Isochrone
 

Does anyone know the answer to this ATPL question and why?

When a course is plotted at minimum time route, one passes from the air isochrone to the corresponding ground isochrone by applying to point K (original spot) a vector KK' which is equal to:

a. wind at K.

b. wind at K'.

c. mean wind from the preceding ground isochrone.

d. mean wind up to the next ground isochrone.

This is part of the Operational Procedures syllabus and refers to a technique now thankfully lost in the mists of time.

ISO means 'the same as' or 'equal', CHRONOS means 'time' so an ISOCHRONE is a line of equal time.

The simplest way to illustrate this is with a series of diagrams, for simplicity I've put the expanation in a PDF file here

.... and I think the answer is (d), though you could argue you pass not from the air isochrone but from the preceding ground isochrone to the origin for the next isochrone by moving from K to K'.

Can you believe this rubbish?

While I was doing my groundschool, the OP instructor said this had come up a number of times and his advice was "Too confusing and irrelevant - just remember, wind at K"!

This is at odds with Alex's answer but luckily it didn't come up for me anyway ;)

Hope it doesn't for anyone else.....

Oh well, maybe they'll never let me in to SODCAT after all. I'll trawl through the textbooks and get confirmation.

Oh dear, my feedback questions say the answer is (a) Wind at K :rolleyes:

Well they can't all be right. The reference would be the old RAF Manual of Air Navigation.

Probably something last used by Sir Francis Drake, more like!

What utter nonsense JAR continues to be concerned with....

PS- Doesn't Basil know?

I will be tempted to query this one if it comes up.

I was once told that if I didn't remember being taught how to answer a question, to answer (b). The theory being that if most of the examinees answered the same option wrongly, the mark would be credited to all.

This may well be true but if answer (b) is obviously complete nonsense then I doubt that anyone would pick it.

J_D

I have the aforementioned Nav manual, once I have translated it from 1960's RAF into some sort of English I will try and give you an answer, could be some time! I can prob narrow it down to c or d, and most prob d although they appeared to be back plotted from destination, so that could be a spanner in the proverbial.

D - I reckon.

You are trying to convert what happens in the air to what happens on the ground - this is done by adding/subtracting the effect of wind.

Mean wind is always used and, as you are looking ahead, surely it is the mean wind up to the next isochrone.

Well, that's the "applying logic" approach, anyway!

BTW - although I pilot, I reckon that having a grade B 'O' level pass in Air Navigation permits me to have Honorary Life Membership of SODCAT.

I put a request for help on the Military Pilots forum here.

The two problems we have encountered are that the RAF navigators used to work out minimum time routing by flying pressure levels, which we don't do, and they also back plotted the route from destination. The technique we are describing seems to be a simplified version and it's difficult to get an answer other than by applying logic, as moggie has done. The consensus seems to be answer (d).

I think this question originated in France. Ian Seager, from Flyer, is going to make some enquiries there. If we get anything new I'll get back to you.

Reliable informed by a reliable nav (using diagrams and short words) that it is, in fact, answer a) - wind at K.

Thanks moggie, is that his opinion or does he have a reference?

Alex, thanks for all your help, the Operational Procedures ATPL exam was yesterday for us in Florida and the question didn't come up.

PS. DJ at Atlantic still reckons it's the wind at K!

Alex - he did look it up but I can't remember where.