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

[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 .

[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!

[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]

Just to stir the pot.

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]

Clarity – Nearly.

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:

Originally Posted by J3Piper cub

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.