CFM56 Variable Thrust Options

[No Fly Zone]

I'm missing some part of the story about current generation jet engines, that of variable thrust. Since the CFM-56 (in variations) is a common engine and powers many frames from both big producers, I'll use that as my example. CFM's own website lists a range of thrust options for each of its variants on the '56, even some ranges for specific air frames. I get the part about adequate thrust for the weight, some extra for high/hot applications, but I do not understand how this is accomplished. If the engine's thrust range is (random numbers), 27000# to 33000#, is this set in advance? If max available is set at 28000#, is that based on airframe and MTOW numbers or something else. In a crunch, perhaps an engine failure after Vr, can the pilot get a little more?
I well appreciate that running engines at something less than their maximum output can extend their lives and service intervals, but I do not understand why engine providers provide a range of thrust outputs for a given sub-model of an engine. Can someone help me understand this? Does max available power setting affect engine cost per hour, even if the operator chooses to use less than max for most take-off operations?
Put differently, how is engine time paid when airline X uses higher thrust than airline Y - in actual operations? And lastly, if a typical engine is 'set' to run at a higher maximum thrust than its identical twin, on a differenet airplane, is it expected wear faster or need more frequent service? How finely is this detail recorded and how does it affect engine time charges?
I know that is a mouthful. Those who know/understand the finer details will understand my question; others, please just read along and learn..:8. And, Thank you!! :D

[c100driver]

A quick explanation for you.

The basic engine is built for a certain max thrust.

The airframe manufacturer designs and will certify the aircraft for a range of thrust setting. For example you could have a B735 with 18.5K or 20K however the B734 may be 24K or 26K. You cannot get a 26K B735 or a 18.5K B734 as the are not certified.

The Aircraft Manufacturer will limit the max thrust due to perfromance limits such as the size of the rudder, length of the rudder arm. I.e. More thrust needs bigger rudders. The minimum thrust is limited by runway length for operation and climb requirements.

The owner/operator choose which thrust option from the range available that they need based on a study of cost v performance required. They then buy the Aircraft Flight Manual supplement for the performance needed and then a "derate plug" from the engine manufacture. If the operator buys too much thrust they pay more for the engine, if they don't buy enough they may have to leave payload behind. It is a pure cost/benefit exercise.

The "derate plug" is fitted to the engine. This tells the engine it's operating power. This is an operational and certification limit and cannot be changed without the aircraft AFM and certification change.

The pilots may also have "fixed derate" available via the FMC which cannot be higher than the "derate plug". A B733 may have a 22K engine but the option for pilot selectable down to 20K via the FMC. Either "rating" thrust may be further reduced but the assumed temp method (ATM) if allowed by your regulator.

A large number of airlines operate an engine lease called "power by the hour". The airline and the engine owner negotiate how they will operate the engine, derate, ATM etc and then derive a price that the airline will pay for the thrust they use. Some times it is a mix of low derate to max thrust within the engine life that is agreed and if the airline does more max thrust then a penalty applies. Likewise at overhaul time if the engine wear is beyond the expected then the airline has to pay a penalty.

Good engine management will have lower costs due to increased engine life. The CFM56 has had on wing life of over 30,000 hours with some operators.

[No Fly Zone]

@C100Driver: Thanks. That is one of the best responses I've ever seen and much appreciated.:D If I've got it right, when a PBTH operator leases a (hypothetical) 25K engine, plugged at 22K, 1) the pilot cannot go beyond 22k - no matter what and 2) of course, the engine wear will be less than if operated at 24k or maxed at 25k - but such is simply not possible. ANd of course, less-than-max thrust is appropriate for some airframes, all using the same engine, perhaps down-rated (plugged) to fit both the airframe and the buyer's (user's) needs. I can imagine the extensive calculations and flight trials necessary to certificate each available option for a given airframe. (Ouch!)
On point: When an operator buys a new airplane (of a fleet), and rents their thrust from the engine manufacturer under a PBTH program, the cost/benefit calculation must be a complicated one.
Thanks again for the sharp reply! ;)

[barit1]

No Fly Zone asked:
In a crunch, perhaps an engine failure after Vr, can the pilot get a little more?

A very bad idea, because of Vmca (or Vmcg, pre-rotation). The thought of returning to earth flying sideways holds no appeal to me.

If using REDUCED thrust, via ATM, as opposed to DERATE, then all is well, because Vmc's have been calculated based upon the rated thrust per the rating plug.

[tdracer]

One reason why the higher CFM56 ratings are not certified on the shorter 737s is that, with the shorter fuselage, there isn't enough tail authority to overcome the pitch-up moment that you get with increasing thrust.


Since we've gone to FADEC, the 'overboost' available by firewalling the engine has been minimal (there is some "headroom" to make sure you can get to rated thrust even with minor air data sensor errors, but we're talking less than one percent N1) unless you select Alternate mode. As I've noted above, that may not be a good idea...