This is from the F4AF 1.08 manual addendum:
Using Helmet Mounted Sight with IR missiles
Helmet Mounted Sight (HMS) has been improved for patch 1.08 in both displayed symbology and
functionality.
Specifically, the way that HMS is cueing IR missiles is corrected. Previously, padlocking a target with
4 key was the only way to get HMS to designate target for IR missile. This is now corrected, so that
HMS reticle, which is drawn in the center of the screen, needs to be placed over the intended target
and IR missile seeker will try to acquire and lock onto it. This can be done either manually or with
padlocking.
Using HMS is really easy (that's the point of it!) if you follow these rules:
1. You have to be in an HMS equipped plane, which includes all F-16 CCIP and MLU variants in
2005 and 2010 theaters.
2. You have to be in 3D-cockpit, Padlock or Extended-FOV views and have IR missile selected
as a current AA weapon.
3. Press U key to uncage seeker from Radar cueing and switch to HMS cueing. If your plane
has HMS you will now see HMS cue reticle in the center of the screen. At the center of the cue
will be the missile seeker diamond and offset from center will be a number in degrees showing
how far away from the aircraft boresight axis (angle off-bore) your current view direction is.
You can use this angle display to estimate how probable your missile shot is at any moment.
Unfortunately IR missile seekers provide no information on the range to the target so this
information can not be displayed.
4. After uncaging the missile and slaving it to HMS view direction, missile will try to lock onto the
first target it finds and remain locked onto it as long it has good IR "tone". At this point you can
look away and the missile will not drop its current target. While you look away a line extending
from the center of the HMS cue will point in the direction that missile seeker is looking at.
To force missile to drop current target you have to cage it with U again and then uncage it to
be able to search for another target. In case the missile loses target lock on its own - either
because target moved outside its seeker gimbal zone or its IR signature is too small (too far
away) - missile will go hunting for next target in the direction that you are looking at.
5. Once you're satisfied with the relative position of your target and the missile has a solid lock,
all that remains is to fire the missile.
To use the next missile in HMS mode, press uncage again and repeat the procedure.
Using HMS in padlock view (4 key) and Extended-FOV view (5 key) will automatically cue the missile
to the current padlock target. In 3D-Cockpit view (3 key) you have to manually slew view and look
close to the direction of the target for missile to acquire it.
Using Track IR to pan the view - apart from making looking around an easier exercise than using
Joystick POV hat - has the added benefit of becoming a true point and shoot combination in Falcon.
Note that HMS will work with all IR missiles although its full potential is realized only when used
together with high angle off-bore capable missiles such as Aim-9X (90° off-bore angle), Aim-132 (90°)
and Python-4 (70°). For comparison Aim-9M has off-bore angle of only 30°.
But beware that opposition also has similar capability - MiG-29 equipped with HMS and AA-11 (R-73
Mod2) is capable of 67° off-bore shot as well and really is a worthy opponent.