Confused over met question

[Pianorak]

I am working through the PPL Confuser's Meteorology questions and am stuck over Q98.
The correct answer is said to be B, ie 35518kt which doesn't make sense to me. The Confuser's explanation is: 340 + 010 + 360 = 710 divided by 2 = 355. Why does the 360 to be added in this instance? :confused:
Any explanation would be most gratefully received.

[Evo]

OK, looking at the Confuser... :)

at 50N, 0230W the wind at 2000ft is 340/10kts. At 50N 0230E the wind at the same altitude is 010/25kts. You need to know the wind at 50N 0W, but nothing is given on the Form 214.

The best you can do is interpolate (i.e. average) the two values. Average wind is (10+25)/2 = 17.5 kts (rounds to 18 kts). As for the heading, there are 30 degrees between 340 and 010, so you take half the difference which is 340 + 15 (or 010 - 15 if you prefer) which is 355. Hence answer B, wind of 355/18kts. Simple, I hope.

Confuser answer just overcomplicates it. :rolleyes:

[Pianorak]

Thanks Evo –

I fail to see the logic. If you interpolate the wind speed, ie 10 plus 25 divided by 2 surely you should do the same with the wind direction, ie 340 plus 10 divided by 2 which is 175. And the Confuser did precisely that dealing with Q87 – hence my confusion. Thanks anyway.

[bookworm]

And so emerge the pitfalls of taking arithmetic averages of vectors... I've long had a bee in my bonnet about this one.

Pilots are taught (and the CAA writes exam questions that supports the teaching) that if you want to average two wind velocities you take the arithmetic mean of the speeds and the arithmetic means of the directions.

For similar wind velocities, this is a pretty good approximation.

Mean of 240@10 and 250@12 is pretty close to 245@11.

However, once you get to less similar velocities, you start running into big difficulties. If the velocities 'span' 360, you're bound to be in trouble.

Mean of 340@10 and 020@10 is not 180@10.

You can get round that by adding an arbitrary 360 degrees (when? -- well when it looks right of course! :)). But it's not just about that discontinuity.

Try

Mean of 020@30 and 160@30. It is not 090@30. It's 090@10.

Why? Well when you take averages of vectors you have to add them up as vectors, and that means putting the arrows end to end. In this case, the way to do it is to break the vectors into components. The first has a northerly component of almost 30 knots and an easterly component of 10. The second has a southerly component of almost 30 knots and an easterly component of 10. The northerly and southerly components cancel and we're left with an average of 10 knots easterly.

Does it matter? Well I reckon that if I'm on a long easterly flight, I'd like to get the tailwind right within 20 knots, wouldn't you? :)

[Evo]

Pianorak

Forget the maths for a bit. Think about what makes sense. If the wind at one point is from the north west, and at the second point from the north east which is more likely at a point in between - wind from the north ... or the south?

[Speedbird252]

Evo hits the nail on the head, this aint rocket science, interpolation should suffice, im not in any way disputing what bookworm has explained, but it seems bleedin` complicated way of arriving at a mean for two wind velocities.

Speedy(with correct heading):ok:

[Pianorak]

Evo and Bookworm - Many thanks taking the trouble of dealing in detail with my question. You see my problem: I seem to approach such tasks with too mechanistic a mind. Must try and muster a bit more common sense.

[Airbedane]

I've just read this thread. Although Bookworm is correct when he says adding vectors 020/30 to 160/30 does not produce xxx/30, vector addition does not apply to the question asked. Interpolation is what's required, and interpolating between 020/30 and 160/30 will always give xxx/30.

Pianorak: it would appear you have no problem with interpolating the speed, just direction. For the direction, draw a diagram and estimate the direction of the answer. Then do the maths. All the methods described above should work, but if they give an answer too far away from your estimate, then you know you've done something wrong. For interest, this technique is called a 'gross error check', and if you haven't used it already, it should come in handy in your navigaiton training - estimate the expected heading, then compare the estimation with the computed answer - if the results are different, find out why.

All the Best with your exams,
A

[Evo]

I think that vector addition is just confusing the issue - keep to linear interpolation, but apply common sense and understand that if the values on Form 214 differ significantly then your estimate may be invalid.

Bookworm's example of


Mean of 020@30 and 160@30. It is not 090@30. It's 090@10.


is strictly correct but a bit misleading. Say you're flying from Bournemouth to Manston. They're both reporting 30kts wind. Do you really expect Shoreham to be at 10kts? It might be, but with the vector addition example you're assuming that the only influences on the wind at Shoreham are the winds at Bournemouth and Manston. With linear interpolation you're assuming the wind direction and magnitude varies smoothly along your route. The important thing though is that they're both making assumptions to estimate the wind. Neither method is telling you the actual wind - you could always phone Shoreham... ;)

[bookworm]

Evo, Airbedane

You're correct -- I leapt in quickly without sight of the specific question. Interpolation in the way that you suggest may be the best that one can do without further info -- though it's still very unreliable. You can easily find significant counter-examples on a 214. Looking at isobars helps a great deal.

But I stick to my guns about the ludicrous approach taken in teaching and examining this area. I remember a question on a CAA met exam asking for the average wind over a route for which the only option available was an arithmetic mean of speed and direction. It was clearly inappropriate.

[Pianorak]

Airbedane quote: ”For the direction, draw a diagram and estimate the direction of the answer. Then do the maths.”

Have just returned from another flying lesson and discussed the issue with my FI. Well, he did exactly as Airbedane suggested, ie common sense and then some more.
Thanks everybody for pitching in – much appreciated.


Edited to add the following info:

I think I am right in saying that neither Trevor Thom nor the Oxford Aviation Training CD “Aviation Meteorology” deals with this question. The PPL Confuser provides the correct answer but no explanation.
As so often in the past: Ppruners to the rescue!

[DRJAD]

To echo Evo ...

Essential in this business to gain a visual view of what the situation is. I.e. the wind is going to come from the vector mean (i.e. in between) the two wind vectors you're given.

But, always try to visualize what is going on - you'll build situational awareness, which is vital when you are flying in busy airspace (dealing with other traffic, wind, where you want to go, and how you must get there.)

[Pink_aviator]

Well
I can't of course help ,
but i thought i was nearly ready ,
ie like thursday ,
to take my met exam ,
but reading this thread ,
i realise i'm not,
i think i will just give up and open nav book instead

PINK-AVIATOR
PINKSTER TO MY FRIENDS

[Pianorak]

Pinkster – It’s now my turn to say: Don’t give up. Remember you advised me recently not to shoot myself. Well, I didn’t – am still in the land of the living.

I too feel I am not ready but shall have a go at the exam on Friday. Please, please do likewise and just have a go. You are allowed two or three fails before they send you to Gatwick.

This will be my fourth exam and I find it the most complex, confusing, difficult and frustrating subject on God’s earth. All those grand statements – only to be demolished forthwith by dozens of exceptions to each and every rule and statement made. And I don’t think I am particularly thick, having scored 100, 97.5 and 90 per cent in the previous exams. AARRGGHH, please let me pass the Met exam and I promise not to sulk if they won’t let me go solo before the new year.

[Evo]

Pinkster - don't let me scare you off Met by waffling on about vectors. Met's a good exam, probably the only one that's useful in day to day life. Rather than bury your head in a book just look at the METARs and TAFs each day, and the forms 214 and 215, try and work out what you think it all means and then compare it with what happens during the day. After a while the weather will start doing the things you expect and you'll know you've got it :)

If you do go off and do Nav, just remember that it cannot be hard ... or Navigators wouldn't be able to do it :)

[Keef]

Pinkster and Pianorak

Don't look at Nav and Met as something to learn parrot-fashion. They are both "practical" subjects, a bit like language: once you can speak it, it's a doddle. Common sense will get you a long way, once you've got your brain round the basics.

There isn't that much "detail" stuff that you need to know, it's mostly about interpreting.

For met, that means "interpret what you see on the charts". Download a 214 and a 215 every day or so, and look at each to see what it's telling you. Ask yourself a few questions about the weather at various places, then look to see if you got it right.

I find that the 214 and 215 (or their continental equivalents) tell me pretty precisely what to expect, and whether or not to go flying. Then I look at the TAFs and METARs for home, destination, and major places enroute to confirm the diagnosis. That's "coarse flying" but it works!

[Airbedane]

Bookworm - re your post on 17th Aug - I certainly agree about the ludicrous approach taken with the current PPL exams.

About a year ago, some of my students were having difficulty with the Air Law exam. I had a go at it and failed! I've been a professional aviator for some 34 years and a private pilot for 36 - how have I survived for so long without the so called essential knowledge required to pass PPL level air law? DOn't get me wrong, some of it 'is' essential - rights of way, lights, signs, etc - but some of the sylabus is just way over the top.

We seem to be loosing the common sense approach to aviation that I was brought up with - "ignoring the essentials and taking the trivia by the throat" as we used to say in my military days.

Oh well, I'm just glad I don't have to take todays exams!

A

[Pianorak]

Keef and Evo - Forms 214 and 215 aren't the problem. They are fine and I can interpret them correctly - which probably won't be enough to get me through the exam. What I find so frustrating is that Thom and the Confuser deal with quite different areas and I am just wondering which side the exam paper is going to come down on. As to Oxford Aviation CD: excellent but way OTT and I almost wish I hadn't touched it.

I should add that I am a great believer in attending Ground School. Unfortunately due to circumstances outside my control I am unable to attend the one preparing for the Met exam.

[Evo]

Don't expect to see the exact same questions (unlike Perf & Planning!) but the Confuser is very similar to the Met exam.

[Pianorak]

Evo - Bad news indeed! Am managing near-100 per cent with Thom's specimen questions but only just over 80 with the Confuser. Hey ho! :{

[Evo]

80% is a pass, innit :)

Seriously, you've got the important bit right here


Forms 214 and 215 aren't the problem. They are fine and I can interpret them correctly


the difference between 80% and 100% is probably details like being able to identify some dumb Met chart symbol (back in the real world you can just look up if you need it).

[GroundBound]

Pianorak

I found the met exam one of the hardest. Also, it was the subject I least understood, despite reading and re-reading.

I don't want to frighten you, but, for me, the met exam was the one which was least like anything explained by Thom or like the examples from the Confuser. :{

There were two questions which I had never seen before, and several others were phrased somewhat differently.

I was able to answer about 14 of the 20 questions, without too much trouble, but I had to spend a lot of time over the other six. For some, of them I just worked on eliminating the obvious wrong answers, and picked the most probable from what was left, and crossed my fingers.

Strangely, I only failed the form 214 example question, which was a completely new example. When the instructor tried to explain why the answer was area one not two, I said his explanation would apply to either area, and he agreed - so it does seem it is not straight forward. :uhoh:

In the end, my preparation was to be sure I could answer all the questions in the Confuser, and then hope for the best.

I wish you the best of luck with it - however, once you've passed it don't just forget about it - try to use your knowledge, and other's experience to fly safely. And keep reading - it eventually begins to make some sense. ;)

GB

[Pianorak]

GroundBound <<I found the met exam one of the hardest. Also, it was the subject I least understood, despite reading and re-reading.>>

Ditto, ditto!!! :*

<<I don't want to frighten you, but, for me, the met exam was the one which was least like anything explained by Thom or like the examples from the Confuser.>>

Well, that cheered me up no end. :{

Got to laugh though – for heaven’s sake, it’s only an exam. Anyway, this might be my chance to go and get to know Gatwick. :O :O :O