How much do you know about the altimeter in your airplane?

What Exactly Does An Altimeter Measure?

Altimeters measure height above particular pressure levels. To do this, they compare the pressure of outside static air to the standard pressure of 29.92" Hg of air at sea level. Air is denser at sea level than aloft, so pressure decreases as altitude increases (and vice versa).

In most flights below FL180, your goal is to set your altimeter so that it reads out your aircraft's height above Mean Sea Level (MSL), but more on that in a bit...

How Does It Work?

A standard altimeter contains a stack of sealed aneroid wafers with an internal pressure of 29.92" Hg. These wafers expand and contract based on the static pressure inside the casing of the altimeter. This static air enters the casing through a tube attached to the static ports on your airplane. The chamber is otherwise sealed, so only static air from directly outside the airplane enters the chamber.

"A higher static pressure presses down on the wafers and causes them to collapse. A lower static pressure (less than 29.92" Hg) allows the wafers to expand" (FAA). Mechanical linkages connect the movement of these wafers to needles on the interior face of the altimeter. Compression of the wafers translates to a decrease in altitude, while expansion translates to an increase in altitude.

Reading The Altimeter

Reading a standard 3-hand altimeter is easy. The long pointer measures altitude in intervals of 10,000 feet (2 = 20,000 feet). The short, wide pointer measures altitude in intervals of 1,000 feet (2 = 2,000 feet). The medium, thin pointer measures altitude in intervals of 100 feet (2 = 200 feet).

What Happens When Air Pressure Isn't Standard? (29.92" Hg)

Across the globe, or even across a few miles, different air pressure can have a dramatic effect on altimeter settings. As you fly from high-pressure weather systems to low-pressure systems (or vice versa), you need to adjust your altimeter to get an accurate Mean Sea Level (MSL) altitude reading on your altimeter.

You reset your altimeter to match local, nonstandard station pressure readings, using the Kollsman window on your altimeter. This is usually done every 100NM for aircraft flying below FL180.

Where do you get the altimeter settings from? If you're flying VFR, you can tune in to a nearby ASOS or AWOS weather station at an airport. If you're getting VFR flight following from ATC, or if you're on an IFR flight plan, ATC will give it to you periodically. And if you have ADS-B or Sirius XM weather, you can pull up the altimeter setting for a nearby airport.

Every .1" Hg is equivalent to 100 feet in altitude. So, let's say you took off with an altimeter setting of 29.96" Hg. 150 miles into your flight, the pressure dropped to 29.70" Hg. The altimeter would be off by approximately 260 feet in altitude if you didn't make any adjustments. (29.96 - 29.70 = +.26).

If you kept flying at your indicated altitude (let's say 3,500' MSL) without adjusting your altimeter, you'd be 260 feet low. (Remember the saying "high to low, look out below).

But by dialing the new altimeter setting in the Kollsman window, or the altimeter setting of a glass-panel aircraft, you'll read out an accurate MSL altitude.

What About Glass Cockpits?

Electronic Flight Displays (EFDs) do things a little differently. Altimeter readings are generated by an Air Data Computer (ADC), which uses the same static air input to measure altitude. However, the static air never enters a diaphragm the same way it does in a traditional altimeter. "The ADC computes the received barometric pressure and sends a digital signal to the PFD to display the proper altitude readout" (FAA).

The ADC uses the same basic concept as a traditional altimeter, but with fewer moving parts.

What else do you want to know about altimeters or other flight instruments? Tell us in the comments below.