AARRRGGHHHH NAV Computer

[PompeyPaul]

I was taught this yesterday and was happily calculating headings in order to get the track I want. I've gone back to it tonight and can't remember how to do it. Can anybody refresh my memory ?

1. I select the back of the computer
2. I line the dial onto the direction of wind (220 degrees)
3. I draw a line downwards to the extent of the wind (30 kts)
4. I move the card in the back to indicate TAS (100 kts)
5. I then select my required heading (170 degrees)
6. I look at the distance off track, (16 degrees)

I can then remember I need to have 2 things match up, the distance off track and something else.

When they both line up I read the heading off of the top of the computer and the speed which is read from the location off of the cross off of card vertical line.

From the above example I wrote down 184 degrees and 76 kt. If I set this again I can see that yes 184 degrees reads of 76kts but I can't remember the 2 things I need to line up. One of them is the degrees to the left or right of the end of the wind line, but I can't remember what the 2 things that must match up are.

Any help appreciated - I am really pulling my hair out in frustration. I had this all nailed yesterday.

As an extra bit of info, I remember it was itterative. I had to check distance of track, check it lined up, then move it over again, check the distance of track. I just can't remember what the 2 things I had to compare was

Please save my sanity!

[neilia]

Hi Paul,
Don't worry, you'll get the hang of it eventually... though it might be a messy journey of tears and tantrums... ;)
Steps 1-6, all good.
Next step is to "balance the drift". What you're basically trying to achieve is to have the drift line that your wind dot appears on reading the the same as the drift lined up with your desired track round the top of the wheel.
Really easy way to do this... Follow steps 1-6. Then slide the card to get your wind dot at the top of the graph paper, and draw a line vertically downwards from your wind dot. Now slide back up to where you were with the central dot over 100kts TAS. Now turn the wheel until the line you've just drawn parallels the drift lines. You should end up with the wind dot showing 13 deg port drift. Look up at the top and you'll see your desired track of 170 magically lined up against 13 deg port drift, giving you a heading to fly of about 184 deg.
Not sure if I explained that too clearly, but hope it helps!

[Mark 1]

Draw the wind dot UP
Set the track index to your desired track
Slide the wind dot over your TAS arc
Read GS over centre dot
Read heading correction under wind dot, apply to track to get hdg.

Wind dot down method requires iteration, is less accurate and takes longer to do.

[tangovictor]

after the exams, do people still use this machine ?

[neilia]

Well, yup, you still gotta work out what heading to fly to account for the wind. I'm sure a lot of people use rule of thumb a lot of the time, but I had whizz wheel so thoroughly hammered into me throughout my PPL that I'm quite happy using it. Having said that, the Aussie one I use in real life is round, compact and altogether more convenient than these CRP things.

[Flash0710]

Not used one since....

xxxxxxx

luv

f

[BigEndBob]

Mark 1, nice to see someone suggesting the proper method!
...and the one slagged off by the Pooleys manual supplied with the CRP-5?
is it the 'professional' one ...thats less accurate than its cheaper version.

[foxmoth]

Draw the wind dot UP
I would suggest that if PompeyPaul has been shown wind down then all you are doing is adding to the confusion, this is one of those debates that you will get 50/50 and goes on forever!
Using wind down, my own way of remembering this is -
Wind down from center
You then want everything in the following place:-
TAS under the center dot.
Heading at the top
GS under the wind arrow
Track against drift at the top
Drift the same amount and on the same side at the top and under the wind arrow
Using this you can find any two unknowns if you have the rest.
i.e. in the normal problem (find hdg/gs) you do not know the heading so you cannot put it at the top, you therefore put in an approximate heading (most people start with the track) and 'balance' the drift to give you the correct heading & gs.
So, just remember where everything should go, then put everything in where it should be as much as you can then you can find anything else you need, even with a bit of fiddling.

[CPilotUK]

Hi Paul,

I note from a previous post of yours, you are using study manuals by Jeremy Pratt.

Rather than confuse yourself with the different methods that others have so kindly suggested, have a look at the chapter: Navigation Principles: The Triangle of Velocities. This should be around page 29 of the Navigation section. The methods and explanations, coupled with examples and wonderful illustrations, make the topic very easy to understand.

[PompeyPaul]

I was wrestling with it this morning and I can now remember what the last step was. Foxmoth was correct.

7. Rotate the wheel so that the TRUE heading lies underneath the top of the white wheel (INDEX)
8. Rotate the disc so that the amount of drift (i.e. the horizontal distance of the end point of the wind) matches the amount of drift at the TOP of the white circle (the true heading is underneath the part labelled drift). When those 2 matches (i.e. 20 or 30 or whatever it is) you then read off the heading from underneath INDEX.

Rather than try any new approaches I managed to get to the one I was shown and it's all ops normal again.

I was trying to read the Jeremy Pratt book last night but it may of well as been written in Chinese!

Thanks again

[ST-EX]

Hi Paul,
I agree that the flight computer can be a bit of a nightmare. I was taught to use the wind down method, but always got confused-I found that the wind up was easier for me(I use the CRaP-5, not that it makes any real difference)! Try this:
Find magnetic heading and groundspeed given the following conditions:
Magnetic course 095°
True airspeed 98 kt.
Magnetic wind 150° at 20 kt.
Rotate the plotting transparency so the magnetic wind direction of 150° is opposite the True Index.
Move the sliding card so the grommet is over a convenient speed arc, such as 100.
Using the speed arcs, place a dot 20 kt. up from the grommet. This represents the wind speed, also called a wind dot. If the grommet is on the 100-kt. speed arc, the dot should be placed on the centerline at the 120-kt. speed arc.
Rotate the plotting transparency so the magnetic course of 095° is opposite the True Index.
Move the sliding card so the wind dot is on the 98-kt. speed arc, which is the TAS.
The WCA is determined by the wind dot and the wind correction lines. In this example, the wind dot is 10° to the right of the center.
+R
Magnetic heading (MH) = MC WCA.
-L
In this problem, MH = 105° (95 + 10).
Groundspeed is read under the grommet; it is 85 kt. in this example.
Under the given conditions, the magnetic heading is 105° and the groundspeed is 85 kt.
Hope this helps!