QNH correction for approach

[novicef]

Hi everyone,

I have just changed over to Jeppesen charts and I am now getting confused about the applied QNH correction for approach minima.

For example if you are flying into a main airport with ATIS you can fly down to the actual aero QNH minima with no correction, same goes for when using QNH off the TAF (use forecast QNH).

However, what is the correction if any when using Actual Area QNH (given by ATC)for an approach?

(I understand the 50ft correction for forecast area QNH)

Thanks in advance
Nf:)

[THE IRON MAIDEN]

Actual QNH applies to an approved source. ATIS, AWIB. or if your RPT, your man or woman on the ground.
Forcast QNH applies to what you get off your TAF.

Jepp put the two on there, where ASA make you take the 100' yourself.

The 50' is PEC (pressure error correction) and applies to an ILS.

Cheers Maiden

[novicef]

Thanks Ironmaiden

But is there a correction when using Actual Area QNH (given by ATC) for the approach?

[Maggott17]

Hey NOVICEF,

go to a reputable flying school and get an Instrument Rating.

Then you may understand the answer to your question.:ugh:

Every IFR pilot knows how to deal with the type of QNH available at his/her destination.

Its as basic as applying power before you commence your take-off run.

[THE IRON MAIDEN]

No, not unless its the ILS.

ASA plates only put the forecast QNH minima on their plates, the correction you might be thinking of is taking 100' off those minimas for Actual.

Jepp make it nice and easy. read off which one you have, actual or Forecast.

Maiden.

[Victor India]

Novicef,

Regarding the minima used at "main" aerodromes (like SYD, MEL, ADL etc) - on an Airservices approach plate, there is no shading on the minima, because tower is assumed to be active H24 and accurate QNH is assumed to ALWAYS be available.

For other aerodromes (which may not have accurate QNH sources) - you will find shading on the Airservices plate. With forecast TAF (Terminal) QNH, use the published minima. With an ACCURATE QNH (AWIS, Unicom, CAGRS etc), you may lower the minima by 100', and I believe Jepps show both minima on their plates.

Your question about "actual area QNH"... mmmn not sure what you mean. As far as I understood, there is no such thing. Maybe an ATC reader can help? I believe they just read the area QNH off the forecast.

You are right in saying that if using area QNH, one adds 50' to the minima.

So, for example at Dubbo, using the RW23 VOR, the Airservices minima is 1450 (for the S-I minima). As it is shaded, if you have an accurate QNH ('fresher' than 15mins old), you could use 1350'. On the other hand, if using area QNH (forecast/actual if there is such a thing), you would need to use 1500'.

I don't use them, but I imagine the Jepp plate for the same approach would show two minima - one of 1450' for forecast and one with 1350' for actual QNH.

Anyone else care to comment re : the idea of an "accurate area QNH"?

Cheers

VI

[Bendo]

AIP ENR 1.5 Ss 5 Application of Aerodrome Meteorological Minima

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 (-100')
b. The forecast Terminal QNH (minima as published), or
c. the forecast area QNH (add 50')

That's all there is to it, mate. :ok:

[THE IRON MAIDEN]

all the airlines in the world use Jepps. ( or atleast the vast majority)

surely they are certified to be correct. I know Casa put the clause in with with reference to exams. (probably to scare newbies into buying ASA products over Jepps)

[flying-spike]

I can see where you got your name from.
Clearly not every IFR pilot knows this otherwise he wouldn't be asking.

[ITCZ]

>> maggot17
Every IFR pilot knows how to deal with the type of QNH available at his/her destination.

Ah, maggot, if only that were true.:ugh:

And why is it that one has to look for a 'good' IFR school? Sign of the times! Go easy on him mate, there are some pilots of large kero burners that could not tell you the difference!

>> novicef

This is not a Jepp v AsA issue. I think you have not had this bit of altimetry properly explained to you. Changing to the Jepp presentation has possibly revealed the misunderstanding.

Refer to Bendo's post above. He is on the money. There are 3 types of QNH for your altimeter, not 4.

You asked:
what is the correction if any when using Actual Area QNH (given by ATC)for an approach?

If ATC ever gives you an "actual" area QNH, reply by demanding an "actual" case of beer on arrival! There is no such thing as an "actual" area QNH. :=

Jepp MET AU-1 Definitions
AREA QNH - A forecast altimeter setting which is representative of the QNH of any location within a particular area.

Area QNH are forecast for Area QNH Zones (AQZ) which normally match ARFOR boundaries (Jepp MET AU-11).

"representative of the QNH of any location" means + or - 5HPa. If the QNH of any location is likely to vary more than 5HPa from the forecast area QNH during the forecast period, the MET office will subdivide the AQZ/ARFOR and provide one area QNH for each of the smaller areas.

e.g subdivide ARFOR area 30 to make two areas, one east of a line MIA to ML, one west of the line.

Why 5HPa? Well, 1 HPa = roughly 30' so 5 HPA = roughly 150'

which is... the maximum possible difference between the Actual Aerodrome QNH (AsA chart -100') to Area QNH (AsA chart +50').

:cool:

Make sense?

[Counter-rotation]

Maggot17 - You are officially banned (by me :ok: ) from ever asking another pilot a question about anything - Jeezuz mate. Yes it's an important point but I can bet you only know because someone taught you. Same goes for me.

Novicef, as someone else said BENDO got it right.

I'll say it again here, with some further elaboration. There's a lot of words here, but it's really quite simple when the "penny drops"...

NON PRECISION APPROACHES

Three flavours of QNH exist:

1) Area QNH - available pretty much everywhere all the time.

If you set this, take the "Forecast Terminal QNH" minima from you Jepp plate, or the published minima on your Airservices plate, and add 50'. That's your minima.

2) Forecast Terminal QNH - available if you have a valid TAF, and your ETA falls within the period covered by the QNH groups at the bottom (NB it may be a TAF with 24 hour validity - that does not mean you have 24 hours of QNH information on the TAF... a minor point but worth knowing).

If you set this, use the "Forecast Terminal QNH" minima from your Jepp plate, or the published minima on your Airservices plate. That's your minima.

3) Actual Aerodrome QNH - this is the sort you get from an ATIS, AWIB, Tower, approved met. observer etc. In the majority of locations you won't have it. Remember, it is valid only for 15 minutes too. Don't copy the ATIS at 50 miles, bore in, sit in the hold, etc. etc. then fly the approach with a stale QNH, to a reduced minima (not in a renewal anyway :ouch: )

If you set this, you can:
(1) For Jepp plates - use the "Actual Aero QNH" minima.
(2) For airservices plates WITH SHADED BOXES - subtract 100' from the printed minima. (If no shading, use the published minima.)

PRECISION APPROACHES

Everything above applies exactly the same, but now...

There is also the question of PEC. This is an instrument installation correction and not to do with QNH. It is "another" kind of minima "adjustment", and is:

+50 feet, or the published figure from the aircraft's AFM.

With most precision approaches (ie. ILS), you will have access to "Actual Aerodrome QNH" - but there are cases when you won't, and anyway it is important to still understand the theory behind it all.

What a mouthful all that is... But as I said, be not afraid - you will get it with just a little exposure - you probably already have. Sorry if this post came over as patronising. But maybe someone else will read it one day and it will help them to "get it".

CR.

[Spearmint]

Good on ya "Maggot"

Way to give the bloke a hand.

It's oxygen thieves like you that stop questions & queries, no matter how simple or complicated, being discussed without ridicule or reprimand.

Have you ever heard of the notion: "There are no stupid questions" ?

Wake up to yourself, have a good look in the mirror & try to think that perhaps everyone else isn't as good as you apparently think you are and are obviously not.

:yuk:

[Tempo]

Found the answer so post deleted