"Steven P. McNicoll" wrote in
ink.net:

You are if you're being compensated for it.

Wrong again. You're being paid to instruct; not to fly.

You don't even need a current medical unless you're acting as a
required crewmember.

Irrelevant.

Actually, it is. FAR 61.23 states that you must must hold at least a
second-class medical certificate when exercising the privileges of a
commercial pilot certificate. The FAA had to make a special case in
FAR 61.23 in order to state that you don't need a medical unless the
instructor is a required crewmember (i.e., you're not being paid to
fly).

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