An aircraft altimeter is a cockpit instrument that shows a pilot how high the airplane is above a reference level, usually sea level. It matters because safe flying depends on knowing altitude during takeoff, cruising, approach, and landing. Most traditional altimeters do not measure height directly.
They measure air pressure and convert it into an altitude reading.
Understanding Aviation: The Altimeter
Inside a mechanical altimeter are sealed, flexible metal capsules called aneroid wafers. Almost all air has been removed from them before the instrument is built. The wafers sit in a case connected to the aircraft static ports, which sample outside air without using the rushing air at the front of the airplane.
As the surrounding pressure falls, the wafers expand slightly. Linkages magnify this tiny movement and turn the hands on the dial. A digital display may look different, but an air data computer still begins with the same pressure information.
The instrument needs a clear static source. Ice, dirt, insects, water, or damaged plumbing can make the displayed altitude unreliable.
Pilots set a local pressure value in the small adjustment window before departure and whenever air traffic control provides a new value. This makes the dial relate more closely to mean sea level in that area. Weather systems matter because high pressure and low pressure change the shape of the atmosphere.
A pilot flying from an area of high pressure toward lower pressure without updating the setting will be lower than the altimeter suggests. This is remembered as high to low, look out below. The risk is greatest near terrain and during an approach, when a small vertical mistake can remove an important safety margin.
Temperature creates another important difference between indicated altitude and true altitude. The pressure pattern used for calibration assumes a standard temperature profile. On a very cold day, pressure levels are compressed closer together.
An airplane following a given indicated altitude can therefore be physically lower than expected. Mountain airports, winter operations, and instrument approaches require particular care for this reason.
Warm air produces the opposite tendency, though it does not remove the need for accurate settings. Pilots use published temperature corrections in some conditions and add extra clearance when planning near high ground.
Students often meet the same idea in weather reports, hiking watches, and phone sensors. A sudden pressure change can make a barometric watch report a false climb or descent even when its wearer stays still. In an airplane, pilots compare the altimeter with other evidence.
Radar altitude measures distance above the surface directly, but only over a limited range and it is not a replacement for the main altitude system. GPS can provide a height estimate based on a mathematical Earth model, which may not match the sea level reference used on an aviation chart. When learning this topic, keep the reference level clear.
Indicated altitude, pressure altitude, true altitude, and height above the ground describe related but different things. Confusing them is a common source of errors.
Key Facts
- An altimeter measures static air pressure, not altitude directly.
- Air pressure decreases as altitude increases, so lower pressure usually means higher altitude.
- Standard sea level pressure is 29.92 inHg or 1013.25 hPa.
- Pressure altitude is the altitude shown when the Kollsman window is set to 29.92 inHg.
- A 1 inHg error in the altimeter setting can cause about a 1000 ft altitude error.
- Indicated altitude depends on the pressure setting: correct setting gives a better estimate of height above mean sea level.
Vocabulary
- Altimeter
- An instrument that uses air pressure to estimate and display an aircraft's altitude.
- Static pressure
- The pressure of still outside air measured by the aircraft's static port.
- Aneroid wafer
- A sealed flexible metal capsule inside an altimeter that expands or contracts as outside pressure changes.
- Kollsman window
- The small display on an altimeter where the pilot sets the local pressure reference.
- Pressure altitude
- The altitude indicated when the altimeter is set to the standard pressure of 29.92 inHg or 1013.25 hPa.
Common Mistakes to Avoid
- Thinking an altimeter measures distance from the ground, which is wrong because it normally estimates altitude from air pressure relative to a pressure setting.
- Forgetting to set the Kollsman window before flight or approach, which can make the indicated altitude hundreds of feet too high or too low.
- Assuming higher pressure always means higher altitude, which is wrong because pressure generally decreases as altitude increases.
- Confusing indicated altitude with pressure altitude, because indicated altitude uses the selected pressure setting while pressure altitude uses the standard setting of 29.92 inHg.
Practice Questions
- 1 An aircraft altimeter setting is 30.12 inHg, but the pilot accidentally leaves it at 29.92 inHg. Using 1 inHg = about 1000 ft, estimate the altitude error in feet.
- 2 A pilot sets the altimeter to 29.92 inHg and the instrument reads 6500 ft. What is the aircraft's pressure altitude?
- 3 Explain why two airplanes at the same true height could show different indicated altitudes if their Kollsman pressure settings are different.