Convergency and conv angles
 

Hi there. I have a question that need some explanation.

The following 3 positions are given:
A) 0238N 17832E
B) 0317N 17615W
C) 0249S 17905W

Q1 - What is ch lat B - C?
Q2 - What is d long A - B?

Q3 - . A, B and C represent islands forming the HARRY ISLANDS.
A group of tourists is chartering a plane to take them to THE WESTERN HARRY ISLAND. To which of the islands would you take them?

What is the formula for finding change of latitude (Q1) and what are they looking for in Q2? Answers should be given in degrees and minutes.

My book says Convergency = Change of Longitude x Sine Latitude, but I don´t even know where to start :-/

Change of latitude is simply the difference between the initial latitude and the final latitude. If both latitudes had be North or if both had been south we would simply subtract the initial latitude from the final latitude. But in this case one is north of the Equator and the other is south of the equator. So the change in latitude is the sum of the two (0317 + 0249) = 0606. C is south of B so the change in longitude is 6 degrees 6 minutes to the South. Ch long is usually expressed in minutes because the ground distance (or departure) between any two points of longitude at the same latitude = Ch Long (in minutes) x Cos latitude.


Dlong is the difference between the final longitude and the initial longitude. As with change in latitude, if they were both at the same side of the reference point (in this case 180EW) we would subtract the initial from the final. In this case they are at opposite sides of the reference 180EW so we normally add them together. But in this case the shortest route is going East from 17832E past the 180EW to 17615W. From 17832E to 180EW is a change of 1 degree 28 minutes to the East. And from 180EW to 17615W the change is 3 degrees 45 minutes to the East. Adding the two together gives a total change of 5 degree 13 minutes to the East.


To go from A to C we must go from 17832E past 180EW to 17905W. This is a change of 2 degrees 23 minutes to the East. So to go from C to A we must go 2 degrees 23 minutes to the West. So the most westerly of the three points is island A.

I'm really worried. I fly aircraft around the world, have been for 30!years, and I don't even understand the question, let alone the answer.:p

Its all about the 180 meridian.:cool:

Dauphin - why mention convergency?

As BOAC says,

If you understood the question you wouldn't need to fly your aircraft around the world to get from Harry A to Harry C.

- it seems to be a very good job that the FMC understands.:rolleyes:

And if candidates were required to use FMCs in the ATPL theory exams it would make the exams much more relevant.

Is this question actually, for real, on an exam for pilots? What's the point?

gf- I'm afraid it is another 'PPRune puzzle' - DD appears to be a CPL-IR yet does not know sem to know what happens to longitude at 180E/W. That is the 'point' of the question, which is a very basic one.

Makes you wonder......................perhaps they don't teach Nav any more for the CPL?

BOAC, sadly no longer do they teach the PRINCIPLES of navigation. Even if we never actually used Mercator Sailings in daily aviation, there was a reason why we were forced to master the theory. Some navies and coast guards share a similar attitude and no longer teach celestial to skippers and navigators. Such stuff is seen as unnecessary now that we have GPS even in our mobile phones.
It is interesting to watch how Gen X and Y can usually make the FMC sit up and beg, but when it behaves badly because of either finger trouble or a glitch they don't pick up errors that should be immediately obvious.


Good on the OP for his/her interest in the subject.

Indeed, Mach, but
"sadly no longer do they teach the PRINCIPLES of navigation." then why are 'they' asking?

"I'm really worried. I fly aircraft around the world, have been for 30!years, and I don't even understand the question, let alone the answer.http://images.ibsrv.net/ibsrv/res/sr...ies/tongue.gif"

After 42 years in pro flying this one had me flummoxed!

So, in an attempt to get the big picture on this, I sourced a large orange and a felt-tip pen and prepared to draw in the points and the proposed route.

And then I thought, "sod it, life's too short" and put the pen away, peeled the orange and ate it and put the peel on top of the central heating boiler to dry it off so's it would be ready for firelighting duty.

Q. Am I taking life too seriously by recycling the peel as bio-mass? :confused:

Ah! The magic of Mercator’s Sailings. Once learned never forgotten.

Those formulas forever etched into one’s brain:

DLong/DMP = Tan Track & DLat sec Track = Distance

Learn those little beauties and with a set of Norris Tables and you can solve most navigation calculations. :D:D