I have a couple questions regarding the differences between the CFM56-7B22 and CFM56-7B24 engines, particularly in relation to the -700.  The context of these questions are primarily coming from the available planning software more commonly using the CFM56-7B22 (TOPCAT and soon PFPX) while the PMDG -700 uses the CFM56-7B24.  Since the routes I've been aiming to fly are more akin to the -700 (closer to SWA's -3/500's in appearance), I'd love to be able to accurately plan for the -700.  So my questions:

 

1. Is there a context to understand why different people are using different engines as their basis?  Was one more common earlier and now the other is more common?  Or did this purely come down to individual choice by the developers.

 

2. What type of fuel burn difference would be encountered?  In other words will PFPX based on the -7B22 be of no use or are they similar fuel burn rates for similar amounts of thrust produced?  I assume given the different maximum thrust, the burn is quite different at maximum thrust.  However, if cruise only needs say 17k pounds of thrust (completely made up), do both engines burn about the same rate to produce that 17k (making cruise predictions similar)?

 

3. If I understand this correctly, when I derate a -7B24 from say TO to TO1 I turn in in to a -7B22 essentially and then from TO to TO2 essentially a -7B20.  So if I get takeoff performance data fro the -7B22 that is say TO1 at 41C assumed temp, then using TO2 41C on a -7B24 is pretty darned close at least?

 

4. I'm pretty sure there's no way currently to pick a -7B22 on the PMDG -700 right now, but are there any hints we might be able to in the future with an SP?

 

...and with all of that said:

 

5. How have any of the -700 pilots out there been planning takeoff performance and any thoughts how PFPX might be used given the different engine types?  I'm sure hoping to get -700 profiles with the -7B24 for TOPCAT/PFPX at some point, but that is likely to take some time.

 

Thanks for any insight.  Even if all you have is some general education on the differences between these engines, I'd appreciate it.

 

Eric Szczesniak

 

 

1. Is there a context to understand why different people are using different engines as their basis?  Was one more common earlier and now the other is more common?  Or did this purely come down to individual choice by the developers.

 

In real life 737s - depends on what airline needs, wants and wishes to pay for.

In sim, developer preference.

 

 

 

2. What type of fuel burn difference would be encountered?  In other words will PFPX based on the -7B22 be of no use or are they similar fuel burn rates for similar amounts of thrust produced?  I assume given the different maximum thrust, the burn is quite different at maximum thrust.  However, if cruise only needs say 17k pounds of thrust (completely made up), do both engines burn about the same rate to produce that 17k (making cruise predictions similar)?

 

Should be quite close.

 

 

 

3. If I understand this correctly, when I derate a -7B24 from say TO to TO1 I turn in in to a -7B22 essentially and then from TO to TO2 essentially a -7B20.  So if I get takeoff performance data fro the -7B22 that is say TO1 at 41C assumed temp, then using TO2 41C on a -7B24 is pretty darned close at least?

 

Quite close, can't say "same" though, even if it might be.

 

 

 

5. How have any of the -700 pilots out there been planning takeoff performance and any thoughts how PFPX might be used given the different engine types?  I'm sure hoping to get -700 profiles with the -7B24 for TOPCAT/PFPX at some point, but that is likely to take some time.

 

The (real) 700 I fly in sim the most, has 20K engines. I force TO1 in TOPCAT and TO2 in PMDG and let the AST do the rest. It is plenty close enough for me.