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ALTIMETER, QNH, QFE, QNE AND TRANSITION LEVELS
Firstly there are differences around the world and I will try to explain the best I can.
FSX Mainly uses the North American system as this is where the simulator was designed, although it does give options for different measurements for different parts of the world as you'll see further down.
QNH/ALTIMETER
In the US and Canada this is called the Altimeter Setting or Altimeter for short and is measured in Inches of Mercury(In).
Everywhere else in the world this is known as QNH and is measured in Hectopascals (HPa).
Both are the same and show the regional or local air pressure at mean sea level (MSL).
Altimeter is QNH
QNH is Altimeter.
I will refer to this as QNH in the rest of this post, it's less letters to type.
When QNH is set your aircraft altimeter will show your altitude above Mean Sea Level.
You can get the QNH by listening to the ATIS or looking at the FSX map as below.
If QNH is set then when sitting on the threshold your altimeter will read the airfield elevation and not ZERO.
For instance if the airfield is 200ft above mean sea level (like at Manchester UK) your altimeter will read 200ft when sitting on the runway threshold.
If your in the Himalayas at Lukla (which is half way up a mountain at 9300ft), then you altimeter would read 9300ft while on the threshold with QNH set.
So in theory if you set QNH in a float plane and start at an airport, your altimeter would show the airfields elevation while your on the threshold.
If you then take off and land in the sea the altimeter should read ZERO.
The pictures below show QNH set in both HPa and In of mercury.
You can see in the pictures above that the map says the airfield elevation is 270ft.
With the correct QNH set for this airfield the Altimeter is also showing 270ft and not Zero.
An easy way to remember QNH is Nautical Height
QFE
QFE is used everywhere but the US and Canada.
If you set the QFE into your altimeter then it will read ZERO when you are sitting on the threshold.
This is handy if you are in the circuit (pattern) at an airfield and you want your altimeter to read ZERO as you touchdown and not the field elevation.
The pictures below show QFE set in both HPa and In of mercury.
Notice that with QFE set the aircraft altimeter is reading Zero.
The problem of using QFE is that all obstacles (High rise buildings, mountains, and airspace) are measured from Mean Sea Level (MSL).
**Example.**
Your charts show controlled airspace starts at 2500ft.
You want to stay clear of controlled airspace and plan to fly at 2300ft.
The airfield elevation is 500ft
You set QFE so when your on the ground your Altimeter reads ZERO instead of 500ft (the field elevation) if you had set QNH .
You take off and fly around the airfield at 2300ft.
Your Altimeter reads 2300ft so your think you are 200ft below controlled airspace, but because you have set QFE you are actually at 2800ft MSL and 300ft inside the controlled airspace.
If you had set QNH your altimeter would still have read 2300ft but you would have been 200ft below and outside of the airspace.
An easy way to remember QFE is Field Elevation
QNE
This is the odd one out.
QFE and QNH are both the 4 digits you enter into your altimeter in which to calibrate it.
QNE Is the actual reading in feet when you set your altimeter to 29.92In or 1013.25HPa.
Also known as the Standard pressure setting.
**For instance **
QFE 1021HPa
QNH 1016HPa
QNE 12500FT
All IFR aircraft above the transition level would set the Standard pressure setting which would show them QNE.
TRANSITION LEVEL
If your flying in the United States or Canada this is easy as the transition altitude is 18,000 ft end of.
In Europe and elsewhere the transition altitude varies depending on what is around the aerodrome and can be as low as 3,000 ft.
The transition levels can be found on the aerodrome charts.
Above the Transition Level a pilot would set the standard pressure setting into their altimeter (29.92" or 1013.2).
Once this is set ATC will give height information in Flight Levels(FL) and not in feet.
For instance FL240 not 24000ft.
Below the Transition Level altitudes are given in feet with the local QNH.
**For instance**
The Transition level is 5500ft.
UNI123 Cleared for take off, climb to altitude 3000ft, local QNH 1003.
Aircraft sets 1003 into the altimeter, departs and climbs to 3000ft......
UNI123 Continue climb Flight Level Eight Zero (FL80).
UNI123 Would then set 1013 or 2992 into their altimeter and climb to 8000ft.
You never get a Flight Level below FL180 in the US and Canada as that is the transition level, and there is never a lower transition level.
IN FSX YOU CAN ALWAYS SET THE QNH BY PRESSING B.
I've tried to keep it as simple as possible and hope I've helped to try and explain Barometric pressure, Transition altitudes and regional differences.
Firstly there are differences around the world and I will try to explain the best I can.
FSX Mainly uses the North American system as this is where the simulator was designed, although it does give options for different measurements for different parts of the world as you'll see further down.
QNH/ALTIMETER
In the US and Canada this is called the Altimeter Setting or Altimeter for short and is measured in Inches of Mercury(In).
Everywhere else in the world this is known as QNH and is measured in Hectopascals (HPa).
Both are the same and show the regional or local air pressure at mean sea level (MSL).
Altimeter is QNH
QNH is Altimeter.
I will refer to this as QNH in the rest of this post, it's less letters to type.
When QNH is set your aircraft altimeter will show your altitude above Mean Sea Level.
You can get the QNH by listening to the ATIS or looking at the FSX map as below.
If QNH is set then when sitting on the threshold your altimeter will read the airfield elevation and not ZERO.
For instance if the airfield is 200ft above mean sea level (like at Manchester UK) your altimeter will read 200ft when sitting on the runway threshold.
If your in the Himalayas at Lukla (which is half way up a mountain at 9300ft), then you altimeter would read 9300ft while on the threshold with QNH set.
So in theory if you set QNH in a float plane and start at an airport, your altimeter would show the airfields elevation while your on the threshold.
If you then take off and land in the sea the altimeter should read ZERO.
The pictures below show QNH set in both HPa and In of mercury.
You can see in the pictures above that the map says the airfield elevation is 270ft.
With the correct QNH set for this airfield the Altimeter is also showing 270ft and not Zero.
An easy way to remember QNH is Nautical Height
QFE
QFE is used everywhere but the US and Canada.
If you set the QFE into your altimeter then it will read ZERO when you are sitting on the threshold.
This is handy if you are in the circuit (pattern) at an airfield and you want your altimeter to read ZERO as you touchdown and not the field elevation.
The pictures below show QFE set in both HPa and In of mercury.
Notice that with QFE set the aircraft altimeter is reading Zero.
The problem of using QFE is that all obstacles (High rise buildings, mountains, and airspace) are measured from Mean Sea Level (MSL).
**Example.**
Your charts show controlled airspace starts at 2500ft.
You want to stay clear of controlled airspace and plan to fly at 2300ft.
The airfield elevation is 500ft
You set QFE so when your on the ground your Altimeter reads ZERO instead of 500ft (the field elevation) if you had set QNH .
You take off and fly around the airfield at 2300ft.
Your Altimeter reads 2300ft so your think you are 200ft below controlled airspace, but because you have set QFE you are actually at 2800ft MSL and 300ft inside the controlled airspace.
If you had set QNH your altimeter would still have read 2300ft but you would have been 200ft below and outside of the airspace.
An easy way to remember QFE is Field Elevation
QNE
This is the odd one out.
QFE and QNH are both the 4 digits you enter into your altimeter in which to calibrate it.
QNE Is the actual reading in feet when you set your altimeter to 29.92In or 1013.25HPa.
Also known as the Standard pressure setting.
**For instance **
QFE 1021HPa
QNH 1016HPa
QNE 12500FT
All IFR aircraft above the transition level would set the Standard pressure setting which would show them QNE.
TRANSITION LEVEL
If your flying in the United States or Canada this is easy as the transition altitude is 18,000 ft end of.
In Europe and elsewhere the transition altitude varies depending on what is around the aerodrome and can be as low as 3,000 ft.
The transition levels can be found on the aerodrome charts.
Above the Transition Level a pilot would set the standard pressure setting into their altimeter (29.92" or 1013.2).
Once this is set ATC will give height information in Flight Levels(FL) and not in feet.
For instance FL240 not 24000ft.
Below the Transition Level altitudes are given in feet with the local QNH.
**For instance**
The Transition level is 5500ft.
UNI123 Cleared for take off, climb to altitude 3000ft, local QNH 1003.
Aircraft sets 1003 into the altimeter, departs and climbs to 3000ft......
UNI123 Continue climb Flight Level Eight Zero (FL80).
UNI123 Would then set 1013 or 2992 into their altimeter and climb to 8000ft.
You never get a Flight Level below FL180 in the US and Canada as that is the transition level, and there is never a lower transition level.
IN FSX YOU CAN ALWAYS SET THE QNH BY PRESSING B.
I've tried to keep it as simple as possible and hope I've helped to try and explain Barometric pressure, Transition altitudes and regional differences.
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