Temp's and QNH's in a TAF

[anothertwit]

Hi guys,

Can anyone explain why the first forecast Temp and QNH given in a TAF are only valid for 90mins while the other three sets of forecast values are valid for 3 hours? :sad:

[mcgrath50]

I struggled hard with getting my head around this one until someone told me to think about it in 90 min blocks. Eg; The 2nd QNH lasts 90 mins before and after the time.

A diagram makes more sense

TAF valid from 0100.

T 24 20 18 24

0100 24 0230 20 0530
|----------|---------!----------|


See how it's 90 mins either side of the middle time? Hopefully this helps.

If you are asking why they do it this way? No idea, convention I guess.

[Nirak]

The times on a TAF are hours only, because it is a prediction onto the future. No Met man (unless he has heavenly powers) can "see" that accurately into the future. The temp and QNH forecast therefore covers a block where the start time is the 'earliest' this phenominon will happen (before the published hour) and the end time is the 'latest' it is predicted to happen (slightly after the published time)

Remember: The future is the worlds best kept secret.

A METAR, on the other hand, is recent 'history'. It has happened already. It was physically measured during the 10 min prior to its publication time, on the hour e.g. 0800 UTC and, at International airports also on the 1/2 hour e.g. 0800, 0830, 0900, 0930 etc

A SPECI will give you the exact time of observation, hours and minutes, because it is 'history' also, measured physically or observed.

Read the AIP Chapter 3.5 MET section. You will now understand it better..:ok:

[anothertwit]

not sure what your getting at Nirak? i understand these values are only a "prediction", that's the reason i called them "forecast values"

What i'm asking is why the first of the 4 values given is only valid for 90mins? This would mean that in a 12hour TAF the last 90mins of it's validity period would has no valid "forecast" Temp or QNH. :ugh:

Seeing as most of the TAF's i see are valid for a 12 hour period I thought this was an topic. :confused:

[Nirak]

E.g. the 0900 TAF:

The forecasted Temp and QNH can happen from as early as 0730 onwards but can also occur as late as 1030. The prediction covers the three hours bridging / saddling the forecasted time.

With the last Temp and QNH: It can happen from 90 min before the 12 h TAF ends, but it can also happen as late as 90 min after the end (13,5 hrs from the start time of the TAF), but that will then be the updated start time of the new 12 h TAF (TAF's are updated every 6 hrs and therefore overlap, but the futher in the future you go, the more inaccurate and vague they become.

That is why you have to fuel and plan for an alternate that has much better wx and less chance of a mishap. TAF's are for planning.

During your flight you compare the predicted with the actuals (METAR) to determine how accurate the Metman was, and if needed replan, refuel, re-route or divert.

If you look at the TAF:
TAF YXXX 110835 110921 ....

TAF was finalised and published on the 11th of the month at 0835 UTC
covering the period 0900 UTC to 2100 UTC

0835 UTC is the exact time it was published by the Metman (hours and min)

The actual TAF period is just in hours because it can happen either slightly before or slightly after, no Metman can predict right down to the minute, not humanly possible. That is why there is that "bridging / saddling" time covering 90 min before and after.

[Awol57]

I always believed they were the forecast values for that particular time. So in a 0000 TAF the first set is the forecast values at 00, the next set at 03, 06 and 09. Then you can interpolate the values in between. I couldn't find a reference in AIP or on the BoM website.

[Nirak]

The principle is explained somewhere in that chapter.

I do not have an AIP here with me at the moment, will have to search a bit for the exact page ref.

[Awol57]

Found your reference Nirak


Gen 3.5-31 12.16

I can't copy and paste for some reason at the moment, thanks Adobe.

[Nirak]

Yep

GEN 3.5 (12.16 and 12.17)

Encompasses both sides of the forecasted hour, for the reasons I have given: The Metman is human :ok:

[muffman]

I'm not sure I understand nirak's point about the forecaster being human. But I will disagree about interpolating between the QNHs. Each forecast QNH is valid for the specified times. There is no interpolating.

Someone probably has more details, but to add something else to the discussion, the temp/QNH section of the TAF format changed a couple of years ago from each temp/QNH being valid for 3 hours starting at the beginning of the TAF's validity, to the present system where the first is valid for 90 minutes and the rest being valid for 3 hours each.

The reasoning, I believe, is that under the previous system, the potential for error was somewhat greater. By way of example, imagine the QNH is decreasing from 1013 at a rate of 1hPa per hour. At the end of each 3 hour block, there is a 3hPa error. Under the new system, the forecast is being made for the central time in each 3 hour block, so the error in that example would be only +/- 1 hPa.

As for the first one only being 90 minutes and the last 90 minutes of 12 hour TAF being unforecast, it's just how it has to be in order to maintain the +/- 1.5 hour system. The final 90 minutes is not an issue for most operations because you need to obtain a forecast within 60 minutes of your departure and few of us are still flying 11 hours later. And if you are, it is likely you could obtain an updated forecast in flight to cover that period.

I disagree with the suggestion about the future being impossible to predict and all that stuff. When it comes to these QNHs, it really is fairly predictable and as a result of that, we are able to fly instrument approaches using only a forecast QNH and the only penalty is adding a 100ft (3hPa) buffer. So with all the risk assessment that goes into instrument procedure design, they have decided they will always be within 3hPa of the correct QNH. Not bad. Even the area forecast QNH is guaranteed within +/- 5.

[Captain Sand Dune]

Now that that's sorted, can anyone explain howthese ALT Correct vs Temp. charts in the front of DAPs work? I remember when the old DAPs had a table in the front - much easier. Then they were taken out, and now we have this new "improved" version.
Does it apply to precision approaches only, or to all approaches?
The example talks about not only adjusting the DA but also the IAF. What about the intermediate ALTs?
How will ATC know what adjustments the pilot has made to the IAF ALT when cleared to the IAF for an approach?
How will other pilots know what ALT adjustments other pilots have made to their IAF ALTs when in Class G?

[Nirak]

Oh, I forgot ! Pilots walk on water, therefore they are not human and can predict the future right down to the minute!!!

I guess the Metman's forecast is always right, pilots complain about inaccuracies purly as a matter of it is a nice chance from complaining about the airline food

I have to deduct then that the Japanese are human too, the fact that they could not predict the earthquake beforehand :E

Getting an inflight updated forecast will make the data more accurate, because accuracy deteriorate the further you predict into the future.

Getting an actual from the tower or the ground staff during the descent will be even more accurate, leaving you with only temperature, instrument and calibration error on the altimiter only

No heavenly powers needed, because 100' is added as a safety margin for determining your minima. Sorry to make your godly appearance evaporate in the mist :mad:

[The Green Goblin]

Just circle the highest temp, and the lowest QNH and plan accordingly.

:ok:

[Nirak]

:ok: Green Goblin

Always assume the worst case scenario and you will error on the safe side !! :)

[FGD135]

Off topic I know, but can somebody answer this:

If planning an IFR flight to a destination that does NOT have a TAF, is there a requirement to plan for an alternate (for the reason that there is no TAF)?

Assume that the destination in question is one of those that NEVER have TAFs issued for it (as opposed to those destinations that do have TAFs, but whose coverage is for less than 24 hours).

[Agent86]

FGD135 ...
AIP GEN 57.1.3 (page 88 of the PDF from the web)

When an aerodrome forecast is not available or is "provisional", the pilot in command must make provision for a suitable alternate that has a firm forecast.

[muffman]

Nirak...dude. Are you Charlie Sheen? That was quite a rant! At first I thought you had no idea what you were talking about. Now it's crystal clear.

[anothertwit]

Nirak, three words buddy.....puff puff pass! :hmm:

[Nirak]

anotherwit

You are welcome to do so, mate. The choice is yours, just remember, terra firma is down there somewhere below.......'terain, terain, pull up, pull up'

[PyroTek]

I have to deduct then that the Japanese are human too, the fact that they could not predict the earthquake beforehand :E

Funny thing is, those in Tokyo had 90 seconds warning of said earthquake:}

[FGD135]

AIP GEN 57.1.3 (page 88 of the PDF from the web)

When an aerodrome forecast is not available or is "provisional", the pilot in command must make provision for a suitable alternate that has a firm forecast.

Wrong! I know it says exactly those words in the AIP, but in the case of aerodromes that NEVER have TAFs issued for them, that is not what is meant.

Try again.

[Agent86]

FGD ..OK try this one.

Enroute 1.2.3
For flights to a destination for which a aerodrome forecast is required and cannot be obtained or is "provisional", the flight is permitted to depart providing an alternate aerodrome meeting all the requirements specified in ENR1.1 Section 57 is provided.


Cannot be obtained ... can also mean there aint one to get!

Now it all comes down to english interpretation .. That's what Lawyers are for ...not pilots

[Icarus2001]

that is not what is meant.

Your certainty of this is based upon what exactly?

[Counter-rotation]

FGD 135...

Got to agree with Icarus and Agent - and I have run up against this in the past. The argument "well most pilots don't do that" or "there's never a TAF for there, so use the ARFOR", or something else that sounds similar, is not the basis upon which I'm going to conduct my operations.

You can if you want to, that's up to you.

[Capt Fathom]

If there's no TAF, there's probably no Navaids (eg station strip).

So more than likely, you'll need an alternate anyway!

[patienceboy]

In response to the original question, just think of it as each temp/QNH being valid 90 mins either side of their forecast time (except for the first one because that is a spot forecast for the same time that the TAF commences, so you only move forward 90 mins from there). E.g. for a TAF commencing at 0600, the published QNH values are valid for spot times of 0600, 0900, 1200, 1500 (every 3 hours). You use whichever QNH that your arrival time falls within 90 mins of. E.g. ETA 1325 – you would use the 1200 QNH. Six minutes later and you would use the 1500 QNH. Hope that helps.

Wrong! I know it says exactly those words in the AIP, but in the case of aerodromes that NEVER have TAFs issued for them, that is not what is meant.

Try again.

I don’t agree. It is not sensible and doesn’t make any logical sense. If an ARFOR was acceptable for determining alternate requirements, wouldn’t you just use that if the TAF wasn’t available or was provisional? So in the event that a certain TAF is provisional (cavok) and the corresponding ARFOR is fine, the aerodrome with the provisional forecast would need an alternate but a busted bush strip 3 nm away that is never issued with a TAF wouldn’t?
How could you tell if the crosswind at your destination was to exceed the maximum for your aircraft based on an ARFOR? How could you tell if any wx deteriorations were inter or tempo?

If there's no TAF, there's probably no Navaids (eg station strip).

So more than likely, you'll need an alternate anyway!

... unless cloud if forecast less than scattered below LSALT +500ft and vis greater than 8km.

[muffman]

Personally, I don't find it that hard to interpret. I think the entire TAF requires a certain level of knowledge to interpret correctly and the temp/QNH section is no different. Adding more QNHs and temps without actually increasing the precision of the forecast would seem counter intuitive to me. But hey, I just drive aeroplanes around :8

[Nirak]

muffman

I can see you are understanding the issue now clearly, well done !!:ok:

[muffman]

Why must you continue adding so little with so many words? :ugh: A waste of an increasingly valuable IP address.

[FGD135]

If there's no TAF, there's probably no Navaids (eg station strip).

So more than likely, you'll need an alternate anyway!

Getting warmer!

[Capn Bloggs]

Getting warmer!
Are you geniunely interested in people's opinions or is this just a test?

[Berner]

This one gets dragged out every now and then but never seems to get settled. Is anyone aware of this ever coming up in a CASA investigation or court case and having some sort of precedent set?

[Capn Bloggs]

Is anyone aware of this ever coming up in a CASA investigation or court case and having some sort of precedent set?
I don't understand why it would. The regulation is pretty clear. If one doesn't understand it, fair enough, ask, as the original posters has, but it is hardly so vague that it could be challenged in court or be used to explain a serious operational incident.

[Icarus2001]

Bloggs it does go to the level of literacy amongst our ranks that some cannot make simple sense of "If an aerodrome forecast is not available..."

Judging by some of the poorly thought out, illogical arguments seen on this forum and the poor use of english grammar and spelling not surprising really.

On the TEM thread apparently it is all a waist(sic) of time.

[FGD135]

Are you geniunely interested in people's opinions or is this just a test?

Just a test. A test to ascertain how many pilots truly understand alternate aerodrome requirements.

I have not yet found a pilot that understands the rule regarding TAFs.

Everybody seems to think that, unless you have a TAF for your destination, you must provide for an alternate. But this understanding is mostly wrong.

It is easy to see why everybody has this erroneous understanding. It is because of that poor wording in the AIP. Here is that wording:


AIP GEN 57.1.3 (page 88 of the PDF from the web)

When an aerodrome forecast is not available or is "provisional", the pilot in command must make provision for a suitable alternate that has a firm forecast.


I am limited for time at the moment and may have to come back later, but for now, I will begin the explanation of what the rule really is, and why it is.

That AIP wording only applies to aerodromes that NORMALLY have a TAF issued for them - and that includes aerodromes with "part time" TAFs.

It does NOT apply to aerodromes that NEVER have TAFs issued for them. This aspect is not well communicated in the AIP wording.

If the aerodrome NEVER has a TAF issued for it, you do NOT need to provide for an alternate (for the TAF reason - you may still need the alternate for other reasons, however. See below).

If the aerodrome NORMALLY has a TAF - including those with TAF coverage that is not round the clock - then the above AIP wording applies (i.e: yes, you must provide for an alternate if the TAF has not been issued, or has been marked "provisional").

To understand how and why this is the intent of the rule, it is necessary to look at ALL the rules relating to alternate requirements (especially those related to reasons of weather, but also those related to navaids).

I will come back to this thread later, but for now, will pose a question that may help to resolve this question instantly:

Regarding weather aspects, for non instrument approach aerodromes, the rules only require nomination of an alternate when the forecast cloud is more than SCT below the final route segment LSALT + 500'.

Application of this above rule, by day, will often mean that the provision of an alternate is NOT necessary. And, given that these are non instrument approach aerodromes, there usually won't be a TAF for them.

So, the question is:

If it is as simple as NO TAF = ALTERNATE REQUIRED, then why does the above rule exist?

Ignore all the above re RPT. I have never learned the rules re RPT, but believe they different to what I have quoted above.

[Capn Bloggs]

Off topic I know, but can somebody answer this:

If planning an IFR flight to a destination that does NOT have a TAF, is there a requirement to plan for an alternate (for the reason that there is no TAF)?

Assume that the destination in question is one of those that NEVER have TAFs issued for it (as opposed to those destinations that do have TAFs, but whose coverage is for less than 24 hours).

Just a test. A test to ascertain how many pilots truly understand alternate aerodrome requirements.
Off topic, and a test for pruners toboot. When you knew the answer all along. Get a life. :=

[anothertwit]

As the original poster of "this" thread, my question was not to explain how it worked, as this is very simple, but WHY it is like it is? muffman's first post is the only reply that seems to have anything to do with my question. In true PPRuNe thread drift fashion.....here we are. :ugh:

All good though, I love a good discussion/debate. Especially when we can all learn or rather relearn something that can add to safety. :ok:

anywho back to the banter :ouch:

FGD135, what about the 8k's vis? seems your test has some flaws!!!:=

[patienceboy]

FGD135.

You might quote ATC Flight planning 1.2.1:
“A forecast must be either a flight forecast or an area forecast with an aerodrome forecast for the destination and, when required, the alternate aerodrome. For a flight to a destination for which a prescribed instrument approach procedure does not exist, the minimum requirement is an Area Forecast”

No problem, so we can still depart without a TAF for our destination, but I don’t believe this absolves us of ATC Airports and Ground Aids 3.1.3 (Alternate Aerodromes) which clearly states that “When an aerodrome forecast is not available or is provisional, the pilot in command must make provision for a suitable alternate that has a firm forecast”. There is no getting around this one!

Regarding weather aspects, for non instrument approach aerodromes, the rules only require nomination of an alternate when the forecast cloud is more than SCT below the final route segment LSALT + 500'.

Application of this above rule, by day, will often mean that the provision of an alternate is NOT necessary. And, given that these are non instrument approach aerodromes, there usually won't be a TAF for them.

So, the question is:

If it is as simple as NO TAF = ALTERNATE REQUIRED, then why does the above rule exist?

ATC Airports and Ground Aids 3.3.2 (a) “… a flight may be planned under IFR by day to a destination aerodrome which is not served by a radio navigation aid without the requirement to provide for a suitable alternate, provided that: a) not more than SCT cloud is forecast below the final route segment LSALT +500ft and forecast visibility at the destination aerodrome is not less than 8km…” How could you tell the forecast visibility at the destination aerodrome with only an Area Forecast? These alternate minima are based on a TAF as required by 3.1.3 above. The only reason they refer to the “final route segment” is to calculate an aerodrome alternate minima which should get you visual by your LSALT.

ATC Airports and Ground Aids 3.2.1 (d) also requires an alternate if the forecast crosswind or downwind component is more than the maximum for the aircraft (including gusts). How could you comply with this without a TAF?

In summary, you need a TAF for your destination. You are permitted to depart without a TAF if your destination is an aerodrome without an instrument approach procedure, or the normal TAF is not available or provisional, but you must carry fuel for a suitable alternate with a firm forecast.

[FGD135]

patienceboy,

There is no getting around this one!

As I said in an earlier post, that rule applies only to aerodromes that normally have TAFs issued for them. You are reading the wording way too literally. There is a subtlety in the wording that you are not picking up. Here is that wording again:

When an aerodrome forecast is not available or is provisional, the pilot in command must make provision for a suitable alternate that has a firm forecast.

Note the words "not available". Such wording is not the same as "does not exist". In everyday communication - written and oral, formal and informal - the expression "not available" implies that the particular thing is usually available.

If you went to the government and said to them "I would like to take up your offer of the free Ferrari now please", would they say "that is not available", or would they say "that does not exist"?

What would the implication be if their response was the former?

That expression - "not available" - is used in another meteorologically related place in the AIP. I quote that section here. Note how, in this case, the overall intent is much more clear (given our prior knowledge that Area Forecasts are valid for daytime periods only):

From AIP GEN 3.5:

10.3 Forecasts for Flights - Valid Area Forecasts not Available

10.3.1 Route forecasts required for flights for which valid Area Forecasts are not available will be supplied subject to the prior notification specified in the following table: ...

You said:

How could you tell the forecast visibility at the destination aerodrome with only an Area Forecast?


The area forecast will tell you if all, or part of the area, has visibility less than 10 km. The whole point of the area forecast is to warn you of things like this. Don't expect it to give you a list of individual visibilities at individual aerodromes.

The area forecast will also allow you to glean whether the wind at your destination is such that an alternate may be required, but again, this information will not be presented directly for individual aerodromes.

Here's another question for you. Take a look at AIP ENR 1.1-57.2.12. About IFR flights, it says:

b. For aerodromes with an instrument approach procedure where an aerodrome forecast is unavailable or is "provisional", the pilot in command must make provision for a suitable alternate.

The question is:

Why does the directive only apply to aerodromes with an instrument approach? If your understanding is correct (i.e, NO TAF = ALTERNATE) then why is the instrument approach qualification in there?

Another way to consider the question of whether "NO TAF = ALTERNATE" is true or not is to look at the effects of such a rule and ask whether those effects could be intended by the regulatory authority.

That poorly worded passage in the AIP refers to both VFR and IFR flight.

Under your understanding, a VFR flight to a small destination 50 NM away (e.g farm strip), in the middle of the day, without a cloud in the sky, would have to provide for an alternate (because there is no TAF).

Does that sound like the kind of effect the regulator was looking for?

Under the correct interpretation of the rule, however, the VFR aircraft would only need the alternate if that flight was at night.

[patienceboy]

FGD135

Here's another question for you. Take a look at AIP ENR 1.1-57.2.12. About IFR flights, it says:

b. For aerodromes with an instrument approach procedure where an aerodrome forecast is unavailable or is "provisional", the pilot in command must make provision for a suitable alternate.

The question is:

Why does the directive only apply to aerodromes with an instrument approach? If your understanding is correct (i.e, NO TAF = ALTERNATE) then why is the instrument approach qualification in there?

You have to read this in conjunction with (a) and in the knowledge that it is talking purely about meteorological alternate minima. It is basically saying that for aerodromes with an instrument approach you can use the published alternate minima, unless the forecast is unavailable or provisional (which is a redundant clause since we already knew that). The reason it refers to “instrument approach procedure” is because aerodromes without a procedure simply wouldn’t have a published alternate minima. I am not defending the way this is written in any way! See below.

http://i1188.photobucket.com/albums/z411/patienceboy/IFRmetminima.png

Another way to consider the question of whether "NO TAF = ALTERNATE" is true or not is to look at the effects of such a rule and ask whether those effects could be intended by the regulatory authority.

I agree! Under your interpretation we could fly to an aerodrome with multiple instrument approaches, a provisional TAF and a valid area forecast and we would require an alternate - yet we could fly to an aerodrome never issued with a TAF, no instrument approach or navaids and the same area forecast and no alternate is required. That doesn’t make sense.

Under your understanding, a VFR flight to a small destination 50 NM away (e.g farm strip), in the middle of the day, without a cloud in the sky, would have to provide for an alternate (because there is no TAF).

No he wouldn’t. See below.

http://i1188.photobucket.com/albums/z411/patienceboy/50nmVFR.png

When an aerodrome forecast is not available or is provisional, the pilot in command must make provision for a suitable alternate that has a firm forecast.

Note the words "not available". Such wording is not the same as "does not exist". In everyday communication - written and oral, formal and informal - the expression "not available" implies that the particular thing is usually available.

Section 16 lists the “locations for which TAFs are issued”

http://i1188.photobucket.com/albums/z411/patienceboy/TAFavailability.png

Given that this is a list of aerodromes “for which a TAF will be available”, couldn’t one reasonably conclude that a list of aerodromes which don’t feature here would be a list of aerodromes for which a TAF will not be available?

The area forecast will tell you if all, or part of the area, has visibility less than 10 km. The whole point of the area forecast is to warn you of things like this. Don't expect it to give you a list of individual visibilities at individual aerodromes.

The area forecast will also allow you to glean whether the wind at your destination is such that an alternate may be required, but again, this information will not be presented directly for individual aerodromes.

I don’t agree that an area forecast can be used to calculate meteorological alternate minima. This is a major point. An ARFOR is a general forecast for a very large area to be used enroute. In addition to points already alluded to, ENR 57.2 (Alternate Aerodromes – Weather Conditions) continually refers to weather at the “destination”.

For example, I don’t agree that the area forecast gives you enough information to decide whether the crosswind or downwind will exceed the maximum for your aircraft. The wind at 3000ft very rarely resembles the wind on the ground with any degree of accuracy, and local winds can vary greatly due to terrain, proximity to coast etc. I have copied a random area forecast here:

http://i1188.photobucket.com/albums/z411/patienceboy/ARFOR.png

This is great info enroute, but what is the wind forecast on the ground at our destination? 070/30? What about gusts? Smoke over land reduces visibility to 8km, but then this “may" be reduced further in thick smoke near fires. Where are these fires? Are they near my destination? Does this mean I need an alternate? Maybe I should just carry fuel to get somewhere with a firm aerodrome forecast. That makes sense.

[blacknight]

If you are using an area forecast as there is no TAF available then the criteria required at the destination are VMC. Would it not then be possible to overfly and look at the windsock and isn't that what "they" expect you to do?
The biggest problem with using an area forecast is deciding if your destination is in a particular area used in the forecast eg. on the western slopes or on the ranges south of xxx. Can become sticky.
It's ok to say just plan for an ALT if in doubt but sometimes fuel and weight become a big issue and out west it's a long way to another destination that does not require an ALT. I know of quite a few aerodromes without TAF service but they have an aid or an RNAV approach.

[FGD135]

patienceboy,


You have to read this in conjunction with (a) and in the knowledge that it is talking purely about meteorological alternate minima.

You missed the point of my question. You seem to be missing the point of most things and not thinking critically about what is before you.

Yes, the heading to that passage (57.2.12) implies that what follows is purely about alternate minima, but then you get to section (b) and you find that that section is not actually about alternate minima.

Sections (a) and (c) cover all the possibilities (all two of them) for IFR flight. Section (b) does not contribute anything at all relating to alternate minima.

Section (b) does not need to be there, and the question of why it is there is irrelevent.

But what is relevent is the implication carried by the words of (b).

According to you - a believer that "NO TAF = ALTERNATE" - the words "with an instrument approach" are in that section by error. Here is that section again, this time with the erroneous words (according to you) highlighted in red:

b. For aerodromes with an instrument approach procedure where an aerodrome forecast is unavailable or is "provisional", the pilot in command must make provision for a suitable alternate.

According to you, the inclusion of those words in red was an error on the part of CASA.

-------------


No he wouldn’t. See below.

Should I have said 51 NM instead of 50 NM? Ok, try this:

Under your assertion (that "NO TAF = ALTERNATE"), a VFR flight, in perfect daytime weather, to a small farm 51 NM away must provide for an alternate (because the place has no TAF (and no navaids)).

-------------

Given that this is a list of aerodromes “for which a TAF will be available”, couldn’t one reasonably conclude that a list of aerodromes which don’t feature here would be a list of aerodromes for which a TAF will not be available?

That is NOT a list of aerodromes for which a TAF will be available.

That is a list of aerodromes for which a TAF MAY be available. For some aerodromes, the TAF coverage is not "full time" (i.e. the TAF covers daylight hours only).

If the aerodrome has a TAF during the day only, then, during the night hours, the TAF is ... wait for it ...

NOT AVAILABLE

-------------


Under your interpretation we could fly to an aerodrome with multiple instrument approaches, a provisional TAF and a valid area forecast and we would require an alternate - yet we could fly to an aerodrome never issued with a TAF, no instrument approach or navaids and the same area forecast and no alternate is required. That doesn’t make sense.


At long last we are getting to the logic behind the rules. I'm glad you raised this because it shows you are at last starting to think about things, rather than just reading words from a page.

It's all about the navaids.

Yes, it appears that navaid aerodromes impose greater requirements than non-navaid aerodromes but this is not actually true. In practice, the average GA pilot will carry an alternate for non-navaid aerodromes far more frequently than he will for those with navaids.

And it would probably be true to say that, in the majority of cases where the average GA pilot is carrying an alternate for a non-navaid aerodrome, he is unaware he is doing so!

An aerodrome with a navaid will tend to be one that does not require an alternate, and can be nominated as an alternate. Such an aerodrome WILL have a TAF routinely issued for it.

But the TAF may not cover the night hours. The TAF is then considered "not available" and that rule (about which we are debating) comes into play.

That rule, as we well know, is this one:

When an aerodrome forecast is not available or is "provisional", the pilot in command must make provision for a suitable alternate that has a firm forecast. (AIP GEN 57.1.3)

The logic behind the rule is about addressing the fact that, despite having an instrument approach, without a valid TAF, the aerodrome is not satisfactory as a place that can serve as an alternate for other aerodromes.

Please note that, for simplicity of discussion, I have deliberately left out runway lighting related alternate requirements.

[Icarus2001]

Yes, it appears that navaid aerodromes impose greater requirements than non-navaid aerodromes but this is not actually true.

Mmmm, then of course RNAV(GNSS) approaches...
b. For aerodromes with an instrument approach procedure where an aerodrome forecast is unavailable or is "provisional", the pilot in command must make provision for a suitable alternate.


Navigation aid is not even mentioned in that sentence is it?

So dirt strip in the GAFA with an RNAV approach but no TAF WILL REQUIRE an alternate.

[FGD135]

So dirt strip in the GAFA with an RNAV approach but no TAF WILL REQUIRE an alternate.

Not necessarily. If it is day, and the weather is good, then no, you do not need to plan an alternate. That the place has an RNAV approach makes no difference.

By "weather is good", I mean the forecast weather satisfies the alternate requirements, which you can find in AIP.

[patienceboy]

You missed the point of my question. You seem to be missing the point of most things and not thinking critically about what is before you.

I certainly am thinking critically. I can see your argument clearly. I would happily change my interpretation of the legalities (not my practice), but you are not addressing my points.

I'm glad you raised this because it shows you are at last starting to think about things, rather than just reading words from a page.

You say that as if I have been regurgitating GEN57.1.3; which I have not. I have given my best efforts to addressing your questions – which didn’t turn out to be questions at all.

That is NOT a list of aerodromes for which a TAF will be available.

That is a list of aerodromes for which a TAF MAY be available. For some aerodromes, the TAF coverage is not "full time" (i.e. the TAF covers daylight hours only).

If the aerodrome has a TAF during the day only, then, during the night hours, the TAF is ... wait for it ...

NOT AVAILABLE

Aerodromes and Categories for which a TAF will be Available is the actual name of the list, which is why I pasted a copy and put that section of the name in quotation marks.

Should I have said 51 NM instead of 50 NM? Ok, try this:

Under your assertion (that "NO TAF = ALTERNATE"), a VFR flight, in perfect daytime weather, to a small farm 51 NM away must provide for an alternate (because the place has no TAF (and no navaids)).

Only if his cruise speed is less than 100kt.

http://i1188.photobucket.com/albums/z411/patienceboy/NoForecast30mins.png

Please refer to the point I made in post 41 regarding ARFORs and meteorological alternate minima (specifically wind). How could you meet all of the requirements of ENR 57.2 without a TAF? If an ARFOR cannot be used to meet ALL of the requirements of ENR 57.2 this is all academic and you simply need a TAF – which would certainly agree with 57.1.3 at face value.

[patienceboy]

Blacknight

If you are using an area forecast as there is no TAF available then the criteria required at the destination are VMC. Would it not then be possible to overfly and look at the windsock and isn't that what "they" expect you to do?

Yes, that is what you might do when you arrive at the airfield. However, at the planning stage you will need to refer to a forecast to determine whether you need to load fuel for an alternate or not.

The last thing you want is to rock up to a strip in the middle of nowhere with no fuel to divert and be unable to land.

[blacknight]

patienceboy
Fair point!

[FGD135]

Aerodromes and Categories for which a TAF will be Available is the actual name of the list, which is why I pasted a copy and put that section of the name in quotation marks.

But did you not give any thought to the fact that, for most of the aerodromes on the list, for half the day, the TAF will not be available?

Are you back to just reading words without thinking, or are you raising semantics in order to obfuscate?

Given your quibble over the 50 NM vs 51 NM, I think it is the latter.


Only if his cruise speed is less than 100kt.


Another attempt at obfuscation. What you are referring to is a rule about whether a forecast is required for a flight, which is a totally different thing to whether an alternate is required for a particular destination.


Please refer to the point I made in post 41 regarding ARFORs and meteorological alternate minima (specifically wind). How could you meet all of the requirements of ENR 57.2 without a TAF?


I addressed this question of yours in an earlier post. I said then:


The area forecast will also allow you to glean whether the wind at your destination is such that an alternate may be required, but again, this information will not be presented directly for individual aerodromes.

For an aircraft's crosswind or downwind limits to be exceeded - on a sustained basis - there has to be some suitable meteorological phenomena in the area (or nearby). Examples of such phenomena include cyclones, tropical lows and fronts.

If any of those are present, the ARFOR will mention it - along with enough specifics of associated wind direction and speed to make possible the assessment of alternate requirements.

[patienceboy]

Another attempt at obfuscation. What you are referring to is a rule about whether a forecast is required for a flight, which is a totally different thing to whether an alternate is required for a particular destination.

The point of this is not to quibble, but to show that remote VFR flights hopping between strips are not unfairly disadvantaged by the alternate requirements – as was your assertion.

When questioned on an ARFORs suitability for determining meteorological minima (specifically wind) you said:

For an aircraft's crosswind or downwind limits to be exceeded - on a sustained basis - there has to be some suitable meteorological phenomena in the area (or nearby). Examples of such phenomena include cyclones, tropical lows and fronts.

If any of those are present, the ARFOR will mention it - along with enough specifics of associated wind direction and speed to make possible the assessment of alternate requirements.

I don’t agree that the ARFOR is accurate enough to comply with the requirements of ENR 57.2. As I mentioned earlier, 57.2 continually refers to “weather at the destination”, not “weather within x hundred miles of the destination”.

The winds forecast are an average for a very large area and are winds aloft – not surface winds. Local geography alone can cause winds at a specific location to be wildly different from those on the area forecast. For a light aircraft with a maximum crosswind component of 15kt, it hardly takes a cyclone to require an alternate!

The AIP also includes the words “Note: wind gusts must be considered”. Limits do not have to be exceeded on a "sustained basis". The ARFOR doesn’t give you this information. If the wind forecast at 3000ft was an easterly at 35kt, and your destination runs North/South – what will be the wind on the ground and do you need an alternate? Also refer to the ARFOR I posted earlier with a similar question re visibility.