An unserviceability in an aircraft may be 'legal' when the aircraft is employed in private operations but not 'legal' if the same aircraft is employed in another classification of operations.
If your question, VT, is whether you can conduct a charter flight in aircraft X from A to B and, having suffered an unserviceability in an instrument or piece of equipment required for a charter flight, you're allowed to conduct a private flight in aircraft X from B to somewhere else, the answer is: Yes, subject to some provisos.
The provisos are that:
- the instrument or piece of equipment must not be required to be serviceable to employ the aircraft in a private flight
- the unserviceability must be recorded in the maintenance release or approved alternative for the aircraft,
- the instrument or piece of equipment must be placarded unserviceable, and
- most importantly, the flight must be in the private category.
[I]f a non-essential piece of equipment for a private VFR flight fails away from home (a light bulb, for example), and you can still conduct the flight according to 20.18.10.2, and the aircraft happens to be in the charter category, then does that mean a mercy flight has to be declared to get home?
No, provided:
- the unserviceability of the bulb is recorded in the maintenance release or approved alternative for the aircraft,
- the instrument or equipment is placarded unserviceable (if it's just a bulb, it might just be "Landing light not working" sticker under the landing light switch), and
- most importantly, the flight home must not be a charter pretending to be private.