They usually fit better without the base, and sometimes actually cannot be used with the base on a plane, if the seatbelt buckle is blocking the latching mechanism. Remember that in a plane, the seatbelt buckles in the center, not at the side.
and you don't need the base.....if you've never used it without b4, practice before you leave & bring your directions with you so you can use it in your rental car without the base.
snugride is rated to be used with or without the base.
amie