The weather might be marginal somewhere along your route. You’re instrument rated, but you’re concerned that filing IFR will result in a much longer trip, or maybe you’re worried that you’ll hit ice at the required IFR altitudes. Assuming that you don’t cancel the trip (possibly the best choice), what do you do?

1. file IFR, because you can always cancel and finish VFR if the weather’s not IMC; or
2. file VFR, because you can always get a pop-up IFR clearance if the weather closes in.

I’ve seen both of these pieces of advice many times on aviation lists, and have sometimes heard them from other pilots, but I know from personal experience that they’re both wrong.

If you’re trapped under a lowering ceiling, you can’t always get a pop-up IFR clearance. First, you’re probably well below the minimum IFR altitude and have no way to climb VFR; even worse, you’re probably too low for ATC to see you on radar or even to hear your radio call. The only option is to cross your fingers and climb (illegally) through the cloud until you’re high enough to get into the system. That’s a scary option, especially near high terrain.

If you’re IFR, you might just as easily be trapped on top. Maybe there is a safe amount of VMC below you, but you have to have a way down to it before you cancel and continue VFR. Maybe you can shoot and approach and break off down low, depending on where you are, but it will frequently happen that once you start IFR, you find yourself effectively trapped in the system, maybe with ice-filled clouds below you.

So you can’t, always. As with much of flying, the pat answers aren’t that helpful, and pilots of small, piston planes are left with difficult choices that flight instructors (who rarely fly long cross-countries) and turbine pilots might not understand.