Ground studies question

[neil_1821]

I have 2 exams left to take, these being both meterology and general navigation, i seem to be getting through the questions ok but i seem to stumble on a few, and they seem to be of the same type, so if anyone could help me with either one or both of the questions i'd be grateful.

Q) Given: FL120, OAT is ISA standard, CAS is 200 kt, Track is 222°(M), Heading is 215°(M), Variation is 15°W. Time to fly 105 NM is 21 MIN. What is the W/V?
A) 050/70kt

Q) Given: Compass Heading 090°, Deviation 2°W, Variation 12°E, TAS 160 kt. Whilst maintaining a radial 070° from a VOR station, the aircraft flies a ground distance of 14 NM in 6 MIN. What is the W/V °(T)?
A) 160/50kt

I can get close to the answers but no-where near close enough to hazard an educated guess:ugh:not that i want to be able to guess that answer, it would be nice to know how to work it out

[GaryS]

1.

CAS = TAS (200)

TRACK (T) = 207

HEADING (T) = 200

DRIFT = 7 STARBOARD

G/S = 105/21*60 = 300

CRP-5

Set the middle dot on TAS (200)

Set true heading at the top (200)

Locate the ground speed arc and mark a dot 7 degrees starboard

Rotate the disk so that the dot is directly below the middle dot

Under true heading is the wind speed direction

And to find the speed count the boxes between the two dots

I may be wrong :ugh:

[wbryce]

My CRP 5 is in the car so I can't test the numbers out for you!
But essentially you want to use the altitude and OAT temperture to convert your CAS into TAS. You then have a distance and time to work out your groundspeed.

Now for the CRP 5 bit, convert the figures all to true, as your final answer (wind direction) is true. Now going from memory, set your true heading and put the blue dot on your calculated TAS, from your calculated heading/track you should have worked out your drift and whether this is port or starboard. Go out horizontally to your drift line and go up/down to your calculated GS line and mark a dot and rotate the CRP window until the marked dot is below your CRP DOT which should still be set on your TAS. The heading window should give you wind direction and the gap between your marked dot and the CRP dot is your wind speed.

[BobC]

Neil
Gary's reply to your first question has the correct methodology except for his calculation of TAS. CAS = TAS only in ISA conditions at sea level. For this question you have to work out the ISA temperature at 12,000 ft (-9C) and then calculate TAS in the normal way. I get 240 KTAS, and using this I get a W/V of 052/70

Similar method for your second question: heading (T) = 090 - 2 = 088(M) + 12 = 100(T); track (T) = 070(M) + 12 = 082; so drift = 18 left; GS = 14 NM in 6 min = 140 kt. Put all that on the CRP as in the first question and the W/V comes to 160/52.

[GaryS]

Sorry if I confused you, i'm still studying :ok:

[neil_1821]

thanks for the replies, i'll give it a go tomorrow on a fresh brain

:ok:

[NavPilot]

GaryS's 2 things in your method need correction,
1. Make sure you get the TAS though, using CAS, PA and the OAT.
2. The center dot will be on GS, not TAS. If you've got TR on the Index, it'll always be GS in the center dot. TAS-HDG, GS-TR
I get 240 KTAS, and using this I get a W/V of 052/70

Ditto..

For Q 2, similar, just that the TAS is already provided.
I get 159deg at 50kts for that one.

Off HDG/TAS, TR/GS and W/V, you need any 4 values to get the remaining 2. I trained myself using different combinations of those 4 given parameters to calculate the other 2.

[neil_1821]

Navpilot

Are you saying that with TAS placed with a HDG it will give you your GS and TR? And visa versa?

If so that seems a good trick to try and learn :ok:

[NavPilot]

Affirm.
I remember it quite simply as...
1. TAS/HDG - What's happening inside your cockpit. The HDG is what you maintain to counter the drift caused by the wind.TAS is basically your ASI indication corrected for xyz...
2. TR/GS - What's actually happening on the ground. TR is the line you're describing on the ground AFTER the wind effect. GS is the speed you're actually making over the ground.Remember the stuff about a tailwind comp adding to your groundspeed, while a headwind comp slowing you down, or increasing your time to destination..?

Are you saying that with TAS placed with a HDG it will give you your GS and TR? And visa versa?
that would be provided you're given W/V. When you have to find W/V, it goes the same way, only this time use the two sets of friends- TR/GS and HDG/TAS, to get the winds affecting all the mayhem !

:ok:

[BobC]

BEWARE - BEWARE. Potential confusion between "wind up" and "wind down" methods. Using the "wind down" as described by Gary (and used in most, if not all, JAA ATPL instruction I believe), the centre dot always represents TAS.

[Nozwaldo]

Neil,
Did the calculations last night with CRP-5 and completely agree with the answers given by BobC but did manage to get 050/70 for the first. Are you having difficulty with the CRP-5, or are you not getting to grips calculating true headings & track from the information given? You could alternatively try drawing the vectors with an appropriate scale for speed and use a protractor to draw the drift angle, just remember to use the same reference for 360 degrees when measuring the wind angle. Once you have drawn the vectors for TAS/HDG, TRK/GS (from the same origin) just join the ends of both to give the wind vector which blows you from HDG to TRK. It's a bit long winded but may help you visualise the situation when doing it on the CRP-5, I wouldn't try it in the exams though. I also use the wind down method on the CRP-5 with TAS under the dot.

Noz