A question (or comment request) to our resident senior MD-80 pilot forum user:
In planes like these, that lack RAT, I've always thought it would be a good idea to keep the APU running during the take-off and climb to some "safe" altitude and fire it up again before starting the approach, again at a "safe" altitude, as to be able to keep most systems on-line and attempt an engine restart in the event of a dual engine failure, "safe" meaning precisely that you have time (altitude) to, while gliding, asses the situation, start the APU, bring the systems on-line and attempt an engine re-start with some margin of altitude to spare. As a minimum, the APU will be running whenever the plane is not "clean".
I know that it can be done. However, AFAIK that is not the usual practice. The APU is typically turned off after engine start and turned back on after landing. I know this practice saves a bit of fuel, but still...