks a lot to you both for helping us so well.

Have a good day,
Emmanuel Cordier.

[Keith.Williams.]

To calculate the Earth convergence we would use sin mean latitude.

But to calculate Grid Direction we need to use chart convergence.

On a Polar Stereographic this is:

Longitude with sign reversed in the Northern Hemisphere.

Longitude in the Southern Hemisphere.

The numbers 95 and 101 in the diagrams show that the examiner has used this method.

Had he/she used sin latitude then the numbers would be 90 and 96.

You are correct in saying that the correct diagram fo an ADF would be the bottom left one. I suspect that the examiner included it to confuse the candidates. It looks much more like the real situation.

[manucordier]

Ok it's all clear now.

Thanks a lot.

Emmanuel.