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49 CFR 830.5 requires immediate notification for in-flight fires.
Does that mean there must have been visible plasma? Would the presence of smoke alone be enough to report that there was a fire, or would something that simply smolders not meet the standard of an in-flight fire?
No, there does not need to be visible plasma, and smoke alone is enough to constitute a fire onboard an aircraft.
FAA Advisory Circular 120-80A contains the following definitions that indicate that a visible flame is not the only definition meaning "fire" pertaining to aircraft. In my experience, it's reasonable to expect that the FAA's definitions in this case would fall in line with the NTSB's definitions.
Hidden Fires:
Fires that are “hidden” are not readily accessible, may be difficult to locate, and are more challenging to extinguish. Some examples of hidden fires would be fires behind sidewall paneling or in overhead areas.
Smoldering Fire:
Combustion without a visible flame and a slow combustion rate are characteristics of a smoldering fire. A smoldering fire left unattended or an incompletely extinguished fire can ignite and grow into a larger, uncontrollable fire in a short time.
Suppressed Fire:
A fire that has been partially extinguished which may or may not have visible flames. A suppressed fire, if not extinguished, may reestablish itself and grow into a larger, uncontrollable fire in a short period of time.
