How the altimeter works

How do you read a three-pointer altimeter?

This altimeter has three hands, like a clock with an extra one. Read them from the smallest to the biggest unit:

Example: the short hand about halfway between 3 and 4, the long hand halfway between 4 and 5, the thin hand just past 0. That is 3,450 feet, the starting reading on this page.

What does the altimeter setting do?

Air pressure at sea level changes from day to day and from place to place. The setting tells the altimeter what today’s sea-level pressure is, so it can show your height above sea level correctly. Air traffic control and weather reports give the local altimeter setting.

Turning the setting up makes the altimeter read higher, about 1,000 feet for each inch of mercury, or 10 feet for each 0.01. With the right setting, the altimeter reads the airport’s elevation when you are on the ground there.

In the United States, pilots flying at or above 18,000 feet all set the standard 29.92, so everyone up there measures height the same way.

How does it work inside?

The altimeter is connected to the static port, which feels the still air pressure outside. Inside is a stack of sealed, thin metal capsules called aneroid wafers. They hold a fixed pressure inside.

1. 1
   As you climb, the outside pressure falls and the wafers expand.
2. 2
   As you descend, the pressure rises and the wafers are squeezed smaller.
3. 3
   Gears turn that movement into the turning hands.

Indicated, true and other altitudes

• Indicated altitude: what the altimeter shows with the current setting.
• True altitude: your real height above mean sea level.
• Absolute altitude: your height above the ground below you.
• Pressure altitude: what the altimeter shows with 29.92 set.
• Density altitude: pressure altitude corrected for non-standard temperature. It is how high the aircraft "feels" and decides how well it performs.

What can go wrong?

High to low, look out below. Flying from high pressure into low pressure without resetting the altimeter makes it read higher than you really are. The same happens flying from warm air into colder air. Either way, the ground is closer than the dial says.

• Misreading the hands. Mixing up the short hand and the long one gives a reading 1,000 feet or more off. Read every hand, every time.
• An old setting. Update the setting when air traffic control or a weather report gives a new one.
• A blocked static port. The altimeter freezes at the height where it blocked and stops showing climbs and descents.