Weld command in Rhino (if similar to other 3D apps) will convert 2 vertices to a single vertex and delete any connecting link between the 2. But the weld statement in a Poser cr2 file is meant to prevent breaks or gaps between adjacent groups during bending operations. If these breaks occur in your airplane model, it's a case of non-identical vertices at group boundaries (as in the earlier problem with your airplane wing), or the lack of proper group locations and welds. In the final case where your airplane splits apart at the nose, this is because there's no stationary intermediate group to weld the two side parts together. If your airplane splits at the nose, then model it like a human form, with head, chest, arms, hip and legs.