Aussie METAR / TAF "T" and "Q" lines..?
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VIS in a MetarAWS
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.
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.
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.
PPruNaholic!
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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
Andy
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.
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.
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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.
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.
Use the most current info you have.
Seems to me that the TAF takes priority
AIP ENR 1.7 para 2 says:
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).
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.
…
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.
Well I for one would much rather be illegally alive than legally dead. 
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.
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.
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!
I’m merely quoting what the AIP appears to require you to set.
Curse you Aussie Andy!
When you live....
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.
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.
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.
Is everybody following this ???
This is the sort of good discussion we can get on here!
DoD
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.
Is everybody following this ???
This is the sort of good discussion we can get on here!
DoD
When you live....
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!
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!
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:
Let’s assume a TAF says for an aerodrome in the area says:
Your ETA MRY 0445UTC.
If that’s all the information you have, what do you set on the altimeter at top of descent into MRY?
(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: 1020
ARFOR AMEND AREA FORECAST 062000 TO 070500 AREA 21
….
AQNH AREA QNH 19/22
AREA 21: 1020
TAF TAF YMRY 061816Z 2008 25010KT CAVOK FM04 04010KT CAVOK T 08 14 18 21 Q 1020 1019 1017 1014
If that’s all the information you have, what do you set on the altimeter at top of descent into MRY?
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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.
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.
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)
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)
