Sylvain writes:

Pilots flying without a license, this one is easy: none of them (ok,
some might be picky and point out that folks flying with a certificate
issued under 14 CFR 61.75 do indeed need a license in addition to their
certificate, if their country of origin issues one).

as for folks flying without a medical; well, you have all the glider
pilots to boot, and they have been doing that for a long time, no
problem; and pilots with sport pilot certificates. (I don't know about
ultralights pilots but I reckon they don't need one either); To that lot,
you can also add a number of instructors, since a valid medical is not
required to instruct a student who is already certificated.

Actually, I meant how many pilots are flying illegally (because they lack a
valid license or a current medical, for example)?

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