Definition
A pre-flight or in-flight verification that an aircraft's Automatic Direction Finder (ADF) receiver is correctly indicating the bearing to a Non-Directional Beacon (NDB), performed by tuning the station, identifying it by its Morse code identifier, and confirming the needle points to the station from a known position.
Plain English
Checking that the aircraft's radio compass is pointing to the right ground station before you rely on it for navigation. You tune the station, listen for its Morse code identifier to confirm it is the right one, and make sure the needle is pointing where it should from where you know you are.
Context Anchor
Used before relying on an NDB for instrument navigation, especially before flying an NDB approach.
Derivation
NDB stands for Non-Directional Beacon -- 'non-directional' because the beacon transmits its signal equally in all directions, leaving the aircraft's receiver to work out the bearing back to it. 'Accuracy check' simply means confirming the equipment is reading correctly before it is trusted.
Why Pilots Care
Confirms that NDB bearings are trustworthy, preventing lateral navigation errors on approaches or en-route segments that could lead to controlled flight into terrain or airspace violations.
Intuition Check
An NDB accuracy check does not adjust the beacon or prove that every indication will be perfect. It simply confirms that your aircraft’s direction indication agrees with a known direction closely enough to be trusted.
Example Sentence 1
Before commencing the NDB approach, the pilot tuned the beacon, identified the Morse code, and performed an NDB accuracy check by confirming the needle pointed toward the station from the holding fix.
Example Sentence 2
During the NDB accuracy check over the known landmark the pilot observed a ten-degree error and elected to use an alternate navigation aid instead.