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QNH altimeter settings

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S70BlackHawk created the topic: QNH altimeter settings

So I was flying IFR into Mackay from Rockhamton at 6000 FT to conduct the RNAV RWY 32 Via Bavam (4700FT). Tower was open and therefore the Class D started at 4500 Feet below the Class C I was in. Upon reaching the terminal area and just before handoff to tower at 25Nm Mackay I received ATIS and set the local QNH instead of Area QNH that Centre had passed to me earlier. I hadn't reached my true top of descent point yet.

The other pilot said, I would of failed an IPC for that (because I set Local QNH before the top of descent point). I said, but I'm setting the local QNH within 100Nm as per AIP and prior to the IAF for the approach I was about to fly. And because I've received ATIS and reported I've have received it, then when I contact tower they should expect that I would of set local QNH. After all, QNH is a required readback, and required to be set prior to the IAF for the approach.

He said no, because I was still in Class C Brisbane centre's airspace and therefore should be still cruising or descending in their airspace on Area QNH.

I don't think this is correct, and MATS states that ATC will take into account aircraft using Local QNH or Area QNH. Also the Area QNH is not to differ more than 5 HPa, so the maximum error could only be 150 Feet.

I also said, that once in the terminal area and received AWIS from a Class G non-towered aerodrome I would also set local QNH even before reaching overhead an aid such as a VOR reversal approach. Which was wrong too. I am supposed to wait until top of descent strictly???

Have been flying for almost 20 Years and seen the change from previous AIP altimeter setting to the current one. Maybe he is still thinking the old rule applies? He used the justification that "why does Brisbane Centre give you Area QNH?" so that whilst in their airspace as an IFR aircraft I need to be on Area QNH.

Would appreciate some input for this one as I have my IPC due again and don't want to fail!
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bobtait replied the topic: QNH altimeter settings

Thanks for you interesting post. This has always been a thorn in my saddle. If you read the AIP ENR 1.7 para 2.1.2 (a) & (b) along with the diagram on AIP ENR 1.7 Page 4, it clearly states that you should use Area QNH for cruise ONLY if the Local QNH of an aerodrome within 100nm is NOT available. That is to say, the Area QNH is intended to be a back up, and is used only if the Local QNH within 100nm is not available.

For a great many flights, the Local QNH within 100nm will be available either through AWIS, ATIS or even a TAF. I've always found it a bit weird that, if the Local QNH within 100nm is available, the Area QNH MUST NOT be used.

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S70BlackHawk replied the topic: QNH altimeter settings

I agree Bob. And he even agreed it was a grey area. And not every FE would fail you. But what happens if I come across him that does!

He mentioned Area QNH is different to the diagram as it refers to “forecast QNH.”

Go figure.
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Stuart Tait replied the topic: QNH altimeter settings

Just point out the highlighted section

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bobtait replied the topic: QNH altimeter settings

Yep. Area QNH is a forecast anyway. In fact it's an average of the forecast QNHs throughout the specified area.
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S70BlackHawk replied the topic: QNH altimeter settings

So to provide some closure on this topic. I managed to convince the other party that AIP states that you can use Local QNH, OR Area QNH. The big ticket item is the text that says OR.

You can't fail someone on the OR part if they selected one OR the other. Plus ADSB uses Pressure Altitude output so air traffic control take into consideration the different altitude settings.

He said AIP is not a head of power document. But, I found the Part 91 draft for altimeter settings, which is going to be a head of power, and funnily enough it was a copy and paste out of AIP which states OR for the Forecast TAF QNH, Local QNH and Area QNH. Now they just need to ammend the MOS for the altimeter setting for an instrument approach proficiency check to make sure you don't have rogue flight examiners applying their own opinion to the matter.

Because if you don't set the local QNH on descent and level out at an MSA then set a QNH that will make your altimeter lower than the MSA, for example, then that's an instant fail for being below an LSALT!!!!

I'm going to stick with the, I'm within the terminal area of 30Nm and getting ATIS/AWIS etc... and set QNH. During the cruise I'm going to use Area QNH or Terminal QNH as told by ATC... which is what I've been doing for 20 Years.

And if any FE wants to fail me on an OR statement, then a complaint will be raised with CASA.
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Carello replied the topic: QNH altimeter settings

The following would suggest that you would continue to use Area QNH if the local QNH within 100nm was from a high elevation (above 1000') report.

More to the point, high elevation QNH's are not used to derive the forecast Area QNH. This means that the local QNH at a high elevation aerodromes could differ from the Area QNH by more than 5mb - in theory.


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S70BlackHawk replied the topic: QNH altimeter settings

If it does differ than more than 5, the area will be subdivided. I see your point though about high elevation airfields. But that would stress the use of local QNH over the area QNH in this case. As it will, and always will be more accurate than a forecast average. ☺️ Area LSALT calcs are quite conservative. So you won’t be hitting any terrain in a hurry.
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Carello replied the topic: QNH altimeter settings

"If it does differ than more than 5, the area will be subdivided."

No, the area will not be subdivided. High elevation QNH's are simply ignored when calculating the forecast Area QNH.

Recall that QNH is one approximation of SLP pressure, there are others like QFF.



QNH is based on station level pressure (QFE) corrected for the height of the station above MSL. This correction assume an ISA atmosphere in the vertical column from the station to SL. At high elevation stations the QNH approximation can differ significantly from the real SLP (QFF) on very hot or cold days. At low elevation stations (< 1000') the difference is not so significant.
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