Calibrated altitude is the altitude reading from your altimeter after it has been corrected for two specific errors: instrument error (imperfections in the altimeter mechanism itself) and position error (distortions in air pressure caused by the aircraft’s shape and flight conditions). It sits one step above “indicated altitude,” which is the raw number you read off the altimeter dial, and represents a more accurate picture of how high you actually are.
To make sense of calibrated altitude, it helps to understand why your altimeter can’t just give you the right number in the first place, and where calibrated altitude fits among the several types of altitude pilots work with.
How an Altimeter Can Be Wrong
A barometric altimeter works by measuring the static (ambient) air pressure outside the aircraft and converting that pressure into a height reading. The lower the pressure, the higher the altitude. Simple enough in theory, but two categories of error creep in before that number ever reaches the dial.
The first is instrument error. The altimeter is a mechanical device with springs, gears, and aneroid capsules (small sealed metal chambers that expand and contract with pressure changes). Manufacturing tolerances, temperature shifts, and the imperfect elasticity of those capsules all introduce small inaccuracies. The FAA requires periodic testing under 14 CFR Part 43, Appendix E, and the allowable scale error is defined at various test altitudes. On the ground, if your altimeter reads more than 75 feet off from the known field elevation, it’s considered questionable and should be sent for repair.
The second is position error, sometimes called installation error. Your altimeter gets its pressure reading from a static port, a small opening on the fuselage. Ideally, that port would sense the true freestream air pressure. In reality, the aircraft’s own shape speeds up or slows down the airflow around its body, changing the local pressure right where the port sits. Engineers pick a location on the fuselage where this distortion is minimal, but it can never be eliminated entirely. Worse, position error isn’t constant. It changes with airspeed, angle of attack, flap settings, aircraft weight, and acceleration. Each aircraft type has a known set of position error values published in its flight manual, along with correction charts pilots can reference.
From Indicated to Calibrated Altitude
Indicated altitude is whatever number appears on the altimeter face after you’ve dialed in the current local altimeter setting (the barometric pressure reported by a nearby station). It’s a useful starting point, but it still contains the instrument and position errors described above.
Calibrated altitude removes those two errors. For many general aviation flights at moderate speeds and altitudes, the difference between indicated and calibrated altitude is small, often within a few dozen feet. But the correction matters more in certain flight regimes. At unusual angles of attack, at very high or very low speeds, or with flaps and gear extended, position error can grow significantly. Helicopter pilots face an additional source of position error from rotor downwash disturbing the airflow around the static port.
In older aircraft with traditional “steam gauge” instruments, the pilot applies corrections manually using tables from the aircraft flight manual. In modern cockpits, an Air Data Computer takes raw pressure inputs from the pitot and static systems, applies correction algorithms and calibration factors, and outputs a corrected altitude to the flight displays. The pilot sees the result automatically, though the underlying process is the same.
Where Calibrated Altitude Fits Among Other Altitudes
Aviation uses several definitions of altitude, each serving a different purpose. Understanding the hierarchy helps clarify what calibrated altitude actually accomplishes.
- Indicated altitude: The raw reading on the altimeter after setting the local barometric pressure. Contains instrument and position errors.
- Calibrated altitude: Indicated altitude with instrument and position errors removed. A more accurate reading, but still assumes standard atmospheric conditions.
- Pressure altitude: The altitude displayed when the altimeter is set to the standard pressure of 29.92 inches of mercury (1013.4 millibars). This is what all pilots use above 18,000 feet, where everyone flies on the same reference to maintain safe vertical separation. It’s also the starting point for performance calculations like climb rate, fuel burn, and true airspeed.
- Density altitude: Pressure altitude corrected for actual temperature and humidity. This is what the airplane “feels.” On a hot, humid day at a sea-level airport in Florida, density altitude could reach 5,000 feet, meaning the aircraft needs a longer takeoff roll and climbs more slowly, even though the field elevation is near zero. Pilots use density altitude to assess real aerodynamic performance.
- True altitude: Your actual height above mean sea level. This is what calibrated altitude is trying to approximate, though additional corrections for non-standard temperature are needed to get there precisely.
Each type builds on the one before it. Calibrated altitude is the critical second step: cleaning up the mechanical and aerodynamic noise so that further corrections (for pressure and temperature) start from an accurate baseline.
Why the Altimeter Setting Matters
Calibrated altitude assumes you’ve set the correct local barometric pressure in the altimeter’s Kollsman window, the small adjustable scale on the instrument face. The FAA requires pilots below 18,000 feet to update this setting using a reported value from a station within 100 nautical miles. On piston aircraft, pilots typically update it every 10 to 15 minutes to keep pace with changing weather.
Getting this wrong has real consequences. Every inch of mercury of error in the altimeter setting translates to roughly 1,000 feet of altitude error. Setting 29.90 instead of 30.90 would put you 1,000 feet lower than your altimeter indicates. The old aviation saying captures the danger well: “Going from a high to a low, look out below.” Flying from a region of high pressure into low pressure without updating your setting means your true altitude is dropping while your altimeter stays the same.
Other Errors Calibration Doesn’t Fix
Calibrated altitude specifically addresses instrument and position errors. Several other error sources remain even after calibration, and pilots need to be aware of them.
Density error comes from variations in atmospheric pressure and temperature that differ from the standard atmosphere model the altimeter is built around. This is what pressure altitude and density altitude corrections handle separately. Hysteresis is a lag effect caused by aneroid capsules that don’t respond instantly to pressure changes. During rapid climbs or descents, the capsules retain their previous shape briefly, causing the altimeter to read behind the actual altitude. This effect fades shortly after leveling off. Reversal error occurs during abrupt pitch changes when false pressure spikes at the static port momentarily push the altimeter reading in the wrong direction before it corrects itself.
At higher altitudes and speeds, compressibility error becomes relevant. Air compressed in the pitot tube inlet at altitudes above 10,000 feet and calibrated airspeeds above 200 knots can distort pressure readings. This primarily affects airspeed indicators rather than altimeters, but it’s part of the broader pitot-static system that feeds altitude data.
None of these errors are exotic edge cases. They’re present to varying degrees in every aircraft’s pitot-static system. Calibrated altitude handles the two most consistent and correctable ones, giving pilots and onboard computers a clean starting point for everything else.