De-rated power settings question

[Andy Rylance]

I can just not get my head around de-rated take offs.

I am under the wrong impression here... please help me out...

If you are taking off from a 1c in temperature airport, then you need less power to take off as the air is denser.

If you are taking off from a 38c in temperature airport, then you need more power to take off as the air is less dense.

Therefore by my calculation the rotation of the engine will need to be a lot more to push more air into the compressor to get the necessary thrust in a hotter environment.

So I must be mis-reading things when a de-rated take off is one where the engine power is reduced in hotter climates?!

Help - I am wondering lonely as a confused cloud...

[ppppilot]

The engine power rate is related always to the ISA temperature. Temperatures below ISA will give you max power. From ISA to ISA+40º engine performances are gradually degraded because of EGT limits, that is to say "You can not use full power" then you use reduced or derated power always function of the EGT MAX.
Considere the same air mass. As its temperature increases you may add less temperature (=fuel burned) to reach max EGT.
Aerodynamics works the same with ISA. So, when you TO below ISA to ISA you can always use full TO power and have more lift, resulting on LESS RWY needed for a given TO weight. If you adjust your performances to the full lenght of the RWY and the obstacles at the TO path then you will use less power, less EGT and more rwy.

[mutt]

de-rated take off is one where the engine power is reduced in hotter climates?
Not quite right, we operate in a summer environment where temperatures of 40-50C exist. We dont reduce the engine power because of these temperatures, in fact we generally seek the highest installed engine power available.

However, when the takeoff weight is not limited, we have the ability to reduce takeoff thrust, we do this by fooling the engine into thinking that the outside air temperature its hotter than it actually is, ie, its 45c and we tell the engine that its 50c, the power delivered is therefore less than it would be at 45c... hence reduced.

Make sense?

Mutt

[mutt]

Temperatures below ISA will give you max powerWhat engine type are you talking about? I'm used to getting max rated thrust upto ISA+15.

From ISA to ISA+40º engine performances are gradually degraded because of EGT limits, that is to say "You can not use full power" then you use reduced or derated power always function of the EGT MAX.
Not really correct, if the OAT is 10/20/30/40C, that is still considered FULL POWER, its only reduced if there is a difference between OAT and the temperature used for the takeoff/thrust calculation.

Mutt

[Andy Rylance]

Ok I am getting some of this....

one of the limiting factors is the Exhaust Gas Temperature, so putting very hot air into an engine you cannot add as much heat to the air compared to if it was cold - ok got that seems to make sense.

Now this "fooling the engine into thinking..." part... well why does an engine have to be fooled? Surely modern Flight Management Computers you can say "I am at this weight today, which is lighter than usual, the outside temperature is this, so work out how much power/thrust I need to get off the ground..." Why do you have to start putting in false figures to fudge the result so you get what you want?

[FE Hoppy]

Hi Andy. To get the idea of de-rate lets forget about FMC and clever software.

First lets think about a modern Gas Turbine.
It has a normal TO thrust.
This thrust lets say is 20000lb.

So the engine is designed such that when the pilot or autothrottle system puts the thrust levers to the TO position the engine accelerates to a speed that deliverers 20000lb.

However if the air temperature is too high(density too low) the engine will reach its maximum operating temperature (TGT, ITT, EGT) before it reaches the speed that equates to 20000lb thrust. The engine is now temperature limited and the maximum thrust it can produce reduces as the air temperature increases(density reduces).

lets say that at all temperatures below and up to ISA (+15 at sea level) our engine can produce 20000lbs but as the temperature becomes higher than ISA the thrust reduces, Say 19000lb at isa +5 (+20) 18000lb at ISA +10 (25).


Today the temperature is below ISA so I will get 20000lb thrust when I set Take Off thrust.
My runway is 10000ft long.

So I calculate that today I can take off with a weight of 35000kg and achieve all the take off requirements.

I find that my actual take off weight today is in fact 30000kg.

I have more thrust than I need to achieve all the requirements. So I can choose to use all 20000lb of thrust and have very very good performance or I can use less than normal TO thrust and just meet all the requirements.

I choose to reduce the thrust to save engine life (money)

How much thrust do I need?

I calculate that at my weight I only need 19000lbs of thrust.

How do I set 19000lbs of thrust? If I put the thrust levers to TO because the temperature is low I will get 20000lb. I could put the thrust levers somewhere below the TO position but the problem with this is it's tricky to set them exactly and it will take time to adjust them to the correct setting.

But I know that if the temperature was isa +5 (20) if the thrust levers are at TO I would get 19000lbs.

So I cheat and tell the engines its ISA +5. So I can put the thrust levers to the TO position and I will get the 19000lb thrust I need.

This way I always use the TO position for take off. And I can use this method to always set just the amount of thrust I need to take off no matter what my weight is.

Notes.

1) The engine has to do a bit of clever calculation because even though I told it the temp was 20, the air entering the engine is 15.

2) There are maximum limits to how much I can reduce thrust so some time I will have a lot more thrust than I need.

[Spanner Turner]

I'll have what he's having! FE Hoppy is bang on the mark.



FE Hoppy,

Without a doubt that is one of the easiest and clearest answers to a question that i have seen on the prune yet. I knew what needed to be said(even had a crack at typing a draft) but sometimes getting what's in your head and onto a computer screen for a wordwide audience to read can be a little difficult. Well done. Clear, concise and to the point without going too deep and getting too technical.

:ok: :ok: :ok:

I'll just add that the method of de-rating the engines desribed above is called the "assumed temperature" method of de-rate.

Some aircraft/engine combinations have settings for specific de-rate levels. 767 for example. You'll find settings for "de-rate 1" and "de-rate 2" which limit thrust levels by a set percentage. This de-rate can also be used for climb thrust de-rate levels in addition to take-off.

[oz in dxb]

We haven't discussed the differences between de-rated thrust and assumed temperature method.

De-rate is a fixed thrust setting. Eg. T/O, rating 1 and rating 2 thrust levels.
That means if you use a de-rated thrust such as rating 1 or 2, it is almost like strapping on a smaller engine. You then have associated limits for that thrust rating e.g V speeds and maximum thrust.
Some airlines use de-rated T/O, but almost all use de-rate for the climb.

Assumed temperature method (ATM) is a thrust reduction based on not needing a full thrust T/O. Again you trick the engine into thinking that the outside air temperature is hotter than it really is. Therefore you have a lower thrust.

ATM can also be used with de-rates.

People sometimes use de-rate when they acutally mean ATM. Big difference!

[Spanner Turner]

oz in dxb,

i was editing my post to touch on the difference between the two methods - you beat me to it.

:ok:

[ppppilot]

Ok folks. First of all sorry for my poor English. I would be clearer using Spanish, French or Italian.
All of you have explained it really good and easy.
Mutt, you should read a little more about engine performances. ISA reference temperature is 15º celsius measured at Alicante (a city in Spain at sea level, crossed by the Greenwich meridian) in a special day of the year. The ideal atmosphere considers that temperature is reduced from there 2º celsius each 1000 feet elevation (Aprox).
When Rolls Royce says that an engine gives you a max of 20000 lb. is related to that point in Spain at SL, that special day of the year with 15 degrees Celsius. You can use full power every time and every place. Even at 40º or 80º Celsius. but RR does not guarantee 20000lb. Let say 15000lb. FE Hoppy has explained it easy and clear, better than me. So you are using full power but you are not using full thrust. There it is the difference. Full power but less thrust (reduced due to EGT limits)
The second point I would like to explain is that my friend Andy Rylance is mixing some concepts and I was trying to explain him the differences. He said:
If you are taking off from a 1c in temperature airport, then you need less power to take off as the air is denser.
Less power, only if you considers same atmosphere, elevation, rwy length used, TO weight etc. Less power than if the temperature with the same conditions is 20ºC and you use the same length of rwy.
From another point of view. Same airport, same conditions (TO weight, airplane etc.) and 1ºC (lets suppose ISA) In real life you may use full power (TO pwr) obtaining full thrust. If with the same conditions there is 20º C you will use also full TO pwr but you will not obtain full thrust, even you have seen the same EGT, and you will lift off at a point well beyond the one at 1ºC.
The use of derated power and how it works has been very well explained by my colleges.
Nice flights to everybody.

[barit1]

ATM is also known as flex thrust, because assumed temperature can be adjusted to fit a variety of conditions, runway & TOGW permitting.

Derate is a fixed decrement (-4%, -10%, etc) and separate AFM performance charts are provided giving certified aircraft performance for each derate.

Derate may be used whenever TOGW will allow.

Flex or ATM may NOT be used with contaminated runway.

Here is a prior thread (http://www.pprune.org/forums/showthread.php?t=283152) with more detail.

[ppppilot]

Barit1 you are right. I have been flying B757 and it has 3 options for the TO. TO pwr, Derate, and flexible. TO power states for full power. There are two kinds of derated power TO1 and TO2, reducing TO power by a 12% and a 20% respectively. Flexible is used as usual, regarding on the current rwy performance analysis, introducing the max assumed temperature possible in the FMS.
You also have 3 options for the climb. Max climb power, derated climb1 or climb2 that reduces the climb power by a 6 and 12% respectively.
The A340 has only full TO pwr or flex, but for the climb the ultimate fms has derated1 or d2.
I have used other TO options as CLB power under certain conditions on the CRJ200 and water&glycol injection on the Fairchild metroliner.

[groundfloor]

For the 340 you have; de-rate, flex and TOGA (Take-off Go Around) thrust.

De rate is used in very very cold conditions to produce LESS thrust so in case of an engine failure on the ground you can control the swing...so less thrust (slower acceleration) allows for a heavier take off weight. Ask the Finns and the Swedes. Pop VMCG into a search..NB your operator MUST have authority for this.

Flex thrust is thrust corrected for temp. Ie the maximum take off weight at 46 deg is 350 000KG. If we weigh 350 tons on a 25 deg c day we tell the engines to produce power as per a 46 deg c day..accelerate slower but quiet safe.

TOGA if runway is contaminated or required for performance... TOGA take -offs in lightweight big jets are reasonably exciting!:}

[mutt]

Mutt, you should read a little more about engine performances:{:{:{:{

Have a look at the thrust chart for your B757, tell me at what temperature does the thrust start to decrease? ISA or ISA+15C?

What do you understand by the Flat Rating of an engine?

If you are used to A340's, what is Tmin?

Mutt

[411A]

Mutt, you should read a little more about engine performances. ISA reference temperature is 15º celsius measured at Alicante (a city in Spain at sea level, crossed by the Greenwich meridian) in a special day of the year.


Alicante, Spain
Latitude: 38°16' 55" N
Longitude: 0°33' 29" W
Elevation: 142


Hmmm, before lecturing an aircraft performance specialist about engines, perhaps ppppilot might perfer to get the facts straight....:rolleyes:

[barit1]

Most (but not all) modern engines are flat-rated up to approx. ISA+15 - above that temp, thrust is dialed down to protect EGT.

And ISA is a purely arbitrary definition (http://en.wikipedia.org/wiki/International_Standard_Atmosphere) - I don't see why longitude has anything to do with it. It dates back to an early definition of worldwide average SL air temperature (15C or 59F).

BREAKING NEWS!!! think the latest worldwide average temp promulgated is 14.9 C or thereabouts -

[757_Driver]

Mutt, you should read a little more about engine performances
:{:{:{:{

Have a look at the thrust chart for your B757, tell me at what temperature does the thrust start to decrease? ISA or ISA+15C?


indeedy, Mutt is on the money, my 757 charts show the engines (rb211s) flat rated until isa (or isa +3 - can't remember - haven't got any performance charts to hand - but certainly not ISA +15)

TOGA if runway is contaminated or required for performance... TOGA take -offs in lightweight big jets are reasonably exciting!:}

oooh yeah baby - hold onto the tail and enjoy the wild ride.

Windshear reported is a case were we use TOGA - took a 757 with about 10 pax and 10 tons of fuel the other day (70 tons with 39 tons of thrust), - the VSI only goes up to 6000fpm and it was pegged - for quite some time. :}:}:}

[john_tullamarine]

ISA reference temperature is 15º celsius measured at Alicante

a chap learns something new every day ...:}


It (ISA) dates back to an early definition of worldwide average SL air temperature (15C or 59F)

I suspect (from aging memories of reading on the history of the matter) that the stats upon which the ISA is postulated are based on North American profiles dating back quite a few years, now.

It is important to accept that it doesn't make any difference what standard atmosphere you use (and there are a number in common use) so long as everyone talking the same subject are using the same standard atmosphere. In general it makes good sense to base discussions on a standard atmosphere which bears some resemblance to the typical atmosphere of interest but, at the end of the day, it is just a convenient means to the end of reducing performance from a given set of ambient conditions to some (arbitrary) standard conditions for the purpose of discussion and comparison ...

[Andy Rylance]

Thank you, thank you, thank you, gracias, merci etc. to all... it has helped a great deal. I was just getting confused but the clear explanations have really helped work my head around this one.

:D

[chornedsnorkack]

Besides, if you try to put a huge fan and compressor (rigid hub, no variable pitch) rotating at the N1 and N2 and N3 limit speeds in, say, ISA-65 Celsius conditions at Yakutsk or Yellowknife at -50 Celsius and sea level, the dense cold air propelled by the fan might, apart from pushing the plane off the runway in one-engine-out case, break something else. Like generate excess pressure in combustion chamber, or break pylon, wing etc. The engine, pylon and wing strength are fitted to the rated thrust - so if the air is denser than ISA+15, the fan and compressor must be slowed down to keep the thrust within what the engine can endure.

Do turbopropellers also have derate to prevent generation of excessive forces in cold and dense air?

[ppppilot]

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 (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:E) and I have not seen any derateNice flights to everybody

[groundfloor]

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:)

[ppppilot]

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

[Andy Rylance]

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/areatec/airbus_sfo/24derated.htm

http://www.pilotosdeiberia.com/images/tecn/airbus_sfo/24derated/mcdu1.gif

[mutt]

Which horse are you talking about? It always makes sense to state aircraft type with specific explanations like that.

[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...
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.

[411A]

It means that max thrust will be reduced above 30 celsius, but the important information is the value of the Flat rated max thrust...


Ah, not especially.
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.

[barit1]

sispanys ria sez: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.

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.)

[ppppilot]

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.

[Chris Scott]

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..." :}

[sispanys ria]

to PPPPilot

The Piaggio's engines doesn't haveanything original. They are PT-6 A 66, and have TBO of 3000 or 3500 hrs... This engine is also used for other aircrafts.

[mutt]

handbook of Perf A graphs Fair play to you Chris, I never had to use those charts for real, so i have respect for those that did. I wouldnt take you up on that wager :):), I hate losing :)

The assumed temperature must not be lower than the flat rating temperature Why not?

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?


Mutt

[FE Hoppy]

mutt:
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 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!!

[Chris Scott]

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.** R, it must be reduced to VR.[/SIZE]] And don't forget VMCG...

Quote from mutt:
[I]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).] :uhoh:

[john_tullamarine]

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.

[mutt]

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?

the 25% reduction is applicable to the Rated Thrust or De-Rated ThrustThats 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?

I don't think computers are capable of doing thatYep 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

[Chris Scott]

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!

[Fredairstair]

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.

[Old Fella]

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.

[Fredairstair]

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.

[Intruder]

If the calculation via the Assumed Temp method gives you a target N1 of 98.3% and the fixed derate gives you a target N1 of 98.3%, they give EXACTLY the same thrust and EXACTLY the same margin!

However, assumptions and procedures differ, so your margins will vary. For example:

Your company procedure is to use standard derates. You go into the book and find that at Derate 1 for your conditions, N1 is 98.3% and the max TOGW allowed is 300.0. Derate 2 has an N1 of 96.8% and a max TOGW of 275.0.

If your TOGW is 275.0, you can use Derate 2 with the minimum legal margin, or Derate 1 with more margin. If your TOGW is 275.1, you must use Derate 1, and your margins will be significantly higher than if your TOGW was 300.0.

Now say your company procedure is to use a manual or computer-assisted Assumed Temp Reduced thrust. If the margins used by the company are the same as those used in the max TOGW calculation of the derate tables, and your TOGW is 275.0, it will give you an N1 of 96.8%. If your TOGW is 275.1, it might give you 96.9%.

If the company uses other margins, all bets are off.

[Old Fella]

Hi Fredairstair. I am not able to compare the "De-Rate" method I used to use against the "Assumed Temperature" method as I have never used the latter. Reading other posts I think I would rather use the old fashioned "De-Rate" method I used, in as much that having determined the thrust required, for the conditions and runway length available, to safely take-off at a given BRW it was simply a setting of the "De-rated" thrust determined with the thrust levers. If for any reason whatsoever extra thrust was felt required, max thrust (up to flat rated thrust up to 29C) could be applied by simply advancing the thrust levers. No presetting of FADEC or engine components came into it. We had a maximum of three levels of "De-Rate" and it could be used at all our ports with the exception of 07/25 at Sydney and at Gatwick. We also had a "Bump Thrust" available, if required, to get us out at weights above those to which we would be limited with normal Max thrust.

I am sure there are some variations between respective methods, however they are all aimed at achieving as long between engine overhauls as is possible.

[AtoBsafely]

On the B744 that I fly we have:

TO, TO1 and TO2 available with TO1 being a 10% derate and TO2 a 20% derate.

Additionally assumed temperature can be used to give a flex thrust up to 25% less than the rating selected.

So, for example, we could achieve an 80% thrust takeoff using any of the ratings. But the V1 and Vmcg that we use would be different for each rating, because if we manually push the thrust levers fully forward we will get the thrust rating that we set.

We cannot use a flex thrust on a contaminated runway.

The company computer automatically calculates the "optimum" rating and assumed temp, unless we override it by demanding a particular TO rating.

[Denti]

That seems to be pretty much the same as in the 737s. We have three fixed thrust settings, all with their own VMCg/a limitation and own performance data (for the 700/300 its 22k, 20k as derate 1, 18k as derate 2) and on top we can use assumed temperature to furter reduce the available thrust.

The catch is that with a fixed derate you have to be careful about your minimum control speeds, simply advancing the thrustlevers to full certified thrust can lead to very unpleasant surprises if you are below the relevant minimum control speed.

A notebook tool calculates what thrust setting, assumed temperature are used and if we used improved climb performance, however we can of course override some settings if we deem that necessary.

Still, it is weird to use 3500m of a 4000m runway until you rotate and our ATCO friends certainly don't like that either.

[Intruder]

The other "catch" is that tables for the fixed derates are based on the assumption that you SHALL NOT push the throttles any further if you lose an engine at V1! That is a REAL significant difference between fixed derates and assumed temp reduced thrust!