Figure: ILS approach with wind, from G450 Aircraft Operating Manual, §2B-18-00, figure 10.
If you put me and another pilot in a room and ask for opinions on caged versus uncaged, you will get three answers. That's because I have two:
No matter how you operate, you should know that when the drift angle exceeds 12°, the airplane is headed further left or right than either the flight path vector or the ghost vector are telling you.
What follows are quotes from the references listed below, but most of this is pure technique, shown in blue.
Figure: HUD Display Uncaged Mode, from Eddie's note.
[G450 Aircraft Operating Manual, §2B-18-20, ¶4.A., page 46.] In the uncaged mode, the FPV can move up to ±12° laterally and vertically within the display.
The big advantage to the uncaged mode is that your eyes will always be directed to where the airplane is headed. During an approach to a runway with a large crosswind, however, the flight path vector will reach its limit and you must remember to look still further.
Figure: HUD Display Caged Mode, from Eddie's note.
[G450 Aircraft Operating Manual, §2B-18-20, ¶4.B., page 46.] In the caged mode, the FPV is locked into the center of the display and only vertical movement is allowed. No horizontal movement (i.e., drift angle) is displayed. This keeps the display in an easily readable and flyable position during high crosswinds that could result in an uncaged FPV being displayed at the extreme left or right of the display. If the FPV is caged, and the actual velocity vector position differs laterally by 1.5° or more from the caged position, the ghost FPV is displayed in the actual velocity vector location and shows drift angle as well as climb and descent angles. The ghost FPV is also limited to ±12° laterally and vertically. It flashes when it has reached its limit.
The advantage to the caged mode is that everything is where you expect it to be, all the time. Everything, that is, except the runway.
Figure: HUD Display Clear Mode, from Eddie's note.
[G450 Aircraft Operating Manual, §2B-18-20, ¶4.C., page 46.] The clear mode removes many of the displayed symbols to give the pilot better visibility. Airspeed, altitude, heading, radio altitude, FPV, and FD, as well as other necessary symbols, are not removed. The clear mode is activated and deactivated by pushing and releasing the touch control steering (TCS) button on the pilot’s yoke. Pushing and releasing the TCS button toggles the display between the normal and clear modes. The operation of the TCS function has not been changed. It activates any time the TCS button is pushed.
This is a good way to get ready for landing after you have disconnected the autopilot. If you press TCS with the autopilot engaged you will have to handfly for the duration.
Figure: G450 Display Controller HUD Menu Uncaged, from Eddie's aircraft.
[G450 Aircraft Operating Manual, §2B-18-10, ¶6, page 8.] Pushing this key toggles the HUD display between the caged and uncaged modes. The caged and uncaged modes determine how the FPV moves within the display. In the uncaged mode, the FPV can move up to ±12° laterally and vertically within the display. In the caged mode, FPV movement is restricted to vertical motion only and a ghost FPV is also displayed.
Extracted from: G450 Procedures & Techniques / HUD - Symbology.
Figure: G450 HUD Flight Path Vector, from Eddie's notes.
[G450 Aircraft Operating Manual, §2B-18-20, ¶2.A., page 14.] Displacement of the FPV laterally indicates drift. Full-scale deflection indicates 12° of drift. The lateral and vertical motion of the symbol are restricted to ±12° in the uncaged mode, and 0° laterally in the caged mode. The FPV flashes when it is at its limit. The FPV becomes active when the IAS is 63 knots or greater, disabled when IAS is 60 knots or less.
Figure: G450 HUD Ghost Flight Path Vector, from Eddie's notes.
[G450 Aircraft Operating Manual, §2B-18-20, ¶2.A., page 14.] The ghost FPV is displayed when the caged mode is selected on the DC HUD menu, and the drift angle (the difference between the actual velocity vector and the caged FPV) is 1.5° or greater. The ghost FPV is used during high crosswind conditions. This means the pilot can observe the actual flight path while the FPV symbol and associated symbols are caged to the center of the display. The ghost FPV flashes and remains in view when it reaches the lateral limits of the display.
When the flight path vector is uncaged, it points to where the aircraft is headed up to the limits of the viewing angle on the HUD and then it flashes to let you know it would actually be further left or right if it could. Even at these limited extremes it can be difficult to see so you have the option of caging the flight path vector to the center of the HUD. In this case, the ghost flight path vector shows you where the aircraft is actually headed, again up to the limits of the viewing angle of the HUD.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.