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scotchegg

Altimeter settings

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Having never piloted in RL my theory knowledge is apalling so please don't scoff (too much) at this very basic question, but I've noticed different weather systems and their pressures interfering with the Altimeter and want to know how, when, and by how much I should set it to maintain an accurate reading of my altitude.

 

In layman's terms... :P

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In RL there are basically 3 altimeter settings: QNE, QFE and QNH.

 

The tower, as well as the weather messages contained in METAR and TAF reports will give you QNH. This is the setting that will, when in your kollsman window of the altimeter ( the one where the numeric reading of the pressure is displayed ) you set it, give you the altitude you are at referred to mean sea level pressure for that region.

 

Altimeter settings other than those provided by the tower and METAR strings are regional, and you update your info as your flight progresses.

 

With a QNH setting, your aircraft in the rw, your altimeter will read the altitude of the rw (above mean sea level ). Now, as weather systems travel across a given place on Earth, pressure varies. If for instance you are at LPPT rw 03 with a QNH reported by the tower of 1020 Mbar, you will read approximately 374' in your altimeter, but should a low pressure system progress towards that region, and the pressure drop to, 1003 Mbar, and you let the altimeter setting stay at the original 1020 Mb setting, the result would be the altimeter starting to read a higher altitude. In the standard atmosphere there is a 1 hpa drop for each 10 m / 30 feet variation in altitude ( aprox ), so, 1020-1003 = 17 would give you about 510' higher than you actually are ( since you're still seating in the rw ....), and your altimeter would read 884' 

 

This is why the metoffice that gives this info to the tower will take care to update the wetaher ( including pressure ) observations and reduction to QNH at the local aerodrome / region....

 

Now, this is also why pilots know about that old aviation saying: "From high to low, watch out bellow!!!".  In the above scenario, if you were flying from a region with a QNH alt setting of 1020 into another one where the pressure was lower, at those 1003 Mb, you would actually "think" you were flying around 510' higher than you really are!!!!  Later on this message I'll give you the other side of this old saying - the temperature side of that same saying....

 

Well, in the USA QNH is used, if I'm not wrong, up to 18000' QNH. In most parts of the World a much lower altitude is the limit to use the QNH altimeter setting. That altitude is called the "Transition Altitude", and it's the one above which you set your altimeter to a new - standard - altimeter setting, designated QNE: 1013 Mb ( aprox. ). This is the mean sea level pressure of the international standard atmosphere. Here in LPPT we have the transition altitude at 4000', meaning that as you cross that altitude your altimeter should have the reading in it's Kollsman window set to 1013 Mb ( 29.92 Hg ), and all aircraft flying around you in that area will, above 4000', be using that same altimeter setting.

 

When an aircraft has it's altimeter set at QNH, it is assuming the local weather system is ISA, so, all aircraft with QNH settings in their altimeters will actually climb / descend ( their autopilots or the crew doing the job, while keeping a given preset altitude ), as they cross higher / lower pressure regions. That is - if you have the alt set at QNH, and you transition from a lower to a higher pressure region, your aircraft will have to climb to maintain the same altitude reading in the altimeter. If you are flying above a "flat" surface ( an Ocean, for instance ), and you have a radar altimeter, you will actually be able to see that you radar alt reading is increasing, while your pressure altitude is maintained.

 

When pilots are in the approach phase of their flight, still above the local transition altitude, they are fying by flight levels, and there is a local transition level, the one bellow which they'll have to set their altimeters from QNE to the local reported QNH. That level is reported as the "Transition Level", and usually given to you by the ATC services.

 

Finally, there's QFE, a pressure setting that will place 0' in your altimeter when you are at the local ground level. We use it a lot in local flights, and in soaring. It's very easy to set because you simply have to set your altimeter so that it reads 0 feet. The pressure you then read in the Kollsman window will be your QFE altimeter setting.

 

At most aerodromes the tower can give you the QFE setting by demand. 

 

And now... something which is very interesting because - NO FLIGHT SIMULATOR ACTUALLY MODELS THIS!!!!!  The temperature side of the "From High to Low Watch Out Bellow" saying.... ( well, Aerowinx PSX already models it, as Hardy Heinlin once asked me if I knew of any simulator that modeled it, and told me he took care to include it in PSX :-)

 

Air masses vary in density. The colder the denser. If we imagine the points that are at a given pressure level ( say those at 500 Mb ) we will understand that they to not belong to a flat surface, more realistically to the surface of a sphere because we have to take into account that the atmospheric surfaces, just like the surface of our Earth, are subject to the same gravity force, and hence they're "spherical as well"... But the points at 500 Mb will actually belong to a rather complex surface called a geopotential level. The Geopotential is lower on higher density - colder - regions and higher at zones with higher airmass temperatures. (*)

 

So, we can imagine a scenario where an aircraft is flying from A to B, imagine to simplify that they're both over an Ocean, and the QNH at both places is exactly the same, and we are in USA airspace (transition alt = 18,000' ). They have their altimeters set to QNH, and are maintaining a given altitude, say 10,000', BUT, place A is hot, and B is cold, so the atmosphere above B will actually look compressed / shrink, and the aircraft while maintaining the 10,000' reading, common to both places QNH altimeter settings, will actually be descending while travelling from A to B!  So, from a higher temperature zone to a lower one, watch out bellow as well :-)

 

(*) Just a last note, because ELITE Jet ( ELITE's MD-81 procedural simulator ) includes some sort of modeling of this, in the form of "Tropopause Height", which can be set in one of the menus....

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So, we can imagine a scenario where an aircraft is flying from A to B, imagine to simplify that they're both over an Ocean, and the QNH at both places is exactly the same, and we are in USA airspace (transition alt = 18,000' ). They have their altimeters set to QNH, and are maintaining a given altitude, say 10,000', BUT, place A is hot, and B is cold, so the atmosphere above B will actually look compressed / shrink, and the aircraft while maintaining the 10,000' reading, common to both places, will actually be descending while travelling from A to B! So, from a higher temperature zone to a lower one, watch out bellow as well :-)

 

The UK and some other countries are divided into Altimeter Setting Regions to avoid this. All aircraft en-route in a region use the same setting The setting is based on  lowest forecast value of QNH,  with a margin to ensure safe terrain separation.

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The UK and some other countries are divided into Altimeter Setting Regions to avoid this. All aircraft en-route in a region use the same setting The setting is based on  lowest forecast value of QNH,  with a margin to ensure safe terrain separation.

 

Yep, I know about it Gerry, but the purpose of my note was to say that no flight simulator ( for the PC ) ever modeled the shrink of the atmosphere due to temperature ( and hence density ) changes, even at same QNH levels. In other words, no sim models ( well PSX does, but not yet released... ) the Geopotential Levels!!!

 

In your example in the UK, nothing would have changed since, at both regions the Area QNH ( just as the QFE for aerodromes at the same level ) would be the same, while in fact the "height" at the colder region would be lower..., and that's the problem... This can be particularly nasty when flying in / towards mountainous regions. 

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In the UK, the National Meteorological Office calculates the lowest forecast QNH in each region. These values are available hourly for the period H+1 to H+2.

 

As I said, they include a margin to ensure if an aircraft using this setting flies at an altitude calculated to give 500ft vertical separation over a mountain, then it will have at least that separation.

 

It also ensures that all aircraft in the region are using the same setting. This avoids problems with aircraft with different settings flying on conflicting courses.

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USA, GPS, WAAS.......and mountains

 

Before takeoff, set the altimeter to the airports elevation. If getting a setting from ATC or an automatic reporting system, it should be very close, to the airports elevation.

 

 

 

In flight, on a cross country; I used satellite weather, in addition to uplinked weather reports. This would show as text, on my GPS screen. Some settings are an average, but work very close. My GPS is using WAAS, which has corrections, to maintain an accurate altitude. Whenever I'd get a current in flight altimeter update, the aircraft's altimeter and the GPS's altimeter would match, and then slowly drift apart. I'd use the six-pac page on the Garmin, which duplicates the looks of the plane's round gauge. It was interesting to see the dials duplicate each other.

 

 

 

For seperation from other aircraft, use the "current" altimeter settings from ATC, when available. For accuracy against mountain peaks, use the GPS/WAAS altitude. They can eventually vary by several hundred feet.

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Wow, thanks guys, that's a lot of interesting reading you put up there. It seems then that a lot of RL effects are not currently modeled, but for the ones that are in XP itself the only way I have of getting current settings is from ATC?

 

I know the default GPS in XP isn't very good, but does it have an altimeter reading?

 

Also for example on the Jetstream, MU2, or any of the Carenado planes, how can I change the altimeter setting? I can see the reading on some of the dials, but can't see where to change it.

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I know the default GPS in XP isn't very good, but does it have an altimeter reading?

 

 

I don't think so :-( But there are excellent rumors about something being done about it for future 10.X releases :-)

 

ATIS should give you a reading if you're at an aerodrome and tune to it's frequency, but I don't use X-Plane's ATIS:-/ - I'm too lazy...

 

It works like this...

 

1) In the Map window, Zoom to see the aerodrome reference point and click over it to get the info you see displayed in the next screenshot, here for LOWI. Take note of the first ATIS frequency - 126.02 in this case:

 

 

 

2) In the aircraft, use the COMM1 or COMM2 frequency setting knob(s) to set the frequency you noted above. X-Plane's way of dealing with this kind of input is clumsy :-/  On a more up-2-date model like the Mu-2 in this example you place your mouse near different regions of the knob and the arrows will change size indicating they're ready to adjust units or the decimal part... You then drag the mouse with the left button pressed to set it :-/

 

 

 

 

 

In MSFS / P3D we have that magic "B" key, that once hit will automatically set your altimeter to the local reading in QNH.

 

Ah!  Don't forget to enable the COMM channel to go into your headphones!!!

 

 

 

I really prefer to display some Data in the upper left corner of the screen, and that includes the atmospheric pressure already QNH-calibrated and the instrument reading. To set it you go to the Settings / Data Input & Output menu and tick the boxes bellow:

 

 

 

When you return to the aircraft you now have the corresponding fields of information in your upper left corner of the display screen :-). The one bellow is your altimeter setting, and will change as you adjust it...

 

 

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Thanks once again Mr Monteiro, that's exactly the kind of idiot's guide I was looking for! I just plain didn't notice before that the altimeter reading and the barometer are linked so they both change when the dial is turned.

 

That's the scale of fool you're dealing with here :lol:

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It's easy to get confused, and X-Plane's knob interface logic doesn't help either :-/  

 

Most would rather prefer to be able to use the mouse wheel, like in FSX...

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