I think there is some confusion here and since I am currently studying ADP logic on 747s lets see if I can clear this up.
All 747 series aircraft carry the same Fairchild Controls ADP. The design is 35 years old, though there is a replacement that is not only more powerful, but MUCH quieter and is found on the 767 & 777.
The ADP is designed to supplement hydraulic pressure during times of low pressure, failure or when the airplane is configured such that a failure may cause an unrecoverable configuration(ie. can't get the flaps/gear up for go around.
It is different on all three engine configurations as far as where the hydraulic pressure must dip to come on automatically. The RR version comes on first and once on remains that way until cycled(this is due to the RB211 having a small core). The GE and Pratt are relatively the same.
Flap movement on ground can make the ADP come on. Anytime the Flap lever is in excess of 25 degrees the ADP is on and running continually. Excessive control input by the pilot can also make them come on. Fuel cutoff or flow failure causes them to come on sensing the impending engine failure.
Some operators OPs manual calls for the ADP to be set to ON during approach. Most set to Auto.
How you guys like the noise coming off of the ADP?