Idle reverse policy on landing

[Centaurus]

For many reasons, individual company SOP may direct idle reverse after touch-down - unless of course good airmanship dictates otherwise. It may be for fuel conservation, perceived engineering costs or noise-restrictions.

In the CFM 56 series (737), idle reverse N1 is the same as idle forward thrust N1 (nominally 23% N1) - except it takes a bit of fiddling to get the reverse thrust levers at idle reverse without inadvertently slipping into forward thrust. The theory of selecting idle reverse only is that instant full reverse is available should things look a bit dodgy and the end of the runway is coming up faster than planned.

The problem then arises of the relatively long delay in spool-up to max reverse from idle reverse (23% N1) Try it in the simulator and you are looking at 6-8 seconds before full reverse is reached. Because of the extended spool-up period, by the time full reverse is attained the airspeed has probably dropped below 80-100 knots when reverse is not as effective.

I understand N1 remains at high (in-flight) idle with thrust levers closed at touch down and if reverse has not been selected within four seconds after touch-down, the power automatically decays down to ground idle, which works out to be around seven percent N1 less than flight idle.

Presumably this is based on the assumption by the designer that if you haven't selected reverse by four seconds after touch-down then you aren't going to use it and so the power drops back to ground idle (less residual thrust).

The point is this. If company policy is to use only idle reverse for landings then crews should ensure the reverse levers are correctly positioned to give at least (say) ten percent above ground idle in order to permit rapid spool up to max reverse if needed. The spool-up time from ground idle reverse (nominally 23% N1) to full reverse, is significantly longer than the spool-up time from (say) 35% N1 reverse thrust. By selecting (say) 35% N1 reverse thrust after touch-down, the landing roll safety factor is increased with little discernible increase in noise footprint and/or fuel consumption.

Any thoughts?