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.
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.