A bit more background to the RR Trent 892/884 used by CX on the 773:
The 777-300 initially came to CX with Trent 884 engines.
Although the Trent 884 thrust rating is adequate to cover takeoff, MCT and go around thrust requirements, its climb thrust is marginal.
After discussion with RR and Boeing, an agreement to use the Trent 892 max climb thrust rating was reached, with takeoff thrust remaining at 84,000 lb.
At the time of the agreement, the only thrust ratings available for the 777-300 were the Trent 884 and Trent 892, so the only way to implement the use of Trent 892 max climb thrust was to use the Trent 892 rating plug.
In order to comply with the 84,000 lb takeoff thrust rating certification, the 777-300 fleet was restricted to only use TO1 and TO2 (TO1 closely approximates 84,000 lb thrust).
In the longer term a unique thrust rating was created to incorporate the take-off thrust of the Trent 884, with the climb thrust of the Trent 892.
Hence the Trent 884B thrust rating:
Takeoff
- Same rating as Trent 884 TO (equivalent to Trent 892 TO1)
Climb
- Same rating as Trent 892
MCT
- Hybrid rating Trent 884/Trent 892
- Below 21000' same as Trent 884
- 21-25000' Trent 884-Trent 892
- above 25000' Trent 892
Go Around
- Same rating as Trent 884
Clear as mud?