Definition
A style of altimeter display in which altitude is shown on a rotating drum (a cylinder with numerals printed around it) visible through a small window, usually combined with a single pointer that indicates hundreds of feet against a fixed scale. As the aircraft climbs or descends, the drum rotates to show thousands and ten-thousands of feet directly as numbers, while the pointer sweeps the dial for the finer reading.
Plain English
An altimeter that shows your altitude partly as numbers in a little window (like an odometer) and partly with a needle on a dial. You read the big numbers off the drum and the smaller ones off the needle.
Context Anchor
Seen in descriptions of sensitive altimeters and in cockpits that use an altimeter with rotating altitude numbers.
Derivation
Named for the rotating drum inside the instrument — a small cylinder with numbers printed around it that turns behind a window, similar to the number wheels in a car's odometer.
Why Pilots Care
Provides a clear, quick-to-read altitude display that reduces the chance of misreading during instrument flight.
Analogy
Think of a car odometer combined with a clock face — the odometer-style window gives you the big number, and the clock hand gives you the precise position between marks.
Intuition Check
Do not think of a musical drum. In this context, a drum is a rotating numbered wheel used to display altitude.
Example Sentence 1
The Cessna's panel had a drum-type altimeter, so I read the thousands directly off the window and used the pointer for the hundreds.
Example Sentence 2
The student practiced reading both drum-type and three-pointer altimeters before the instrument checkride.