De-rated power settings question
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Hi everybody. I am old. Not too old to rock and roll but too young to die. When I was young I heard a curious thing about ISA, maybe only a legend, and I wrote into my brain’s hard disk. I am a pilotàmy brain is not used, so I don’t remember it very clear. ISA is an agreement between different countries, for using it as a reference. It is an ideal state of the atmosphere, where the temperature and altitudes can be mathematically predicted. The question is, for that ideal it is not used the altitude. It uses height that ideally must match. Everybody knows that usually, height does not match with altitude; it is lower at the Equator and higher at the poles. So the ideal atmosphere determination team decided that the reference should be at the Greenwich meridian when it reaches the Mediterranean Sea (Sea Level) in a day at the springtime with the rest of standard parameters. That occurs close to Benidorm in Alicante, not at LEAL with 142’ elev. I mentioned only as a curiosity. I am expert on nothing. Only enjoying in pprune (the best place) helping and learning from other pilots. Mutt don’t get mad. You are very right. The B757 I was flying uses RB 211-535E4 44100lb up to ISA+14 SLS. There it goes a very interesting link to RR engines data:
http://www.rolls-royce.com/media/packs/200702-civilaerospace.pdf
The A340-300E uses CFM 56-5C4 34000lb flat rated to ISA+15 at SLS.
The only thing I was questioning is: those manufacturers only guarantee FULL THRUST at ISA SLS. Or up to ISA+15 or ISA+22 or ISA+30 as you said, depending on the certificate of the engine. But only full thrust is at “SLS” that states for sea level, 1013.2, Benidorm, 14,9ºC and all that stuff. After that SLS, you can use the words full power, but not full thrust, and to explain the Andy’s question we must have this very present. I apologize for my words Mutt.
Groundfloor I have not seen any A340 with a derate TO option in the FMS or a pushbutton like in the B757, so the Finns and the Swedes must derate using the flex option. If you have other information, please post it.
With respect to turbopropellers I have flown from Fairchild the MetroIII MetroII and Merlin IV, from CASA the C235 and from Fokker the F50 (the modern fokker
) and I have not seen any derateNice flights to everybody
http://www.rolls-royce.com/media/packs/200702-civilaerospace.pdf
The A340-300E uses CFM 56-5C4 34000lb flat rated to ISA+15 at SLS.
The only thing I was questioning is: those manufacturers only guarantee FULL THRUST at ISA SLS. Or up to ISA+15 or ISA+22 or ISA+30 as you said, depending on the certificate of the engine. But only full thrust is at “SLS” that states for sea level, 1013.2, Benidorm, 14,9ºC and all that stuff. After that SLS, you can use the words full power, but not full thrust, and to explain the Andy’s question we must have this very present. I apologize for my words Mutt.
Groundfloor I have not seen any A340 with a derate TO option in the FMS or a pushbutton like in the B757, so the Finns and the Swedes must derate using the flex option. If you have other information, please post it.
With respect to turbopropellers I have flown from Fairchild the MetroIII MetroII and Merlin IV, from CASA the C235 and from Fokker the F50 (the modern fokker
) and I have not seen any derateNice flights to everybody
Last edited by ppppilot; 18th Feb 2008 at 16:06.
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From the horses mouth so to speak...340...cut from airbus manuals
FLEX TAKE OFF:
Takeoff at reduced thrust is permissible only if the airplane meets all applicable performance requirements at the planned takeoff weight with the operating engines at the thrust available for the assumed temperature.
Thrust reduction must not exceed 25 % of the full rated takeoff thrust. To meet this requirement, the flexible temperature must not be higher than ISA + 40°C (T MAX FLEX).
The assumed temperature must not be lower than the flat rating temperature or the actual OAT.
Takeoff reduced thrust is not permitted on contaminated runways.
Takeoff at reduced thrust is allowed with any inoperative item affecting the performance only if the associated performance shortfall has been applied to meet all performance requirements at the takeoff weight, with the operating engines at the thrust available for the flex temperature.
DERATED TAKE OFF:
Derated takeoff may be used when the takeoff weight is limited by VMCG, enabling benefit to be taken from the reduction in VMCG associated with the new rating.
The use of flexible thrust is not permitted when derated thrust is used. Moreover the level of derate is entered on the MCDU PERF TO page in the DRT TO/FLX TO field.
When a derated takeoff is performed, selection of full takeoff thrust by setting thrust levers at TOGA is not permitted below the speeds specified in engine failure procedure (FCOM 3.02.10 page 4)
The use of derated takeoff is allowed on dry, wet and contaminated runway.
The minimum control speeds VMCG and VMCA are reduced for two reasons :
The derated thrust is lower than the maximum takeoff thrust
The effect of temperature on VMCG and VMCA is taken into account (which is not the case for takeoff without derate, due to the flexible takeoff concept)
The effect of the derate on the maximum takeoff weight is different depending on whether VMCG or VMCA is limiting. Indeed the effect on maximum takeoff weight is the result of a thrust decrease (downgrading the takeoff performance) and of a VMC decrease (improving the takeoff performance). As VMCG only concerns the accelerate stop distance, the VMCG decrease by far compensates the thrust loss. The VMCG limited weight is then improved by derating.
But as VMCA mainly concerns the airborne phase of the takeoff, the effect of the thrust decrease is more important and not compensated by the effect of a lower VMCA. Therefore derated takeoff would not improve TOW if VMCA limited.
When VMCG limited, an optimum derate can be determined as shown below.
Hope this helps
FLEX TAKE OFF:
Takeoff at reduced thrust is permissible only if the airplane meets all applicable performance requirements at the planned takeoff weight with the operating engines at the thrust available for the assumed temperature.
Thrust reduction must not exceed 25 % of the full rated takeoff thrust. To meet this requirement, the flexible temperature must not be higher than ISA + 40°C (T MAX FLEX).
The assumed temperature must not be lower than the flat rating temperature or the actual OAT.
Takeoff reduced thrust is not permitted on contaminated runways.
Takeoff at reduced thrust is allowed with any inoperative item affecting the performance only if the associated performance shortfall has been applied to meet all performance requirements at the takeoff weight, with the operating engines at the thrust available for the flex temperature.
DERATED TAKE OFF:
Derated takeoff may be used when the takeoff weight is limited by VMCG, enabling benefit to be taken from the reduction in VMCG associated with the new rating.
The use of flexible thrust is not permitted when derated thrust is used. Moreover the level of derate is entered on the MCDU PERF TO page in the DRT TO/FLX TO field.
When a derated takeoff is performed, selection of full takeoff thrust by setting thrust levers at TOGA is not permitted below the speeds specified in engine failure procedure (FCOM 3.02.10 page 4)
The use of derated takeoff is allowed on dry, wet and contaminated runway.
The minimum control speeds VMCG and VMCA are reduced for two reasons :
The derated thrust is lower than the maximum takeoff thrust
The effect of temperature on VMCG and VMCA is taken into account (which is not the case for takeoff without derate, due to the flexible takeoff concept)
The effect of the derate on the maximum takeoff weight is different depending on whether VMCG or VMCA is limiting. Indeed the effect on maximum takeoff weight is the result of a thrust decrease (downgrading the takeoff performance) and of a VMC decrease (improving the takeoff performance). As VMCG only concerns the accelerate stop distance, the VMCG decrease by far compensates the thrust loss. The VMCG limited weight is then improved by derating.
But as VMCA mainly concerns the airborne phase of the takeoff, the effect of the thrust decrease is more important and not compensated by the effect of a lower VMCA. Therefore derated takeoff would not improve TOW if VMCA limited.
When VMCG limited, an optimum derate can be determined as shown below.
Hope this helps
Last edited by groundfloor; 21st Feb 2008 at 19:47.
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I have been searching, and from MSN302 you can use Dxx instead of Fxx in perf page of the FMS. In my company we are not allowed to use it. Good explanation. Thanks groundfloor.
Tailwinds to everybody
Tailwinds to everybody
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Thanks to all again. Just found this. and explains VMCG and VMCA
This webpage actually shows what the Flight Management computer screens for different inputs.
http://www.pilotosdeiberia.com/areat.../24derated.htm
This webpage actually shows what the Flight Management computer screens for different inputs.
http://www.pilotosdeiberia.com/areat.../24derated.htm
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ISA + 15 flat rating doesn't make sense to me.
It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
If the AC is overpowered, operations above 30 celsius won't become an issue, while a less powered AC won't be able to operate safely, and this, with the same flat rating value. Flat rating just gives the value at which the aircraft will suffer thrust losses, but doesn't give any clue about the AC thrust values.
Another example is the Piaggio avanti which is using 47 % of the available thermodynamical power of the PT 6 (850 shp out of 1800 thermodynamical hp), giving a flat rating of ISA + 37, meaning it won't loose power in very hot environment.
It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
If the AC is overpowered, operations above 30 celsius won't become an issue, while a less powered AC won't be able to operate safely, and this, with the same flat rating value. Flat rating just gives the value at which the aircraft will suffer thrust losses, but doesn't give any clue about the AC thrust values.
Another example is the Piaggio avanti which is using 47 % of the available thermodynamical power of the PT 6 (850 shp out of 1800 thermodynamical hp), giving a flat rating of ISA + 37, meaning it won't loose power in very hot environment.
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It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
Why make things more difficult than they need to be?
For example, the L1011 with RB.211-524B02 engines has, as standard at many airlines, runway analysis takeoff charts supplied that permit ops at sea level to 54C (ISA+39), so the assumed temperature method of calculating MTOW is very straightforward....even without the fancy FMC's of today.
Piece of cake.
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sispanys ria sez:
That's not too unusual as turboprops are upgraded. Turbomachinery improvements provide much more available SHP, but the gearbox becomes the limiting factor, resulting in flat rating up to a remarkably high OAT. (There are Vmc issues as well that may hold SHP to a legacy limit.)
Another example is the Piaggio avanti which is using 47 % of the available thermodynamical power of the PT 6 (850 shp out of 1800 thermodynamical hp), giving a flat rating of ISA + 37, meaning it won't loose power in very hot environment.
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The piaggio avanti it is not a good example of anything but originallity. It uses the more extrange engine I have ever seen, where the exaust are thrown over the propellers. I have heard they need to be replaced or overhauled every 500 hrs. Ferrari has one with their logo based at Bolonia.
Quote from sispanys ria:
ISA + 15 flat rating doesn't make sense to me.
It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
[Unquote]
I would wager that there is a high proportion of pilots who have little idea what the thrust ratings of their engines are, without looking them up. We just don't need to know it. The people who calculate the Performance graphs/charts/tables/FMC-algorithms, of course, DO.
411A is right, charts or tables have been a quick and accurate way of calculating the Flex Temps and V-speeds. On my last type (I retired 6 years ago), TMAX-FLEX was ISA+57, and our tables went right up to that level. There was shading to show that anything above TMAX was for Assumed (Flex) Temperature calculations only, not for actual OAT, and that the lowest permitted Flex Temp was TREF (the top end of the flat-rating).
In an earlier incarnation, on VC10s, we copilots used to calculate the Flex Temp (we called it something else) and all the other T/O performance data from a handbook of Perf A graphs (like you might still do in an ATPL exam?). Sounds difficult, particularly at some third-world airfield in the middle of the night, but we used to get very good (and quick) at it, using a proforma provided for the purpose. If the aeroplane was very light, even TMAX (we didn't have TMAX-FLEX in those days) permitted a range of possible V1s, and some of the more pernickity captains woud ask us to calculate them. If the departure airfield had no engineering facilities, the lowest V1 would be chosen, so we had to make sure it was above VMCG.
"You guys have it easy nowadays..."
ISA + 15 flat rating doesn't make sense to me.
It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...
[Unquote]
I would wager that there is a high proportion of pilots who have little idea what the thrust ratings of their engines are, without looking them up. We just don't need to know it. The people who calculate the Performance graphs/charts/tables/FMC-algorithms, of course, DO.
411A is right, charts or tables have been a quick and accurate way of calculating the Flex Temps and V-speeds. On my last type (I retired 6 years ago), TMAX-FLEX was ISA+57, and our tables went right up to that level. There was shading to show that anything above TMAX was for Assumed (Flex) Temperature calculations only, not for actual OAT, and that the lowest permitted Flex Temp was TREF (the top end of the flat-rating).
In an earlier incarnation, on VC10s, we copilots used to calculate the Flex Temp (we called it something else) and all the other T/O performance data from a handbook of Perf A graphs (like you might still do in an ATPL exam?). Sounds difficult, particularly at some third-world airfield in the middle of the night, but we used to get very good (and quick) at it, using a proforma provided for the purpose. If the aeroplane was very light, even TMAX (we didn't have TMAX-FLEX in those days) permitted a range of possible V1s, and some of the more pernickity captains woud ask us to calculate them. If the departure airfield had no engineering facilities, the lowest V1 would be chosen, so we had to make sure it was above VMCG.
"You guys have it easy nowadays..."
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handbook of Perf A graphs
, I hate losing
The assumed temperature must not be lower than the flat rating temperature
exceed 25 % of the full rated takeoff thrust
Mutt
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mutt:
The first applies to our E-jets. Any Assumed temp below Min Assumed Temp(flat rated temp) will not produce a reduction in thrust. If you look at the figures for an assumed temp below ISA+15 (ISA+20 for your jets I think) the N1 reduced is always the same a s the N1 for To thrust at the ambient temp.
As for the second point I'm with you. the 25% reduction is applicable to the Rated Thrust or De-Rated Thrust.
As for d and x graphs, many moons ago in deapest Africa a Nimrod crew were a bit limited so they pulled out the books the night before and 2 pilots and 2 FEs each did a calculation. In the morning we ehrrr.... they compaired the results and used the most limiting of the 4 different answers they had.
Thank the lord for computer generated runway analysis!!
Quote:
The assumed temperature must not be lower than the flat rating temperature
Why not?
Quote:
exceed 25 % of the full rated takeoff thrust
Why is this restriction applicable to the full rated takeoff thrust, why not the rating in use?
The assumed temperature must not be lower than the flat rating temperature
Why not?
Quote:
exceed 25 % of the full rated takeoff thrust
Why is this restriction applicable to the full rated takeoff thrust, why not the rating in use?
The first applies to our E-jets. Any Assumed temp below Min Assumed Temp(flat rated temp) will not produce a reduction in thrust. If you look at the figures for an assumed temp below ISA+15 (ISA+20 for your jets I think) the N1 reduced is always the same a s the N1 for To thrust at the ambient temp.
As for the second point I'm with you. the 25% reduction is applicable to the Rated Thrust or De-Rated Thrust.
As for d and x graphs, many moons ago in deapest Africa a Nimrod crew were a bit limited so they pulled out the books the night before and 2 pilots and 2 FEs each did a calculation. In the morning we ehrrr.... they compaired the results and used the most limiting of the 4 different answers they had.
Thank the lord for computer generated runway analysis!!
Evolution of Flex-Thrust Performance Calculations
Quote from mutt:
Quote:
handbook of Perf A graphs
...I never had to use those charts for real...
I doubt any airline jet-pilot will ever have to use graphic charts again. Pity in a way, because, like any pictorial representation of a process - e.g., a systems diagram, or flow chart - they could be an aid to understanding and monitoring the "big picture". Just in case anyone is wondering, they were NOT runway-specific, so the first part of the process was to extract the runway details from an airfield list: Elevation; Slope; TORA; EMDA (ASDA); TODA; and any obstacles affecting the NTOFP (net T/O flight-path, after the end of the clearway). The graphs could be re-used indefinitely, being covered in sticky-back plastic, so we could use a sharp china-graph pencil - but fine ball-point was more practical.
Later. on the B707, we still used charts, but they combined runway, WAT and NTOFP considerations on to one chart, and were runway-specific. So the only remaining variables were QNH, OAT/Flex-Temp, W/C (wind-component, head or tail), and any permiissible contaminant. [You had to extract V-speeds elsewhere.]
TABLES are runway-specific; and incorporate V-speeds in the same 'box' for each temp, W/C, and resulting weight. The only corrections required are the QNH, and any permissible R/W contaminant. The trouble is that each weight is the RTOW for a specific OAT/Flex-Temp and W/C, and the speeds therefore assume you are at that weight. If you are lighter, the V1 is still valid, but the VR and V2 have to be calculated for the actual weight.** [If the quoted V1 is higher than (actual) VR, it must be reduced to VR.] And don't forget VMCG...
Quote from mutt:
Quote (from groundfloor):
The assumed temperature must not be lower than the flat-rating temperature
Why not?
Good question. It's always been so! I see that groundfloor correctly added "or the OAT", something I wrongly omitted in my post. But using a lower temperature shouldn't matter because, if you use TREF, you are already at Rated Thrust; so using a lower assumed temperature shouldn't make any difference. Equally, if you use something below OAT, I doubt that a modern engine would try to give you more than Rated Thrust. BUT I wouldn't want to try it out! In the old days, American jet engines COULD be "over-boosted" by opening the thrust levers too far. It was the job of the F/E (much-missed flight engineer)
to pre-calculate the appropriate EPR, P7, (or whatever power indication applicable to the engine), and set the correct take-off thrust on the pilot's command.
Actually, using OAT or TREF as the assumed temperature is pointless - you might as well select Rated Thrust in the conventional manner. [The flat-rating concept did not apply, by the way, on the Conway-powered VC10. The first engine I operated with it was American - the JT3D on the B707.]
FE Hoppy has answered mutt's other point - I've never used de-rated thrust, so can't comment. Re graphs: yes, but practice makes (nearly) perfect, and a limiting take-off out of Entebbe (old R/W) or Nairobi used to concentrate the mind wonderfully... We always used to use the "thickness of the pencil" on the safe side. I don't think computers are capable of doing that!
** [Unless you are using optimised performance (capitalising on surplus runway length by using increased VR and V2 for a better climb gradient).]
Quote:
handbook of Perf A graphs
...I never had to use those charts for real...
I doubt any airline jet-pilot will ever have to use graphic charts again. Pity in a way, because, like any pictorial representation of a process - e.g., a systems diagram, or flow chart - they could be an aid to understanding and monitoring the "big picture". Just in case anyone is wondering, they were NOT runway-specific, so the first part of the process was to extract the runway details from an airfield list: Elevation; Slope; TORA; EMDA (ASDA); TODA; and any obstacles affecting the NTOFP (net T/O flight-path, after the end of the clearway). The graphs could be re-used indefinitely, being covered in sticky-back plastic, so we could use a sharp china-graph pencil - but fine ball-point was more practical.
Later. on the B707, we still used charts, but they combined runway, WAT and NTOFP considerations on to one chart, and were runway-specific. So the only remaining variables were QNH, OAT/Flex-Temp, W/C (wind-component, head or tail), and any permiissible contaminant. [You had to extract V-speeds elsewhere.]
TABLES are runway-specific; and incorporate V-speeds in the same 'box' for each temp, W/C, and resulting weight. The only corrections required are the QNH, and any permissible R/W contaminant. The trouble is that each weight is the RTOW for a specific OAT/Flex-Temp and W/C, and the speeds therefore assume you are at that weight. If you are lighter, the V1 is still valid, but the VR and V2 have to be calculated for the actual weight.** [If the quoted V1 is higher than (actual) VR, it must be reduced to VR.] And don't forget VMCG...
Quote from mutt:
Quote (from groundfloor):
The assumed temperature must not be lower than the flat-rating temperature
Why not?
Good question. It's always been so! I see that groundfloor correctly added "or the OAT", something I wrongly omitted in my post. But using a lower temperature shouldn't matter because, if you use TREF, you are already at Rated Thrust; so using a lower assumed temperature shouldn't make any difference. Equally, if you use something below OAT, I doubt that a modern engine would try to give you more than Rated Thrust. BUT I wouldn't want to try it out! In the old days, American jet engines COULD be "over-boosted" by opening the thrust levers too far. It was the job of the F/E (much-missed flight engineer)
to pre-calculate the appropriate EPR, P7, (or whatever power indication applicable to the engine), and set the correct take-off thrust on the pilot's command. Actually, using OAT or TREF as the assumed temperature is pointless - you might as well select Rated Thrust in the conventional manner. [The flat-rating concept did not apply, by the way, on the Conway-powered VC10. The first engine I operated with it was American - the JT3D on the B707.]
FE Hoppy has answered mutt's other point - I've never used de-rated thrust, so can't comment. Re graphs: yes, but practice makes (nearly) perfect, and a limiting take-off out of Entebbe (old R/W) or Nairobi used to concentrate the mind wonderfully... We always used to use the "thickness of the pencil" on the safe side. I don't think computers are capable of doing that!
** [Unless you are using optimised performance (capitalising on surplus runway length by using increased VR and V2 for a better climb gradient).]
Last edited by Chris Scott; 23rd Feb 2008 at 11:55. Reason: ** Optimised VR and V2. Typo. More typos. "Vref" corrected to read "Tref".
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As for d and x graphs ..
.. those were the days ... guess 99.9% of folk these days have not the slightest idea of what x and d means ...
they compaired the results and used the most limiting of the 4 different answers they had
Similar to one where ATC requested a wind check in the wee hours (in the olden days when people carried, but rarely used, prayer wheels) ... we all did our sums and decided that embarrassment would be minimised by averaging the answers and reporting L&V.
.. those were the days ... guess 99.9% of folk these days have not the slightest idea of what x and d means ...
they compaired the results and used the most limiting of the 4 different answers they had
Similar to one where ATC requested a wind check in the wee hours (in the olden days when people carried, but rarely used, prayer wheels) ... we all did our sums and decided that embarrassment would be minimised by averaging the answers and reporting L&V.
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Our E-Jets are ISA+15C, its the other end of the envelope that they are playing with. (But as i havent seen the new envelope yet, i stand to be corrected). I know that the N1 is always constant below the min assumed temperature, but my question is, why do we have to use it? Why complicate the procedure more than it needs? Isnt it realistic to have the crews to exactly the same calculation everytime and just accept that below the Min Assumed, the thrust wont change?
Thats what i was getting at, we operate some aircraft with 3 ratings, we can apply assumed derates of 25% to all of them, so in reality we can reduce thrust by around 45%.
Chris, what VREF are you talking about?
Yep they are, in fact the pencil is so sharp that Boeing will offer you 4-8000 kgs additional takeoff weights in some hot/high airports, same aircraft, same AFM, just using a sharper computer powered pencil
Mutt
the 25% reduction is applicable to the Rated Thrust or De-Rated Thrust
Chris, what VREF are you talking about?
I don't think computers are capable of doing that
Mutt
Well spotted, mutt...
Quote from me:
Actually, using OAT or VREF as the assumed temperature is pointless - you might as well select Rated Thrust in the conventional manner.
Quote from mutt:
Chris, what VREF are you talking about?
Good to know someone is paying attention. It was a typo - I meant TREF. In case I've confused anyone, I had better explain. If my memory serves, TREF is the highest temperature for the maximum thrust rating (the right end of the flat part of the line). On the day, OAT can be higher, or lower.
[Hope no one minds, but I'm going to EDIT my post to remove that error.]
Quote from mutt:
Yep they are, in fact the pencil is so sharp that Boeing will offer you 4-8000 kgs additional takeoff weights in some hot/high airports, same aircraft, same AFM, just using a sharper computer powered pencil. [Unquote]
Sorry for any confusion, but I was not suggesting that the old manual method enabled us to get a HIGHER RTOW than a computer might calculate. Quite the reverse.
That's why I wrote: "We always used to use the "thickness of the pencil" on the safe side. I don't think computers are capable of doing that!"
That was in response to FE Hoppy's anecdote about the Nimrod crew, who got 4 different answers to the manual calculation, and sensibly decided to use the most conservative (least optimistic) figures.
When a human is running a "pencil" horizontally along the graticule of the graph, it's very easy to slip the line - up or down. It's usually safer to err on the 'down' side...
I say again, a computer would never do that. So a computer may remove that EXTRA margin of safety that you or I might instinctively apply, but which we would not admit to our company commercial department! The regulations, of course, already build in fat in most cases (e.g., only 50% of a headwind to be taken into account, etcetera..).
But, when you load the data into the FMC performance-computer, make sure you don't over-estimate the strength of that headwind!
Actually, using OAT or VREF as the assumed temperature is pointless - you might as well select Rated Thrust in the conventional manner.
Quote from mutt:
Chris, what VREF are you talking about?
Good to know someone is paying attention. It was a typo - I meant TREF. In case I've confused anyone, I had better explain. If my memory serves, TREF is the highest temperature for the maximum thrust rating (the right end of the flat part of the line). On the day, OAT can be higher, or lower.
[Hope no one minds, but I'm going to EDIT my post to remove that error.]
Quote from mutt:
Yep they are, in fact the pencil is so sharp that Boeing will offer you 4-8000 kgs additional takeoff weights in some hot/high airports, same aircraft, same AFM, just using a sharper computer powered pencil. [Unquote]
Sorry for any confusion, but I was not suggesting that the old manual method enabled us to get a HIGHER RTOW than a computer might calculate. Quite the reverse.
That's why I wrote: "We always used to use the "thickness of the pencil" on the safe side. I don't think computers are capable of doing that!"
That was in response to FE Hoppy's anecdote about the Nimrod crew, who got 4 different answers to the manual calculation, and sensibly decided to use the most conservative (least optimistic) figures.
When a human is running a "pencil" horizontally along the graticule of the graph, it's very easy to slip the line - up or down. It's usually safer to err on the 'down' side...
I say again, a computer would never do that. So a computer may remove that EXTRA margin of safety that you or I might instinctively apply, but which we would not admit to our company commercial department! The regulations, of course, already build in fat in most cases (e.g., only 50% of a headwind to be taken into account, etcetera..).
But, when you load the data into the FMC performance-computer, make sure you don't over-estimate the strength of that headwind!
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Hi folks,
If we're talking about margins, is it fair to say that a fundemental difference between a "derate" and an "assumed temp" reduction is that with a derate you have no margin, i.e. what you select is what you've got - 20k say. Whereas with an assumed temp reduction, the calculation is inherently conservative, so that if you reduce by say, 20%, the reality is you have a little more in hand.
Could someone remind me why this is so? (in English!!)
Cheers.
Fred.
If we're talking about margins, is it fair to say that a fundemental difference between a "derate" and an "assumed temp" reduction is that with a derate you have no margin, i.e. what you select is what you've got - 20k say. Whereas with an assumed temp reduction, the calculation is inherently conservative, so that if you reduce by say, 20%, the reality is you have a little more in hand.
Could someone remind me why this is so? (in English!!)
Cheers.
Fred.
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De-Rate
When de-rated thrust is used on "non-electronic" types such as the B747 Classics, it simply means a thrust setting of less than max thrust available is used. It is not "set in concrete" and if required, for any reason, the thrust levers can be advanced to give max thrust available. Pretty bloody simple really, and purely manual.
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G'day Old Fella,
If that was a reply to my question, thanks but that wasn't quite what I was getting at.......
I'll try to phrase the question better:
If you had two identical a/c, and on one you selected a derate of say, 20%. On the other you selected an assumed temp reduction that equated to a 20% reduction. I think that on the second a/c, (the one with the ass temp) you actually have more power than you have on the first a/c (the one with the derate) due to the way it's calculated - for example, it's not really 60 degrees OAT outside.
This is leaving aside the issues with advancing the T/L in either case should you require full power. (and whether that's a good idea with a derate)
Can you help with an answer?
Regards,
Fred.
If that was a reply to my question, thanks but that wasn't quite what I was getting at.......
I'll try to phrase the question better:
If you had two identical a/c, and on one you selected a derate of say, 20%. On the other you selected an assumed temp reduction that equated to a 20% reduction. I think that on the second a/c, (the one with the ass temp) you actually have more power than you have on the first a/c (the one with the derate) due to the way it's calculated - for example, it's not really 60 degrees OAT outside.
This is leaving aside the issues with advancing the T/L in either case should you require full power. (and whether that's a good idea with a derate)
Can you help with an answer?
Regards,
Fred.