The flight path vector (eg aircraft track, flight path up/down or level) is calculated by the IRS and can be displayed on the PFD. This symbol is known as 'the bird'. you can also project on to the PFD a radial, and an angle up or down - 'the cage'. you would normally do this on a non-precision approach using the inbound radial and 3 degree descent angle. You then manually 'fly the bird' - the aircraft's calculated track - in to 'the cage' - the required track and angle of descent. It works very well.