Assumed temperature thrust reduction B737NG

[PitotTube]

Hello,

Can someone please expain to me about the assumed temperature thrust reduction on the 737NG. Does the required takeoff thrust decrease with increased temperature? I doesn't make sense to me.

thanks,
pt

[alatriste]

HI pt, maybe I didnot catch your question but the reason for using assumed temperature is to decrease thrust in order to save engine life and manteinace cost
ATM (assumded temperature) is always higher than OAT and therefor if we are cheating the engine is because we want to use less power. The higher the ATM the lower the to thrust.

I do apologize if you already know this, it is possible that I misunderstood you. Regards.

[Maximum]

As Alatriste says, you are basically finding the highest allowable OAT at which you could take off given your actual take off weight. This is the assumed temperature.

This temp, when input to the FMC, will reduce your take-off thrust, (ie, the N1 value for take-off will be lower), preserving the engines. The higher the assumed temp, the greater the thrust reduction.

Obviously full thrust is still available if needed. Hope that helps.

[mono]

Hmmm,

Reply quality not too good so far ;) (not that mine will be any better)

Most (if not all) jet engines are flat rated. This means that the makers have guaranteed that they will produce at least a certain minimum thrust up to a certain temperature at full throttle positions.

Above this temperature (which is around 22 degs C give or take a few degrees) the rated thrust will reduce (less dense air less available thrust).

When using an assumed temperature for T/O, a temperature above the flat rated temperature is selected which is input into the FMC/TMC (the temperature selected is based on a/c weight, available runway, actual OAT, etc) This fools the computers into thinking the engine cannot produce max rated thrust due to high OAT and instead uses the rated thrust for the temperature selected. This is then used for performance calculations and autothrottle thrust settings. When the throttles are advanced and A/T selected (push TOGA) the levers will advance to the level where the reduced thrust setting is established and be held there until another thrust mode is selected.

The reason for this is to reduce engine wear and so increase engine life. In the event that there is an engine failure or obstacle clearance is required the throttle levers can be manually advanced (firewalled) to acheive full rated thrust.

Hope this helps.

[PitotTube]

Thank you very much for your replies!

I am still a little bit confused. I am stuck in my mind by thinking that if an assumed temperature like OAT 45 C is set in the FMC, it would have to increase the performance of the engine?? Since it would be a hot day with degraded performance the engines have to work harder, no?

For me I would have better understanding if I would set -20C because that would mean that the engines wouldn't work as hard to get the plane airborne, ?? I am confused.

Please help me get a better understanding of this.

thanks,
pt

[SuperRanger]

pitot,

i believe your 737NG's engines are flat-rated at ISA+15°C. this means the engine will produce it's max power at ISA+15°C. At any higher or lower temp, the eng will produce less thrust (thats why the lowest assumed temp is ISA+15°C).

say you are taking off at OAT 30°C. the thrust produced by the eng will be higher than if say the OAT is 45°C, right? by entering an assumed temp of 45° in the FMC, you are merely 'fooling' the FMC to think that the OAT is 45° instead of 30° thereby producing less thrust.

if you enter an assumed temp of less than ISA+15° (30°C), the thrust will remain at the current OAT setting. therefore, you will not get any thrust reduction as the FMC is not program as such. (try it in the acft!)

SR

[mono]

Sigh :hmm: ,

Pitot, It is not possible to increase the performance of the engine above flat rated temperatures, because there is no more performance available.

At full power settings the engine is guaranteed to produce a certain minimum thrust UP TO IN THE CASE OF THE CFM56 ON THE NG ISA + 15. above this temperature the thrust will DECREASE because of various engine parameter limitations. The engine may be EGT, N1 or N2 limited depending on the conditions of the day. The FMC 'knows' this so when a high assumed temperature is used it reduces thrust to the appropriate value to compensate. Thus helping ensure the engine does not over boost or over temp.

[alatriste]

HI AGAIN PITOT TUBE.

Cause of your last post it seems you understand the reason for using derated takeoffs. An example will help:

Just figure out you are going to take off at your airfield and OAT is 15ºC, EPR max for this temperature is 1.96. Your maximum operational weight (OAT 15ºC) is 162.000 lbs so you are allowed to take off that heavy. But today your airplane is almost empty and just weighs 114.000 lbs.
In this scenario you can advance throtles full forward to EPR max 1.96 and you will become airbone at the first third of the runway, no problem BUT it does not make any sense to request maximun stress to your engines, if you are leaving 2/3 of the runway unused.
Now we can think of cheating the engine, instead of use the OAT (15ºC= 1.96 EPR) we are going to use NOT a real temperature(OAT) but a fake one (assumed).
IF the runway is long (e.g. 12.000 ft) we could take off with 114.000 lbs if the OAT were 50ºC, and therefore 50ºC could be a good assumed temperature (fake), the engine output will be NOT 1.96 EPR but a lower one 1.82 EPR.
Using this criteria we are saving engine life (not stressing to max) while not compromising safety .
REGARDS

[Fresca]

It looks to the that pitottube has a bit of a misunderstanding, if not then my answer is not needed.

Jet engines are flat rated to a certain teperature, in the case of CFM-56 it is ISA+15°C or 30°C on a standard day.

What this means is that the engine is capable of giving out full thrust up to that temperature, but if it gets hotter than that, it will give out less thrust (due to less denisty and less energy we can put into the air before reaching the ITT (inlet turbine temperature) maximum limit).

Therefore, if we don't require the maximum engine thrust for T/O, then we tell the engine that it is hotter than it actually is, the engine knows that at this high (assumed) temperature it can not give us the full engine thrust, reduces the thrust accordingly.
Less thrust=lower RPM=lower temperature in the core=less strain on the engine=more engine life.

One last note though. If one whishes to use use assumed temp reduction, one must always use temperature that is higher than the flat rated temp on the engines, otherwise it will still give us full thrust.

Regards Fresca

[PitotTube]

Thank you very much everybody for your explanations. It all makes good sense to me now!

Have a great weekend,
pt