hoggsnortrupert posits:
I would on occassion start engines with Electric pumps off, as it is a sure way of testing if the associated engines Mechanical pump is working before getting into the air.
Well, I can guarantee that if the engine mechanical pump is NOT working, you won't get the engine started, much less airborne.
Think about it: How much pressure does the electric pump(s) produce? And what is compressor discharge (or burner) pressure at idle, and at takeoff? The mechanical pump must overcome that air pressure, otherwise compressed air would backflow into the fuel system!
The engines I know have mechanical pumps that produce upwards of 1000 psi; this not only runs the engine, but drives internal servos in the fuel control unit, variable stator servos, etc. The electric pumps won't come close to handling this job.