The discussion above started me thinking what it costs to 'buy the higher engine rating' and to operate it? I'm working a bit in the dark here, and if anyone has better figures, they would be welcomed. The cost difference probably comes down to fuel, maintenance and capital.
FUEL
I'm guessing that the difference in fuel consumption between the different motors in service must mainly be at takeoff, since they have the same thrust at cruise at altitude. SNECMA data on max. climb thrust @ 35 000 ft and Mach 0,8 and ISA and engine installed, for all ratings, is 5,960lbs.
I have no idea if there is a difference in the fuel used in climb. Any inputs welcome. I estimated the difference in fuel use between engines with different ratings assuming a constant SFC of 0.36 lb/hr/lb and no derating on takeoff. If the difference in fuel use between the different ratings can be simplified to 5 minutes takeoff thrust (and I'm not sure if this approach is valid), then the extra fuel burn per flight based on US$1.30 per US gallon (about AUD$0.45 per litre), is
CFM56-7b18 19500lbs US$0 base case
CFM56-7b20 20600lbs $6 extra over base
CFM56-7b22 22700lbs $18 extra over base
CFM56-7b24 24200lbs $ 27
CFM56-7b26 26400lbs $ 40
CFM56-7b27 27300lbs US$ 45
MAINTENANCE
It costs more to maintain the higher rated engines. The higher engine ratings go hand-in-hand with a lower EGT margin, and that generally means that the shop visit comes up earlier. The best I could find on the CFM engine data is:
ENGINE TAKEOFF POWER EGT margin
CFM56-7b18 19500lbs 145
CFM56-7b20 20600lbs 145
CFM56-7b22 22700lbs ?
CFM56-7b24 24200lbs 125
CFM56-7b26 26400lbs 85
CFM56-7b27 27300lbs ?
I have better data for the V.2500 motor. The issue of workscope at the shop visit is complex (sounds like the mechanic at my local garage, only he uses shorter, more punchy words). But it comes down to 19,000 hours on the wing use before the first visit for the 22,000lb motor and 12,000 hours for the 30,400lb motor. The maintenance varies of course by the ratio of engine flight hours to engine flight cycles – this data is based on 2 EFH/EFC.
The run to the second shop visit is about 70% of that. Engines with average cycle times around 1 EFH will only achieve 12,000 EFH because most damage is being done at takeoff. Hot environments also worsen the wear. At the lowest engine ratings though, the deterioration rate of EGT margin per 1000 EFH is pretty low, and its other factors cause removals rather than EGT margin. That gets complex to model and I've left out its discussion here; also there is no published discussion about how often they replace the spark plugs, but I suppose that must be buried in the calculations somewhere.
Anyway, the cost per flight hour for the different ratings can be calculated as:
Model V2522 V2527 V2533
Thrust rating 22,000 26,600 30,400
EGT margin 90-115 70-80 40-60
$US/EFH $103 $122-131 $150-161
So it costs about $30-50 per flight hour in extra maintenance for the higher rated engines. The CFM data indicate more EGT margin at higher power ratings than the V2500, so the difference in maintenance costs might not be as great for the CFM 56.
CAPITAL
I have no idea at all on what the different ratings cost, but my suspicion is that the fairly simple process to uprate them, outlined by DDG above, doesn't mean they are cheap to buy. I'm guessing here, and wonder if anyone has better figures, but they probably price the upgrade in terms of $/lb thrust. So if the engine cost about $4 million for say 20000 lbs thrust, then that is $200 per lb, and an uprate of 1000-2000 lbs might be charged at something like the $US 200,000 level. Or the 27,300 lbs rating engine might cost some $800,000 more than the 19,500 lbs rating. Anyone got more information?
TOTAL
For an aircraft doing 3000 flight hours per year and 1500 flight cycles, I estimated that changing from the 22700lb motor to the 26400lb motor costs (in US$ per year terms):
Maintenance $90,974
Fuel $31,928
Capital cost $26,600 (actually increasing lease costs)
TOTAL $149,502 per motor per year.
In a 30 aircraft fleet, with 2 motors per airframe, that's about $9 million per year.