QNH in a TAF

[flyingpom]

I haven a question re QNH from a TAF. The AIP states the TAF values relates to 3 hr blocks from the start of the forecast period i.e HH HH+3 HH+6 HH+9

Now i have seen some references that note these values are actually only valid across 90 min blocks e.g

If we have a forecast from 0200 the TAF QNH values reading 1010 1011 1012 1013

Do I read that as as

Time

0200 - 0330 is 1010
0330 - 0630 is 1011
0630 - 0930 is 1012
0930 - 1230 is 1013

Or

0200 - 0500 is 1010
0500 - 0800 is 1011
0800 - 1100 is 1012
1100+ is 1013

?

[compressor stall]

(Very) technically neither. :cool:

AIP GEN 3.5 12.17.3 has your answer quite clearly. There is an amendment bar on the last half of the paragraph, so it's possible that people haven't caught up on the clarification.

[coarsepitch]

Bob Tait explains it quite well on his website: Private Pilot Licence (PPL) - Errata (http://www.bobtait.com.au/books/errata/103-private-pilot-licence-ppl)

Essentially, given the latest amendment to AIP, you're now able to simply interpolate between QNH and/or temperature values. Much easier than the 90 minute blocks they used to relate to!

[flyingpom]

thanks for the clarification

[Kelly Slater]

So the answer is

0200 - 0500 is 1010
0500 - 0800 is 1011
0800 - 1100 is 1012
1100+ is 1013

give or take a minute with linear interpretation between.

So at 0200 the QNH is 1010
At 0300 the QNH 1010.33 so you use 1010.
At 0400, you are two thirds of the way in so the QNH would be 1010.67. Do you use 1010 or 1011?

There are people who will say that QNH must always be rounded down but is there a reference?

At 0430 the QNH is 1010.83. What do you use now.

The answer is almost never Quite clear.

[laser650]

Just a refinement of my interpretation
HH being the commencement hours of the TAF, in this case HH 0200, HH+3 is 0500......
Lets also insert temperatures for the example, 28 31 33 34

So for the relative Time QNH Temp, I use a simple layout for interpolation as below-

0200 1010 28
0500 1011 31
0800 1012 33
1100 1013 34

So now in the above scenario we may simply interpolate, for eg, with an ETA of 0400 I would use a QNH of 1011 for estimated altimeter settings(or 1010 for the performance charts for my estimated RTOW BRW), and a temperature of 30 degrees.

I hope this clears some confusion :O

[compressor stall]

There are people who will say that QNH must always be rounded down but is there a reference?


Yes, it's in the AIP. Funnily enough it's in the Meteorology Section, and by a massive coincidence in the chapter labelled QNH. One would never think to look there.

[Creampuff]

So we’re back to precisely where we started on when the 3 hour brackets apply, :D but we’ve added interpolation.

Of course, lots of people weren’t aware of how the old (new) 3 hour brackets applied, and will now inadvertently be restored to compliance on that matter. I wonder whether the same will happen with interpolation …

[OZBUSDRIVER]

Nice thread TAF Questions circa2008 (http://www.pprune.org/dg-p-general-aviation-questions/334231-taf-question-2.html)

And we find ourselves at the beginning of time...but now with interpolation:ugh:

There is a nice little post from ITCZ and a responce from a METO at post28

to qoute-
ITCZ

Thank you for the information you have supplied.

The TAF QNH forecasts are point forecasts for the indicated times, e.g. 00, 03, 06, 09. However, a point forecast is valid for 90 minutes each side of the time point; and pilots should not interpolate. Unfortunately, the rule doesn't work for a half way point, e.g. 0430, in which case you have to choose one of the two point forecasts.

We will reword GEN 12.16.3 to make this clear.

Regards

Peter



Back to the FUture!:}

[Kelly Slater]

This is for Compressor Stall

The AIP states that intermediate QNH readings are rounded down. This is a very far cry from stating that all QNH readings must be rounded down. Check your facts or check your con-descension. You could have put forward your view without trying to put me down.

[compressor stall]

Besides intermediate values, there's only whole numbers. You can't round a QNH that's a whole number. :confused:

Sorry to hurt your feelings, but I have a beef with people asking questions on here that are an AIP or CAO away. The OP built me up (which is why I deliberately gave the reference, not the answer) and your query pushed me into a digital catharsis.

[tweekey]

For examination purposes, the first qnh is valid for 90 mins, the remaining three are valid for 180 mins a piece

[Creampuff]

For examination purposes, the first qnh is valid for 90 mins, the remaining three are valid for 180 mins a piece Not any more, unless the test is against the old (new) rules.

As OZ says, it's back to the future and the old rules are new again! 3 hour brackets, with the first starting at the commencement of the validity period of the TAF.

[tweekey]

Not any more, unless the test against the old (new) rules.

As OZ says, it's back to the future and the old rules are new again! 3 hour brackets, with the first starting at the commencement of the validity period of the TAF.

Well it was 3 weeks ago...

[Kelly Slater]

Compressor Stall you fail again. If the QNH at 0200 is 1010 and the QNH at 0500 is 1011, then by linear interpretation as per the AIP, the QNH at 0330 is 1010.5. Do you use 1010 or 1011 and can you provide a reference that justifies your answer, being aware that your previous assertion was incorrect. And yes it is academic but academic is what is required when doing exams. In real life, it makes no discernible difference.

[Creampuff]

The chronic problem here is a lack of proper education in advance of these kinds of changes.

We’ve gone from 3 hour brackets commencing at the start of the validity period, to 3 hour brackets spanning the 90 minutes before and after each 3 hour period commencing at the start of the validity period (that method was always doomed to failure from a comprehension and compliance perspective, because it’s counter-intuitive), back to 3 hour brackets commencing at the start of the validity period.

But we’ve also gone from interpolation: ‘bad’, to interpolation: ‘good’.

Lucky that keeping up with these changes has zero implications for safety…..

[MakeItHappenCaptain]

Interpolation and QNH

Observed values are rounded down.
ie. Decimal values don't appear on a TAF.
Anyway, 15' isn't going to affect it that much.:rolleyes:

Play nice, kiddies.

Tweeky,

Since when has ASL been able to keep up with rule changes?
Major changes to IFR rules 9 months ago that I bet still haven't been switched in IREX. :hmm:
If you get an answer wrong because of expired rules (your AIPs current, their answer not), you have the right to appeal.

[AerocatS2A]

Kelly Slater, use 1010. A low QNH is "safe" because it will lead to the altimeter under-reading slightly. Presumably this is why the forecast QNHs are rounded down. If you're not happy with that, consider that nowhere in the AIP does it say to round up, but it does say intermediate values are rounded down.

[Kelly Slater]

As the forecast values are already rounded down, further rounding down increases the error, all be it on the safe side.

I am interested in what CASA would consider the correct answer in an exam. Would they mark you wrong if you used standard mathematical rounding rules, that is .1 to .4 round down, .5 to .9 round up, or would either answer be correct.
The AIP states that the intermediate forecast values are rounded down. I know that observations are rounded down.
Can anyone give an AIP reference that says pilots should always round down when interpolating QNH values?

As I have said, people say that QNH values must always be rounded down. I do not dispute that this is correct, I merely want to read the section in the AIP that makes this statement.

I was simply throwing the question out there for those of said belief, I did not expect to be ridiculed. I was hoping for a reference because it is something that I would like answered properly and conclusively, not because it has any bearing on flight safety but because the requirements need to be clearly defined.

The standard altimeter setting is 1013.2, should anyone dispute that you can have decimals in QNHs.

[MakeItHappenCaptain]

Actually it's 1013.25"

There's not going to be answers that are that tight.

The range of answers will be targeted at using incorrect technique, eg. dry vs. wet lapse rates, 120'/hp instead of 30' etc....

AIP doesn't say round down, nor anything about intermediate QNHs.

It says observed values, ie. If their mickey doo baro says 1013.5, they will publish 1013.

[Kelly Slater]

AIP Gen 3.5 12.17.2

"Observed imtermediate values are rounded down."

[MakeItHappenCaptain]

Excellent! Exactly what I've said in my last two posts!

Now, the very next paragraph says to use linear interpolation for values in between the time points...
But does not say to round them down.

Common practice is to round down when utilising interpolated QNH but it is a safe action, not a mandated one. When IFR, if you're using Area QNH or forecast QNH, you straightaway increase minimums by at least 100', but the minima themselves aren't affected.

Again, CASA will not have an exam with;

At 4:30, what will the pressure atitude at Sea Level be?

A) 90'. (for 1010 HPa)
B) 65.1'. (for 1010.83 HPa)
C) 60. (for 1011 HPa)'
as you seem to be concerned so much about.:rolleyes:

KISS priniple!

I suppose if you ask long enough someone will agree with you.

Have whinge about being picked on, but jeez dude, you're gonna have to harden the fcuk up when someone directs a mildly sarcastic remark your way. You've gone a bit Charlie Sheen here. My first post actually leaned toward your interpretation, but don't lash out at anyone who doesn't agree 100% with you.
:cool:

[C-change]

It says observed values, ie. If their mickey doo baro says 1013.5, they will publish 1013.

If anyone's interested, ATIS is also rounded down. If the little red readout says 1012.8 we stick 1012 on the ATIS. If it drops to 1011.9 on the readout, we stick 1011 on the ATIS.

[Kelly Slater]

I don't mind being pointed out as wrong but I object to people using incorrect or unsubstantiated information to make out that I am incorrect or that they are above me.

My part in this thread has gone on long enough and would appear to have served no purpose so c'est la vie.

[Traffic_Is_Er_Was]

As a once certified Met observer (maybe still am - does it ever lapse?) and once Briefing Officer (definately lapsed!) my take is that you should read and apply the section in AIP regarding QNH as a whole. It is included as an explanation as to the decode of QNH in regard to observations (METAR/SPECI) and forecasts (TAF/TTF etc), there is no distinction between the two. The BoM quite clearly always rounds down (and they provide the raw info), AIP Met section reflects their practices. If an interpolation between 2 forecast QNH's gives an intermediate result, then using the rule applied to an actual observation (i.e. a forecast observation) if done at that time with that same result, it would be rounded down. My interpretation of "Observed" in this sense would be any intermediate value that is arrived at by whatever means.