Aussie METAR / TAF "T" and "Q" lines..?

[Aussie Andy]

Hi guys, I don't get to fly at home in Australia very often as I am based in teh UK, so would appreciate if someone could remind me of the answer (or tell me where to look this up!):

Take the following TAF/METAR data for example:
CAMDEN (YSCN)
TAF YSCN 021611Z 1812 23005KT 9999 SCT050
FM01 15010KT 9999 FEW050
T 14 14 18 21 Q 1023 1025 1026 1024
METAR YSCN 021830Z AUTO 23001KT 9999NDV // ////// 14/09 Q1023
RMK RF00.0/000.0
I recall from when I have previously flown in Australia that the "T" line gives temps in degrees C at various time intervals, and the "Q" is obviously QNH at the same times. But what is time interval? I think I remember it as being 4hrs, but now wonder if it was three?

So for example does the above mean that the forecast QNH will be 1023 at 1800Z, 1025 at 2200Z, 1026 at 0200Z and 1024 at 0600Z, or is it some other permutation?

Likewise, on the METARs there seems to be some auto-generated information that I am not familiar with. I beliebe the "RMK RF" relates to mm of recent / total (24hrs?) rainfall, but have no idea what NDV means, not all the "// //////" etc.

Maybe if someone could just point me to the relevant AIP refs?

Thanks!

Andy :ok:

[kair1234]

The temps and qnh's are at 3 hour intervals as i recall.
So first one would be at 1800utc then second at 2100 etc.

And pretty the rainfall is rainfall in the last 10 mins / rainfall total since 0900 local time.

[kair1234]

The temps and qnh's are at 3 hour intervals as i recall.
So first one would be at 1800utc then second at 2100 etc.

And pretty the rainfall is rainfall in the last 10 mins / rainfall total since 0900 local time.

NDV means No directional variation available.

[Creampuff]

kair1234 is correct.

AIP GEN 3.5 - 30/31 paras 12.15, 12.16, 12.18 and 12.19 here: http://www.airservices.gov.au/publications/current/aip/aip48_060831.pdf

UTC + 10 TEMP/QNH:
0400-0700: 14/1023
0700-1000: 14/1025
1000-1300: 18/1026
1300-1600: 21/1024

Nil recorded RF in previous 10 minutes and nil since 0900 local.

VIS in excess of 10k in all directions.

The number of "/"'s are important.

// = WX element of AWS not available
//// = VIS element of AWS not available
////// = cloud element of AWS not available.

In the METAR quoted you didn't get WX or cloud, but you did get VIS (9999NDV)

[Aussie Andy]

Thanks fellas, that's very helpful and much appreciated!

Andy :ok:

[jumpuFOKKERjump]

9999NDV = VIS in excess of 10k in all directions. It means the opposite, "no directional variation can be reported". There is one sensor, the direction it is looking it can see 'more than 10 km', there may be all kinds of crap on every other bearing... Everything else OK. The rainfall is supposed to be stripped from the reports between BOM and us, but often isn't.

[Aussie Andy]

Got it, thanks!

Andy

[Creampuff]

AA: juFj is correct on the 'NVD' point and I was wrong - sorry about that.

Now the tough question juFj: how do we work out the direction in which VIS is 9999?

[Aussie Andy]

Sounds like they don't intend that you work out which direction the vis indication applies: just covering their a&rses :}

Andy :ok:

[topdrop]

AA: juFj is correct on the 'NVD' point and I was wrong - sorry about that.

Now the tough question juFj: how do we work out the direction in which VIS is 9999?

If it's crap from your direction fly a 3NM arc til you find the good vis :}

[Capn Bloggs]

The rainfall is supposed to be stripped from the reports between BOM and us, but often isn't.
Nor should it be. The "last 10 min" value is a good indicator of what's happening/just happened at the airport precip-wise.

[Roger Standby]

The other acronynm that makes its way onto these auto generated metars is "NCD". The modern decode for this is NO CLOUD DETECTED. Not what it used to mean!!!:} :E

[almostthere!]

UTC + 10 TEMP/QNH:
0400-0700: 14/1023
0700-1000: 14/1025
1000-1300: 18/1026
1300-1600: 21/1024

Not quite. The TEMP/QNH are not for block three hour periods rather, at 0400 it should be 14/1023 with a linear change to 14/1025 at 0700.

This would be pretty important at places which have quite large variations during the day, ie cold at night hot in the middle of the day.

[Creampuff]

Do you have a reference for using a linear interpolation of temp and QNH, almostthere!?

I can't recall any document suggesting that, or any instructor saying (for example): It's 0531 puffy, so we should change from 1023 to 1024, 'cause that's half way between the 0400 forecast of 1023 and the 0700 forecast of 1025.

You could be right, but it's the first I've heard of it.

[tobzalp]

Creampuff, you are correct with your first post.

[Awol57]

BoM man told us at a seminar that those values are forecast values at that specefic time. Might not be a linear line, but their prediction is the QNH for that time only. QNH could drop 5 mins later as the cyclone decends, but it was an accurate(??) forecast for that particular time.

[almostthere!]

Do you have a reference for using a linear interpolation of temp and QNH, almostthere!?

No specific reference for a "linear interpolation" however reading the regs "forecast values of QNH are given, valid at three (3) hourly intervals" NOT "valid for three (3) hourly intervals".

As the previous poster has said, I have also been at a briefing from BoM guy that said the exact same thing that it is a forecast at a specific time, starting from the beginning of the forecast and then at three hourly intervals thereafter.

Whilst the interpolation would most often make very little difference, it most certainly would if there was big differences between the three hourly forecasts.

In response to "It's 0531 puffy, so we should change from 1023 to 1024, 'cause that's half way between the 0400 forecast of 1023 and the 0700 forecast of 1025.. what about "It's 0700 puffy, so we should change from using a temperature of 12 to 25 for our TAS calculations." Temperatures and QNH dont just change instantly on the hour so we need to take this into account. Puffy's instructors directions on the QNH seems a lot better than his instructions on temperature!!

[Creampuff]

That’s all well and good, but what I’m trying to work out is what the required procedure is.

My understanding of the altimetry rules is that they are intended, at least in part, to ensure that everyone in the same area uses the same QNH setting, not necessarily the accurate QNH, taking into consideration things like the QNHs in adjacent areas and know aerodrome QNHs.

Let’s assume we’re both going to fly to Grong Grong with an ETA of 0630, and the TAF for Grong Grong forecasts 1010 for 0400 and 1019 for 0700 – there’s a front forecast to move through.

I’m going to have the altimeter set to 1010, unless one of those nice Flightwatch or ATC people tell me otherwise. Are you saying you’re going to set 1018 or 1019?

Your comment about the numbers being merely point in time forecasts is correct, but that cuts both ways. The forecast could be wrong, and neither of us may be even close to the correct QNH. But don’t you think we should at least have the same setting?

Any IFR boys or girls care to comment on what they would have set at 0630 at top of descent into Grong Grong?

As for temp and TAS calculations, the difference is that in most aircraft you are able to measure and use the actual temperature while you’re flying.

[Aussie Andy]

Grong Grong forecasts 1010 for 0400 and 1019 for 0700 – there’s a front forecast to move through.But even so, a 9mb change in 3hrs would be pretty amazing...

Are you saying you’re going to set 1018 or 1019?1mb difference = ~30ft so its not really going to make a mega-difference.

And even if it were 9mb, in must situations ~300' won't spoil your day in VMC (unless very low over the terrain in the first place!?). It may be of more concern if doing a non-precision (e.g. NDB) IAP of course... in which case I would want to have some means of getting the info from FIS or elsewhere... but still I think the idea of a 9mb change is pretty unlikely!?

Anyway, I am happy with the 3hr intervals info above - thanks!!

Andy :ok:

[No Further Requirements]

The other acronynm that makes its way onto these auto generated metars is "NCD". The modern decode for this is NO CLOUD DETECTED. Not what it used to mean!!!:} :E

NCD = another good day at the office!

:ok:

[Pharcarnell]

When quoted from an electronic sensor and not from an observation, the VIS is how well the sensor sees through a piece of air seperating the transmitter and receiver of the sensor, about 430mm. So unless the crappy air is BETWEEN the sections of the sensor and the atmosphere is constant across the airfield, it's not too accurate.

[Awol57]

Area QNH is valid if it's plus/minus 5hPa so the linear interpolation is a moot point anyway, unless the change is 6 or more hPa over the 3 hour period.

I realise you want to be as accurate as possible, but VFR's are allowed up to 100' variance which is a bit over 3hPa anyway. I think in this instance if you were to use the forecast QNH you would be ok.

[Aussie Andy]

I agree -- and you have just reminded me that back home in Australia there are no millibars (had forgotten that!)... must be quite different flying in Hectopascals!! :}


Andy :ok:

[Dookie on Drums]

I would be using Area QNH for my let downs as it's more current than a TAF's QNH which could be several hrs old and then some.

Given the sometimes large differences between Area and TAF QNH I would be using the former source which may prevent me knocking koalas out of trees whilst shooting an NDB in solid IMC to the minima. :E

[kair1234]

I realise you want to be as accurate as possible, but VFR's are allowed up to 100' variance which is a bit over 3hPa anyway. I think in this instance if you were to use the forecast QNH you would be ok.

But isn't your 100' variance on your altimeter to do with instrument errors not for setting the wrong QNH.

If your altimeter had an instrument error under reading 100' and you had it set 3hpa too low (another 100ish') and you had drifted up say 50' from your cruise level you could be flying along 250' higher than your true altitude. All it takes is someone coming in the opposite direction with the reverse situation (250' low) and you are history. :}

I believe you should use the most accurate QNH setting you have access to.

[Awol57]

You are absolutely correct regarding instrument error. The point I was trying to make was a 1hPa difference in most circumstance is negligble.

Use the most current info you have.

[Creampuff]

AIP ENR 1.7 para 2 says:2. ALTIMETER SETTING RULES
…
2.1.2 For all operations at or below the Transition Altitude (in the Altimeter Setting region), the altimeter reference setting will be:

a. the current Local QNH of a station along the route within
100NM of the aircraft; or

b. the current Area Forecast QNH if the current Local QNH is not
known.
…
2.3 Local QNH

2.3.1 Local QNH, whether provided by ATS, AWS or Aerodrome Forecast (TAF) or by using the altimeter subscale to indicate airfield elevation AMSL, is used as shown at Figure 1.Seems to me that if the only information you have access to is the relevant TAF and ARFOR, the rules require you to use the QNH in the TAF. Of course the rules are ambiguous because of the word ‘current’, but if that word was interpreted to mean the actual QNH rather than the current TAF QNH, the inclusion of TAF in paragraph 2.3.1 would be a nonsense (an outcome not to be lightly dismissed).

[Dookie on Drums]

Well I for one would much rather be illegally alive than legally dead. :rolleyes:

If the TAF QNH is 1019 and the Area QNH is 1012 for example on a solid IMC day then I would much rather use the Area (usually more current) QNH than use TAF QNH. If I used the TAF QNH then I could/would be flying 210ft lower than I should. Throw in some altimeter error (let's say 60ft) for example and pretty soon you are almost 300ft below where you should be.

Extreme example I know but not impossible either as I have come across it before.

Yeah sure the AIP says you can but I wouldn't follow it blindly. :hmm:

[Creampuff]

Very good points and I wouldn’t presume to tell you how set your altimeter in IMC.

I’m merely quoting what the AIP appears to require you to set.

Curse you Aussie Andy!

[UnderneathTheRadar]

Dookie-on-drums - I see your point but it is an extreme example - and I think the powers-that-be have considered the reasonable possibility/probability of TAF QNHs being incorrect and decided that adding 100' to the minima when an accurate QNH isn't know covers it.

While you are certainly erring on the side of caution, you're also giving yourself larger chance of not getting in.

Not saying you're wrong - just that errors in forecast QNH have already been allowed for.

Out of curisoity, if you've sent QNH 1019 based on the area instead of 1012 based on the TAF, which minima do you use?

Perhaps OzExpat could enlighten us?

UTR.

[kair1234]

Disregard this post

[Dookie on Drums]

Hi UTR,

Sure it is an extreme example but in this particular example I would err on the side of caution rather than be concerned about getting in. The large differences between the 2 QNH's would make alarm bells ring in my head. I am not saying I would use it every time but in this case I would use area QNH.

In answer to your question if you've set Area QNH and elected to use it for your approach then you add 50ft to your forecast Terminal QNH minima (Jepp Terminal 5.3.3)

Also out of interest Jepp terminal 5.3.1 states
" Prior to passing the IAF,pilots are required to set either:
a. the actual aerodrome QNH from an approved source, or
b. the forecast Terminal QNH, or
c. the forecast area QNH "

So you are actually meeting the requirements if you use area QNH should you wish to. :ok:

Is everybody following this ??? :8 :O

This is the sort of good discussion we can get on here! ;)

DoD

[UnderneathTheRadar]

Hi Dookie,

With you all the way in your logic and the rules (although it's interesting that Jepps don't REQUIRE you to use the local/TAF QNH while the AIP does REQUIRE that you do use the forecast TAF QNH) and the alarm bells ringing in your head.

What I'm curious about is, with the example you've given which you said you've encountered, shows that something has gone seriously awry with the TAF. I'm using a JAA definition (from memory) - Area QNH is the lowest value forecast for any spot within the area - but I imagine it's not very different down here. So, with your example, there must have been a change in the area forecast and hence the area QNH to a much higher value which has not been reflected in the TAF.

The reasons I can think of are: a) the area forecast at the arrival airport hasn't changed or b) there is enough margin built into the minimas that there is no scenario where you could find yourself hitting anything while still in cloud.

ATS don't (from memory) issue Hazard Alerts for radically changed forecast TAF QNHs so I have to conclude that the procedure designers have allowed enough fat to cover any likely QNH changes.

Just my rambling thoughts....

UTR.

PS This is fun!

[Creampuff]

I think we’re mixing up three different issues:

(1) discrepancies between the ARFOR and the TAF

(2) which one you're obliged to use

(3) whether you should interpolate the numbers in the TAF

Let’s just concentrate on (3) for the time being.

Let’s assume an ARFOR says:AREA21 (21)
ARFOR AMEND AREA FORECAST 062000 TO 070500 AREA 21
….
AQNH AREA QNH 19/22
AREA 21: 1020Let’s assume a TAF says for an aerodrome in the area says:TAF TAF YMRY 061816Z 2008 25010KT CAVOK FM04 04010KT CAVOK T 08 14 18 21 Q 1020 1019 1017 1014Your ETA MRY 0445UTC.

If that’s all the information you have, what do you set on the altimeter at top of descent into MRY?

[Spodman]

AIP ENR 1.7
2.1.2 For all operations at or below the Transition Altitude (in the Altimeter
Setting region), the altimeter reference setting will be:
a. the current Local QNH of a station along the route within
100NM of the aircraft; or
b. the current Area Forecast QNH if the current Local QNH is not
known.

AIP ENR 1.5 5.3
QNH Sources
5.3.1 Prior to passing the IAF, pilots are required to set either:
a. the actual aerodrome QNH from an approved source, or
b. the forecast Terminal QNH, or
c. the forecast area QNH.

"(1) discrepancies between the ARFOR and the TAF" Use either, or local QNH.

"(2) which one you're obliged to use" You can use either, or local QNH.

"(3) whether you should interpolate the numbers in the TAF" The value shown is the forecast QNH for that 3 hour period, no need to interpolate.

The only real thing to pick between them is the 50' "discount" concept on the IAL mimima. So! The answer in this case - 1017.

[Creampuff]

Looks like there's a discrepancy between AIP ENR 1.7 2.1.2 and AIP ENR 1.5 5.3. (Who'd have thought!) The former suggests you have no choice but to use the TAF, even if you have an ARFOR with a QNH for the period; the latter suggests you have a choice.

I think the former was changed during the NAS adventure, to try to wean people of checking area QNH by radio.

PS: I'd use 1017 too (no interpolation)