Definition
A barometric altimeter that displays altitude using a rotating numerical drum visible through a small window on the instrument face, combined with a single pointer that indicates hundreds of feet on a circular scale. The drum shows thousands and ten-thousands of feet directly as numbers, removing the three-pointer presentation found on older altimeters.
Plain English
An altimeter that shows altitude with a small rolling number window for the larger digits and a single needle for the hundreds of feet, so you read the altitude almost like reading a car odometer with one extra needle.
Context Anchor
Seen in instrument-panel descriptions and FAA illustrations that compare different ways an altimeter can display altitude.
Derivation
Named for the small rotating cylinder, or drum, inside the instrument that turns to show the altitude numbers through a window. The word drum here simply refers to the cylindrical shape of the rotating display element.
Why Pilots Care
It provides a clear, direct numerical readout of altitude that reduces the chance of misreading compared with pointer-style instruments.
Analogy
It works a little like the rolling number display on an older car odometer: the numbers turn into view, and you read the digits directly.
Intuition Check
Do not think of “drum” as a sound-making object here. In this term, a drum is a rotating numbered wheel used to display altitude.
Example Sentence 1
The Cessna's drum-type altimeter showed 6,500 feet, with the drum reading 6 and the pointer resting on the 5.
Example Sentence 2
During the approach briefing the pilot confirmed the drum-type altimeter was set to the current altimeter setting.