| The higher the ratio, the less the amount of air passing through the 'combustion' section which is what you are trying to light, and therefore the lower rotational speed of that assembly.
At low forward speeds the 'normal' starting system is used to spin up the HP section, ie whatever the aircraft uses to provide ground starts - typically the APU. Restart during 'critical' phases as you put it is not immediately essential as the performance on the remaining engine/engines is adequate. Given sufficient altitude/remaining thrust the a/c can always be accelerated to sufficient speed to give an 'air' start. |