The interest rates today generally make the whole thing no big deal to loan yourself money.
The kicker on 401K loans is always that "due immediately" part if you're laid off.
Many companies love to do layoffs near the end of the calendar year which means you already made 80% or more of your salary that year, and then you're hit with a penalized retirement plan withdrawal if you can't pay it back, to add insult to injury in that year's taxes.
And it's a BIG penalty.
Generally, I leave retirement money alone. If my goal/plan was to save for retirement then it stays in a retirement account.
I'm also VERY leery of buying anything on debt that's a depreciating asset. If you're talking about an airplane, it's depreciating by the hour, until you throw a lot of cash at it and rebuild or replace the engine.
If you're cash strapped you could end up paying yourself interest on something you can't afford to repair, and it sits parked until you sell it off at fire-sale prices to get out. And then you'll pay to lower the loan but maybe not pay it off, and keep paying yourself that interest, long after the aircraft is gone.
Caution is in order. How long do you think you'll be at the current job? How fast can you pay yourself off? How much can you afford to fly on top of paying yourself? When will the airplane need an engine overhaul?
I don't like mixing multiple avenues for risk in the same "overall" fiscal deal. An airplane by itself is a risk. A loan on top of an airplane is another risk.
Them's my thoughts. Do as thou wilt, you only get one go at life AFAIK.