Probably a bit late for a topic which may have run it's course, but an interesting topic, particularly for Airbus pilots.
Residual climb is pretty much as the name implies, it's the climb performance remaining after the aircraft has reached it's optimum level.
There are two optimums to be considered in ascertaining optimum operation, (1) Optimum speed for the airframe, and (2) Operation of the engines at optimum speed for Thrust Specific Fuel Consumption (TSFC).
For each and every weight, at each and every level, there is one speed which provides for the optimum performance of the Airframe. For the engines, there is one engine speed (with some variation with altitude for N1/N2/N3 relationship) which provides optimum TSFC, i.e. best thrust output for minimum fuel expenditure for each unit of thrust. A "ball park" RPM for best TSFC is around about 88%-90% N1 (at least for the aircraft that I fly).
At low levels, optimum engine speed produces thrust well in excess of that required for optimum airframe speed, and climb is possible. If this "perfect state" climb were to continue during climb at the optimum TSFC engine speed, thrust would steadily decrease, and at an altitude where optimum TSFC engine speed maintained optimum airframe speed, we are at Optimum Altitude. If this were to continue, a "creep" or "cruise" climb would follow, as further climb would only be available as aircraft weight decreases, typically at about 70 ft/min.
The problem with such a climb is that the last several thousand feet of climb to optimum altitude would be VERY protracted, taking hundreds of miles. The practical solution is to "bump up" the climb thrust to a little above optimum TSFC speed, tolerate a small fuel penalty, but enable a more positive climb to optimum level. Residual climb is usually optimised (by Airbus) at about 400 ft/min, causes much bitching amongst A340 pilots in particular, but provides very good economics by enabling an acceptable climb to optimum level.
Typically, MCR is a little above optimum TSFC engine speed, and MCL somewhat more.
There are often instances where an advantageous wind gradient will warrant a climb above the still air optimum level, and this residual climb capability is useful in climbing those extra couple of thousand feet to a more advantageous level.
An interesting interchange of thoughts within this thread regarding 2 engine Vs 4 engine climb capability. All other factors being equal, the 2 engined aircraft more often than not is capable of much higher initial levels, due to a considerably greater thrust excess. If it is remembered that a 2 engined aircraft approximately must achieve on 1 engine what a 4 engined aircraft does on 3 engines, in normal all engines flight the 2 engined aircraft has a 100% excess of thrust over minimum required, whilst the 4 engined aircraft has a mere 33% excess above minimum, and..... climb performance depends upon excess thrust available.
Regards,
Old Smokey