Definition
A standardized labeling scheme used in a Minimum Equipment List (MEL) that identifies each aircraft system by an ATA (Air Transport Association) chapter number, allowing equipment items to be grouped and referenced consistently across manufacturers and operators.
Plain English
It's the numbering system used in the MEL to organize equipment by which part of the airplane it belongs to — fuel system, electrical system, landing gear, and so on — so each item can be found in the same place every time.
Context Anchor
Seen when using a minimum equipment list to find or describe an item that is not working.
Derivation
Designation comes from a Latin word meaning “to mark out” or “to point out.” That fits this use because the designation points you to the correct aircraft system in the list.
Why Pilots Care
When something is inoperative before flight, the pilot looks it up in the MEL by its system designation. Knowing how the systems are numbered makes finding the right entry fast and reduces the chance of applying the wrong relief or restriction.
Analogy
It works like a folder label. If the label says “lights,” you know to look there for items related to aircraft lighting rather than searching the whole list.
Intuition Check
Do not read designation as a detailed repair description. Here it means the identifying label or code for the aircraft system the item belongs to.
Example Sentence 1
Before departure, the pilot found the inoperative fuel quantity indicator listed under the fuel aircraft system designation in the MEL.
Example Sentence 2
Each aircraft system designation in the MEL includes the number of items installed and the number required for dispatch.