[Porsche-O-Phile]

Technically the pilot is responsible for adequate preflight preparation and planning. IIRC the pertinent regulation reads "availing him/her self of all possible sources of information which pertain to the flight being planned". I forget the exact word-for-word (I'd have to look it up), but that's the jist of it. Based on that "letter of the law", it strongly implies that simply "hopping in and going" is discouraged, if not outright illegal. You'd have a helluva time standing up in court and justifying how you "availed yourself of all possible sources of information" if you "hopped in and went" and something happened, right?

That said, there's a real-world approach to flying that is not necessarily the same as the FAA's idealized version: if a pilot is comfortable with "hopping in and going", that's their business in MOST cases (there's a line past which it violates the "common sense" rule and it would be appropriate for a third party to intervene or say something, kind of like a friend at a bar having had "one too many").

If the pilot flies regularly, knows the airplane and the route, etc. it's normally understood by others that "they're the Pilot-In-Command, they're assuming the liability, it's their call". If I see someone getting ready to take off with full fuel and going somewhere very close by, and it's severe clear good weather for 1,000 miles in every direction, I'm unlikely to say anything if it looks like they're getting ready to "hop in and go". The presumption is that they know what they're doing - unless something looks out of place or it appears that they're about to do something very stupid.

If there is ever an incident though, you can bet that the NTSB (or FAA) will investigate into the extent of preflight planning done. If it ends up that it was a contributing factor, that person is probably looking at some "remedial training", a certificate suspension or both, not to mention civil liability.

In flying, as with most things, a certain amount of common sense needs to be exercised. It may not be appropriate to do two hours of preflight planning for an "over the hill", 15-minute flight from Santa Monica to Van Nuys in VFR conditions, but it's certainly not appropriate to do 5 minutes of preflight planning to do a four-hour, IFR flight from Santa Monica to Reno (for example). The latter example could even potentially qualify as a violation of FAR 91.13 ("careless and reckless operation"), a "catch-all" in the regulations. If I ever heard anyone bragging about doing such a thing, I'd have a few words with them for sure. Possibly even with a local FAA official.

Two truths I learned in aviation:

"That which is legal is not necessarily smart"
"That which is technically illegal is sometimes the smarter choice"

For such an exacting discipline, aviation does have a lot of "grey areas" at times - ones which experience and common sense are the governing principles.