Limited experience (same as yours--my wife was hurt on a trip), one from check-in to the gate, a different one to board. Beware that at our connecting airport they wanted to leave her at the gate with no chair--for a very long connection. She solved the problem by refusing to get out of the chair until they provided another. (The chair is assigned to the pusher, not to the flyer.) Note that there will be a change of chairs at the end of the jetway--standard chairs won't work on a plane, the plane chairs are too unstable for general use. (They're reasonably safe on the plane--there's little opportunity to tip when they're going between seats.)