Definition
A sensitive altimeter that displays altitude using three pointers on a single dial: a long pointer indicating hundreds of feet, a shorter pointer indicating thousands of feet, and a small narrow pointer indicating tens of thousands of feet. The instrument is driven by an aneroid mechanism that responds to changes in static air pressure, with a Kollsman window allowing the pilot to set the local altimeter setting.
Plain English
An altimeter with three hands on its face, each one showing a different scale of altitude. You read all three together to get your full altitude, similar to reading hours, minutes, and seconds on a clock.
Context Anchor
Seen on older or training aircraft instrument panels and in FAA instrument diagrams explaining how altimeters work.
Derivation
Named simply for its three pointers. 'Altimeter' comes from the Latin 'altus' (high) plus 'meter' (measure) -- a device that measures height.
Why Pilots Care
Easy to misread by a thousand feet, which can cause altitude busts or terrain conflicts in instrument conditions.
Analogy
Think of a clock face. The hour, minute, and second hands all share the same dial but mean very different things. On a three-pointer altimeter, each pointer also has a different scale, and reading the wrong one gives you a wildly wrong altitude.
Intuition Check
Do not assume the three pointers are three separate altitude readings. They are three parts of one altitude reading, read together.
Example Sentence 1
Climbing through 9,500 feet, the pilot watched the long pointer of the three-pointer altimeter swing past the 5 while the thousands pointer crept toward 10.
Example Sentence 2
During the climb the student correctly read the three-pointer altimeter passing through 3,200 feet.