Definition
The range of atmospheric icing conditions defined in Appendix C of 14 CFR Part 25, used as the certification standard for transport-category aircraft and their ice protection systems. It specifies the supercooled liquid water content, droplet size (mean effective diameter up to about 50 micrometers), temperature range, and horizontal/vertical extent of icing clouds that an aircraft must be able to safely operate in when certified for flight into known icing.
Plain English
A defined set of icing conditions that an aircraft must be tested and approved against before it can be certified to fly into known icing. It describes the kind of icy clouds the aircraft is expected to handle: how much water is in them, how big the droplets are, how cold it is, and how far the icy area extends.
Context Anchor
Seen in aircraft certification, flight manual limitations, and discussions about whether an aircraft is approved for flight in known icing.
Derivation
Named after Appendix C of Part 25 of the Federal Aviation Regulations, where these icing parameters are tabulated. 'Appendix' comes from Latin appendere, 'to hang on to' — material attached to the end of a regulation that defines specific technical limits.
Why Pilots Care
Determines whether an aircraft is legally and safely approved to operate in icing; affects dispatch decisions and required equipment.
Analogy
It is like a jacket tested for a certain kind of rain. Passing that test tells you something useful, but it does not mean the jacket was tested for every possible storm.
Grounding Statement
Appendix C icing conditions describe a defined test environment, not a promise that all real-world icing is safe to fly through.
Intuition Check
Do not read “Appendix C Icing Conditions” as “all icing conditions.” It means only the specific icing conditions described in Appendix C of the certification rule.
Example Sentence 1
The aircraft's ice protection system is certified to operate within Appendix C icing conditions, but the crew briefed an alternate in case they encountered freezing rain.
Example Sentence 2
Dispatch delayed the flight until the forecast no longer included Appendix C Icing Conditions.