Originally Posted by
LADAN 2ALPHA
Still on take-off thrust, anyone with an idea why we advance thrust levers to 40%, or around 50% for some jets, before setting the full takeoff thrust? Anyone know of any other reasons apart from engine thermal stabilization and making sure that both engines are spooling at the same rate? I can't seem to find any other reasons online...
It's because turbine engines accelerating from idle tend to accelerate slowly and often at different rates. If you set takeoff thrust (or power) from idle, if the turbines don't accelerate at the same rate you can get a massive thrust asymmetry and associated yaw - much more than can be overcome by the rudder at low speed. Either abort, or go off the side...
By advancing the thrust levers to some mid-thrust position gets away from that slow idle response regime that is characteristics of turbines. So the procedure is to set a mid N1/EPR, let the engines stabilize, then advance to TO (or engage autothrottle). Simples.