TraineeWhen an ATC Center gives you an altimeter reading, WHERE is the reading being taken and how is applicable to you at your position and altitude?
Are they reading the barometer where they sit, or can they give you an accurate reading for your altitude?
7 Answers
First OfficerMy understanding is that they are giving you the barometric pressure at sea level at your given location. They measure it at the airfield and adjust it based on the field elevation. All it does is gives a base point to ensure that everyone's altimeter is accurate!
Chief CaptainBrook wrote:
When an ATC Center gives you an altimeter reading, WHERE is the reading being taken and how is applicable to you at your position and altitude?
Are they reading the barometer where they sit, or can they give you an accurate reading for your altitude?
They give you either QNH or QFE.
QNH is the airfield pressure reduced to sea level via ISA (International Standard Atmosphere) figures i.e. 1mb per 27ft. This gives the pilot their altitude above mean sea level
QFE is the airfield pressure. Therefore, when you select the QFE and start flying (from the same aerodrome) you will have a height above the airfield
As an aside, QFF is similar to QNH but it is aerodrome pressure reduced to sea level via the actual conditions rather than ISA conditions
QNE is (strictly speaking) a height that is read on the altimeter when landing with 1013.25mb set on the altimeter. A common misconception is that QNE = 1013.25mb = standard pressure but it's actually a height.
Does that make sense?
Chief Captain🙄
First OfficerOk then ❓
Chief CaptainTailhook wrote:
🙄
There's some more info here: http://en.wikipedia.org/wiki/Atmospheric_pressure
Chief Captain99jolegg wrote:
Tailhook wrote:
🙄
There's some more info here: http://en.wikipedia.org/wiki/Atmospheric_pressure
...and here: https://forum.flyawaysimulation.com/forum/topic/35184/how-not-to-ask-for-help/
CaptainHello fellow aviation enthusiast,
That's a great question! When ATC (Air Traffic Control) provides an altimeter setting, they're essentially giving you the local QNH (Quasi-Nonhydrostatic) reading. QNH is a term used to describe the atmospheric pressure adjusted to mean sea level, so when you set your altimeter to the provided QNH, it will display your altitude above sea level.
The altimeter setting is primarily derived from ground-based weather stations or automated weather observation systems (AWOS) located at airports. These weather stations measure the local barometric pressure and then adjust it to sea level to obtain the QNH value. In some cases, ATC can also access remote weather data for airports that might be closer to your current position, giving you a more accurate reading.
Now, concerning the applicability of the altimeter setting at your altitude, it's essential to understand that the atmosphere's pressure decreases with altitude. The standard atmosphere model assumes that the pressure decreases at a constant rate with altitude, known as the pressure lapse rate. However, the actual pressure lapse rate can vary due to atmospheric conditions, which is why it's crucial to obtain updated altimeter settings throughout your flight.
As you climb or descend, the pressure changes will affect your altimeter's accuracy. Having an accurate altimeter setting is essential for maintaining proper vertical separation between aircraft, as well as ensuring that you're flying at the correct altitude in relation to terrain and airspace restrictions.
In summary, ATC provides altimeter settings derived from ground-based weather stations or automated systems that measure local barometric pressure adjusted to sea level (QNH). Although the pressure lapse rate might not be consistent throughout the atmosphere, obtaining updated altimeter settings during your flight ensures accurate altitude readings, promoting safe and efficient operations.
Safe flying!
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