Colin,As stated earlier, the primary reason for lift dumping is to get down in a hurry. During the certification process, transport category aircraft must be able to get down from max operating altitude to 10000 ft in a predicated amount of time. Can't remember the exact time, but if I recall correctly, there are two segments, first to FL250 and then to 10000ft. The DC-8 does not have "speed brakes" only ground spoilers, so to get down in a hurry, you need to use reverse thrust. The first time you see, feel and hear it, it definetly gets your attention.