Definition
A path and terminator leg type used in RNAV procedure coding that defines a course (C) from a fix flown until a specified DME distance (D) from a navigation aid is reached. The aircraft flies the published course and the leg ends when the DME distance terminator is met.
Plain English
A coded flight segment that says: 'Fly this exact course until you are a certain distance from a specific ground station, then the leg is over.' The course is the path; the distance from the station is the stop point.
Context Anchor
Seen in instrument procedure coding and RNAV/FMS leg descriptions, especially when studying how a published instrument path is built.
Derivation
From ARINC 424 path-terminator coding, where each leg is named by two letters: the first letter describes the path the aircraft flies, and the second letter describes what ends (terminates) the leg. C stands for Course, D stands for DME distance. So 'CD' literally means 'Course to a DME distance.'
Why Pilots Care
It defines an exact segment that keeps the aircraft on the published path until the next fix or turn point.
Grounding Statement
On a CD leg, the distance reading from a specific DME station is what ends the leg.
Intuition Check
Do not read “CD” as a general direct-to instruction. A CD leg means stay on the specified course until a specified DME distance is reached.
Example Sentence 1
The departure procedure begins with a CD leg that has the aircraft fly runway heading until 4 DME from the airport VOR.
Example Sentence 2
The pilot flew the CD leg until the distance readout matched the published value.