Climb gradient after engine failure
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Climb gradient after engine failure
Hi,
I am not familiar with these stuffs, I hope these questions don't seem stupid.
a)Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?
b)Where these numbers (2.4%, 2.7%, 3.0%, …) came from? : Are these empirical numbers or just as a consensus or decision made by certain aviation authorities or administrations (like FAA, ICAO,…)?
Just to make it clear: for example why the climb gradient for the second segment is 2.4% and not 1.2% (twin) or for example why the second segment is usually the most limiting than the third or the fourth?
c) If a turn is required after engine failure during takeoff, does this affect aircraft performance (more/less altitude loss than usual or small/big radius than usual?) depending on the turn into the inoperative engine or into the operative engine (assume it is a twin)?
Feedback appreciated
Regards
I am not familiar with these stuffs, I hope these questions don't seem stupid.
a)Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?
b)Where these numbers (2.4%, 2.7%, 3.0%, …) came from? : Are these empirical numbers or just as a consensus or decision made by certain aviation authorities or administrations (like FAA, ICAO,…)?
Just to make it clear: for example why the climb gradient for the second segment is 2.4% and not 1.2% (twin) or for example why the second segment is usually the most limiting than the third or the fourth?
c) If a turn is required after engine failure during takeoff, does this affect aircraft performance (more/less altitude loss than usual or small/big radius than usual?) depending on the turn into the inoperative engine or into the operative engine (assume it is a twin)?
Feedback appreciated
Regards
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a)Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?
"...The take-off and take-off flight path REGULATORY definitions...." They are regulatory for Aircraft certification. BUT, regarding the airline, when those certification criteria can not be met, the airline is required to publish a proven alternative stratagy (Ie; EOSID).
Where these numbers (2.4%, 2.7%, 3.0%, …) came from? : Are these empirical numbers or just as a consensus or decision made by certain aviation authorities or administrations (like FAA, ICAO,…)?
Just to make it clear: for example why the climb gradient for the second segment is 2.4% and not 1.2% (twin) or for example why the second segment is usually the most limiting than the third or the fourth?
Just to make it clear: for example why the climb gradient for the second segment is 2.4% and not 1.2% (twin) or for example why the second segment is usually the most limiting than the third or the fourth?
c) If a turn is required after engine failure during takeoff, does this affect aircraft performance?
In short, yes it effects performance, which is why the turn is limited to 15 degrees (or another stipulated bank angle). See AMC OPS 1.495 and JAR-OPS 1.495 (or EU-OPS version)
Note that the FAA does not consider this extra vertical margin.
Does that help?
Moderator
To add to PappyJ's post, in summary -
Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?
The Design Standards require minimum still air, out of ground effect, climb capabilities depending on number of engines. These minimum limitations (generally referred to as WAT [Weight for Altitude and Temperature] limits) give you a reasonable probability that, in the event of a critical failure, the aircraft will continue to climb. Note that turbulence and inversions are outside of the square and may cause you to go down rather than up.
On top of the WAT limits, the operator/pilot may need to restrict TOW further to achieve other requirements, such as runway length or obstacle clearance gradients.
Where these numbers (2.4%, 2.7%, 3.0%, …) came from?
The history will be in some ancient ICAO document which I am not able to cite for you. In the nature of ICAO matters, Signatory States then implement ICAO requirements in National requirements. So, for the USA, FAR 25, for example, imposes the WAT limits on the designer for certification purposes.
why the climb gradient for the second segment is 2.4% and not 1.2% (twin)
The original Standard will be based on statistical work resulting in probabilities which meet the normal design philosophies, ie a very small probability that a failure will result in an accident. A twin will have imposed a shallower gradient than a three or four motor machine as the comparative loss of performance is much higher .. for example, considering that climb relates to T-D, loss of half the thrust may result in loss of, say, 70-80 percent of climb performance for a twin.
why the second segment is usually the most limiting than the third or the fourth?
Not necessarily the case. However, given that, once we are up and away, the main concern is terrain .. and that the terrain clearance calculations are based on net performance (gross, or reasonably expected performance less a fudge factor ... 0.8% for twins), terrain generally becomes less of a routine worry the further we are away from the runway. Also, for the majority of runways, most of the terrain problems are reasonably close in to the airport.
If a turn is required after engine failure during takeoff, does this affect aircraft performance
Certainly does. Depending on the aircraft polars the typical climb gradient decrease for a 15 degree bank turn will be something in the region of 0.5 to 0.9 percent. The general restriction to a maximum bank of 15 degrees is due to the significant ramp up in climb performance loss as the bank angle increases. Note that, for some runways, the procedure may require a smaller angle of bank to achieve a larger turn radius.
small/big radius than usual?
variation in turn radius will depend on speed so the range of V2s has to be considered in determining the turning trapezoid area to be considered for obstacle clearance.
turn into the inoperative engine or into the operative engine
not relevant unless you are going somewhat slower than you should be. Main concern with which side the failed engine is on relates to Vmca considerations where the real Vmca depends very critically on bank angle.
when those certification criteria can not be met, the airline is required to publish a proven alternative stratagy
Not the case. The WAT limits MUST ALWAYS be met in the RTOW for the runway on the day. However, if the straight ahead RTOW is not adequate for the airline's commercial desires, it is perfectly appropriate for the airline to investigate turns to see if they can achieve a better weight. A case of matching the loss in climb performance against a possibly better terrain profile and it doesn't always work .. ie sometimes the best weight is straight ahead and you just have to accept whatever the RTOW is. I can recall spending a GREAT deal of time trying to get a better weight out of Gladstone (Queensland) years ago and all I achieved was a lot of practice at drawing curved splays .. at the end of the day I had to admit defeat for the aircraft concerned and go back to the original straight ahead calculations.
Are climb gradient requirements for aircraft certification or requirements that an operator should comply? or both?
The Design Standards require minimum still air, out of ground effect, climb capabilities depending on number of engines. These minimum limitations (generally referred to as WAT [Weight for Altitude and Temperature] limits) give you a reasonable probability that, in the event of a critical failure, the aircraft will continue to climb. Note that turbulence and inversions are outside of the square and may cause you to go down rather than up.
On top of the WAT limits, the operator/pilot may need to restrict TOW further to achieve other requirements, such as runway length or obstacle clearance gradients.
Where these numbers (2.4%, 2.7%, 3.0%, …) came from?
The history will be in some ancient ICAO document which I am not able to cite for you. In the nature of ICAO matters, Signatory States then implement ICAO requirements in National requirements. So, for the USA, FAR 25, for example, imposes the WAT limits on the designer for certification purposes.
why the climb gradient for the second segment is 2.4% and not 1.2% (twin)
The original Standard will be based on statistical work resulting in probabilities which meet the normal design philosophies, ie a very small probability that a failure will result in an accident. A twin will have imposed a shallower gradient than a three or four motor machine as the comparative loss of performance is much higher .. for example, considering that climb relates to T-D, loss of half the thrust may result in loss of, say, 70-80 percent of climb performance for a twin.
why the second segment is usually the most limiting than the third or the fourth?
Not necessarily the case. However, given that, once we are up and away, the main concern is terrain .. and that the terrain clearance calculations are based on net performance (gross, or reasonably expected performance less a fudge factor ... 0.8% for twins), terrain generally becomes less of a routine worry the further we are away from the runway. Also, for the majority of runways, most of the terrain problems are reasonably close in to the airport.
If a turn is required after engine failure during takeoff, does this affect aircraft performance
Certainly does. Depending on the aircraft polars the typical climb gradient decrease for a 15 degree bank turn will be something in the region of 0.5 to 0.9 percent. The general restriction to a maximum bank of 15 degrees is due to the significant ramp up in climb performance loss as the bank angle increases. Note that, for some runways, the procedure may require a smaller angle of bank to achieve a larger turn radius.
small/big radius than usual?
variation in turn radius will depend on speed so the range of V2s has to be considered in determining the turning trapezoid area to be considered for obstacle clearance.
turn into the inoperative engine or into the operative engine
not relevant unless you are going somewhat slower than you should be. Main concern with which side the failed engine is on relates to Vmca considerations where the real Vmca depends very critically on bank angle.
when those certification criteria can not be met, the airline is required to publish a proven alternative stratagy
Not the case. The WAT limits MUST ALWAYS be met in the RTOW for the runway on the day. However, if the straight ahead RTOW is not adequate for the airline's commercial desires, it is perfectly appropriate for the airline to investigate turns to see if they can achieve a better weight. A case of matching the loss in climb performance against a possibly better terrain profile and it doesn't always work .. ie sometimes the best weight is straight ahead and you just have to accept whatever the RTOW is. I can recall spending a GREAT deal of time trying to get a better weight out of Gladstone (Queensland) years ago and all I achieved was a lot of practice at drawing curved splays .. at the end of the day I had to admit defeat for the aircraft concerned and go back to the original straight ahead calculations.
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IIRC there's a segment for which (I believe for a twin) the climb gradient requirement is actually 0%. Can someone confirm o correct this?
Funny, I just had a mental picture of an airplane on 2nd segment pulling G's from time to time during the climb to "meet the instantaneous requirement" 
the interesting thing is that these are only instantaneous gradients, you dont have to maintain them for the complete second segment!
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Who could expand upon John_T's superlative summary, just about the best precis I've seen on this topic!
Mutt speaks the truth, as always, even if the thrust is constant throughout 2nd segment, increasing TAS will steadily reduce the Climb Gradient. The technique that I use is to apply the mid Pressure Height between end of 1st Segment and Minimum Acceleration Altitude, thus, Initial Instantaneous Gradient will slightly exceed that required, Final Instantaneous Gradient will be slightly less, but the Mean Gradient should equal that required. It introduces a little conservatism between beginning and end of 2nd Segment.
To give a short answer to a question which Mutt has asked of other P/E's techniques in applying nominal higher Acceleration Altitudes (e.g. 1000 ft) than the MAA, REQUIRED Gradient is assured until MAA, and a lesser gradient accepted for further climb to the increased 'nominal' Acceleration Altitude (because it's not needed), but ensuring significantly improved vertical obstacle clearance in the 3rd segment. No significant performance penalty should result, excepting the possibility of hitting the 5 minute Takeoff thrust time limit. Most modern aircraft now have a 10 minute limit, so even this is not a real problem.
MarkerInbound, what you say is true, but the reality is significantly better than the minimum certification requirement. Typical Deltas betweem 1st and 2nd Segment Gradients are rarely in excess of 1% (0.9% is the worst that I've worked with), thus, if the aircraft can achieve a 2.4% Gross Gradient in the 2nd Segment (minimum requirement), it's reasonable to assume that 1.4% should be achievable in the 1st Segment. Even if you degrade this by the arbitrary 0.8% Gross Vs Net, we should see no worse than 0.6% in 1st Segment. Hardly rocket performance, but a darned sight better than the nominal "positive" climb gradient. I think that Galaxyflyer once stated a worse than 1% delta, but this is the exception rather than the rule.
Regards,
Old Smokey
Mutt speaks the truth, as always, even if the thrust is constant throughout 2nd segment, increasing TAS will steadily reduce the Climb Gradient. The technique that I use is to apply the mid Pressure Height between end of 1st Segment and Minimum Acceleration Altitude, thus, Initial Instantaneous Gradient will slightly exceed that required, Final Instantaneous Gradient will be slightly less, but the Mean Gradient should equal that required. It introduces a little conservatism between beginning and end of 2nd Segment.
To give a short answer to a question which Mutt has asked of other P/E's techniques in applying nominal higher Acceleration Altitudes (e.g. 1000 ft) than the MAA, REQUIRED Gradient is assured until MAA, and a lesser gradient accepted for further climb to the increased 'nominal' Acceleration Altitude (because it's not needed), but ensuring significantly improved vertical obstacle clearance in the 3rd segment. No significant performance penalty should result, excepting the possibility of hitting the 5 minute Takeoff thrust time limit. Most modern aircraft now have a 10 minute limit, so even this is not a real problem.
MarkerInbound, what you say is true, but the reality is significantly better than the minimum certification requirement. Typical Deltas betweem 1st and 2nd Segment Gradients are rarely in excess of 1% (0.9% is the worst that I've worked with), thus, if the aircraft can achieve a 2.4% Gross Gradient in the 2nd Segment (minimum requirement), it's reasonable to assume that 1.4% should be achievable in the 1st Segment. Even if you degrade this by the arbitrary 0.8% Gross Vs Net, we should see no worse than 0.6% in 1st Segment. Hardly rocket performance, but a darned sight better than the nominal "positive" climb gradient. I think that Galaxyflyer once stated a worse than 1% delta, but this is the exception rather than the rule.
Regards,
Old Smokey
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OS, using AFM-DPI it appears that Boeing use 2.4% at the start of the 2nd segment, but that decreases by the end of the 2nd segment. This is different to your concept of using a MEAN value so that you were above 2.4% for the whole segment.
Mutt
Mutt
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OS, using AFM-DPI it appears that Boeing use 2.4% at the start of the 2nd segment, but that decreases by the end of the 2nd segment. This is different to your concept of using a MEAN value so that you were above 2.4% for the whole segment.
An old chestnut which has been troubling all of us for decades. In Oz, if I go back 30-40 years, the airworthiness and operational rules took opposing views. In general, the Airlines, certainly Ansett, took the view that the conservative requirement applied .. ie not less than WAT throughout the second segment.
My view has always been that, for an approved AFM for the country in which one is working, the certification is the basic authorisation unless the operational rules are more conservative, in which case the latter apply.
If one takes the view that the WAT limit really is a safety net consideration then, at the end of the day, the main concern is to have the net flight path not less than the appropriate clearance from the bumpy bits. It seems to me that a CFIT is a good recipe for really and truly spoiling one's day ...
OS' technique is a tad conservative and that is his/his airline's prerogative.
An old chestnut which has been troubling all of us for decades. In Oz, if I go back 30-40 years, the airworthiness and operational rules took opposing views. In general, the Airlines, certainly Ansett, took the view that the conservative requirement applied .. ie not less than WAT throughout the second segment.
My view has always been that, for an approved AFM for the country in which one is working, the certification is the basic authorisation unless the operational rules are more conservative, in which case the latter apply.
If one takes the view that the WAT limit really is a safety net consideration then, at the end of the day, the main concern is to have the net flight path not less than the appropriate clearance from the bumpy bits. It seems to me that a CFIT is a good recipe for really and truly spoiling one's day ...
OS' technique is a tad conservative and that is his/his airline's prerogative.
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If one takes the view that the WAT limit really is a safety net consideration then
So what OPERATIONAL use is the WAT limit?
Mutt
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So what OPERATIONAL use is the WAT limit?
I suggest none ... other than the safety net consideration that it gives you some likelihood of not descending.
The WAT limit is a certification (rather than operational) animal in the same vein as the maximum structural limits .. just draws a line in the sand beyond which one is not supposed to go.
I suggest none ... other than the safety net consideration that it gives you some likelihood of not descending.
The WAT limit is a certification (rather than operational) animal in the same vein as the maximum structural limits .. just draws a line in the sand beyond which one is not supposed to go.
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Mutt,
Skipping back a few posts, yes, I'm well familiar with the Boeing Instantaneous gradient at commencement of 2nd segment, and other manufacturers cuch as McD stipulating Field pressure as the data entry point.
John_T and I were / are products of the Australian system where there was often conflict between the airworthiness requirements and operational rules were oft at loggerheads on this issue. The operational folk took the viewpoint that using a lower reference Pressure Height "consumed" a portion of the Gross Vs Net Delta as 2nd segment continued and was thus unacceptable. Consequently, reputable operators like Ansett and TAA / Australian took the more conservative approach. (I'm sure that QANTAS probably went the same way).
In practice, there's only a minor penalty, and in 99% of cases can be avoided. For a MAA of, say, 500 ft and an end of 1st Segment height of 100 ft, use of mean 2nd segment PH means using 300 ft AFL, a small penalty. This equates to approximately 10hPa conservatism, and for a B777 amounts to approximately a 2500 Kg penalty. As 99% of Takeoffs are with reduced thrust (ATM or Flex) this penalty may be recovered by a couple of degrees of Assumed Temperature reduction. In the other 1% of cases, there's a penalty, but the good folks at airworthiness at the regulatory authority won't compromise upon any degradation of a climb gradient required for obstacle clearance.
What use is the WAT limit? For my 2c worth, it offers a minimum standard of performance for the aircraft, irrespective of Runway and Obstacle factors which are another issue. Of course, we do know that there are people "out there" who go flying with WAT as the only limit regardless of Runway / Obstacle computations, but there's always a few rotten eggs in the basket.
Regards,
Old Smokey
Skipping back a few posts, yes, I'm well familiar with the Boeing Instantaneous gradient at commencement of 2nd segment, and other manufacturers cuch as McD stipulating Field pressure as the data entry point.
John_T and I were / are products of the Australian system where there was often conflict between the airworthiness requirements and operational rules were oft at loggerheads on this issue. The operational folk took the viewpoint that using a lower reference Pressure Height "consumed" a portion of the Gross Vs Net Delta as 2nd segment continued and was thus unacceptable. Consequently, reputable operators like Ansett and TAA / Australian took the more conservative approach. (I'm sure that QANTAS probably went the same way).
In practice, there's only a minor penalty, and in 99% of cases can be avoided. For a MAA of, say, 500 ft and an end of 1st Segment height of 100 ft, use of mean 2nd segment PH means using 300 ft AFL, a small penalty. This equates to approximately 10hPa conservatism, and for a B777 amounts to approximately a 2500 Kg penalty. As 99% of Takeoffs are with reduced thrust (ATM or Flex) this penalty may be recovered by a couple of degrees of Assumed Temperature reduction. In the other 1% of cases, there's a penalty, but the good folks at airworthiness at the regulatory authority won't compromise upon any degradation of a climb gradient required for obstacle clearance.
What use is the WAT limit? For my 2c worth, it offers a minimum standard of performance for the aircraft, irrespective of Runway and Obstacle factors which are another issue. Of course, we do know that there are people "out there" who go flying with WAT as the only limit regardless of Runway / Obstacle computations, but there's always a few rotten eggs in the basket.
Regards,
Old Smokey
So what use is the WAT limit?
I found it quite useful in The Gulf when climbing into an inversion after T/O.
I found it quite useful in The Gulf when climbing into an inversion after T/O.
Moderator
Indeed, if you are WAT-limited and then climb into a significant inversion OEI you may well find yourself dead in the water at the inversion ie if you know there is a significant inversion then it probably is a good idea to consider going into the WAT charts with the maximum lapse temperature rather than surface temperature if the former exceeds the latter.
Not a requirement but might help you not to spoil your day.
Not a requirement but might help you not to spoil your day.
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I will have to dig into some books, since it is a long time ago.
however, on a pure certification data collection side :
On a V1 cut takeoff, a theodolith is (was) used to measure the angle from the reference point to 400ft, a simple arithmetics would give the gradient; now, if my memory is correct in FAR25 we needed 7 valid points between 2 inflexions of any performance curve, therefore at least 7 measurements, during the life of the test program we were gathering much more data, in order to give "sensible" information, these data were crunched using statisical methods in particular the RMS methodology for Gauss distributions; leading to a sample of around 70% of the mean value for each point.
This being said, the manufacturer can publish any number within the 7 points in the Gauss distribution since he is covered by the RMS buffer; this is why some aircrafts are "flying the book" better than others.
Today, as far as I can see the gradient published in tab data is the one at 400 ft AGL, again this is the RMS gradient, leading to a conservative publication, this is why when asking the manufacturer a special set of data for specific conditions , you usually end up with performance numbers you would have not imagine.
however, on a pure certification data collection side :
On a V1 cut takeoff, a theodolith is (was) used to measure the angle from the reference point to 400ft, a simple arithmetics would give the gradient; now, if my memory is correct in FAR25 we needed 7 valid points between 2 inflexions of any performance curve, therefore at least 7 measurements, during the life of the test program we were gathering much more data, in order to give "sensible" information, these data were crunched using statisical methods in particular the RMS methodology for Gauss distributions; leading to a sample of around 70% of the mean value for each point.
This being said, the manufacturer can publish any number within the 7 points in the Gauss distribution since he is covered by the RMS buffer; this is why some aircrafts are "flying the book" better than others.
Today, as far as I can see the gradient published in tab data is the one at 400 ft AGL, again this is the RMS gradient, leading to a conservative publication, this is why when asking the manufacturer a special set of data for specific conditions , you usually end up with performance numbers you would have not imagine.
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Today, as far as I can see the gradient published in tab data is the one at 400 ft AGL
we do know that there are people "out there" who go flying with WAT as the only limit regardless of Runway / Obstacle computations
The Gulf when climbing into an inversion after T/O
Mutt
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747's DESCENDING after takeoff with all engines running, the new procedure is worth the weight loss.
As Paul Hogan might have observed ... "now that's an inversion".
We used to see much the same result out of Tennant Creek during summer on the F27 ... wet power and next to no AEO climb.
As Paul Hogan might have observed ... "now that's an inversion".
We used to see much the same result out of Tennant Creek during summer on the F27 ... wet power and next to no AEO climb.
OS
Yes, I did, but it applied to a certain beast that didn't meet all the civil requirements. Can you guess?
Regards to turning departures, up to a point (30 degrees) the loss of performance is kind of linear. That is, shallower bank angles lead to less loss of performance but more time getting the required turn equals, or closely equals, a higher bank angle with greater performance loss but less time spent turning. 15 degrees is the standard compromise, I believe, because most FAR 25 planes can do that bank angle at V2 without causing margin problems.
Today, departing Rifle, CO, while I had a APG Company runway analysis for CL60 departure, it was interesting to load the SID gradients and level-off heights. Loading the standard level-off height and noting the EO gradient in the FMS and then reloading the SID "top of gradient" height, it was about 15% loss in gradient going from 1500 QFE level-off to 4400 QFE level-off. Airport elevation was 5540 MSL.
Speaking of the Galaxy, departing Anderson AFB, Guam with a 600 drop to the ocean, we had ex-bomber guys who figured we didn't have to meet AFM minimum climb gradients (WAT limit) because there was nothing to hit. They were disabused of this idea, by me amongst others. True, BUFFs did sometimes disappear after lift-off (level-off?) and reappear several miles downrange.
GF
Yes, I did, but it applied to a certain beast that didn't meet all the civil requirements. Can you guess?
Regards to turning departures, up to a point (30 degrees) the loss of performance is kind of linear. That is, shallower bank angles lead to less loss of performance but more time getting the required turn equals, or closely equals, a higher bank angle with greater performance loss but less time spent turning. 15 degrees is the standard compromise, I believe, because most FAR 25 planes can do that bank angle at V2 without causing margin problems.
Today, departing Rifle, CO, while I had a APG Company runway analysis for CL60 departure, it was interesting to load the SID gradients and level-off heights. Loading the standard level-off height and noting the EO gradient in the FMS and then reloading the SID "top of gradient" height, it was about 15% loss in gradient going from 1500 QFE level-off to 4400 QFE level-off. Airport elevation was 5540 MSL.
Speaking of the Galaxy, departing Anderson AFB, Guam with a 600 drop to the ocean, we had ex-bomber guys who figured we didn't have to meet AFM minimum climb gradients (WAT limit) because there was nothing to hit. They were disabused of this idea, by me amongst others. True, BUFFs did sometimes disappear after lift-off (level-off?) and reappear several miles downrange.
GF
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Question: when using RTOW charts how does one account for a forecast temperature inversion? If I have a relatively high flex I know I have more power available if needed but what if the conditions of the day force me to use TOGA?
However, Derrated-TO is a max. thrust. Can someone with a bit more of experience in Mr. Boeing's aircrafts confirm this?