Abstract: An auxiliary power unit (“APU”) system and methods used to bring such system on-line and off-line are disclosed. An APU generator forming part of an APU system, is brought “on-line” smoothly in order to replace a primary generator operating at a steady-state rotational speed that is interconnected with a load. A controller forming part of the APU system controls rotational speed of the APU and thereby the generator so that it approaches a rotational speed beneath the steady-state rotational speed of the primary generator. The controller then connects the APU generator to the load and the primary generator. This causes the APU generator to motor at the steady-state rotational speed dictated by the primary generator. Thereafter, the controller may increase rotational speed of the APU generator until it provides power to the load and the primary generator may be disconnected from the load.