then why can't you swap an alternator? Or replace a broken landing light switch?
Maybe you can, maybe you can't. Interpretations vary.
FAA legal has issued an LOI stating that the
definition of preventive maintenance is controlling, and
not CFR Part 43 Appendix A, which is to be considered a list of examples, not a comprehensive and limiting list.
From the LOI,
As with the other paragraphs of Appendix A (i.e. , on major repairs and major alterations), the lists are
better viewed as examples of the tasks in each category-they cannot be considered all-
inclusive. There are, no doubt, many "simple or minor preservation operations [tasks]"
and many "replacement of small standard parts not involving complex assembly
operations" performed daily, especially on small general aviation aircraft, that the agency
would consider to be preventive maintenance, though they are not included in the 32 listed items.
Per the CFR definition,
"Preventive maintenance means simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations."
Now, is the alternator a small standard part?
Maybe, though that might be a stretch, but since R&Ring it requires removing and reinstalling the prop, it can hardly be claimed that it doesn't require "complex assembly operations." I'd guess that one's out. What about a landing light switch? Well, the switch itself is almost certainly a small standard part, but for any given plane I don't know whether accessing it requires complex assembly operations.
The FAA seems to have left quite a bit of latitude for owner/operators to decide whether something falls under PM,
and I'd like to leave it that way so please don't ask the FAA for clarification. I've never heard of anyone getting gigged for deciding wrongly and logging something as pilot-performed PM (maybe it's happened), but I suspect that the worst outcome would be requiring an A&P to either inspect or redo the work and then make a new log entry.
Of course, if an owner doesn't closely guard the airplane around the clock, lots of things seem to get fixed by hangar fairies who only work under cover of darkness and never log anything......