What is rarely discussed with regards to spool up time is engine handling if landing on a slippery runway with a strong crosswind. Read the following extract from the Boeing 737 FCTM carefully. Better still, try it in the simulator because it may give you an unpleasant surprise if you are not aware of the marked delay in reverse thrust spool up times.
Under the heading Reverse thrust and Crosswind (All Engines).
As the airplane starts to weather vane into the wind, the reverse thrust side force component adds to the crosswind component and drifts the airplane to the downwind side of the runway. Main gear tyre cornering forces available to counteract this drift are at a minimum when the anti-skid system is operating at maximum braking effectiveness for the existing conditions.
To correct back to the centreline, reduce reverse thrust to reverse idle and release the brakes. This minimises the reverse thrust side force componenet without the requirement to go through a full reverser cycle, and improve tyre cornering forces for realignment with the runway centreline.....when re-established near the runway centreline, apply maximum braking and symmetrical reverse thrust to stop the airplane.
The surprise factor is how long it takes for the engines to spool down to reverse idle from full reverse. In fact it is about 12 seconds or more. Reverse idle in the CFM 56 is approximately 22% N1 which is same as normal idle in forward thrust. Having got the aircraft aligned once more with the centreline of the runway, and you apply full reverse from idle reverse of 22%N1, well that takes at least 8-10 seconds of spool up time.
The thing to keep in mind is you are not selecting reverse thrust from a normal touch-down speed where flight idle of 32%N1 is still in place. It is from ground idle reverse of 22%N1. That lower figure causes a significant delay in spool up time to full reverse. For that reason, the additional landing roll to get the aircraft stopped on a slippery runway under crosswind conditions can be disturbingly long compared to a normal crosswind landing on a dry surface
Last edited by Centaurus; 3rd Mar 2013 at 22:13.