Hi Bob, Richard,

I am working through the navigation workbook for a PPL and am having some difficulties with getting the same answers as the workbook. As an example no 15 on page 13.
Given
TAS: 105
Wind 006/30 T
FPT 251 T
Mag Variation 6W

I start by setting the TAS on 105 and then mark the wind on 006/30
Then I set the TC marker on 251 which gives me a right cross wind of 27kt which equates to a 15 degrees crab angle. - To here all is good...

Now if I follow the CR3 manual, I have to rotate the top (green) disk to the right as the wind is from the right
I have also watched this video on youtube which says the same
however, I know this is wrong as I argue that as the wind is from the right, I need to point the nose of the airplane to the right which will increase the track degrees rather than reduce it - If I follow my argument, then I get 266 + 6 variation = 272 which is the same as your answer.

The second question is related to again what both the manual and the video teaches which is to find the crab angle every time when an adjustment is made until it stays the same ...

in this case, after I have moved to 266 degrees as explained above, and I read the angle again then it is 17 degrees (30 kt right cross wind) - if I follow the instructions from the manual and video, then I adjust the TC with 2 degrees to the right to get 268... now if I add the 6 degrees West Mag variation, I get 274 and not 272?

Am I doing something wrong or is both the CR3 manual and the above video wrong?

Appreciate your help

thanks

The video that you are watching shows how to find the TMG (Track Made Good), whereas Page 13 exercises need you to calculate the heading and ground speed. Look for "Flight Planning with Forecast winds" in the CR3 manual, or flight planing videos

Thank you Wayne ... it makes sense now.

I am still struggling to get the same answer as the workbook e.g.
Full Flight Planning, page 15, No 5

Question:
Flight Planned Track (FPT degrees True): 186
Mag Variation: 13 degrees E
Forecast Wind (degrees T/kt): 274/20
TAS (kt): 100

Find:
Heading (T)
Heading (M)
Ground Speed
ETI (mins) for Distance (nm): 168nm

I get a True Heading of 197 degrees (using the 11 degrees crab angle) - the work book has 198 which I assume is because the 12 degrees angle was used - Do I use the closest one or the highest one?

For ground speed I get 93kt calculated as 98 kt effective speed (using 11/12 degrees) less 5 kt headwind - the workbook has 97 kt

Will you please document all the steps using the CR-3 here so that I can see where I went wrong?

Thank you

It would help those of us who don't have the workbook to hand were you to post the question details ?

Hi John,

Updated

I get a True Heading of 197 degrees (using the 11 degrees crab angle) - the work book has 198 which I assume is because the 12 degrees angle was used - Do I use the closest one or the highest one?

If the figure is midway, pick the integer number which appeals to you. Otherwise, just round off to the closer. It really doesn't matter all that much as the pragmatic accuracy required is less than that level of discrimination. In this case, 197/198 are both fine
.
For ground speed I get 93kt calculated as 98 kt effective speed (using 11/12 degrees) less 5 kt headwind - the workbook has 97 kt

ETAS is around 98 so that is fine. However, the 5 kt H/W looks to be a bit suss. Perhaps you can rework that bit ? My figuring shows it to be a little less, maybe 1-2 kt ? Again, using these analogue devices, you are not going to get superlative precision.

Thanks John,

Now I am worried that my CR-3 is broken because I still get a 4/5kt headwind ...



Understand that the device will not give me 100% accuracy, but given that the exam has a very small margin for error, I am concerned that I will get it wrong if I cannot work out how to get it more accurate

When it comes to the wind calculations the E6B is more accurate than the CR computers. CASA has this worked out and uses the E6b for Commercial and have to use the Cr computers for the ATPL. There will always be a slight difference, in the real world most of cant fly to a 1 knot difference.

Now I am worried that my CR-3 is broken

There doesn't look to be anything terribly wrong with your device.

However, you have now isolated your errors for our comments. That is what we needed - to be able to see the problems on the device and provide a suitable story for you.

I note, on the scanned image of the device that you have positioned the

(a) TAS marker (ie the "10" marker on the original inner C/D scale before the scale was reworked to give the angles for sine/cosine values) is a bit off the TAS value. Pretty trivial but you do need to try to position things as accurately as you reasonably can manage to minimise calculation errors. The general scale accuracy on some of these clones is not all that impressive so we need to exercise what diligent care we can during calculation work. The reason why we need this set accurately is that it sets the scales for both the sine (drift) and cosine (ETAS) multiplications. Those sums need to be accurate if you want to end up with a reasonably good answer,

(b) and you have set 197 against TC. Why ? I know what you have done but not why, specifically. Once I know what your specific reasoning processes were, we can go through why that is not the correct number - that is, I need you to walk me through your logic process for the setting so I can give you a logical contrary story for reflection. Once we do that, and you have some time to reflect upon the story, you will have the process squared away. I have to note that I think the Jepp user guide is dreadfully convoluted and difficult to follow. It all can be explained in a far easier to understand manner.