RB211-535E4-37/12B & 21B - Difference
 

Hi All,
Can you help me please to find a difference between the Engine RB211-535E4-37/12B and RB211-535E4-37/21B? What does 12B and 21B mean? B for my understanding its a Profile, but what does 12 and 21 mean? Is it normal practice if LH has /21B at the end and RH has /12B?

Thank you for any advice.

Despite lots of time working (not an Engineer) on 757's from 1983 sorry haven't a clue - but a pal who was a Chief Engineer on the 757 at Air Europe at LGW should know,
so I will ask him for you.

RB211-535E4-37
RB - General Designation
211 - Specific Engine Type
535E4 - Engine Thrust Variation
37 - B757 Installation

The -12 is an E4 engine, and the -21 is an E4-B

RB211-535E4-37, RB211-535E4-B-37 and RB211-535E4-C-37
The RB211-535E4-B-75 powered the Tupolev TU-204 aircraft.

RB211-535E4 had three Variants, available at thrust ratings from 40,100 to 43,100 lbs take-off thrust.
The first -535E4 variant of 1984 has a rating of 40,100 lbs thrust.

BA, EA, AE, MON, and LTS had all ordered the 535C engines for their first 757's.
Eastern and also Monarch Airlines retrofitted the latest E4 from 1984, onto it's fleet replacing the earlier 535C engines.

The later -535E4-B variant, introduced on the 757-200 in 1989, is rated at 43,100 lbs thrust.
The -535E4-C, powering the larger 757-300, is rated at 43,100 lbs thrust.

The higher-thrust -535E4-B, first went into service with American Airlines in 1989.
The third variant, the 535E4-C, went into service on the 757-300 in 2001, and only 12 757-300s have the 535E4-C, operated by Continental and the now defunct ATA Airlines.

The RB211-535E4-powered 757-200 was certified for 120-minute ETOPS in 1986. In 1990 it achieved certification for 180-minute ETOPs.
The RB211-535E4 achieved the world record for on-wing life without removal on a 757, staying on wing for almost twice as long as any other engine on the airframe.

My Air Europe Pal's reply ---- >>>> Crikey Roger, I just knew them as the 535C and 535E4 models.

Have a look at this EASA Type Data document might help you >
https://www.easa.europa.eu/en/downloads/7699/en

FYI --- Sorry, but that does not refer to 12B or 21B, but on some other AD Docs viewable onlineit does, but does not differentiate the designation as such of what it means.