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QNH

Asked by: 3513 views Helicopter

Hello. 

The more I read about altitude the more it confuses me. I know altitude depends on the weather and barometric pressure.  But I also knew altitude is a height above sea level? And to calculate QNH by handheld weather tracker I need to put in altitude of the helideck. I know our helideck is 220ft above sea. But its altitude on weather reader is constantly changes. So how do I correctly calculate QNH, do i put height of helideck or current altitude readings? 

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1 Answers



  1. Craig on Nov 07, 2019

    My guess is that it would the published helideck altitude above sea level (MSL).

    Your weather tracker probably has a built-in barometer. As I understand it, the formula is: Indicated Altitude = 1,000 x (QNH – B), which is not 100% correct for various reasons, but works. That value on device’s barometer would be the value you use as “B” in the formula. Rearranging the formula and substituting the published helideck altitude above sea level (MSL) for the indicated altitude:

    QNH = (helipad altitude in MSL/1000) + barometer pressure from your device

    http://www.challengers101.com/AltSettings.html

    Just FYI….

    Altitude Above Ground (QFE)
    Altimeter will read “0” feet when on the ground.
    Barometric Pressure for Standard Altimeter Setting (QNE)
    Altimeter set to 29.92 above FL 180.
    Barometric Pressure for Local Altimeter Setting (QNH)
    Altimeter indicates airport elevation above mean sea level (MSL).

    True altitude —vertical distance above sea level (MSL). Airport, terrain, and obstacle elevations on aeronautical charts are true altitudes.

    Absolute altitude —vertical distance above the terrain, or ground (AGL).

    Pressure altitude —altitude indicated when the altimeter setting window (barometric scale) is set to 29.92 “Hg. The altitude above the theoretical standard datum plane, where air pressure (corrected to 15 °C) equals 29.92 “Hg. Used to compute density altitude, true altitude, true airspeed (TAS), and other performance data.

    Density altitude = pressure altitude corrected for nonstandard temperature.
    When conditions are standard, pressure altitude and density altitude are the same.
    If the temperature is > standard, the density altitude is higher than pressure altitude and aircraft performance decreases.
    If the temperature is < standard, the density altitude is lower than pressure altitude and aircraft performance increases.

    Indicated altitude —displayed on the altimeter (uncorrected) when it is set to the local altimeter setting.

    GPS Altitude: A measure take from, and therefore in reference to, the GPS Satellite constellation.

    Changes in pressure and temperature from international standard levels can cause an altimeter to display the incorrect altitude above sea level. QNH or the barometric pressure is a pressure setting used by pilots, air traffic control (ATC), etc., to refer to the barometric setting which, when set in Kollsman’s window of an aircraft's altimeter, will cause the altimeter to read the correct altitude above mean sea level (MSL). My understanding is that the altimeter setting reported by the tower and by automatic weather observation stations (AWOS) are the barometric pressure corrected to sea level. So one actually inputs the sea level pressure into Kollsman’s window so the altimeter will display the correct height above sea level (MSL).

    For example, the ATIS weather reports the QNH altimeter setting as 30.15 at your airport. Your same airport hangar barometer reads a local pressure (B) of 29.45.
    In the plane, you set the reported 30.15 in/Hg (QNH) into the Kollsman window. The altimeter in the plane will indicate:
    Indicated Altitude = 1,000 x (QNH – B)
    IA = 1,000 x (30.15 – 29.45) = 1,000 x 0.70 = 700 feet … indicated altitude (IA) on the altimeter = your current location elevation (MSL).

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