How an Altimeter Works in an Airplane

What an airplane altimeter measures

Most familiar cockpit altimeters are barometric instruments: they infer altitude from the changing pressure of the air outside the airplane. They do not measure distance to the ground, and they do not directly sense the aircraft’s height above a runway or obstacle. Instead, they translate pressure into an altitude indication using an agreed standard relationship between pressure and altitude.

Air pressure generally decreases as altitude increases because there is less air above a higher location. The relationship is not perfectly uniform: temperature and weather systems also affect pressure. A pilot therefore adjusts the instrument’s pressure reference, called the altimeter setting, to account for local atmospheric conditions.

The indicated value is usually expressed in feet in U.S. aviation. It is useful for maintaining vertical separation, following assigned altitudes, and relating the airplane’s position to terrain and airspace. But it is an estimate based on pressure, not a direct measurement of geometric height.

How a barometric altimeter works

Inside a traditional mechanical altimeter are one or more sealed, flexible metal capsules called aneroid wafers. Their internal pressure is kept low and stable. The instrument case is vented to outside air, so pressure around the wafers changes as the airplane climbs or descends.

As outside pressure falls during a climb, the wafers expand. As pressure rises during a descent, they compress. A system of levers, gears, and shafts magnifies these small movements and turns the pointers on the dial. In many round-dial instruments, a long pointer indicates hundreds of feet, a shorter pointer thousands, and another pointer may indicate tens of thousands. The pilot reads the pointers together rather than treating one as the whole altitude.

The mechanism is calibrated against a standard atmosphere, a reference model of how pressure changes with altitude. The setting knob changes the reference pressure used by the instrument; it does not make the airplane move or alter the actual pressure outside. Modern electronic flight displays may show altitude computed from a pressure sensor, but the underlying barometric principle is similar.

Why pilots change the altimeter setting

The pressure value entered in the altimeter is commonly called the altimeter setting. In the United States, a local setting is often provided by air traffic services or an approved weather source. Setting it makes the instrument indicate altitude referenced to mean sea level under the local pressure conditions. This is important when comparing an airplane’s altitude with published airport elevations, terrain elevations, and assigned altitudes.

At higher altitudes, pilots commonly use a standard pressure reference rather than changing settings for each local pressure area. This lets aircraft use a consistent pressure reference when operating in the flight levels. The transition altitude or other procedures determine when pilots change between local settings and standard pressure; pilots follow current charts, clearances, and operating procedures rather than assuming one universal transition height.

A useful example: if an airport’s reported pressure differs from the standard reference, leaving the altimeter at the standard setting while near the airport can make its indicated altitude differ from the airport’s elevation. Correcting the setting reduces that discrepancy. The setting is not a guarantee of exact height above every point on the ground.

Altitude, height, and common misconceptions

Three terms help explain what the display means:

Indicated altitude: the reading shown with the current pressure setting.
Altitude above mean sea level: vertical position relative to the average sea-level reference, as represented by the pressure instrument when properly set.
Height above ground: the distance above the terrain directly below, which varies as the ground rises and falls.

These values can differ. An altimeter showing 5,000 feet does not necessarily mean the airplane is 5,000 feet above the land beneath it. If the terrain is already several thousand feet above sea level, the airplane’s clearance above it is lower.

Another common misconception is that a barometric altimeter gives exact true altitude in all weather. Temperature affects the pressure-altitude relationship: in air colder than the standard model, true altitude can be lower than indicated; in warmer-than-standard air, it can be higher. Pressure-setting errors also matter. Pilots account for these limitations through required procedures, charts, clearances, and terrain margins.

Other altimeters and aircraft systems

Airplanes can carry more than one kind of altitude-sensing equipment because different tasks need different references. A radio altimeter, where installed, sends radio energy toward the ground and measures the return to estimate height above the surface directly below. It is especially useful close to the ground, but its reading is not the same as barometric altitude above mean sea level and may be affected by terrain and the system’s operating limits.

Aircraft also report pressure-based altitude through an altitude encoder or transponder system. That information supports air traffic surveillance and altitude separation. The transmitted value is based on a standard pressure reference, while the cockpit altimeter may be set to local pressure. This distinction helps explain why a controller’s altitude readout and a pilot’s indicated altitude can use different references without representing two different aircraft positions.

Accuracy, checks, and safety

A barometric altimeter can be affected by incorrect settings, instrument defects, blocked or leaking pressure connections, and unusual atmospheric conditions. Before and during flight, pilots use prescribed checks and compare indications with known references when appropriate. They also use the correct current pressure setting and cross-check altitude against other instruments and operational information.

Important practical cautions include:

Use the pressure setting provided for the relevant location and operating procedure.
Do not confuse indicated altitude with terrain clearance or height above a runway.
Allow for temperature and pressure effects when procedures or charts require it.
If an altimeter indication appears unreliable, follow the aircraft checklist and applicable aviation procedures.

Altimeters are a central part of safe vertical navigation, but they are not a substitute for terrain awareness, instrument cross-checks, or following air traffic instructions. Pilots rely on several sources of information together, especially in instrument conditions or near terrain.

Frequently Asked Questions

Does an airplane altimeter measure height above the ground?

A standard barometric altimeter indicates pressure-based altitude, usually referenced to mean sea level when set appropriately. It does not directly measure the distance to the ground; a radio altimeter can provide height above the surface below in aircraft equipped with one.

Why does the altimeter setting change?

Air pressure varies with weather and location. Updating the setting helps the barometric instrument represent altitude relative to local sea-level pressure rather than an outdated or standard reference.

What does “high to low, look out below” mean?

It is a reminder that flying from higher pressure into lower pressure without adjusting the altimeter can leave the aircraft lower than the instrument indicates. Pilots use current settings and established procedures to avoid this pressure-setting error.

Can an altimeter be wrong because of temperature?

Yes. A barometric altimeter follows a standard pressure-altitude model, while actual air temperature can differ. In colder-than-standard air, true altitude may be lower than indicated, so pilots follow applicable cold-temperature procedures and terrain-clearance guidance.

What is the difference between barometric and radio altimeters?

A barometric altimeter estimates altitude from air pressure and normally relates it to mean sea level. A radio altimeter estimates height above the surface beneath the airplane, so its value changes with the terrain below.

Further reading: Federal Aviation Administration. Details can change, so check the official page for anything that matters to you. Spotted a mistake? Tell us. How we write an answer.