What's the procedure to restart the engines in mid-flight of an aircraft like ATR-72 which doesn't have an APU and is started with GPU (external ground power unit)? Pre-flight Airplane Procedures: Why is the "start engines" sequence on a 737 (First start APU, wait for correct EGT, then electrical bus switch, etc) manual? Couldn't it be automated? Is it similar on other planes (B767, A320)?