How to Read an Altimeter (Complete Guide)

Imagine sitting in a cozy cockpit, surrounded by an array of gauges and instruments. Having all those instruments is great—but if you’re not comfortable reading them, they can feel more confusing than helpful.

One of the most important instruments in your panel is the altimeter. It’s not just another gauge; it’s a key tool for staying safe, maintaining separation, and navigating terrain and airspace.

In this guide, we’ll break down what an altimeter measures, how it works, how to read it, and what can cause errors.

Fasten those seat belts and let’s get into it.

What Does an Altimeter Measure?

The altimeter commonly used in light aircraft is essentially a specialized barometer. It measures static air pressure and converts that pressure into an indicated altitude reading.

Although the instrument is physically measuring pressure, its purpose is to show the aircraft’s altitude above a reference level (based on your current altimeter setting).

How Does an Altimeter Work?

Pressure altimeters rely on a simple, reliable principle: air pressure decreases as altitude increases.

Inside the altimeter is a sealed stack of aneroid wafers. These wafers expand as pressure decreases (climb) and contract as pressure increases (descent). The wafers connect to mechanical linkages that move the needles on the altimeter face.

A tube connects the instrument to the aircraft’s static port, allowing the altimeter to sense outside static pressure. The instrument then translates that pressure change into an altitude reading for the pilot.

(source: FAA PHAK )

How to Read an Altimeter

A standard “three-pointer” altimeter uses three hands to show altitude:

• Long hand: hundreds of feet
• Medium hand: thousands of feet
• Short hand: tens of thousands of feet

Tip: Always cross-check all three hands. Misreading the tens-of-thousands hand is a common student mistake and can lead to being off by 10,000 feet.

Altimeter Settings

The altimeter is calibrated to a standard pressure setting of 29.92 inHg (standard atmosphere). Because real-world pressure changes, pilots must update the setting to keep the altitude reading accurate.

You adjust this using the knob and the Kollsman window. During preflight, set the altimeter to the current local altimeter setting so it reads field elevation when you’re on the ground.

Limitations of an Altimeter

A pressure altimeter is extremely useful—but it isn’t perfect. Errors can occur due to:

• Incorrect setting: Failing to update the altimeter setting.
• Nonstandard temperature: Cold air can place you lower than indicated.
• Static system errors: Blocked ports or leaks affect readings.
• Rapid pressure changes: Strong gradients introduce small discrepancies.

Because of these limitations, pilots should cross-check altitude using terrain awareness, visual references, and proper minimums rather than relying on one instrument alone.

Frequently Asked Questions

• What does an altimeter measure?

  It measures static air pressure and converts it into an indicated altitude based on the altimeter setting.

• What is the Kollsman window used for?

  It displays the pressure setting that pilots adjust to match local atmospheric pressure.

• Why does altitude change when I adjust the setting?

  Changing the reference pressure alters how the instrument converts pressure into altitude.

• What’s the most common altimeter mistake?

  Misreading the hands—especially confusing 2,500 feet with 12,500 feet.

• Does temperature affect altimeter accuracy?

  Yes. In cold air, you may be lower than indicated, which is why terrain clearance margins matter.

Takeaways

Altimeters use air pressure to estimate altitude. While reliable, they are affected by pressure settings and temperature—so good pilots always cross-check.

To dive deeper, watch FLY8MA’s video explaining how an altimeter works .

Want to Learn More About Altitude and Flight Instruments?

Check out these related guides:

• Watch Your Attitude: A Complete Guide to Aircraft Attitude Indicators
• Airfoil Camber: Its Effect on Aerodynamics & How It Generates Lift

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