They can split their keel, thats that bone that is the center most of their chest and sticks out somewhat, greys are heavy bodied birds and if they drop like a rock they can split that keel bone, not something you want to happen.
Buffy I am glad to hear you want to leave Echo flighted and it is your decision to make and no one should tell you different, not even your vet for some do think that birds should be clipped, that is hogwash but you can keep a bird flighted and remain safe in your home if you take the precautions to make it so.