# I still don't understand the Thrust Limit assumed temp

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I've read the documentation so many times regarding this but I still don't understand it.

On the THRUST LIM page in the FMC, you need to enter in an assumed temperature for the thrust rating of the engines for takeoff and the initial climb.  In the PMDG 777 Tutorial 1.0 it shows entering a temperature of 65 degrees celsius.  I don't understand why this would be and where he got this number from.

Let's pretend I'm taking off from some place rather cold like Boston, Massachusetts where the temperature right now is roughly between -23 to -10 degrees celsius on a consistent basis every day in the winter.

This is where I start getting confused.  According to the tutorial it states:  "The engines are designed to produce their rated thrust at an actual outside air temperature of 30 degrees celsius (ISA + 15C).  If the temperature is higher than this, the air becomes less dense and the engine produces less thrust at the same N1 setting."

So if the outside air temperature (OAT) in Boston at Logan (KBOS) is let's say -10 degrees celsius, then what number would I put in the  "SEL" (LSK L1) button of the THRUST LIM page?

How do you calculate this?  I'm so lost and I'm probably overthinking this but it's really bugging me.  I want to get this right.

Any help would be much appreciated.

-Andrew Coste

Andrew Coste

James Bennett

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To deal first with how Assumed Temperature works;

A jet engine works by taking a bunch of air molecules in at the front, accelerating them and throwing them out the back. Denser air contains more molecules for a given volume; Newton's laws dictate that the more molecules (and thus greater mass) of air you throw out the back, the more thrust you get. Hot air, naturally is less dense than cold air. So it follows that if you rotate the N1 fan at a given speed (let's say 100%), the amount of thrust created will decrease if you increase the temperature (and thus decrease the density) of the air going in.

Entering an assumed temperature that is higher than the real temperature fools the engine in to only producing the rated thrust for the given temperature. As noted, most engines are 'flat rated' -- i.e. if the engine is rated for 90,000lbs of thrust this is what you will get -- up to ISA +15. So entering a temperature above ISA +15 will result in the engines commanding a lower N1 corresponding to the lower thrust you would get at that higher temperature.

As for what temperature to enter -- this is a calculation that needs to be done based on a number of factors including the length of the runway, obstacles, the real outside air temperature, QNH, wind, takeoff weight and configuration (flaps, packs on/off etc). This can either be done manually with the aid of performance tables and a Vital Data Card, or a piece of software such as TOPCAT can do the calculations for you.

Simon Kelsey

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Hi,

The idea behind the assumed temperature thrust is that the thrust is calculated based by the FMC based on the temperature entered or measured. The higher the temperature, the lower the thrust.

If you don't enter any assumed temperature, the FMC will calculate the thrust based on the actual outside air temperature measures by the probes.

To calculate the assumed temperature for an airport, you need a document called FPPM (Flight planning and Performance Manual) which is unfortunately not provided by PMDG.

In that manual, you have a graph in which you can calculate the maximum t/o weight depending on the OAT, the pressure, the wind, the runway length and the slope.

Therefore starting with your actual weight and the other parameters with which highest OAT you still can takeoff from that runway and therefore use it to calculate your takeoff thrust.

I will an exemple with completely fictionous figures to illustrate:

let's imagine a runway of 3000m with slope 0% and no wind.

10° OAT, the graph gives max t/o weight of 280T (6000ft altitude)

However, my actual weight is 240T. So for the same runway length, I could takeoff with warmer OAT. I go back to the same graph but reverse up to the OAT and find out that with 60° OAT I have the 240T as maximum weight for that runway length.

Now the trick is to make thing to the FMC that is 60° outside instead of 10° hence the assumed temperature.

Hope that exemple illustrate the principle.

Unfortunately, without that graph which is in the FPPM, I don't see how to calculate it otherwise than with one of the tools out there.

Romain Roux

Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

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Would this be the place to ask why, in the Tutorial 1.5, Kyle used both TO-1 and 43° Assumed Temperature? I thought the combination was just for wet runway or below minimum takeoff weight, neither of which apply in the tutorial.

Dugald Walker

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There is a difference between flex thrust (using an assumed temperature) and derated thrust (TO-1 reduces thrust by 10%).  Kyle used TOPCAT to calculate optimum takeoff thrust and it determined that both a flex and derated thrust were appropriate for the runway and conditions. The dual thrust limits are an option depending on operator.

The original post seems to be a question of how-to rather than why and I think that's been answered. The why is simply to reduce engine wear.  The derate method also affects v speed calcs whereas flex allows v speed calcs to assume max thrust I think.  Not sure but I do know each method has advantages and combined is even better in most cases.

Dan Downs KCRP

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So, is the combination implied by "//+43C" in the TOPCAT printout?

Dugald Walker

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TOPCAT normally states D-TO -- e.g.:

```APT  PRWY       POAT  PWIND  PQNH  PMRTW   FLP CONF  THRUST       V1  VR  V2  PTOW
EGLL 27R        +14°C 260/24 1016  252733  FLAPS 5   D-TO1 +55°C  154 154 157 249973

```

As mentioned above, a de-rate and an assumed temperature reduction are two separate things. You can think of a de-rate as being the equivalent of installing a smaller/less powerful engine on the wing, whereas an assumed temperature reduction is just not using all the thrust that would otherwise be available.

In practice, this affects the way that your speeds (particularly Vmca/Vmcg) are calculated. With an assumed temperature, but NO de-rate (i.e. using TO rather than TO1 or TO2), your speeds are calculated based on the full rated thrust of the engine. This means that if, for example, you have an engine failure at V2 you can advance the remaining engine to full thrust if desired, safe in the knowledge that you will be above Vmca and therefore will be able to maintain directional control.

On the other hand, if you use a de-rate, the speeds are calculated based on the de-rated thrust only. In practice, this means that if you are using a de-rate, and you have an engine failure at V2, advancing the remaining engine to maximum thrust may (probably will, particularly on an aircraft like the 777 with such enormously powerful engines) result in a loss of directional control as you will be below Vmca for the full rated thrust. The maximum thrust that can be safely applied is the de-rated take-off thrust (achieved with one click of the TOGA switches).

Why would one use both an assumed temperature and a de-rate? Simple: to achieve a greater thrust reduction. The law dictates that takeoff thrust may only be reduced by a maximum of 25% from the maximum rated thrust: therefore, if you de-rate the engine by 20% (as TO2 does), you can then apply a further 25% reduction upon this de-rated thrust with an assumed temperature, if so desired.

Hope that helps.

Simon Kelsey

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piece of software such as TOPCAT can do the calculations for you

Does TOPCAT support PMDG's B 777?

Elimar Bossini Piratelo

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Hope that helps.

Thank you. Every explanation helps. In the case of Tutorial 1.5, the TOPCAT printout is a bit different from yours and shows:

------------------ CONDITIONS -------------------

TOW 569731 LB FLAPS 15 THRUST TO1 RWY DRY

AIR COND ON ANTI ICE OFF/AUTO

------------------ FULL THRUST -------------------

+20C 635517 OBS(A) 141-147 147 155 3658FT 95.6

---------------- REDUCED THRUST -----------------

+39C 598286 OBS(A) 143-147 147 154 2932FT 92.7

+40C 592512 OBS(A) 143-147 148 154 2841FT 92.5

+41C 586739 OBS(A) 144-146 148 154 2747FT 92.4

+42C 580966 OBS(A) 144-146 148 154 2637FT 92.2

//+43C 575192 OBS(A) 144-145 148 154 2521FT 92.1

+44C NOT AUTH

+45C NOT AUTH

As far as I can see, the manuals mention only wet runways or below minimum takeoff weight as situations for which combined derated thrust and reduced thrust might be considered. In the tutorial, the runway is dry and I estimate the minimum takeoff weight to be roughly 416,900 lbs so I wondered what other reasons would one have for wanting the greater thrust reduction.

Dugald Walker

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One thing to bear in mind is that D-TO and D-TO1, 2 etc are customer options, whereas all 777's have assumed temperature I believe. So for example as far as I know, Emirates does NOT have the D-TO options, although they do have TO-B for the thrust bump. (Handy in Dubai)

Wes Meyer

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Thank you. Every explanation helps. In the case of Tutorial 1.5, the TOPCAT printout is a bit different from yours and shows:

------------------ CONDITIONS -------------------

TOW 569731 LB FLAPS 15 THRUST TO1 RWY DRY

AIR COND ON ANTI ICE OFF/AUTO

------------------ FULL THRUST -------------------

+20C 635517 OBS(A) 141-147 147 155 3658FT 95.6

---------------- REDUCED THRUST -----------------

+39C 598286 OBS(A) 143-147 147 154 2932FT 92.7

+40C 592512 OBS(A) 143-147 148 154 2841FT 92.5

+41C 586739 OBS(A) 144-146 148 154 2747FT 92.4

+42C 580966 OBS(A) 144-146 148 154 2637FT 92.2

//+43C 575192 OBS(A) 144-145 148 154 2521FT 92.1

+44C NOT AUTH

+45C NOT AUTH

As far as I can see, the manuals mention only wet runways or below minimum takeoff weight as situations for which combined derated thrust and reduced thrust might be considered. In the tutorial, the runway is dry and I estimate the minimum takeoff weight to be roughly 416,900 lbs so I wondered what other reasons would one have for wanting the greater thrust reduction.

Ah -- if you look at the too of the printout you'll notice it says FLAPS 15 THRUST TO1 -- indicating the derate. Below that is a FULL THRUST section which gives the numbers for full derated takeoff thrust, and then in the 'reduced thrust' section a selection of assumed temperatures is displayed -- the /// simple indicates the maximum permissible assumed temperature (and thus maximum permissible thrust reduction) and associated V speeds.

As for the combination of derate and assumed temperature -- I have to say I'm a little surprised to see that in the manuals. Most airlines that I'm aware of tend to have procedures requiring the opposite -- i.e. prohibiting the use of reduced thrust on contaminated runways. Overall, as others have alluded, it's one of those things which is very airline-specific -- each operator will have its own procedures. Whilst all will have the assumed temperature facility, not all will have de-rate options fitted and even those that do may have restrictions on its use -- BA, for instance, as far as I'm aware do not routinely use the derates on their 747 fleet even though the option is available (but do use assumed temperature reductions from full rated thrust).

As to why one would want the additional thrust reduction -- as with most things in the airline world, the answer is financial. Takeoff thrust reduction is linked to engine maintenance contracts, and especially so if the engines are leased. There is generally a reward/discount for carrying out reduced thrust takeoffs, and in some cases even penalities for not doing so.

Simon Kelsey

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In a simple sense, assumed temp is just a thrust reduction. In the old days when I flew DC10s, this is how we used assumed temp before the nice computers were involved. This may help you understand from an old school stand point. Basically I run the takeoff charts in order to calculate the max takeoff weight for the given conditions. These charts were runway limiting charts and climb gradient limiting charts. The charts were 5 pages, 1 and 2 was runway limiting and 4 and 5 were climb limiting. Page 3 was the max weight and takeoff flap chart. I run 1 to 2 for runway limit weight and 5 to 4 for climb gradient limit weight using actual outside temp. I enter the chart 3 from both sides and meet in the middle. The point at which the weights intersected gave me the max weight for conditions. If i read straight down from the max, it gave me the optimum flap setting. If i wanted to reduce thrust to save engines/FLEX, I now use the optimum flap setting and read up to my actual gross weight. At my actual weight, I read left backwards through 2 to 1 to get a temp. I read right from my actual backwards through 4 and 5 to get a temp. I then compare the temps and take the lesser of the two. I then dial this assumed temp into my assumed temp selector.  I select the T.O. flex button on my thrust rating selector and it uses this assumed temp to calculate takeoff thrust. Assumed temp is just a fancy way to lie to the engines about outside temp to produce less thrust. I hope I didn't confuse you. This whole process we used in the DC10 to reduce to a safe thrust level, the FMC does in less than a second.

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As to why one would want the additional thrust reduction -- as with most things in the airline world, the answer is financial. Takeoff thrust reduction is linked to engine maintenance contracts, and especially so if the engines are leased. There is generally a reward/discount for carrying out reduced thrust takeoffs, and in some cases even penalities for not doing so.

Well it may be also for ferry flight where such powerfull engines would send an empty T7 "to orbit"!

I did on FsX a ferry flight between Paris Orly (LFPO) to Charles de Gaulle (LFPG) for the maintenance of an AFR 777-3000ER. 20 minutes of flight with 15000kg of fuel and empty load... Even with D-TO2 and max assumed temperature reduced thrust, I still got nearly 4000fpm!

I will try with full thrust, it must be quite impressive!

Romain Roux

Avec l'avion, nous avons inventé la ligne droite.

St Exupéry, Terre des hommes.

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