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.

Originally Posted by j3pipercub

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.

[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?