PPRuNe Forums - Power set or thrust set ?

Quote:

To avoid asymmetric spool up of the engines and the resulting yaw at low airspeed where the rudder has no power.

But rudder pedal nose wheel steering is effective if that happens.
According to the CFM 56 OperationManual for the engine there is a 3 second tolerance in CFM 56 series engines between ground idle of 22% N1 and 40% N1 for the same rate of throttle opening. But between 40% N1 and full power there is only a one second tolerance in rate of spool up for same rate of throttle movement.

This means if the pilot selects TOGA from idle thrust of 22% N1 and one of those engine takes up its three second tolerance between 22% N1 and 40% N! while the other engine spools up to take off thrust without any delay, the aircraft would initially very likely yaw strongly towards the "slower" engine and cause the pilot to lose directional control in the first few seconds of the take off roll due strong asymmetric thrust.

But by first opening the thrust levers to approximately 40% N1 with the brakes off and checking equal rate of spool up then selecting TOGA either manually or via autothrottle the chances of a directional control problem early in the take off roll happening is reduced.

At least one Asian operator I know includes an extra SOP where the PM is required to call "STABLE" when the N1's are equal at 40%N1 during the initial part of the take off roll. This is not a Boeing required call. On receiving the call of "STABLE", the PF selects TOGA and the thrust levers advance as advertised.
On one occasion during simulator training, the PM was quietly advised not to say anything as the PF pushed open the throttles to 40% N1. The PF proceeded to trundle down the runway at 40% N1. Passing the 1000 ft marker still at 40% N1 and nearing 50 knots IAS, the PF looked accusingly across the cockpit to his PM and said "Hurry up - you have forgotten to call "Stable" Readers can draw their own conclusions..

A common mistake made by pilots is to push the thrust levers to half way up the throttle quadrant on brakes release in a mistaken attempt to hurry up the rate of engine spool up which is relatively slow between ground idle and 40% N1 so by the time they press TOGA during an autothrottle take off the N1 are around 60% to 70% N1. This may have its genesis in pilots having previously operated the P&W JT8D engine where the mantra was "Stand'em up" and throttles pushed up to the vertical position to check spool up before applying full thrust on take off.

Having said that, the Boeing 737 FCTM discusses touch and go landings and states" After touch down, the instructor selects flaps 15, sets stabiliser trim, ensures speedbrakes are down, and at the appropriate time instructors the trainee to move the thrust levers to approximately the vertical position (so engines stabilize before applying go-around thrust). When the engines are stabilised the instructor instructs the trainee to set thrust.

The assumed reason for the difference between setting 40%N1 from ground idle of 22% N1 to ensure equal spool up at the low speed following brakes release, and the touch and go where thrust levers are set to vertical to ensure engines stabilised before selecting GA power, is the relatively high speed at the time the touch and go actions are commenced and therefore ample rudder control available to control any asymmetric swing.