A bit of a technical one here, but have heard much about how the triple core Trents and RB-211 suffer much less from degradation in performance and fuel burn than the twin core competitors such as the GE-90.

My question is partly how does this happen, and also what kind of percentage degradation is there, and how do the two sets compare over time fuel burn wise ( I understand the GE-90 has a lowere fuel burn initially), obviously this is pretty vague with various models (and for different aircraft) available, but is there some general rule?
Cheers

I will be the first to admit that there is a lot I do not know about the topic, but perhaps I can contribute some information.

Most airlines have some way to capture actual fuel burn on each flight. This data gets fed back into the computer and a performance factor for each individual airplane is continually updated (the computer calculates planned versus actual). As the burn gets higher, due to engine wear, weight of the airplane changing, dirt accumulating, etc. the flight planning system is fed this data and gives a little more fuel to successive flights. Of course, things like an engine change, or an aircraft overhaul/interior modification cause the calculation to be reset. The initial setting is normally from manufacturer supplied data.

The wear of an engine can be monitored within fractions of percentage points. As I say, I am not an expert, but I believe it would be rare for an engine burn to increase more than 5% due to wear alone, but that can be significant on a long haul flight. I must say that I am unaware of an degradation differences due solely to the number of spools.

Keep in mind also that engines on newer airplanes (330/340/777/etc) have extensive on-board computer monitoring capability. The data captured is very detailed, all the critical parameters and then some. This data can be read in real time at the airline headquarters or wherever their engine monitoring is done. It is possible to see an individual engine deteriorating to the point where it would fail. The trick is to run it as long as possible with on-wing maintenance and then change it before it fails. This capability is an important factor in the success of ETOPS.