interesting article
This is an article from 2006. It relates to a GE90 and not an RR but it's interesting.
"The Unthrustworthy 777
Air Safety Week, Oct 9, 2006
Emergency Airworthiness Directive Issued For The 777's GE90 Thrust Rollbacks
An emergency Airworthiness Directive (AD) issued September 30 stems from two occurrences of engine thrust rollback during takeoff on Boeing 777-300ERs, which are powered by General Electric GE90-115B engines. The concern is that the GE90 engines powering Boeing 777-200LRs and 300ERs could suffer dual thrust reductions on take-off, with potentially catastrophic consequences.
In the two instances cited, just the one engine was affected but because it's software-related, the FAA cites the possibility that "a dual-engine thrust rollback could occur just after V1 (takeoff decision speed after which takeoff is to proceed even after an engine failure). This would result in the airplane not having adequate thrust to complete the takeoff. A dual-engine thrust rollback, if not corrected, could result in the airplane failing to lift off before reaching the end of the runway or failing to clear obstacles below the takeoff flight path."
Trouble-shooting technicians have found that the two cases in which there were single-engine thrust reductions during takeoff were the result of a flawed software algorithm in the Full Authority Digital Engine Control (FADEC). The emergency AD requires operators of GE90-115Bs powering 777-300ERs as well as GE90-110Bs powering 777-200LRs to amend their procedures to prohibit takeoffs at anything less than full-rated thrust. This is being stipulated because the rollbacks are only likely to occur at reduced powers, during the so-called "balanced field" takeoffs. When the power levers are set to maximum, the predictive software is out of the picture. For the foreseeable future, passengers on lightly loaded 777's are liable to think that they're on a shuttle launch."