Introduction to Measuring Height Above Sea Level in Aircraft

The Role of Barometric Pressure in Altitude Measurement

Altitude measurement is a critical aspect of flight operations, ensuring safety and efficient navigation. The height above sea level (ASL) that aircraft fly is primarily measured using barometric pressure. Barometric pressure is directly related to altitude: as altitude increases, barometric pressure decreases. This inverse relationship forms the basis for setting and adjusting the aircraft's altimeter, which indicates the height above mean sea level, specifically either QNH (for altimeter setting) or QFE (for field elevation).

Setting the Altimeter: QNH vs. QFE

Aircraft typically use the following altimeter settings:

QNH (Qelevation Sea Level): This setting is used from sea level up to approximately 18,000 feet MSL. QNH indicates the height above mean sea level. QFE (Field Elevation): This setting is used for landing and takeoff procedures. It indicates the height above the specific airport's mean sea level. QNE (Standard Sea Level Pressure): Above 18,000 feet, pilots use QNE, setting the altimeter to 29.92 inches of mercury (Hg), and refer to altitudes as Flight Levels, e.g., FL 290 at 29,000 feet.

Understanding Barometric Altitude and AGL

In addition to QNH and QFE altitudes, pilots also use Barometric Altitude and Above Ground Level (AGL).

Barometric Altitude: This is the height above mean sea level determined using a barometric altimeter. The instrument measures changes in pressure due to altitude, which are calibrated to indicate height in feet or meters.

Above Ground Level (AGL): This refers to the height above the ground where the aircraft is flying, typically measured using a radar altimeter. AGL is crucial for approach and landing procedures, as it ensures that the aircraft maintains a safe distance from the ground throughout the descent.

For example, during an approach, the minimum descent altitude (MDA) is the lowest safe height at which the pilot can continue the approach without visual references. It is calculated as the known elevation of the runway threshold (e.g., 1,200 feet MSL) plus the required minimum AGL height (e.g., 200 feet AGL). This results in a MDA of 1,400 feet MSL.

Barometric Pressure at Sea Level and Above

At sea level, the weight of the air above one square inch of surface is approximately 14.7 pounds, resulting in a standard pressure of 1013.25 millibars. As altitude increases, the air becomes less dense, leading to a decrease in barometric pressure. Barometers and altimeters are highly sensitive instruments that measure these pressure changes to determine height above sea level.

These instruments need periodic adjustment to ensure accuracy, especially in areas affected by weather or storm conditions. Small windows on altimeters allow for these adjustments, ensuring that the instrument is correctly set for the specific region.

Conclusion

Accurate measurement of height above sea level is essential for safe and efficient aircraft operations. By understanding and utilizing QNH, QFE, and QNE, as well as the principles of barometric pressure, pilots can navigate and land safely, ensuring the well-being of passengers and crew alike.