Quick Answer
An altimeter needs recalibration when the displayed elevation reading consistently differs from known reference points by more than 100 feet, or when the altimeter's pressure reading deviates from the current atmospheric pressure by more than 5 millibars.
Identifying Altitude Discrepancies
To determine if your altimeter needs recalibration, compare the displayed elevation reading to known reference points such as mountain peaks, valleys, or other recognizable geographic features. If the altimeter consistently reads higher or lower than the known elevation by more than 100 feet, recalibration is necessary. For example, if you know the elevation of a nearby peak is 8,000 feet and your altimeter consistently reads 8,200 feet, it’s likely out of calibration.
Understanding Atmospheric Pressure
Atmospheric pressure affects altimeter accuracy, and significant deviations can indicate the need for recalibration. Check the altimeter’s pressure reading against current atmospheric pressure data from a reliable source such as a weather forecast or a pressure altimeter. If the altimeter’s pressure reading deviates from the current pressure by more than 5 millibars, recalibration is recommended. For example, if the current atmospheric pressure is 1013 millibars and your altimeter reads 1020 millibars, it may be out of calibration.
Recalibration Techniques
To recalibrate your altimeter, compare its reading to a known reference point using a GPS device or a secondary altimeter. If the altimeter is set to the wrong pressure setting, adjust it to match the current atmospheric pressure. Additionally, some altimeters can be recalibrated using a calibration process involving a series of ascending and descending flight maneuvers. Consult your altimeter’s user manual for specific instructions on recalibration procedures.
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