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Matt6461
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Engine Derates and top-of-climb power

Mon Jan 23, 2023 12:00 am

I'm wondering whether an engine derated for regional power - A330 Regional for instance - also has lower power at the top of its climb. Or does the derate only apply to absolute thrust values, which would only be relevant in denser air and lower speeds such as at takeoff.

I would guess derate doesn't usually impact ToC performance, as otherwise a derated plane would be severely climb-limited: with lower MTOW, a plane's optimal cruise air density (FL) is practically linear with weight, while the engine's maximum output at altitude is also nearly linear with weight (and actually linear once the isothermic stratosphere is reached).
 
LH707330
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Re: Engine Derates and top-of-climb power

Mon Jan 23, 2023 1:37 am

There's a thread somewhere about derates on here. Most of them wash out in the 8-10k foot range and then resume the normal climb schedule. I'm pretty sure that only applies to the dense air conditions where it gets the most strain. Higher up you want to allow higher settings because the engine is more thermodynamically efficient.
 
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77west
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Re: Engine Derates and top-of-climb power

Mon Jan 23, 2023 5:07 am

Matt6461 wrote:
I'm wondering whether an engine derated for regional power - A330 Regional for instance - also has lower power at the top of its climb. Or does the derate only apply to absolute thrust values, which would only be relevant in denser air and lower speeds such as at takeoff.

I would guess derate doesn't usually impact ToC performance, as otherwise a derated plane would be severely climb-limited: with lower MTOW, a plane's optimal cruise air density (FL) is practically linear with weight, while the engine's maximum output at altitude is also nearly linear with weight (and actually linear once the isothermic stratosphere is reached).


I think it does depend if its a selectable derate or an actual rating plug derate. EG would a GE90-77 actually produce the same cruise thrust as a GE90-94? The B737NG you can select the thrust model 7B22, 24, 27 etc on the MCDU if the airline ordered the option. In this case it washes out above a certain alt I believe.
 
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Matt6461
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Re: Engine Derates and top-of-climb power

Mon Jan 23, 2023 3:56 pm

LH707330 wrote:
I'm pretty sure that only applies to the dense air conditions where it gets the most strain. Higher up you want to allow higher settings because the engine is more thermodynamically efficient.


That's what I'd have guessed but this...

77west wrote:
EG would a GE90-77 actually produce the same cruise thrust as a GE90-94?


Is a good question/point. Does it come down to max allowable engine RPM? This typically occurs at ToC, AFAIK. Probably there are "hard" factors preventing a -77 from achieving the same RPM as -94. So there's probably different kinds of derate - soft (programming) and hard (plugs and other physical differences in architecture).

Assuming that max RPM at ToC isn't impacted by the type of derate, I wonder what the maintenance difference is between say GE90-94 derated only for takeoff to 77k thrust and a GE90-77 with hard limits on engine RPM.
 
Max Q
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Re: Engine Derates and top-of-climb power

Mon Jan 23, 2023 10:48 pm

77west wrote:
Matt6461 wrote:
I'm wondering whether an engine derated for regional power - A330 Regional for instance - also has lower power at the top of its climb. Or does the derate only apply to absolute thrust values, which would only be relevant in denser air and lower speeds such as at takeoff.

I would guess derate doesn't usually impact ToC performance, as otherwise a derated plane would be severely climb-limited: with lower MTOW, a plane's optimal cruise air density (FL) is practically linear with weight, while the engine's maximum output at altitude is also nearly linear with weight (and actually linear once the isothermic stratosphere is reached).


I think it does depend if its a selectable derate or an actual rating plug derate. EG would a GE90-77 actually produce the same cruise thrust as a GE90-94? The B737NG you can select the thrust model 7B22, 24, 27 etc on the MCDU if the airline ordered the option. In this case it washes out above a certain alt I believe.


I thought thrust ratings were pin selectable and could only be changed by maintenance ?
 
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77west
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Re: Engine Derates and top-of-climb power

Mon Jan 23, 2023 11:19 pm

Max Q wrote:
77west wrote:
Matt6461 wrote:
I'm wondering whether an engine derated for regional power - A330 Regional for instance - also has lower power at the top of its climb. Or does the derate only apply to absolute thrust values, which would only be relevant in denser air and lower speeds such as at takeoff.

I would guess derate doesn't usually impact ToC performance, as otherwise a derated plane would be severely climb-limited: with lower MTOW, a plane's optimal cruise air density (FL) is practically linear with weight, while the engine's maximum output at altitude is also nearly linear with weight (and actually linear once the isothermic stratosphere is reached).


I think it does depend if its a selectable derate or an actual rating plug derate. EG would a GE90-77 actually produce the same cruise thrust as a GE90-94? The B737NG you can select the thrust model 7B22, 24, 27 etc on the MCDU if the airline ordered the option. In this case it washes out above a certain alt I believe.


I thought thrust ratings were pin selectable and could only be changed by maintenance ?


There is an option depending on the model and if the airline has purchased it, to have for example on the 738 a 22K 24K and 27K option selectable, which can then be mixed with an assumed temperature derate as well. I believe if you select, say, 22K, you are stuck with that on takeoff, even in the event of an engine failure, to ensure enough rudder authority will be available. So you might have 22K with an assumed temp of 45degC to reduce power even further (but still have the full 22K avail if needed) They wash out higher up on climbout, I believe this is also airline-selectable as to exactly what alt. Certainly by about 20,000ft you are running at max climb power all the way up from my observations (PMDG 737)

The BBJ has a TO-B bump option which I believe will allow the engine to run at max flat rating to higher density altitudes (at the cost of increased wear).
 
Max Q
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 1:12 am

77west wrote:
Max Q wrote:
77west wrote:

I think it does depend if its a selectable derate or an actual rating plug derate. EG would a GE90-77 actually produce the same cruise thrust as a GE90-94? The B737NG you can select the thrust model 7B22, 24, 27 etc on the MCDU if the airline ordered the option. In this case it washes out above a certain alt I believe.


I thought thrust ratings were pin selectable and could only be changed by maintenance ?


There is an option depending on the model and if the airline has purchased it, to have for example on the 738 a 22K 24K and 27K option selectable, which can then be mixed with an assumed temperature derate as well. I believe if you select, say, 22K, you are stuck with that on takeoff, even in the event of an engine failure, to ensure enough rudder authority will be available. So you might have 22K with an assumed temp of 45degC to reduce power even further (but still have the full 22K avail if needed) They wash out higher up on climbout, I believe this is also airline-selectable as to exactly what alt. Certainly by about 20,000ft you are running at max climb power all the way up from my observations (PMDG 737)

The BBJ has a TO-B bump option which I believe will allow the engine to run at max flat rating to higher density altitudes (at the cost of increased wear).



Understand the max thrust ratings can be changed, however I don’t believe this can be adjusted through the FMC by the pilots, this is a maintenance function
 
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77west
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 5:54 am

Max Q wrote:
77west wrote:
Max Q wrote:

I thought thrust ratings were pin selectable and could only be changed by maintenance ?


There is an option depending on the model and if the airline has purchased it, to have for example on the 738 a 22K 24K and 27K option selectable, which can then be mixed with an assumed temperature derate as well. I believe if you select, say, 22K, you are stuck with that on takeoff, even in the event of an engine failure, to ensure enough rudder authority will be available. So you might have 22K with an assumed temp of 45degC to reduce power even further (but still have the full 22K avail if needed) They wash out higher up on climbout, I believe this is also airline-selectable as to exactly what alt. Certainly by about 20,000ft you are running at max climb power all the way up from my observations (PMDG 737)

The BBJ has a TO-B bump option which I believe will allow the engine to run at max flat rating to higher density altitudes (at the cost of increased wear).



Understand the max thrust ratings can be changed, however I don’t believe this can be adjusted through the FMC by the pilots, this is a maintenance function


Yes correct if you mean the max available which on the B738 is usually the 27K option. They can select lower but not higher. But I believe its an option so some airlines may only have assumed temp derates and no "fixed" derates. I believe the B737-700 tops out at the 26K and B736 the 22K option.
 
Okcflyer
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 6:12 am

I find the data in the EASA TCDS rather insightful.

https://www.easa.europa.eu/en/downloads/7641/en for the GEnx

You'll notice there are multiple different max-continuous thrust ratings. Unfortunately, I don't see mention of thrust at TOC.

I assume this also applies at TOC, even though it obviously less thrust than the numbers shown for STD. Selecting the right power rating to optimized between fuel efficiency (more thrust = more efficient) vs time to overhaul. If operating near MTOW, seems the max thrust for TOC is ideal as any reduction would significantly increase your time in climb at sub-optimal levels. If you're near MTOW, you're likely flying long distance and less cycles, so the time to overhaul (cycles) is probably less concerning.

If you're running pretty short segments, then you probably dont want the max continuous rating as shaving a few seconds to a minute or two off an already brisk climb isn't going to save much fuel, but reducing that rate of wear will greatly extend time to overhaul.

I checked to see if there were any combinations max takeoff but less-than-max continuous, which would be targeted to operators with hot/high or short runways. For this case, you may not be heavy, but you need all the performance to rocket off the limited runway, and then cut back once in the air. I didn't find this option for the GEnx, but suspect there may be combinations 737/A320 with such configurations.

What is curious is that the maximum takeoff thrust ratings only produce 305.2 or 305.1 kN of thrust. However, there are two older 75 ratings which produce 306kN of continuous, despite producing less take off thrust than the more-recent 78X rating. Further, the highest takeoff rating -- the most recent -- further reduces max continuous by 0.1 kN. I suspect there were problems at 306kN of thrust -- either over temps or rapid degradation, hence the newer ratings that back off from that maximum a small bit. It's a good example of how razor tight some of these margins are.

Im surprised an insider hasn't answered yet. I'm quite curious.
 
ArcticFlyer
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 6:50 am

From my own experience on the 737 (CL and NG):

When selecting takeoff power it is possible to choose a "fixed" derate, an assumed temperature thrust reduction, or both. Normally we take off with as little thrust as possible (to minimize engine stress/wear) but there are some situations where only one or the other type of thrust reduction is allowed. In particular we are not permitted to use an AT thrust reduction on a contaminated runway but we can still use a fixed derate. AT is always at the crew's discretion but a fixed derate may be required: Due to the lower Vmc (and therefore lower permissible V1) provided by a fixed derate, on short runways it may be (somewhat counterintuitively) possible to carry a higher payload with a fixed derate compared to full thrust due to V1 potentially being several knots slower, reducing accelerate-stop distance.

In any case, when selecting a takeoff thrust reduction the FMC will automatically select one of two reduced-thrust climb profiles but this can be overridden by the crew and it is even possible to increase thrust at accel height if a large takeoff thrust reduction is followed by a max thrust climb. This scenario would only occur if the crew manually changes the climb settings, but I have done this when we're tight on fuel or climbing through icing conditions. The reduced thrust climb profiles are the same regardless of whether a fixed derate, assumed temp or both were used for takeoff and wash out when climbing through 15,000 feet.
 
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77west
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 7:10 am

ArcticFlyer wrote:
From my own experience on the 737 (CL and NG):

When selecting takeoff power it is possible to choose a "fixed" derate, an assumed temperature thrust reduction, or both. Normally we take off with as little thrust as possible (to minimize engine stress/wear) but there are some situations where only one or the other type of thrust reduction is allowed. In particular we are not permitted to use an AT thrust reduction on a contaminated runway but we can still use a fixed derate. AT is always at the crew's discretion but a fixed derate may be required: Due to the lower Vmc (and therefore lower permissible V1) provided by a fixed derate, on short runways it may be (somewhat counterintuitively) possible to carry a higher payload with a fixed derate compared to full thrust due to V1 potentially being several knots slower, reducing accelerate-stop distance.

In any case, when selecting a takeoff thrust reduction the FMC will automatically select one of two reduced-thrust climb profiles but this can be overridden by the crew and it is even possible to increase thrust at accel height if a large takeoff thrust reduction is followed by a max thrust climb. This scenario would only occur if the crew manually changes the climb settings, but I have done this when we're tight on fuel or climbing through icing conditions. The reduced thrust climb profiles are the same regardless of whether a fixed derate, assumed temp or both were used for takeoff and wash out when climbing through 15,000 feet.


Thanks for that, explains it really well.
 
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 8:10 am

LH707330 wrote:
There's a thread somewhere about derates on here. Most of them wash out in the 8-10k foot range and then resume the normal climb schedule. I'm pretty sure that only applies to the dense air conditions where it gets the most strain. Higher up you want to allow higher settings because the engine is more thermodynamically efficient.


Most Boeing models have two catalog options. The climb derates either wash out between 10-12K or 10-30K.
 
kalvado
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 12:24 pm

BoeingGuy wrote:
LH707330 wrote:
There's a thread somewhere about derates on here. Most of them wash out in the 8-10k foot range and then resume the normal climb schedule. I'm pretty sure that only applies to the dense air conditions where it gets the most strain. Higher up you want to allow higher settings because the engine is more thermodynamically efficient.


Most Boeing models have two catalog options. The climb derates either wash out between 10-12K or 10-30K.

What are actual derate parameters? Something internal to engine like N1, fuel flow, temperature? Or engine actually gets altitude information and thrust is limited according to preprogrammed formula?
 
ArcticFlyer
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Re: Engine Derates and top-of-climb power

Tue Jan 24, 2023 5:37 pm

kalvado wrote:
What are actual derate parameters? Something internal to engine like N1, fuel flow, temperature? Or engine actually gets altitude information and thrust is limited according to preprogrammed formula?

The latter. When choosing a reduced thrust climb it is simply an alternative schedule beginning at field elevation and ending at some altitude during the climb (as I mentioned on the 737 it is 15,000 feet) at which point it will meet up with the normal climb schedule. The thrust setting (full or reduced) is of course based on atmospheric conditions but the FMC figures all of that out and transmits the appropriate N1 or EPR to the autothrottles.
 
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 2:19 am

kalvado wrote:
BoeingGuy wrote:
LH707330 wrote:
There's a thread somewhere about derates on here. Most of them wash out in the 8-10k foot range and then resume the normal climb schedule. I'm pretty sure that only applies to the dense air conditions where it gets the most strain. Higher up you want to allow higher settings because the engine is more thermodynamically efficient.


Most Boeing models have two catalog options. The climb derates either wash out between 10-12K or 10-30K.

What are actual derate parameters? Something internal to engine like N1, fuel flow, temperature? Or engine actually gets altitude information and thrust is limited according to preprogrammed formula?


On all Boeing non-737 models climb derates are a fixed percentage of full climb there. There are two climb derate settings - CLB 1 and CLB 2. The precent derates are customer selectable, but they are usually 10% and 20%.

The 737 is a bit different and I’m not as familiar with it.
 
LH707330
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 5:31 am

BoeingGuy wrote:
kalvado wrote:
BoeingGuy wrote:

Most Boeing models have two catalog options. The climb derates either wash out between 10-12K or 10-30K.

What are actual derate parameters? Something internal to engine like N1, fuel flow, temperature? Or engine actually gets altitude information and thrust is limited according to preprogrammed formula?


On all Boeing non-737 models climb derates are a fixed percentage of full climb there. There are two climb derate settings - CLB 1 and CLB 2. The precent derates are customer selectable, but they are usually 10% and 20%.

The 737 is a bit different and I’m not as familiar with it.

Thanks for confirming, that's super helpful. Sounds like the answer to the OP is "derate is for low-altitude and it goes away near ToC."
 
ArcticFlyer
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 5:56 am

BoeingGuy wrote:
On all Boeing non-737 models climb derates are a fixed percentage of full climb there. There are two climb derate settings - CLB 1 and CLB 2. The precent derates are customer selectable, but they are usually 10% and 20%.

The 737 is a bit different and I’m not as familiar with it.

The 737 is pretty much the same. CLB 1 is a 10% reduction and CLB 2 is a 20% reduction, gradually washing out until the normal climb schedule is reached at 15,000 feet.
 
BoeingGuy
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 7:25 am

LH707330 wrote:
BoeingGuy wrote:
kalvado wrote:
What are actual derate parameters? Something internal to engine like N1, fuel flow, temperature? Or engine actually gets altitude information and thrust is limited according to preprogrammed formula?


On all Boeing non-737 models climb derates are a fixed percentage of full climb there. There are two climb derate settings - CLB 1 and CLB 2. The precent derates are customer selectable, but they are usually 10% and 20%.

The 737 is a bit different and I’m not as familiar with it.

Thanks for confirming, that's super helpful. Sounds like the answer to the OP is "derate is for low-altitude and it goes away near ToC."


On non-737 Boeing models it is as I stated above. Baseline is a washout between 10,000-12,000 feet. There’s an option for 10,000-30,000 feet.
 
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Starlionblue
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 11:02 am

LH707330 wrote:
BoeingGuy wrote:
kalvado wrote:
What are actual derate parameters? Something internal to engine like N1, fuel flow, temperature? Or engine actually gets altitude information and thrust is limited according to preprogrammed formula?


On all Boeing non-737 models climb derates are a fixed percentage of full climb there. There are two climb derate settings - CLB 1 and CLB 2. The precent derates are customer selectable, but they are usually 10% and 20%.

The 737 is a bit different and I’m not as familiar with it.

Thanks for confirming, that's super helpful. Sounds like the answer to the OP is "derate is for low-altitude and it goes away near ToC."


That is a good way of putting it. Engines lose thrust with altitude due to lower mass flow in thinner air. Hence why cruise power is pretty near maximum.
 
kalvado
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 2:10 pm

Starlionblue wrote:
LH707330 wrote:
BoeingGuy wrote:

On all Boeing non-737 models climb derates are a fixed percentage of full climb there. There are two climb derate settings - CLB 1 and CLB 2. The precent derates are customer selectable, but they are usually 10% and 20%.

The 737 is a bit different and I’m not as familiar with it.

Thanks for confirming, that's super helpful. Sounds like the answer to the OP is "derate is for low-altitude and it goes away near ToC."


That is a good way of putting it. Engines lose thrust with altitude due to lower mass flow in thinner air. Hence why cruise power is pretty near maximum.

Near maximum by what metric? Certainly not by shaft power or fuel flow. Near maximum achievable at that altitude? From what I get from pilots here, if there is enough power to climb - it makes economic sense to do so..
Would N1 and/or EGT be near maximum long-term values? Or is there another physical limit in the engine?
 
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Matt6461
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 2:37 pm

LH707330 wrote:
BoeingGuy wrote:
kalvado wrote:
What are actual derate parameters? Something internal to engine like N1, fuel flow, temperature? Or engine actually gets altitude information and thrust is limited according to preprogrammed formula?


On all Boeing non-737 models climb derates are a fixed percentage of full climb there. There are two climb derate settings - CLB 1 and CLB 2. The precent derates are customer selectable, but they are usually 10% and 20%.

The 737 is a bit different and I’m not as familiar with it.

Thanks for confirming, that's super helpful. Sounds like the answer to the OP is "derate is for low-altitude and it goes away near ToC."


Thanks for the answers.
 
SteelChair
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 4:54 pm

I know of at least one airline that effectively derates throughout the entire spectrum (well not descent). They derate in cruise by biasing their FMS software to slow down slightly. The argument is that .01 Mach is negligible with regard to on time numbers, airplane hours, and crew pay, but does add up over millions of engine hours by savings through reduced EGT. Pushing off shop visits as long as possible generates savings (per the argument).

It doesn't have to be .01M. Your FMS might fly the airplane at .816 instead of .82. Reduces EGT by an almost negligible amount over millions of engine flight hours.

Getting the pilots to actually slow down is another issue.
 
BoeingGuy
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 5:29 pm

SteelChair wrote:
I know of at least one airline that effectively derates throughout the entire spectrum (well not descent). They derate in cruise by biasing their FMS software to slow down slightly. The argument is that .01 Mach is negligible with regard to on time numbers, airplane hours, and crew pay, but does add up over millions of engine hours by savings through reduced EGT. Pushing off shop visits as long as possible generates savings (per the argument).

It doesn't have to be .01M. Your FMS might fly the airplane at .816 instead of .82. Reduces EGT by an almost negligible amount over millions of engine flight hours.

Getting the pilots to actually slow down is another issue.


That’s not a derate. There is a Cost Index entry in the FMC that will select cruise speed based on the number input.
 
SteelChair
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 5:51 pm

BoeingGuy wrote:
SteelChair wrote:
I know of at least one airline that effectively derates throughout the entire spectrum (well not descent). They derate in cruise by biasing their FMS software to slow down slightly. The argument is that .01 Mach is negligible with regard to on time numbers, airplane hours, and crew pay, but does add up over millions of engine hours by savings through reduced EGT. Pushing off shop visits as long as possible generates savings (per the argument).

It doesn't have to be .01M. Your FMS might fly the airplane at .816 instead of .82. Reduces EGT by an almost negligible amount over millions of engine flight hours.

Getting the pilots to actually slow down is another issue.


That’s not a derate. There is a Cost Index entry in the FMC that will select cruise speed based on the number input.


Thus the use of the word "effectively."

Reduced power is reduced power, no?
 
bluecrew
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 7:35 pm

SteelChair wrote:
BoeingGuy wrote:
SteelChair wrote:
I know of at least one airline that effectively derates throughout the entire spectrum (well not descent). They derate in cruise by biasing their FMS software to slow down slightly. The argument is that .01 Mach is negligible with regard to on time numbers, airplane hours, and crew pay, but does add up over millions of engine hours by savings through reduced EGT. Pushing off shop visits as long as possible generates savings (per the argument).

It doesn't have to be .01M. Your FMS might fly the airplane at .816 instead of .82. Reduces EGT by an almost negligible amount over millions of engine flight hours.

Getting the pilots to actually slow down is another issue.


That’s not a derate. There is a Cost Index entry in the FMC that will select cruise speed based on the number input.


Thus the use of the word "effectively."

Reduced power is reduced power, no?

Not in this case. You're not limiting power, just changing the speed the aircraft cruises at to save a little gas. The engine health effects, if true, are minor and come along for the ride.
I don't believe this would have any effect on long-term engine health, positive or negative, but I will defer to the mechanics on this one.

That kind of stuff really does save a very small amount of gas. It's fashionable to talk about how much you're the new guy removing the olive from the salad, but we're talking maybe a couple hundred pounds? Cost index gets changed all the time anyways.
 
ArcticFlyer
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 8:16 pm

kalvado wrote:
Starlionblue wrote:
That is a good way of putting it. Engines lose thrust with altitude due to lower mass flow in thinner air. Hence why cruise power is pretty near maximum.

Near maximum by what metric? Certainly not by shaft power or fuel flow. Near maximum achievable at that altitude? From what I get from pilots here, if there is enough power to climb - it makes economic sense to do so..
Would N1 and/or EGT be near maximum long-term values? Or is there another physical limit in the engine?

By maximum he means maximum available. At FL350 max available thrust is only about 15-20% of sea level thrust due to reduced air density. As far as parameters go (at least on the engines with which I have experience):
  • N1 increases gradually throughout the climb. On the CFM56 initial climb N1 is usually in the high 80s and increases to about 101% in the mid-30s (max is 106%)
  • EPR (if displayed) increases pretty dramatically throughout the climb. On the JT8D takeoff was usually around 2.1 and initial climb was 1.8-1.9, but this increased to around 2.4 in the mid-30s
  • Fuel flow steadily decreases throughout the climb despite increases in the above parameters
In the FMC there is usually a thrust limit page displaying the various N1 or EPR limits for TO/GA, CLB, CRZ and (max) CON. At high altitude they are all the same number or pretty close.
 
kalvado
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 8:35 pm

ArcticFlyer wrote:
kalvado wrote:
Starlionblue wrote:
That is a good way of putting it. Engines lose thrust with altitude due to lower mass flow in thinner air. Hence why cruise power is pretty near maximum.

Near maximum by what metric? Certainly not by shaft power or fuel flow. Near maximum achievable at that altitude? From what I get from pilots here, if there is enough power to climb - it makes economic sense to do so..
Would N1 and/or EGT be near maximum long-term values? Or is there another physical limit in the engine?

By maximum he means maximum available. At FL350 max available thrust is only about 15-20% of sea level thrust due to reduced air density. As far as parameters go (at least on the engines with which I have experience):
  • N1 increases gradually throughout the climb. On the CFM56 initial climb N1 is usually in the high 80s and increases to about 101% in the mid-30s (max is 106%)
  • EPR (if displayed) increases pretty dramatically throughout the climb. On the JT8D takeoff was usually around 2.1 and initial climb was 1.8-1.9, but this increased to around 2.4 in the mid-30s
  • Fuel flow steadily decreases throughout the climb despite increases in the above parameters
In the FMC there is usually a thrust limit page displaying the various N1 or EPR limits for TO/GA, CLB, CRZ and (max) CON. At high altitude they are all the same number or pretty close.

And what about temperatures? Those should be significant for engine wear. Of course, if temperature is communicated to pilots to begin with...
 
BoeingGuy
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 9:07 pm

ArcticFlyer wrote:
kalvado wrote:
Starlionblue wrote:
That is a good way of putting it. Engines lose thrust with altitude due to lower mass flow in thinner air. Hence why cruise power is pretty near maximum.

Near maximum by what metric? Certainly not by shaft power or fuel flow. Near maximum achievable at that altitude? From what I get from pilots here, if there is enough power to climb - it makes economic sense to do so..
Would N1 and/or EGT be near maximum long-term values? Or is there another physical limit in the engine?

By maximum he means maximum available. At FL350 max available thrust is only about 15-20% of sea level thrust due to reduced air density. As far as parameters go (at least on the engines with which I have experience):
  • N1 increases gradually throughout the climb. On the CFM56 initial climb N1 is usually in the high 80s and increases to about 101% in the mid-30s (max is 106%)
  • EPR (if displayed) increases pretty dramatically throughout the climb. On the JT8D takeoff was usually around 2.1 and initial climb was 1.8-1.9, but this increased to around 2.4 in the mid-30s
  • Fuel flow steadily decreases throughout the climb despite increases in the above parameters
In the FMC there is usually a thrust limit page displaying the various N1 or EPR limits for TO/GA, CLB, CRZ and (max) CON. At high altitude they are all the same number or pretty close.


On Boeing airplanes the THRUST LIM page doesn’t give the actual N1 or EPR or TPR limit. It just allows you select any limit.
 
BoeingGuy
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 9:11 pm

bluecrew wrote:
SteelChair wrote:
BoeingGuy wrote:

That’s not a derate. There is a Cost Index entry in the FMC that will select cruise speed based on the number input.


Thus the use of the word "effectively."

Reduced power is reduced power, no?

Not in this case. You're not limiting power, just changing the speed the aircraft cruises at to save a little gas. The engine health effects, if true, are minor and come along for the ride.
I don't believe this would have any effect on long-term engine health, positive or negative, but I will defer to the mechanics on this one.

That kind of stuff really does save a very small amount of gas. It's fashionable to talk about how much you're the new guy removing the olive from the salad, but we're talking maybe a couple hundred pounds? Cost index gets changed all the time anyways.


Something that I suspect you understand, but many others don’t. A climb derate actually burns MORE fuel than a full climb thrust climb.

This is counterintuitive. It’s because it takes longer to reach cruise altitude. In other words, you spend more time at less efficient altitudes. The decrease in wear and maintenance cost in the long run outweigh the greater fuel consumption.
 
gloom
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 9:37 pm

kalvado wrote:
Starlionblue wrote:
Hence why cruise power is pretty near maximum.

Near maximum by what metric? Certainly not by shaft power or fuel flow. Near maximum achievable at that altitude? From what I get from pilots here, if there is enough power to climb - it makes economic sense to do so..
Would N1 and/or EGT be near maximum long-term values? Or is there another physical limit in the engine?


The metric usually looked at is N1% (as limiting metric, and a good checkpoint to maximum in current conditions). The conditions in cruise (for the lack of oxygen) will be nowhere near ground thrust, definitely.

There used to be a Java based simulator in one of NASA sites, able to simulate CFM56 thrust (I believe CFM56-3B from the Classics, rated at 23.5klbs TO thrust). At 37kft cruise, the engine was maxed out at ~5.2klbs, so I assume 4:1 ratio between TO and cruise is a good "rule of thumb" estimation.

Cheers,
Adam

PS. They don't have CFM56 anymore, but CF6 shows similar behavior.
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/enginesimu/
Last edited by gloom on Wed Jan 25, 2023 9:49 pm, edited 1 time in total.
 
Chaostheory
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 9:38 pm

kalvado wrote:
And what about temperatures? Those should be significant for engine wear. Of course, if temperature is communicated to pilots to begin with...


Not sure what you're trying to get at here but EGT isn't hidden from the crew, at least not in any types I've flown.

Going by the Trent 800 publication recommended below, a 10% derate will results in EGTs a little over 20% lower than a max thrust takeoff.

EGT isn't a consideration at cruise power. Only rare occasions like the A321 and old V2500 engines out of Khartoum in summer would I watch the redline like a hawk.

Matt6461 wrote:
I'm wondering whether an engine derated for regional power - A330 Regional for instance - also has lower power at the top of its climb. Or does the derate only apply to absolute thrust values, which would only be relevant in denser air and lower speeds such as at takeoff.

I would guess derate doesn't usually impact ToC performance, as otherwise a derated plane would be severely climb-limited: with lower MTOW, a plane's optimal cruise air density (FL) is practically linear with weight, while the engine's maximum output at altitude is also nearly linear with weight (and actually linear once the isothermic stratosphere is reached).


Google 'Roll royce derated climb performance'. The pdf you find will answer all your questions.

And no, our Trent 772s used in the regional fleet have full climb thrust available to them, I think, from about fl250.
 
kalvado
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 10:26 pm

Chaostheory wrote:
kalvado wrote:
And what about temperatures? Those should be significant for engine wear. Of course, if temperature is communicated to pilots to begin with...


Not sure what you're trying to get at here but EGT isn't hidden from the crew, at least not in any types I've flown.

Going by the Trent 800 publication recommended below, a 10% derate will results in EGTs a little over 20% lower than a max thrust takeoff.

EGT isn't a consideration at cruise power. Only rare occasions like the A321 and old V2500 engines out of Khartoum in summer would I watch the redline like a hawk.

I am looking at things from completely different side of technical spectrum. I somewhat understand physics of the engine, but have very limited idea of how things are presented in a cockpit.
And from my perspective temperature is something that accelerates degradation big time. For me "derate" means primarily "lets reduce temperature a bit so things last longer".
In that RR paper they are saying same thing, e.g. fig. 12 and 14. Too bad there is no actual temperature scale on those graphs.
 
Chaostheory
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Re: Engine Derates and top-of-climb power

Wed Jan 25, 2023 11:40 pm

kalvado wrote:

I am looking at things from completely different side of technical spectrum. I somewhat understand physics of the engine, but have very limited idea of how things are presented in a cockpit.
And from my perspective temperature is something that accelerates degradation big time. For me "derate" means primarily "lets reduce temperature a bit so things last longer".
In that RR paper they are saying same thing, e.g. fig. 12 and 14. Too bad there is no actual temperature scale on those graphs.


From my A321 days, I think the redline for egt was around 630C with 530 typical when departing reduced/derated ex MAN or LHR. That was with older v2500 engines. The newer A5 standard and CFM B series engines on later variants had fewer issues.

For our GE 777 and 787 fleet (-9/10) egt will run 30-50C lower with derated thrust.
 
SteelChair
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 12:22 am

The "experts" (again, advanced degrees from first rate engineering institutions) who briefed me said that fuel burn (and other costs) was a secondary consideration and any time (even during cruise) you could lower EGT, it was good. Of course the low hanging fruit is takeoff since that is the highest EGT, but they preached lower EGT at climb and cruise also.

IIRC, engines are ~40% of the cost of an airplane and engine overhauls are a huge recurring cost. These guys wanted to do anything they could to preserve engine life. I would add that this particular airline had their own engine shop. If they were a smaller airline, doing power by the hour and other leasing schemes, perhaps the costs would be already baked in and these engineers wouldn't have been so focused on this issue. No need saving money for the lessor if you aren't getting a cut.

This is not an easy topic. There are a lot of tentacles.
 
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zeke
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 12:53 am

77west wrote:
They can select lower but not higher. But I believe it’s an option so some airlines may only have assumed temp derates and no "fixed" derates.


The terminology you are using is unhelpful.

With assumed temperature the maximum thrust is available, you can do your numbers and insert the speeds for an assumed temperature. When lining up previous aircraft reports windshear on departure you elect to select TOGA. Your assumed temp speeds remain valid, and you have max thrust available.

Derate thrust is normally used on short and/or short wet runways it lowers the max thrust for the engine, reduces Vmcg/Vmca, this in turn lowers V1/VR. TOGA is not available for takeoff until the aircraft is above a safe speed, often after flap retraction.

Both assumed temp and derate is temporarily programmed into the FADEC by the pilots through the takeoff page or similar in the FMC.

These takeoff settings have very little to do with the engine flat rating, these are made by engineers on the engine.
 
Chaostheory
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 1:21 am

zeke wrote:

The terminology you are using is unhelpful.



I'm guilty of that too. I should have added context as to whether I was referring to the takeoff or climb phase. Though where engine temps are concerned, the basic point remains the same.
 
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Starlionblue
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 2:15 am

kalvado wrote:
Starlionblue wrote:
LH707330 wrote:
Thanks for confirming, that's super helpful. Sounds like the answer to the OP is "derate is for low-altitude and it goes away near ToC."


That is a good way of putting it. Engines lose thrust with altitude due to lower mass flow in thinner air. Hence why cruise power is pretty near maximum.

Near maximum by what metric? Certainly not by shaft power or fuel flow. Near maximum achievable at that altitude? From what I get from pilots here, if there is enough power to climb - it makes economic sense to do so..
Would N1 and/or EGT be near maximum long-term values? Or is there another physical limit in the engine?


As ArcticFlyer says, I meant that the engines in the cruise are operating near the maximum available thrust at that altitude. Corollary: You can't power yourself out of a stall at high altitude, since there is hardly any excess thrust available.

It is not really accurate that if there is enough power to climb, it makes economic sense to do so. Recommended max altitude (based on 1.3 maneuvering G limit) is almost always higher than optimum. We can climb to REC MAX, but being at OPT is better for fuel burn.

Winds and temperatures also have an effect. If the wind is more favorable at a lower cruise level, that can easily give far higher fuel burn gains that being closer to the "optimal" altitude. The FM has forecast wind and temperature data so OPT is based not only on a pure pressure altitude profile, but also on wind and temp.
 
ArcticFlyer
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 2:47 am

BoeingGuy wrote:
On Boeing airplanes the THRUST LIM page doesn’t give the actual N1 or EPR or TPR limit. It just allows you select any limit.

Can't speak for other Boeings but on the 737 the values are displayed on the N1 LIMIT page.
 
ArcticFlyer
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 2:54 am

kalvado wrote:
And what about temperatures? Those should be significant for engine wear. Of course, if temperature is communicated to pilots to begin with...

I've never flown a turbine aircraft that didn't have EGT gauges. Usually it's only a concern during takeoff but when I flew the Dash 8 the EGT would gradually rise as we climbed and we would start needing to reduce power through about FL200 to stay within EGT limits. Not sure if that was normal or just because our airplanes were old and junky but I haven't observed this on any other type.
 
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Matt6461
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 4:03 am

Chaostheory wrote:
Google 'Roll royce derated climb performance'. The pdf you find will answer all your questions.


Thanks, the pdf is here: https://www.theairlinepilots.com/foruma ... rmance.pdf.

It definitively answers the question regarding ToC thrust - the derates wash out by then.

The PDF is interesting in that it advises airlines to use derated climb, pointing that the marginal cost of very slightly higher fuel burn on 3,000nm trip is far outweighed by the mariginal benefit of greater engine time on wing (thus lower maintenance cost). The article is from 2005. Has the recommendation been implemented? It seems odd if, as the article implies, industry hasn't conducted this cost-benefit analysis yet (as of 2005).

It also seems that, given the relative trends in high engine temps and fuel burn since 2005, the industry would have shifted more towards climb derate as standard practice (though higher oil prices might counteract that calculus). Are climb derates now standard practice?
 
BoeingGuy
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 4:36 am

ArcticFlyer wrote:
kalvado wrote:
And what about temperatures? Those should be significant for engine wear. Of course, if temperature is communicated to pilots to begin with...

I've never flown a turbine aircraft that didn't have EGT gauges. Usually it's only a concern during takeoff but when I flew the Dash 8 the EGT would gradually rise as we climbed and we would start needing to reduce power through about FL200 to stay within EGT limits. Not sure if that was normal or just because our airplanes were old and junky but I haven't observed this on any other type.


To my knowledge the EGT indication is required by a Part 25 regulation.
 
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glen
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 6:57 am

Matt6461 wrote:
It also seems that, given the relative trends in high engine temps and fuel burn since 2005, the industry would have shifted more towards climb derate as standard practice (though higher oil prices might counteract that calculus). Are climb derates now standard practice?


When we received RR powered A330 13 years ago we used derate depending on TOW. On a heavy aircraft we didn't use any derate at all, at medium weights (10 to 20 tons below MTOW if I remember correct) D1 and at lower weights D2.
Today the amount we have to pay for maintenance is heavily depending on how much derate we use and is based on actual measurements. Therefore we should use always D2 unless we have some climb restrictions.
 
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Starlionblue
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 10:43 am

glen wrote:
Matt6461 wrote:
It also seems that, given the relative trends in high engine temps and fuel burn since 2005, the industry would have shifted more towards climb derate as standard practice (though higher oil prices might counteract that calculus). Are climb derates now standard practice?


When we received RR powered A330 13 years ago we used derate depending on TOW. On a heavy aircraft we didn't use any derate at all, at medium weights (10 to 20 tons below MTOW if I remember correct) D1 and at lower weights D2.
Today the amount we have to pay for maintenance is heavily depending on how much derate we use and is based on actual measurements. Therefore we should use always D2 unless we have some climb restrictions.


Same. Always D2 unless you can't meet the restrictions. For a close-in restriction set D1 or no climb derate on the ground. For a cruise level restriction at a boundary, it's a bit more involved. If it's close we go and then "massage" in the climb. The rule being "decelerate, negotiate, derate". Meaning, first set 285 knots or thereabouts and see if you can make the restriction. If that doesn't work, ask for more track miles or a lower final level. If you can't get any of those, remove the derate.



ArcticFlyer wrote:
kalvado wrote:
And what about temperatures? Those should be significant for engine wear. Of course, if temperature is communicated to pilots to begin with...

I've never flown a turbine aircraft that didn't have EGT gauges. Usually it's only a concern during takeoff but when I flew the Dash 8 the EGT would gradually rise as we climbed and we would start needing to reduce power through about FL200 to stay within EGT limits. Not sure if that was normal or just because our airplanes were old and junky but I haven't observed this on any other type.


From what I have heard, the A340 engines would regularly overtemp EGT if it was hot and/or high, and require thrust reduction. :)
 
kalvado
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Re: Engine Derates and top-of-climb power

Thu Jan 26, 2023 2:33 pm

gloom wrote:
kalvado wrote:
Starlionblue wrote:
Hence why cruise power is pretty near maximum.

Near maximum by what metric? Certainly not by shaft power or fuel flow. Near maximum achievable at that altitude? From what I get from pilots here, if there is enough power to climb - it makes economic sense to do so..
Would N1 and/or EGT be near maximum long-term values? Or is there another physical limit in the engine?


The metric usually looked at is N1% (as limiting metric, and a good checkpoint to maximum in current conditions). The conditions in cruise (for the lack of oxygen) will be nowhere near ground thrust, definitely.

There used to be a Java based simulator in one of NASA sites, able to simulate CFM56 thrust (I believe CFM56-3B from the Classics, rated at 23.5klbs TO thrust). At 37kft cruise, the engine was maxed out at ~5.2klbs, so I assume 4:1 ratio between TO and cruise is a good "rule of thumb" estimation.

Cheers,
Adam

PS. They don't have CFM56 anymore, but CF6 shows similar behavior.
https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/enginesimu/

So what I hear so far, is that rotation speed is the first limiting factor at altitude. Probably fan blade tips going too supersonic (someone mentioned they are already supersonic in GE90) would actually reduce thrust.
Temperature seems to be a close second limitation, going up with altitude. A bit counterintuitive for me as fuel burn should drop. Reduced mass flow effect maybe?
And looks like temperature is important enough to make pilots aware of that..
 
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glen
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Re: Engine Derates and top-of-climb power

Fri Jan 27, 2023 7:49 am

Starlionblue wrote:

From what I have heard, the A340 engines would regularly overtemp EGT if it was hot and/or high, and require thrust reduction. :)


Regularly is a bit too much of a word. But with high OAT and TOGA-takeoff, the possibility of an EGT exceedance during takeoff or shortly afterwards is higher. The reason is the engine performance is limited by the engine design.
As long as the overtemp stays within a certain limit for a limited time, the engine can be continued to operate for the rest of the flight.
The EGT margin is decreasing as the engine becomes less efficient due to compressor and turbine blade wear. On the other hand EGT margin can be increased with regular engine core wash and a sufficient warm-up time. While the minimum warm-up time is 2 minutes, it is recommended to warm up 10 minutes before a TOGA-takeoff of with FLX-temperatures close to OAT.
 
LH707330
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Re: Engine Derates and top-of-climb power

Sat Jan 28, 2023 3:21 am

glen wrote:
Starlionblue wrote:

From what I have heard, the A340 engines would regularly overtemp EGT if it was hot and/or high, and require thrust reduction. :)


Regularly is a bit too much of a word. But with high OAT and TOGA-takeoff, the possibility of an EGT exceedance during takeoff or shortly afterwards is higher. The reason is the engine performance is limited by the engine design.
As long as the overtemp stays within a certain limit for a limited time, the engine can be continued to operate for the rest of the flight.
The EGT margin is decreasing as the engine becomes less efficient due to compressor and turbine blade wear. On the other hand EGT margin can be increased with regular engine core wash and a sufficient warm-up time. While the minimum warm-up time is 2 minutes, it is recommended to warm up 10 minutes before a TOGA-takeoff of with FLX-temperatures close to OAT.

Is that the T500 or CFM56-5C that has the issue?
 
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glen
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Re: Engine Derates and top-of-climb power

Sat Jan 28, 2023 9:16 am

LH707330 wrote:
Is that the T500 or CFM56-5C that has the issue?


The CFM engine
 
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zeke
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Re: Engine Derates and top-of-climb power

Sat Jan 28, 2023 3:01 pm

Starlionblue wrote:
From what I have heard, the A340 engines would regularly overtemp EGT if it was hot and/or high, and require thrust reduction. :)


Nope, not regular at all. We would have had a NTC in the front of the tech log if the engine was having high egts. Compressor washers helped a lot.
 
Chaostheory
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Re: Engine Derates and top-of-climb power

Sat Jan 28, 2023 6:30 pm

zeke wrote:
Starlionblue wrote:
From what I have heard, the A340 engines would regularly overtemp EGT if it was hot and/or high, and require thrust reduction. :)


Nope, not regular at all. We would have had a NTC in the front of the tech log if the engine was having high egts. Compressor washers helped a lot.


Our engineering division was recognised by the regional GE rep for having one of the healthiest engine fleets in the industry. Amongst the metrics mentioned, we had one of the lowest rates of EGT exceedance on the 321/cfm. Engine washes at least once a month and no single engine taxi.
 
Chaostheory
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Re: Engine Derates and top-of-climb power

Sat Jan 28, 2023 10:12 pm

Another consideration in the use of derate during climb is exposure to atmospheric contaminants. If you're flying in heavily polluted areas then it probably isn't wise to use a greater derate prolonging your time down low. The newer engines, especially the Trent 1000, seem to be more vulnerable to pollutants.

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