Economics of FLEX T/O
 

Hello,

perhaps someone could enlighten me: firstly, is it true that in absence of any climb restriction, it is more economical in terms of total fuel burn, to use "high power" until the cruise level? In this case, usage of FLEX is motivated only by lower engine maintenance cost? If so, I would assume that the impact of T/O thrust setting on maintenance $$$ is known quantitatively? Could someone provide an example?

Thanks

pax2908
 

It is more economical in fuel terms to use max take-off power followed by maxclimb power until reaching optimum econ cruise level. (You achieve your econ level earlier and therefore spend longer at that level)

However the additional maintenance costs at max rated power far outweigh any fuel cost savings; for now anyway.

There may come a point where the cost of fuel could increase to such an extent that it would be more economic to use max power settings - such a fuel price hike would need to be quite significant I'm given to understand before this was the case.

It is interesting that from a 'global warming perspective' none of the green lobby appear to have latched on to this one. If they did they would then probably claim that airlines were destroying our environment in order to save cash whilst totally ignoring the additional load on the environment by the production of 'hot end' parts for gas turbines.

Regards
Exeng

Thanks. You said that "additional maintenance costs at max rated power far outweigh any fuel cost savings", this is also what I would have guessed; but how do we verify this... if derated thrust is used most of the time? Or, maybe cargo ops use max thrust often enough in which case some data are available?

Would that any ATC environment would allow unresticted max climb!

Doesn't any power-by-the-hour type contract with the engine manufacturers contain a sliding scale of charges according to how much time is spent at or close to full power? That would be your cost right there.

PBTH contracts will yield the numbers. If your airline calculates 'cost index', then the variables of fuel cost and maintenance cost are integral to that calculation. The 'revenue' side of that calculation, e.g. the cost of a missed connection by the pax versus the increase in fuel burn/ maintenance cost by flying faster to make the connection, is a more complex variable. FLEX is also about maintaining engine EGT margin at high power, i.e. Take Off Thrust; however, the recent attempts (abandoned by most A340-300 operators?) at EGT margin retention on the CFM56 (the so called '-P' project), shows that you can get the EGT gauge to show a healthy margin against the limiting figure without getting a retainable increase in payload or improvement in fuel burn.

I see... by now of course I am not sure I understand everything :) But essentially, you say that what the operator pays, is what's written in the contract. And, the engine manufacturer has the actual data and knows how much it actually costs...

Thanks !

I could have been a little clearer with my post in retrospect. My point about Cost Index is that, if your Ops Eng people caclulate CI for a specific flight, then varying the CI will vary the Climb speed schedule and hence the Time to height and the Distance to TOC with the obvious impact on Fuel used in the climb etc. Neither everyone uses PBTH contracts, nor do you believe everything a manufacturer or leasing company tells you about performance.