Definition
An Automatic Direction Finder (ADF) instrument whose compass card (the circular dial showing 0°–360°) can be manually rotated by the pilot using a heading-set knob, so the aircraft's current magnetic heading can be aligned to the top of the dial. With the card set to heading, the needle then points to the magnetic bearing to the tuned non-directional beacon (NDB), rather than only the relative bearing.
Plain English
An older ADF instrument where you can turn the outer ring of numbers by hand. By turning the ring so your current heading sits at the top, the needle then shows the actual compass direction to the radio station, instead of just the angle from the nose of the aircraft.
Context Anchor
Seen in older instrument panels and in instrument-flying discussions about using ADF equipment to navigate to or from a radio station.
Derivation
"Rotatable" simply means the card can be turned by hand. The key idea is that the dial is movable rather than fixed, which separates this instrument from the simpler fixed-card ADF where 0° (north on the card) is permanently locked to the nose of the airplane.
Why Pilots Care
Lets the pilot read the magnetic bearing to the NDB directly without adding or subtracting heading, reducing mental workload during navigation.
Intuition Check
Rotatable does not mean self-updating here. The pilot turns the card by hand; the ADF pointer still responds to the selected station.
Example Sentence 1
After turning to the new heading, he rotated the compass card on the ADF so 045 sat under the index, then read the magnetic bearing to the NDB straight off the needle.
Example Sentence 2
With the rotatable compass-card ADF aligned, the crew could track the NDB without first calculating relative bearing from the heading indicator.