How Do Aircraft Determine Altitude?

During a flight operation, it is crucial that the pilot is always aware of their altitude, as the distance they are from the ground can affect various factors ranging from engine performance to general safety. While one could possibly estimate their height from visuals, such readings are nowhere near accurate and may not be possible during the night, while flying through clouds, or while having any other visual obstructions. To measure the height at which an aircraft is travelling, pilots rely on the altimeter, which is a flight instrument that utilizes outside air pressure to determine height.

While there are a variety of altimeter types available to pilots, the most commonly used instruments are barometric. As a general rule of thumb, air density and pressure of the atmosphere both decrease as one travels upwards, and the altimeter can directly measure this change to provide accurate readings. For a standard barometric altimeter, aneroid wafers are contained within a sealed chamber which is connected to the static port of the aircraft. As an aircraft ascends or descends, static pressure from outside of the aircraft is fed through the static port, causing the wafers to either expand or collapse. As the wafers adjust, a needle display on the instrument moves accordingly, allowing the pilot to be aware of their altitude. With standard weather conditions, the altimeter will typically change at a rate of 1.00” HG per 1,000 feet of climb.

While the altimeter is fairly accurate in reading changes of altitude by measuring air pressure, it itself cannot know exactly where the ground is located. Generally, pilots will set the instrument to function with a “zero feet” setting at sea level, allowing for the most accurate reading that can also be compared with other data. Nevertheless, altimeter settings may change slowly during the flight as weather, temperature, and other factors adjust. To maintain the accuracy of the instrument, pilots will need to be in period contact with air traffic controllers so that they can fine-tune their altimeters correctly throughout the flight.

Beyond barometric altimeters, there are also other types of altitude instruments available for pilots to use, each providing their own method of measurement. Radio altimeters are a type of instrument that utilize radio signals to determine height, and this is achieved through having radio signals sent towards the ground to bounce back up and be processed for height calculation. Generally, radio altimeters are only effective for up to 2,500 feet above ground, and they are typically more expensive. As a result, radio altimeters are most often used by larger aircraft such as airliners as a backup system for safety.

The vertical navigation system is another altitude measurement device that may be used for aircraft, and such instruments utilize GPS and satellites to determine height. At their current development, vertical navigation systems are not precise enough to be a primary system, but they can still be useful for obtaining altitude data nevertheless. Additionally, systems such as the instrument landing system (ILS) may also be used for receiving some altitude information as well. With the instrument landing system, pilots can be aware of factors such as their height as they approach a runway, utilizing radio navigation from beacons to conduct a safe landing. Devices such as the autopilot system also benefit from such readings, as altitude data can be fed to the autopilot system to hold altitude during flights.

Ensuring that your flight instruments function correctly is paramount to the operations of the aircraft and its systems, and one should always replace their parts and systems periodically and as needed to ensure safety and compliance. When you are in need of premium aircraft parts that you can rely on, let the experts at ASAP NSN Hub Services help you secure all that you are searching for with ease. ASAP NSN Hub Services is a premier online distributor of aviation components, providing customers access to over 2 billion new, used, and obsolete items. Get started on the purchasing process today and receive a personalized quote in 15 minutes or less.


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April 30, 2021

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