Planes have pretty disastrous failure modes, and useful lifespans in the decades. An accelerated test cycle seems... unwise.

How much time do you spend testing some mildly complex web app, by the time you're done with it? You're probably testing a ton as you go along, but a lot of stuff in an airplane can't be tested until the whole thing is assembled.

Then consider that even a smallish airliner is about a thousand times more complex than your web app. The software running on just one of the many computers alone is vastly more complex than your web app, not to mention the huge quantity of complex and often custom-designed mechanical equipment and such, plus the intersections of all these various pieces.

Then add in the fact that bugs that get through testing in your web app probably mean you have to fix it and redeploy, while bugs that get through aircraft testing result in dead passengers.

Three years seems pretty reasonable to me.

Not for a passenger jet.