The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Clouds
Fronts
Visibility and Fog
Aircraft Icing
Thunderstorms
Altimetry
Weather Reports and Forecasts
Practice Exam

Pressure Error

Much like temperature, pressure changes as we fly from A to B. Typically we use the QNH from the nearest airfield and continually update our altimeter subscale setting as we fly cross country, however this is not always possible. In more remote areas there may be 50 miles or more between airfields and we may end up using a somewhat inaccurate pressure setting for a period of time.

The same rule of thumb applies: High to Low – Look Out Below! As we fly from higher to lower pressure, our true altitude will decrease if we maintain a constant indicated altitude on our altimeter. Flying from lower pressure to higher pressure, our true altitude will increase.

The rule of thumb is 30ft per hectopascal. So if the QNH decreases by 15hPa and you don’t update the altimeter subscale, you will be 450ft lower than indicated (30ft x 15hPa = 450ft).

Many countries also have Altimeter Setting Regions (ASR). The ASRs are marked on the VFR charts and you can request the current ASR pressure from any Flight Information Service Officer (e.g. London Information). The pressure given will be the lowest forecast QNH for the entire ASR for the current one hour period (starting on each hour).

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