Definition
A method of suspending the magnetic compass card so that it hangs below its pivot point, allowing it to swing freely under the influence of gravity and align with the Earth's magnetic field. Because the card hangs like a pendulum, it tips when the aircraft accelerates or decelerates, which is the source of the compass acceleration error.
Plain English
The compass card is hung from a pivot so it dangles down and can swing, much like a pendulum. This lets it stay roughly level and point at magnetic north, but it also means the card tips around when the airplane speeds up or slows down.
Context Anchor
Seen in magnetic compass error discussions, especially when explaining acceleration error on east or west headings.
Derivation
From the Latin pendulus, meaning 'hanging' -- the same root as pendulum. The compass card literally hangs below its support, so it behaves like a weight on a string and tilts when forces other than gravity act on it.
Why Pilots Care
This mounting produces acceleration error, causing the compass to show a false turn during airspeed changes on east or west headings.
Analogy
It is like a hanging lamp in a moving vehicle. When the vehicle speeds up or slows down, the lamp may swing even though the vehicle has not turned.
Grounding Statement
When the airplane changes speed, the hanging compass assembly can lag slightly and tilt, creating a temporary compass error.
Intuition Check
Do not read “pendulous-type mounting” as meaning the compass is poorly mounted or freely flopping around. It means the compass assembly is deliberately hung with its weight below the support point so it can settle level under gravity.
Example Sentence 1
Because of the pendulous-type mounting, the compass card tilts during acceleration and produces a temporary false heading indication.
Example Sentence 2
The pendulous-type mounting in the wet compass is designed to reduce some errors but introduces acceleration error as a trade-off.