altitude setting

[BrendAn]

when we are doing preflight checks we always switch to atis and put in the latest qnh .

if you are flying out of the same place all the time why not just turn the altimeter to the known height, like outside our hangar it is 130 ft asl.

wouldn't that be the same result as the atis advice or am i thinking this wrong.

[Red]

If your altimeter is calibrated and correct and you are parked at a spot that has the same elevation as at the centre of the longest runway (or you know for sure elevation at your position) then yes it should match.

I fly mostly in and out of farm strips with of course no ATIS so the procedure I adopt is as you mention dial in known elevation before taxiing then check when Im in vicinity of a airport with ATIS that it matches

Saying all that, anywhere that has an ATIS is likely to require you speak to them before departure and that info will be given anyway

 

(speaking of UK, though should be same..... I must remember to add that disclaimer)

Edited October 14 by Red

[BrendAn]

7 minutes ago, Red said:

If your altimeter is calibrated and correct and you are parked at a spot that has the same elevation as at the centre of the longest runway (or you know for sure elevation at your position) then yes it should match.

I fly mostly in and out of farm strips with of course no ATIS so the procedure I adopt is as you mention dial in known elevation before taxiing then check when Im in vicinity of a airport with ATIS that it matches

Saying all that, anywhere that has an ATIS is likely to require you speak to them before departure and that info will be given anyway

 

(speaking of UK, though should be same..... I must remember to add that disclaimer)

Thanks red.  Atis is an automatic broadcast that runs 24/7. You just tune in to get wind speed and direction ,temp ,dewpoint and qnh.

[Red]

Yes ATIS - Automatic Terminal Information Service, Generaly found only on large airfields with commercial activity in my country, do smaller aerodromes have it in Aus?

 

[BrendAn]

7 minutes ago, Red said:

Yes ATIS - Automatic Terminal Information Service, Generaly found only on large airfields with commercial activity in my country, do smaller aerodromes have it in Aus?

 

i think its on most of the larger regional airfields.   we only have it because its at the raaf base 10km away.

[RFguy]

Brendan

The answer is .. yeah you can just set it to the known height- that's good enough for circuit work. There may be some difference with the published local QNH, perhaps a hecto either way.

 

Once you get airborne though,  flying hemispherical altitudes (4500,5500,6500,7500 etc)  you are expected to be flying on the area QNH, regardless of what your altimeter /setting was at the aerodrome. 

and ATC separation requires / expects  this. 

 

At Cowra, I just set the altimeter to the airport height 970', and then if going somewhere, once established, I set the altimeter to the area QNH (which I will have got from my weather and NOTAM checks before flight).

or, you can call up Centre on the radio, and request the area QNH.  When in the vicinity of my destination, I'll set the altimeter to the AWIS broadcast QNH, if AVBL.

 

[IBob]

I had a altimeter fail very gradually: the first indication of that was when I set the QNH correctly preflight , it did not show the correct field elevation.

Over a number of days it drifted further and further out.

I would guess that one of the two internal bellows or cells sprung a tiny leak, and on inspection it looked as though one of them had not been put through a final plating process.

[IBob]

Edited October 15 by IBob

[BrendAn]

30 minutes ago, IBob said:

They are amazing instruments when you think how they react to such minute pressure changes.

[facthunter]

IVSI's are better. they have a rattler in them to prevent hysteresis..    Nev

[BrendAn]

15 minutes ago, facthunter said:

IVSI's are better. they have a rattler in them to prevent hysteresis..    Nev

[Bosi72]

On 14/10/2024 at 10:04 PM, BrendAn said:

when we are doing preflight checks we always switch to atis and put in the latest qnh .

if you are flying out of the same place all the time why not just turn the altimeter to the known height, like outside our hangar it is 130 ft asl.

wouldn't that be the same result as the atis advice or am i thinking this wrong.

yes you can set altimeter if you know exact elevation of your hangar, and you should be able to confirm accuracy at the thresholds, reference points. 

Assuming this is in ywsl, I am curious if all elevations are between 62ft and 93ft, are you sure 130ft  is right?

 

The main reason for setting qnh from atis/awis is to have everyone's altimeter on the same pressure (elevation). When flying in the levels above 10000ft, everyone is on 1013.

Edited October 15 by Bosi72

[BrendAn]

9 hours ago, Bosi72 said:

yes you can set altimeter if you know exact elevation of your hangar, and you should be able to confirm accuracy at the thresholds, reference points. 

Assuming this is in ywsl, I am curious if all elevations are between 62ft and 93ft, are you sure 130ft  is right?

 

The main reason for setting qnh from atis/awis is to have everyone's altimeter on the same pressure (elevation). When flying in the levels above 10000ft, everyone is on 1013.

I am not sure about your elevations. I thought the western apron was 130 and  runway 9 is 90 ft . I had a Dynon panel and a steam gauge and I always set both to the awis qnh.  We fly 500 ft circuits at 620 on the altimeter.  What have you got for yltv

It is 180 ft there.

[BrendAn]

9 hours ago, Bosi72 said:

yes you can set altimeter if you know exact elevation of your hangar, and you should be able to confirm accuracy at the thresholds, reference points. 

Assuming this is in ywsl, I am curious if all elevations are between 62ft and 93ft, are you sure 130ft  is right?

 

The main reason for setting qnh from atis/awis is to have everyone's altimeter on the same pressure (elevation). When flying in the levels above 10000ft, everyone is on 1013.

Would there be that much difference between the RAAF base and west sale. We use the RAAF base qnh. It's only 10 km away. 

[BurnieM]

From Garmin Pilot;

the reference point at East Sale is 23ft and West Sale is 93ft

 

[Garfly]

 

 

 

1 hour ago, BrendAn said:

Would there be that much difference between the RAAF base and west sale. We use the RAAF base qnh. It's only 10 km away. 

 

Brendan, this is the extent of your local QNH Area as shown in OzRunways

(if that's what you were asking about):

 

 

CLICK FOR FULL REZ:

 

 

Edited October 16 by Garfly

[BrendAn]

 It covers a large area . More than I thought it would.

How can area 30 have one qnh when the weather pattern of eastern Victoria is different to Melbourne and western Vic.

 

[BurnieM]

I guess it does not really matter if it is wrong as long as altimeters in your general area are equally wrong.

[BrendAn]

Just now, BurnieM said:

I guess it does not really matter if it is wrong as long as altimeters in your general area are equally wrong.

It's probably me that's wrong. I haven't flown for a couple of months. I know the runway is 90 at our end .  The hangar is quite a bit higher . I will ask one the other blokes that are based  there. 

Bosi derailed my thread by pouncing on my mistake if it is one .

The question was if you know the height can you just dial in the altimeter instead of using the broadcast qnh. And I now know I can. 👍

[mkennard]

10 minutes ago, BrendAn said:

 It covers a large area . More than I thought it would.

How can area 30 have one qnh when the weather pattern of eastern Victoria is different to Melbourne and western Vic.

 

Because unless it's a nice High Pressure system, they do break it up but that gets fun since you have to find the places they are talking about for the boundries and times for the different QNH. 

[Garfly]

35 minutes ago, BrendAn said:

 It covers a large area . More than I thought it would.

How can area 30 have one qnh when the weather pattern of eastern Victoria is different to Melbourne and western Vic.

 

Yeah, well Area QNH is often an approximation of local QNH at a given time and place.

The main aim being that all aircraft are on the same pressure datum to keep clear of each other vertically whilst enroute.

But, of course, the Area QNH can't be too far out because we need to avoid terrain as much as each other (below the transition level).

So the Area QNH changes regularly to track the changing atmospheric pressure.

 

And yes, you do get the accurate Local QNH by dialling up your known field elevation but that's just for that exact time and that local area.

I guess it's good to keep in mind that arriving or overflying aircraft are likely to be on Area QNH (at uncontrolled fields).

 

 

 

Edited October 16 by Garfly

[facthunter]

The forecast will give you the area QNHs you require and the times that apply.. If you are on the ground at a place where the ATIS gives the QNH that information lets you determine whether YOUR altimeter is within tolerance.   I think it applies at the ARP. (Aerodrome reference point). Make allowance for where you are Parked. Nev

[Ironpot]

consult the bible … er AIP:

1.1 General
1.1.1 Whenever an accurate QNH is available and the aircraft is at a known elevation, pilots must conduct an accuracy check of the aircraft altimeter(s) at some point prior to takeoff.
Note: Where the first check indicates that an altimeter is unserviceable, the pilot is permitted to conduct a further check at another location on the same airfield; for example, the first on the tarmac and the second at the runway threshold (to determine altimeter serviceability).

1.4 Accurate QNH and Site Elevation

1.4.1 A QNH can be considered accurate only if it is provided by one of
the following:

a. AAIS;
b. ATC;
c. ATIS;
d. AWIS;
e. CA/GRS; or
f. WATIR.
Note: QNH contained in an authorised weather forecast must not be used for checking the accuracy of a pressure altitude system.
1.4.2 Site elevation must be derived from aerodrome survey data that is authorised in writing by either CASA or an NAA, or supplied in writing by the relevant aerodrome operator.

 

2.2 Area QNH
2.2.1 Area QNH is a forecast value which is valid for a period of 3 hours and normally applies throughout an Area QNH Zone (AQZ).
2.2.2 Area QNH Zones will be subdivided, if necessary, to meet the following standards of accuracy:
a. Area QNH forecasts are to be within ± 5 hPa of the actual QNH at any low-level point (below 1,000FT AMSL) within or on, the boundary of the appropriate area during the period of validity of the forecasts.
b. Area QNH must not differ from an adjoining Area QNH by more than 5 hPa.

 

 

 

 

 

[facthunter]

One hPa is 32 feet.. One Millibar in the old language. ALL this presupposes the ICAO standard atmosphere applies but if done correctly the separation with other aircraft vertically is preserved. .  Can't be relied upon to clear High terrain.

  Q  If you got into a pressurised plane and both Altimeter subscales show 1013.2 and reading nothing like the actual Aerodrome height what is the likely cause?    Nev

[Garfly]

Nev, could it be that both pilots neglected to set QNH on approach?