Seconded on the details selling it. I had to design a bunch of train car impacts a while back and ran into similar problems with getting things to sound massive. Don't think that your pitch-shifted layers will make the entire sound - they can provide some low body, but all that high tinkly, full-res stuff is also really important.
My quick tips in addition to what's been said here:
1) Picture what's actually happening to the plane in slow motion and map it out if it will help you structure your crash. Like, maybe first there's the plane hitting water, so you need a large splash. Then there's the metal of the plane crumpling - deep metal stresses and groans, maybe some high creaks or write tensiony snaps. Glass breaks. A fuel tank explodes. Bubbles rush up as some stuff starts to sink down, etc.
2) Don't be afraid to use some distortion. Hearing a real impact like that close up would probably deafen you instantly, so go big.
3) Jump from as quiet as you possibly can to as loud as you possibly can as quickly as you possibly can. There's no better way to make something sound loud than using all the dynamic range you have and making it as transient as possible. Once that initial 'hit' is done, you can bring up a bunch of squashed/limited layers, but I wouldn't bother compressing the initial hit at all. Maybe have the first part of the impact be the bass so that people's ears get a chance to warm up a tiny bit before the real nasty hits come in though.
4) Kinda goes without saying, but try to hit the full frequency spectrum - sub basses, low groans, low mid explosions, mid creaks and tears, mid high creepy dry ice-y sort of metal sounds, high glass. That'll give you even more perceived loudness and make sure that no matter what else is going on in the mix (e.g. music), there are the necessary layers available to the mixer to make your hit sound as big as possible at that moment.
Impacts that long are kinda like doing a mini mix session - you will want to be constantly taking your audience's ears from one detail to another in very short amounts of time. So choose your details and make sure to give them space to speak, one by one.
Post up when you're done! Would love to hear it.