Chizh provides a very good description of the FLOOD mode. However, I am not sure that its purpose is only for close combat. In the F-4 fighter from the Vietnam war, the AIM-7E version did not have a monopulse receiver antenna, and it needed CW illumination for every launch. So, all launches of AIM-7E missile in Vietnam were guided by FLOOD mode CW illumination - both in close combat and at long range.

The CW illuminator is separate from the main radar antenna, both in F-4 and in F-15. You can see in this photo, the small, non-moving horn antenna CW illuminator attached above the main antenna of APG-70 radar in the F-15E:

http://www.f-15estrikeeagle.com/refe...s/radar/03.htm

(If you look carefully, you can see an even smaller horn antenna in the top part of the phased array - the smaller horn is for sidelobe compensation, the larger horn is for CW illumination.)

The new AIM-7M version has a monopulse antenna, like the R-24R, R-27R, etc. So, it can use the aircraft's radar lock directly and separate CW illumination is not necessary. Nevertheless, as I understand, the FLOOD mode is retained for:

(1) compatibility with older versions of AIM-7
(2) emergencies when the fighter is not able to lock the target

I agree that theoretically, the FLOOD mode sends less energy to the target and should have a shorter range. But to say that this shorter range is 10 km, or 50 km? - I don't know. Maybe I'll learn.

Hope this helps,

-SK