AIP semantics re QNH

[compressor stall]

AIP ENR 1.5 5.3.3

Where the forecast area QNH is used, the minima must be increased by 50ft

OK my question is this:

Do you increase the minima by 50 ft when you are using the the QNH for, say area 60, as you have on the forecast you got before your flight.

OR

Do you increase your minima by 50 ft, as Centre give you the Area QNH at TOPD which is in effect a forecast area (ie forecast area 60) QNH.

A confused CS :confused:

[Tempo]

Hi there,

I dont have a set of Jepps handy but I am under the impression you essentially have to increase your minima any time you are using a QNH that is not from an approved source. There is a definition in there of what an approved source is but I remember it saying something along the lines of ATIS, AWIB, ATC etc. Therefore, any time you are using a TAF or ARFOR for your QNH source then the 'FORECAST QNH' minima must be used.

Anyone else???

[Jet_A_Knight]

My understanding of this CS, is that if you are using forecast AREA QNH for a terminal area, not the forecast QNH on a TAF, then 50' gets added to the minima. On Jepps that means 50' is added to the forecast QNH minima, and on DAPs, the minima that is shown in the greyed box.

If anyone knows better, I stand to be corrected.....

I didn't appreciate the way that this change was 'slipped in' to the amendment without any notification. AA should provide a summary of changes or additions for each amendment, just like you get with the CAR's/CAO's...

[Jamair]

Funnily enough, this one came up on my last CIR renewal....as I am given to understand it, the requirement is that if using a forecast Area QNH then you add 50' to the MDA; a QNH from an approved source ie ATC or ATIS et al you use the listed MDA. Through ommission of any contradicting direction, the actual or forecast Area QNH is used for any Terminal that doesn't have an approved QNH source. I find the subject to be a too little grey for my liking; too much subjective opinion can be made on its interpretation. Then again, odds are that in practical terms, 50' ain't gunna make or break too many approaches in Oz....:\

[compressor stall]

Hmmmm, yes, but the question still remains...

You are conducting a NPA into Black Stump ALA for which there is no TAF issued.

Top of Descent CTR give you "XYZ, No IFR Traffic for descent. Area QNH 1014".

Do you then add 50' to your minima?

I am arguing no because you are using actual area QNH (not forecast area QNH).

CS

[scrambler]

my understanding was. area qnh, or forecast area qnh add 50 ft. reliable source (atc, aws) or valid aerodrome forecast no need to add it.
I havent looked it up for a while so stand to be corrected

[QSK?]

Compressor stall:I am arguing no because you are using actual area QNH (not forecast area QNH)
There is no such animal as an actual area QNH (what this means is that ATC don't read it off an instrument. It is provided to them by the Bureau of Meteorology, just like it is provided to a pilot through the briefing material).

What ATS gives you TOPD is the forecast area QNH which is a representative QNH which is within +/- 5 HPa of any forecast QNH for any airport located within the relevant area forecast. What ATS gives you should be the same forecast area QNH that you see at the bottom (or top) of your ARFOR.

So the rules re MDA are:

Actual QNH available (from ATC/CAGRS/ATIS using airport met instrumentation): MDAs may be reduced by 100ft

Forecast terminal QNH (from TAF): MDAs as published (shaded boxes).

AREA QNH (from the relevant ARFOR or verbal from ATC): ie no TAF available for airport. Published MDAs have to be increased by 50 feet.

[compressor stall]

QSK

Thanks for that. If ATC might had access to more real time data then that position (which I took for argument's sake) could be right.

It is still ambiguously worded, and in the absence of the info that you provide, I challenge anyone to explain out exactly what one should do.

It could do with a clarifying note (Along the lines re MDA reducing by 100' with actual QNH: "Note METAR QNH does not meet this requirement).

CS

[QSK?]

CS:

I agree it can be confusing and should be reworded (maybe you should write to ASA?).

Just to clarify my earlier point. The reason for the extra 50 ft on MDAs when using Area QNH can be seen through the following example:

Assume the Area QNH for say, Area 85, is 1024. This means that the actual QNH at every airport within Area 85 should be within +/- 5 HPa of 1024.

Now , assume you were flying into an airport (Elev 0 ft) that had an IAL procedure (MDA 500 ft) but no TAF, you should have 1024 set on your ALT on descent (which you took off your ARFOR or were given it by ATC).

After you land at the airport, you notice (after putting the AD elevation on your alitimeter and reading the subscale) that the actual QNH at your airfield was, in fact, only 1020! If you had not adjusted the published MDA by 50ft, this means that your height above the ground at the MDA was actually only 380 feet above the ground (or 120 feet lower than the published MDA of 500 feet in this example).

So by adding the extra 50 feet, the pilot get some protection against variations between the forecast area QNH and the actual QNH at the airfield. But, as indicated previously, the extent of the pressure variance between AD QNH and the forecast Area QNH should never be more than 5 HPa.

Also remember that even if you are given an actual QNH (off met instruments) by ATC, and have adjusted your MDA by 100 feet in response, this QNH is only valid for 15 minutes, after which time the MDA would have to raised again by 100 feet if you had not completed the approach and there was no further update available (not very practical I know).

Also refer to AIP ENR 1.7-3 for more info on Area QNH.

[ITCZ]

Stallie, hold your horses mate! I'm not so sure that we should hit Mr Airservices over the head for ambiguity on this one.

If a METAR QNH was available, then it would also be available to the BoM forecaster in addition to readings at all the other sensors in the area and all the other stuff the BoM wizards have available. They would then be able to issue aerodrome QNH for that aerodrome, available to you pre-departure or through Flightwatch. You would not be faced with the situation you describe.

It is the unavailability of destination aerodrome pressure readings that would lead us to using the 'add 50 feet' rule.

QSK, on the right track, but I'd put it differently...

Its all based on two premises....

First..
The IAL procedure designer does not want you going below a certain FIXED altitude during the approach. So buffers for variables such as allowable altimeter error (i.e. IFR altimeter tolerances) and variation between the QNH that the pilot has and what the station pressure actually is are built in.

Second....
1 HPa (millibar) = 30' altitude.

So...
100' of altitude change is a bit over 3 HPa change.
150' of altitude change is 5 HPa change.

To help, right click and open this chart in a new window (http://www.airservicesaustralia.com/pilotcentre/aip/dap/PECNB01-079.pdf). Its the Aeropelican NDB. At ELEV=5' its pretty much sea level.

Now, say you are inbound to Aeropelican with a valid TAF, which you would be sure of having about 99.99% of the time.

Lets say the forecast QNH for your arrival time of 0800 UTC was 1013.

Unbeknownst to you, the BoM forecaster didn't apply his full met wizardry this morning, and now the pressure at YPEC is really 1010.

Now, if you fly to the MDA of 820' on your setting of 1013 from the TAF, you won't be 815' AGL. You will be 3HPa lower, almost 100' lower in fact. So you will be about 715' AGL.

Not a problem, Mr Airservices has that factored.

Now lets say that inbound to YPEC, you were able to obtain an Actual QNH from an approved source, and that it is less than 15 minutes old so the setting you wrote on your scratchpad has not changed significantly since you obtained it. Viola! 1010 Hpa.

The Circling Minima box is shaded. You can lower your MDA to 720', about 715' AGL.

So the shaded/not shaded MDA is a buffer for possible inaccuracy of the forecast QNH.

But....

The next day you go to YPEC and during the night a lightning strike knocked out the aerodrome wx station pressure sensor (whatever). You have just used your AVFAX briefing to mop up the vomit from the young pax who had too much red cordial, or whatever. So you certainly don't have an actual QNH less than 15 mins old, you don't have a Forecast QNH. All that you have is the Area QNH passed to you by a helpful C172.

The area QNH must be +/- 5 HPa for all aerodromes within the area (if not the area will be subdivided).

This buffer should be applied to any MDA that you might use.

+/- 5 HPa is the same as +/- 150'

or

An additional 50' buffer above the 100' buffer that already exists in the chart's MDA.

So if the Area QNH the C172 gave you is 1015, and sneaky old YPEC is actually 1010 without you being able to tell, you will not go below the 715' AGL that Mr Airservices decided was safe for you.

[QSK?]

ITCZ:

Thanks very much for that detailed description, mate. I am now enlightened even further!

Your eloquence is outstanding and, you're right, you expressed it far better than I!!

I hope stallie is no longer confused.

Cheers QSK?