CSD vs VSCF
 

What's the difference between a CSD and a VSCF?

Why under Uk CAA you must remain within 45 minutes from a suitable airfield?

link to the 737 site...

On the Boeing 737-300/400/500, initially a great idea, the VSCF has proved to unreliable. To the point where some operaters have removed them.
The Constant Speed Drive is basically a Hydraulic motor and a pump in one, this is to allow the varying input speeds from the engine during it's operation to give an almost constant output speed to the generator to supply the 400Hz required for the aircraft systems.
The Variable Speed Constant Frequency is a self contained DC generator with an integral electronic package which converts the power to AC and a stabilised 400Hz, during all phases of engine operation.

http://www.esw-wedel.de/dt/download/vscf.pdf

The common term for the VSCF used to be " Very Seldom Completes Flight "...........

It might have been a large slice of exaggeration, however, when BA retro fitted them to their fleet of 737-400s, the reliability was abysmal. Cue another expensive fleet mod programme to re-fit CSDs and AC generators..:ugh: :ugh: