May 17, 2020

CFM 56–3 Rating Model Change

Today we are all looking for the possibilities to optimize cost of operations and minimize expenses trying to get business and provide a service to other industries for cargo/passenger transportation from point A to point B. Doing engine condition monitoring and providing engine support to our customers operating Boeing 737 Classic in different parts of the world and flying B737 in a different configuration we see very different engine deterioration rate.

Basically we have the following picture on 737–300/400/500:

For -300 basic option is CFM 56–3B1 or -3B2 operated at 20,1k lb
For -400 basic option is CFM 56–3B2 or CFM56–3C1 operated at 22,1k lb
For -400F or high gross weight is CFM 56–3C1 operated at 23,5k lb
For -500 basic option is CFM 56–3B1 operated at 18,5k lb

We see that the market is mainly proposing CFM 56–3C1 options after PR or partial PR, which means that there is a possibility for a shop to get any customer operating Classic aircraft.

But is it relevant for operator to use -3C1 configuration as the only available option?

Engine Thrust Rerate

CFM56–3 engines do not have rating plugs, that is why thrust of an engine model is changed in the following 3 ways:

  1. Requires a Boeing service bulletin with instructions for FMC and Auto- throttle Computer pin-program change, and Airplane Flight Manual coverage
  • CFM56–3-B1 FMC can be set to derate auto-throttle travel to maintain 18,500 lbs thrust but pilot can physically push throttle to 20,100 lbs thrust
  • CFM56–3B-2 FMC can be set to derate auto-throttle travel to maintain 20,100 lbs thrust but pilot can physically push throttle to 22,100 lbs thrust

2. Throttle box on MEC has a throttle lever mechanical stop

  • 737–500 is restricted to 20,100 lbs thrust

3. Engine conversion

Benefits of rerate /conversion

Rerate permits to extend engine on wing life, to optimize LLP management and minimize parts degradation (temperature).

When you need to extend engine on wing life you can recover performance and to continue to use engine when EGTM=0 or less at higher thrust. Example for a CFM56- 3C1, there’s an increase of the EGTm as follow when an engine rerate:

  • 3C1 23.5K: reference
  • 3C1 22K: +20°C of EGTm
  • 3C1 20K: +64°C of EGTm
  • 3C1 18.5K: +89°C of EGTm

Also you might consider that Rerate/conversion allows engines Intermix as per specific Boeing recommendations.

  • Rerate 3B1 or 3B2 for 20,1 K
  • Conversion 3B1or B2 with 3C1 (with same thrust)

Engine model conversion

Engine model conversion required to get a possibility to install the engine on the aircraft where TCDS or intermix SB do not allow use available model. Model conversion is done by hardware replacement. The full procedure is prescribed in relevant SB.

  • 72- 804 3B1/3B2 to 3C1
  • 72–805 3B1/3C1 to 3B2
  • 72–806 3B2/3C1 to 3B1
  • 72–0121 3B1 to 3B2
  • 72–0418 3C1 to 3B1/ 3B2

Conversion of 3B1 or 3B2 to 3C1 requires minimum of 11°C EGTM as well as it will require to do check of P/Ns or replacement of the following parts: MEC, PMC, VSV actuator, HPTCCV Timer & Electrovalve, VSV F/B Coupling, VSV Mount Bracket, VSV F/B Cable.

Estimated Conversion man hours and TAT

Our team can perform conversion in our maintenance locations in US, Ukraine and UAE, as well as we can do it on-site for you. To convert your -3B1 to -3C1 in our shops it will take 2 days and only 28 m/h. And to re-rate -3C1 to -3B1 or -3B2 can be done in 20 m/h only. We propose our partners to make evaluation of their operational demand and to give our advise on the best option for their operations to make it safe and cost effective.

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