What is the difference in equipment used to generate an ATIS, AWIS and METAR?

[LongLats]

Say you're going into somewhere, a class D aerodrome, say Coffs Harbour. The tower is closed, so there's no ATIS available. My understanding is that you can contact ATC to request the latest METAR information to get a understanding of what to expect, but you can't use the QNH they give you from the METAR.

AIP ENR 1.5-33 5.3.1 states that 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

It goes on to talk about the 100ft reduction with an actual QNH from an approved source, then clarifies what an approved source is (ATC, ATIS, AWIS and CASA approved met observers) and adds a note, "METAR QNH does not meet this requirement". So if the METAR QNH isn't covered as an approved source, the only choices we seem to have left are:
b. the forecast terminal QNH, or
c. the forecast area QNH

So the QNH generated from the METAR is obviously unreliable compared to the QNH you'd get from an ATIS or AWIS.

My question is then, how is the METAR generated, and if it is from an automatic weather generating device at the aerodrome, why is it not as reliable as the QNH gathered from somewhere with an AWIS, where it IS an approved source? And what equipment do the guys in the tower have access to, to allow them to distribute an ATIS. Obviously better equipment than what is being used to generate the METAR I presume?

In short: what is the difference in equipment used to generated an ATIS, an AWIS and a METAR?

[Power]

I remember an ATO telling me once, when doing my instrument rating, that the data from the METAR is older than 15 mins when you read it so thats why you couldnt use it to take 100ft off the minima.

[mikewil]

I remember an ATO telling me once, when doing my instrument rating, that the data from the METAR is older than 15 mins when you read it so thats why you couldnt use it to take 100ft off the minima. I am pretty sure this is the reason.

I think the information is all generated by the same weather station, it just has to do with the fact that METARs can be older than the live information that the station is broadcasting.

[LongLats]

I remember an ATO telling me once, when doing my instrument rating, that the data from the METAR is older than 15 mins when you read it so thats why you couldnt use it to take 100ft off the minima.

Yeah, I understand that with QNH from a METAR or a TAF, you're not going to be able to take the 100ft off the minima, but the AIP suggests you can't use the METAR QNH AT ALL. It lists 3 sources of QNH that need to be set prior to passing the IAF, and METAR QHN isn't one of them.

I'm curious as to why that's the case.

[nitpicker330]

Good point, the TAF's we see for Australia don't always have forecast temps and QNH's on them. Why can't you use a METAR that is no more than 30 mins old?

Seems silly.....:{

[Capt Fathom]

Why can't you use a METAR that is no more than 30 mins old?


If a front goes through, there could be large variation in the QNH over a very short time.

[LongLats]

If a front goes through, there could be large variation in the QNH over a very short time.

I thought about this too, but why is a TAF QNH that is forecast in 3 hour intervals still okay to use? Surely the METAR QNH that was worked out 30mins ago will be closer than the forecast TAF QNH if you're arriving somewhere, say 5 hrs after the TAF began, but 20 mins after a METAR is generated.

[nitpicker330]

Forecasts are really only nothing more than a "guess" about what the QRH will be in the future and we all know how inaccurate the BOM's forecasting can be at times!!
The time groups leave a big gap with respect to changes. Surely a METAR could be used with some common sense reading of the Trend over the last couple of hours?

[j3pipercub]

Automets can't be used at all for 'actual' QNh. There is no problem using them for forecast QNH minima.

j3

[LongLats]

Automets can't be used at all for 'actual' QNh. There is no problem using them for forecast QNH minima.

So the METAR QNH meets the requirement of point b. in AIP ENR 1.5-33 5.3.1, "b. the forecast terminal QNH"?

Also, I'm not sure what you mean by an automet. But an AWIS is automatically generated and that is absolutely a source of actual QNH where 100ft can be taken off the minima.

[Nautilus Blue]

Yeah, I understand that with QNH from a METAR or a TAF, you're not going to be able to take the 100ft off the minima, but the AIP suggests you can't use the METAR QNH AT ALL. It lists 3 sources of QNH that need to be set prior to passing the IAF, and METAR QHN isn't one of them.

I'm curious as to why that's the case.

Could it be an unintended consequence of a sloppy wording? i.e. it can't be used for "a. the actual aerodrome QNH from an approved source" (for the reasons above) but should be covered by "b. the forecast terminal QNH" even though it isn't a forecast. Maybe someone simply forgot a METAR isn't a forecast when writing the rule?

edited to add "automet" might be an AUTO METAR?

[LongLats]

Could it be an unintended consequence of a sloppy wording? i.e. it can't be used for "a. the actual aerodrome QNH from an approved source" (for the reasons above) but should be covered by "b. the forecast terminal QNH" even though it isn't a forecast. Maybe someone simply forgot a METAR isn't a forecast when writing the rule?

Yeah possibly. It seems to be pretty confusing to most of the community so I've emailed the BoM. Will post their response here.

[j3pipercub]

Yeah sorry guys, automet= Auto METAR (on the hour and half hour). Someone I fly with rang BoM and asked if the AWIS and AUTO METARS are derived from the same equipment. The answer was that yes they were, the decision to have an AWIS on a frequency or phone number was the airports decision. Cannot vouch for the veracity of info, just what I was told.

So for clarification, AWIS is fine for actual terminal QNH minimas (updates every transmission cycle), provided obtained in 15 minutes etc.

AUTO METARS cannot be used for actual QNH, just forecast terminal QNH minima (due to the fact that they are only 'snapshots' of the weather every 30 mins).

[Capn Bloggs]

AUTO METARS
There is no such animal. It doesn't matter how the information is sourced, a METAR is a METAR is a METAR. Even if ATC give you the "automet Bulla at 0530", how do you know that that QNH wasn't fed into the system at 0523?

Personally, I don't see why a time limit of say 30 minutes couldn't be applied to allow use of the METAR QNH to use the standard minimums. The "-100ft" prohibition is because even though a METAR may be issued at say 2200, the QNH may well have been taken 10 minutes before that, chewing into the 15 minutes.

AUTO METARS cannot be used for actual QNH, just forecast terminal QNH minima
No, a METAR QNH cannot be used for anything.

Forecasts are really only nothing more than a "guess" about what the QRH will be in the future and we all know how inaccurate the BOM's forecasting can be at times!!
The time groups leave a big gap with respect to changes. Surely a METAR could be used with some common sense reading of the Trend over the last couple of hours?
No, if the actual QNHs were drifting outside the issued TAF, the BOM would revise the TAF.

[morno]

No, a METAR QNH cannot be used for anything.

Sure about that Bloggs?

I'm not in any mood to be reading regs, but if that's the case, maybe you'd better tell 3/4 of the pilots in Australia.

I've used it thousands of times, just not as the actual QNH.

morno

[j3pipercub]

Hey Bloggs, I'll take the comment about AUTO METARS, bit pedantic, but taken.

Just with regard to your comment on QNH source. Are you telling me that you would use 'linear interpollation' to determine QNH source from a 2 hour old TAF as opposed to using a QNH from an automatically generated METAR? And do you trust the forecaster to re-issue the TAF in time if the QNH is outside that stated in the TAF?

[Capn Bloggs]

Morno, OK, in the context of the thread. :ok:

Are you telling me that you would use 'linear interpollation' to determine QNH source from a 2 hour old TAF as opposed to using a QNH from an automatically generated METAR? And do you trust the forecaster to re-issue the TAF in time if the QNH is outside that stated in the TAF?
I don't make the rules, I just follow them. As I said, I think that the METAR QNH could be used (within limits, of which there are currently none - is a METAR QNH that's 59 minutes old OK for you? Or maybe one that has not been updated for say 75 minutes?).

The good part about people sticking to the rules is that they (the rules) are more-quickly shown to be an ass, and then they get changed. :D

[j3pipercub]

I would love to see the reference for Metar QNH not able to be used for forecast QNH. If you could provide me with that, I would be appreciative as I cannot find any reference. I am aware of the note at the end of 6.3.2.

j3

[Capn Bloggs]

I would love to see the reference for Metar QNH not able to be used for forecast QNH
There isn't one. You must set what's listed in the original post, nothing else.

[j3pipercub]

Ok, thanks. Just one last hypothetical question if you don't mind:

You are on descent to YMON (middle of nowhere) after very diligently calculating, through linear interpolation, that the Forecast Terminal QNH will be 1011. It is 2.06 am and ATC let you know that there has been a METAR issued for YMON, automatically generated, issued 2.00am. QNH now 1016. What QNH do you set through transition?

[underfire]

How would you use the information from an automated MET system that is constantly broadcasting the METAR data?

(ie no tower control and on a coded approach procedure)

[Capn Bloggs]

What QNH do you set through transition?
As dictated by AIP (see the original post).

How would you use the information from an automated MET system that is constantly broadcasting the METAR data?
As dictated by AIP (see the original post).

[j3pipercub]

Ok, thank you. So you would rather be on a forecast QNH that could put you well below even the actual QNH minima, instead of using the recent QNH from an auto weather station, got it.

That sounds like a very reasonable, practical and sensible approach to flying :D.

[Bladeangle]

If that was the case it would probably be a SPECI, either way you cannot use the QNH from it, legally anyway.

In that case, I would use the AREA QNH and add 50' to the forecast minima (unless you had a ground agent that could relay the phone AWIS by radio, or a blue tooth headset and telstra coverage)

I often hear Qlink going into Coffs on the overnight flight asking center for the latest METAR as the tower closes pretty early there. Im assuming this would be to ascertain wind direction and get an appreciation of the ceiling?

[Awol57]

From my observations at work:

The AUTO METAR is a snapshot of the conditions at the time it is published. As ATC we get 1 sec data so we use that to update the ATIS. I believe the AWIS is also updated each time it starts it's broadcast. The AWIS is airport equipment, but all data is sourced from the BoM gear.

As others have alluded to, it's not uncommon for the data to be wrong by the end of the half hour. However why it doesn't have some allowance I do not know. In 10 mins the most I have seen the QNH change is by 3hPa. So I understand it not being an "actual" QNH but surely it would help you get an idea.

[LongLats]

Bloggs and j3pipercub you guys are having the exact same argument that I've had with a stack of guys in the last couple of weeks. The opinions are cut pretty square down the middle.

j3pipercub what you're saying is the logical, rational argument which is that you just use the most recent QNH, because, hell it just makes sense.

Capn Bloggs is saying what a LOT of other pilots agree with which is that the AIP specifically states 3 places to retrieve your QNH and it is:
a. the actual aerodrome QNH from an approved source, or
b. the forecast terminal QNH, or
c. the forecast area QNH
AIP ENR 1.5-33 5.3.1

I think we can all agree that the METAR QNH does not conform to point a. or c. in the above paragraph. Now some people may say that the METAR QNH qualifies as a forecast terminal QNH, therefore meeting point b. Others disagree and say that a METAR is a report, not a forecast and therefore cannot meet the requirements of point b. above.

Both valid arguments I think. Which is why I have contacted the BoM and will let you all know what they say as soon as they get back to me. Meanwhile I think we can all agree it's another poorly written mess that has pilots setting different QNHs all around the place. Dangerous stuff.

[TOUCH-AND-GO]

You are on descent to YMON (middle of nowhere) after very diligently calculating, through linear interpolation, that the Forecast Terminal QNH will be 1011. It is 2.06 am and ATC let you know that there has been a METAR issued for YMON, automatically generated, issued 2.00am. QNH now 1016. What QNH do you set through transition?

Set Aerodrome elevation! :E

[Capn Bloggs]

Meanwhile I think we can all agree it's another poorly written mess that has pilots setting different QNHs all around the place. Dangerous stuff.
I don't agree. The rules are quite clear. The only "mess" here is that some think that setting the METAR QNH is OK, when it's not.

Whether the current rule is practical or needs updating is entirely another matter.

Whatever the BOM says will be irrelevant to the rules; CASA makes them and I would be approaching it as to why there are perceived anomalies.

[Nautilus Blue]

Shouldn't the TAF, METAR, area QNH and current QNH from the aerodrome sensor all be within 5 HPa? Otherwise there should be amended TAF/area QNH and/or SPECI, any of which would be directed/broadcast by ATC to IFR/VFR. Admittedly in decades of passing amended TAF's to pilots, not one has ever asked for the QNH's (or temperatures). We also religiously pass area QNH's to aircraft on descent to G, whether they use it or not I don't know.

If the minima is designed to allow area QNH to be used it must have a fair margin built in. I've seen area QNH's with a 5HP split on different sides of the same aerodrome!

[Wally Mk2]

It's generally accepted that the spoken word is used for the QNH as actual.
Remember nothing stops the PIC from adding to any Min due all sorts of reasons one being an uncertainty or accuracy of a any given QNH.

ALL Inst App's have been surveyed with a lot of 'fat' built in just for this very reason & other reasons such as instrument inaccuracies as well as pilot abilities/techniques.

Wmk2

[Bladeangle]

If the minima is designed to allow area QNH to be used it must have a fair margin built in. I've seen area QNH's with a 5HP split on different sides of the same aerodrome!

Yep it does, 100' for the use of forecast terminal QNH, and an additional 50' (on top of the forecast QNH minima) for use of area QNH

Plenty of fat!

[Nautilus Blue]

Makes sense, 150' = 5 HPa, which seems to be the range for QNHs. Or is that happy coincidence?

[Kharon]

The original question posed by LongLats is a very good one; the Nautilus Blue postings get awful close to a best correct reasoning (for my money). It's a fair bet that this is just another example the problems we face. This sort of confusion or debate simply should not happen. The trick is to find a 'suitable platform' and get it sorted. Who to talk to and how fast could it be resolved? seem valid questions – perhaps a REPCON, drag the ATSB into it. Should the setting of 'best available' QNH for minima on any IMC operation be even remotely ambiguous?.

On a practical level though, consider the differences from an operational standpoint. Lets go to Darwin, during the wet and use the Mk1 'turbo prop' as an aircraft. It's pisitively possing down, 60 hold everywhere and you are to be dispatched to a remote, no aid, no approach, no TAF field 90 minutes away. The nearest alternate, with an aid and AWIS is 60 miles away and needs 60 holding. Lets work on a 240 knot ground speed and use 30 feet per Millibar (I know, but Hectopascals, well).

Ok, at Darwin you know the altimeters are good and the QNH valid, so off we go. Ten minutes later 1013 set and cross checked, flight levels for the next hour. Approaching top of descent we get area QNH and traffic. Lets set grid LSALT at 2500' and we know there are terrain and 'obstacles' to consider. IF the area QNH is (unusually) 5 Mb wrong, which way will it be wrong?; either way its +/- 150 feet. Down we go to 2500, is there anything to hit at 2350? and can we get visual from 2650?. Would the AWIS QNH from 60 miles away be of any 'practical' assistance?.

No luck today , no breaks at '2500' indicated; so off we go to ALTER 60@240=15 minutes later we are approaching the aid, setting up for approach; lets say the TOD area QNH is 1015, AUTO AWIS is calling 1010, the TAF is indicating 1012.5. Circling minima 1960', elevation say 1200'. That's theoretically 760' AGL @ minima against the 'correct' MSL reading. Using 1015 Mb, 1013 or 1010 the difference between the lowest and highest go around indicated ALT is 'minima' +/- 150 feet; at worst you have 610', at the other worst 910'. The use of TOD area QNH theoretically reduces the 'deficit' to ~ 75' +/- (2.5 Mb).

Potential problems, can the notion of being altitude 'fat' at minima be a temptation to 'pinch' a few feet. Put the ALTER cloud base at 800', (with some rain for fun) and think about fuel remaining; 60 holding + fixed reserves = nogat, tasol. The second shot is going to have to count.

I'd like to see an official answer and, if required a change to the AIP, wouldn't you.....Safe skies and all that. Interesting thread - ain't it.

Hat, coat – Taxi.

[Capn Bloggs]

No Kharon. You have access to the AWIS. Use it's QNH. The issue is that you cannot use the METAR QNH, regardless of where it came from.

It's not rocket science and the rules are quite clear, as stated in the OP. It's just that some want to do something different. Fair enough. Get CASA to change the rules.

[Bladeangle]

The second shot is going to have to count.

Thats a completely different story if your in that situation.

Re the use of METAR QNH, if there were any changes to be made to the AIP accommodating its use, I don't see why its use couldn't be allowed (if the changes were made) so long as the METAR issue time is within 15min of your arrival.

Or, increase the frequency of issuing of a METAR to say every 10 or 15mins. Problem is every man and his dog will be requesting the latest METAR from ATC.

But then if you have NAIPS on your smart phone, you could get it yourself. But if you've got phone coverage, just call the AWIS.

How many aerodromes are there without at least a phone AWIS?

[Capn Bloggs]

I don't see why its use couldn't be allowed (if the changes were made) so long as the METAR issue time is within 15min of your arrival.
Minus the time it has taken to actually create the METAR. Certainly, with human METARs, the actual readings could have been taken some minutes before the issue time, reducing the 15 minute window. Perhaps even with METARs created with auto info, there is a delay between the readings time verses METAR-issue time.

That will be why you cannot use a METAR QNH...

[Kharon]

Agree LongLats. I think we can all agree that the METAR QNH does not conform to point a. or c. in the above paragraph. Now some people may say that the METAR QNH qualifies as a forecast terminal QNH, therefore meeting point b. Others disagree and say that a METAR is a report, not a forecast and therefore cannot meet the requirements of point b. above.

Agree J3. So you would rather be on a forecast QNH that could put you well below even the actual QNH minima, instead of using the recent QNH from an auto weather station, got it.

Agree N. Blue. Shouldn't the TAF, METAR, area QNH and current QNH from the aerodrome sensor all be within 5 HPa?
Agree Bloggs. The issue is that you cannot use the METAR QNH, regardless of where it came from.

That's why METAR was not mentioned. But why?, as NB indicates, is it's use specifically Verboten. Forecast is an educated best guess, reported is based on fact (as was). So, for the sake of further clarity why not make it so that provided the METAR QNH is within 5 Hpa of forecast QNH, for the time, the METAR QNH is acceptable for use, at pilot discretion.

[Capn Bloggs]

But why, as NB indicates, is it's use specifically Verboten.
By accident or by design. Who knows. But as important as your "within 5mb" is when were the readings for that METAR actually taken? That is the critical issue and the one which, I suspect, dictates the current prohibition on use of METAR QNH.

Back to J3 a while ago:
So you would rather be on a forecast QNH that could put you well below even the actual QNH minima, instead of using the recent QNH from an auto weather station, got it.
Yes I would, It's legal and I've got a better chance of getting in. That's the aim of the exercise, isn't it?

Now some people may say that the METAR QNH qualifies as a forecast terminal QNH, therefore meeting point b.
That's half the problem. People don't know the difference between a METAR and a forecast, or are slack enough to twist the obvious to suit their own ends. Sorry, nobody with half a brain could think a METAR qualifies as a forecast terminal QNH. In decades of flying I never come across anybody who's thought that.

[Kharon]

AIP GEN 3.5-8. 4 - Meteorological reports.

4.1. Aerodrome weather reports: are reports of observations of meteorological conditions at an aerodrome. The reports are generated by automatic weather stations (AWS) and may have manual input by approved observers. Manual input of visibility, weather and cloud is for an area within a radius of approximately 5NM of the aerodrome. E&OE.

4.1 1. Owing to the variability of meteorological elements in space and time, to limitations of observing techniques and limitations caused by the definitions of some of the elements, the specific value of any of the elements given in a report shall be understood by the recipient to be the best approximation to the actual conditions at the time of the observation. E&OE.

I think they've got it now Bloggsy – handing over.

[j3pipercub]

Thanks Bloggs,

"Yes I would, It's legal and I've got a better chance of getting in. That's the aim of the exercise, isn't it?"

Wow ok, that sounds prudent.

j3

Ps It's never been proven I actually had a brain to begin with.