Understanding Altimeter Readings – A Comprehensive Guide

What is an Altimeter, and How Does It Work?

An altimeter is an essential flight instrument for measuring an aircraft’s altitude—its height above a specific pressure level. This sensitive barometer, it operates by detecting shifts in atmospheric pressure as the aircraft climbs or descends. First tested by pilot Jimmy Doolittle in 1929, the pressure altimeter remains a cornerstone of the modern cockpit, providing vital vertical position information.

At the heart of a standard altimeter lies a sealed housing containing a stack of flexible, airtight metal capsules known as aneroid wafers. A tube connects this housing to the aircraft’s static port—an external vent designed to sample undisturbed air pressure. This setup allows the pressure inside the altimeter’s casing to equalize with the ambient atmospheric pressure at the aircraft’s current altitude.

The measurement process depends on these aneroid wafers. As the aircraft ascends, external static pressure drops, allowing the sealed wafers to expand; conversely, a descent increases external pressure, compressing them. A sophisticated system of mechanical linkages and gears then translates these subtle physical movements into the rotation of needles on the altimeter’s face, which is calibrated to convert pressure changes directly into an altitude reading in feet.

Understanding Altimeter Settings: QNH and QFE

While an altimeter measures pressure changes, it requires a correct reference point to provide a meaningful altitude reading. Since atmospheric pressure fluctuates with weather systems, pilots must calibrate their altimeters to a local pressure setting, ensuring all aircraft in an area share the same reference. This is accomplished by dialing the correct pressure value into the altimeter’s Hollman window.

Two primary settings govern this calibration: QNH and QFE. Think of them as two different ways of answering the question, “How high are you?”

• QNH represents the barometric pressure adjusted to mean sea level (MSL). When set, it displays the aircraft’s altitude above MSL, which is the standard for en-route flight to ensure safe separation from terrain, obstacles, and other traffic.

• QFE represents the atmospheric pressure at a specific airfield. Setting it displays height directly above that airfield (reading zero on the runway), a feature useful for takeoffs, landings, and local traffic patterns.

The practical difference is straightforward yet important. Imagine an airport with an elevation of 1,000 feet above sea level. With the correct QNH set, an altimeter on the runway would read 1,000 feet. If the pilot then set the QFE, the altimeter needle would move to read 0 feet. Both readings are correct—they just reference different starting points.

QNH – Altimeter Setting for Sea Level

Pilots set the QNH by turning a knob to adjust the pressure value in the Hollman window. This adjustment is precise: increasing the setting by 0.01 inches of mercury (ING) raises the indicated altitude by 10 feet, enabling precise adjustments.

QFE – Altimeter Setting Above Airfield Elevation

The primary application for QFE is during the approach and landing phases. Because it indicates height above the airfield elevation (AAE), it improves a pilot’s situational awareness. Mental calculations become unnecessary, as the altimeter directly displays height above the touchdown zone. This immediate feedback is extremely valuable for maintaining a stable approach path and ensuring a safe landing, particularly in low visibility.

Because QFE is tied to a specific airfield’s pressure, its use is confined to the local traffic pattern and is unsuitable for en-route navigation, where the standardized QNH is required.

Factors Affecting Altimeter Readings

Because an altimeter infers altitude from air pressure, its accuracy is subject to atmospheric changes. Two major factors are variations in barometric pressure and temperature.

The primary factor is a change in local barometric pressure. When an aircraft flies from a high-pressure to a low-pressure area without an altimeter adjustment, the instrument incorrectly indicates a climb, showing an altitude higher than reality. This error can be significant—roughly 1,000 feet per inch of mercury—and is the origin of the aviation mantra: “From high to low, or hot to cold, look out below.” To counteract this hazardous error, pilots must regularly set the current local QNH.

Air temperature affects accuracy. Altimeters are calibrated against the International Standard Atmosphere (ISA), which assumes a sea-level temperature of 15°C and a predictable temperature lapse rate. When the air is warmer than standard, it’s less dense, causing the altimeter to read lower than the true altitude. Conversely, colder, denser air makes the altimeter read higher, posing a significant risk of reduced terrain clearance. Pilots mitigate this by applying temperature corrections, an important procedure during instrument approaches in cold weather.

Beyond atmospheric variables, minor inaccuracies can also arise:

• Instrument error: Inherent mechanical imperfections within the altimeter.

• Position error: Distorted airflow around the aircraft’s static port.

To account for these known errors, which can vary with airspeed, pilots consult correction charts in the Pilot’s Operating Handbook (POH) and cross-reference their readings with other sources like GPS altitude.

Calibration and Maintenance of Altimeters

To address errors from non-standard atmospheric conditions, altimeters require regular calibration and maintenance. These procedures are necessary to synchronize the instrument with local atmospheric pressure, and maintain its reliability.

The main tool for in-flight calibration is the Hollman window, a small, adjustable sub-scale on the altimeter’s face. Using this, the pilot inputs the current local barometric pressure (e.g., the QNH setting from air traffic control or an automated weather station).

Beyond the pilot’s routine adjustments, altimeters undergo periodic, thorough maintenance to ensure their mechanical integrity. An important procedure is the vacuum test, where technicians verify that the instrument does not leak or lose more than 100 feet per minute under specific test conditions.