From the PMDG 737NG AOM, TAKEOFF Sec 1 Page 13:

Frank,There are two ways to perform a reduced thrust takeoff in the NG. You can either select a derated thrust (derate 1 or derate 2 on the N1 LIMIT page of the FMC CDU), or enter an assumed temperature into the 1L SEL/OAT position on the same page. By entering an assumed temperature that is higher than the actual outside air temperature and higher than the temperature at which the engines produce a fairly constant thrust level, you will be reducing the engines' thrust output.PMDG does not provide any performance data for which you can determine what derate level will allow for a safe takeoff. However, if you work through a takeoff performance analysis using the PMDG data and you find that your takeoff weight is well below the maximum allowable takeoff weight, that gives you an indication that it is safe to derate.To choose an assumed temperature thrust reduction takeoff, use the PMDG supplied takeoff data to find the maximum temperature at which you can safely take off. You can insert temperatures up to this temperature as your assumed (also called selected) temperature.Unfortunately, the use of the PMDG data is somewhat limited in that it does not identify what airport pressure altiude and takeoff flap setting it is good for.As a simple example, let's say you have a 737-600 loaded to 123,000 lb and you want to take off on a 7000 foot runway under no wind conditions, and the OAT is 18C. From the runway length limit weight table for the 737-600, the runway limit weight is 159,800 lbs and the climb weight limit is 150,100 lb. From PMDG's Limitations manual, the 737-600's maximum takeoff weight is 127,000 lbs. Since your actual weight is well below the runway limit and climb limit weights, and is less than the limitation weight, this is a candidate for a reduced thrust takeoff. To find the assumed temperature, note that for an OAT of 50 C, the climb limit weight coincides with your actual weight. So, you can use 50 C as your assumed temperature.Hope this helps,Don S.

For both DF's 737 and the 767 PIC, I always use a LOWER tempature when inserting data into the FMC to get a reduced thrust take-off. I often put 00 C into the take-off page line.

Yup, flex to temp is the way to go... If I've understood it correctly, the derate thrust option is a big no no! This actually lowers the max-effect of the engine, and if you (by mistake) use derate thrust together with flex thrust, you will be very very underpowered...rgrds fredrik

A LOWER temperature will not give you a reduced thrust takeoff. The temperature you enter is used by the engine's power management system as the OAT. Engines are flat rated (i.e., produce about the same amount of thrust) up to a given temperature. This is usually about 84 degrees F for the engines on most Boeing airplanes. At higher temperatures, thrust is reduced. This means that if you enter a temperature lower than 84 degrees F, or lower than actual, you will not get reduced thrust.Don S.

The derate option is simpler. In the real world, a separate set of performance tables are provided (although not in our world by PMDG), so it is easy to determine whether and what level of derate can be used. Also, again depending on the takeoff situation, a combination of derate and assumed temperature reduced thrust is okay to use.In the real world, the maximum thrust reduction you are allowed to get from the assumed temperature reduced thrust method is 25%. If you can actually take off with less thrust, you would use a combination of derate and assumed temperature reduced thrust.Don S.

Don,I've been away from flightsim over the weekend, hence the late reply. Many thanks for your comprehensive answer. Now I understand better the factors that would allow a reduced takeoff.:-) BR,Frank

Frank, again, Don.Can you explain the logic behind the way in which engines are rated, please? Does it mean that an aircraft needs less thrust to take off in warmer weather?Thanks for a very informative thread!Cheers,Frank

Don? Perhaps in real life but when I insert a lower temp into both DF's and PIC's FMC, I get a lower N1% for take-off. I do this every single time otherwise I'm pulling 25 degrees and 5000+ fpm to hold V2+15 with the N1 it gives me. I consider that a little un-realistic.Either way, I always assumed a higher temp means less performance hence more power needed by the engines for take-off. I guess not eh.Still, inserting a lower temp works for me! Perhaps PMDG's FMC is different.Jonno.

"Either way, I always assumed a higher temp means less performance hence more power needed by the engines for take-off."Hi Jonno,Yes, I thought that was the case, also. Hence my question about engine rating to Don.BR,Frank

Reduced Thrust take off:In order to reduce the thrust you must input a higher temp than the OAT and as someonw has already mentioned higher than the flat rating temp which is 26 Celcius. The physics in laymans terms is that the higher the temperature outside the higher the EGT will be. So the N1 computer says to itself that in order to prevent over temping I will reduce the amount of power that you get at take off. This is done by reducing the TOGA N1 value. So for example yesterday I did a full power take off out of Leeds where the temperature was around 8 Celcius, we could derate but only just as Leeds is fairly short but we opted to go full thrust due to the high winds. The Takeoff N1 was computed at 93% The trip was to Malaga where the temp was +18 Celcius. Inputting the OAT into the FMC gave a TOGA N1 of 91%. The runway at Malaga is fairly long and with no obstacles we were able to derate as well and enetered an assumed temperature of 40 Celcius. Effectively we a telling the computer it is warmer outside than it actually is. The N1 computer is fooled into thinking that the air outside is warmer and so computes an N1 which protects the engine from over temping at take off power. In this case we ended up with a TOGA N1 of 89.4%. So as you can see from the above example we use the N1 computer to work out the max N1 for a given OAT, we then look in the performance manual and see whether we can reduce the thrust and if so by how much. This fools the computer into thinking its hotter than it is giving us a lower N1 for take off and therefore reducing both the N2 and the EGT's. This in turn pays dividends by not stressing the engine as much and effectively extending the life of the faithful donkey under the wing.One poster suggests he puts in 0 degrees in the FMC and then goes off into the wild blue yonder well if that works it is a fault in the code. The engine is flat rated as stated in another post so below 26Celcius it will put out its max rated power and thats that. The danger is that by putting in a lower temp than it actually is you will fool the N1 computer into thinking that it is colder outside and thus it will drive the TOGA N1 higher than it needs to be and thus you risk overtemping the engines! The 300 series aircraft I fly have only manual OAT input for the FMC so if I were to input an OAT lower than normal then we really would be risking melting the engine and thus ending my flying career rather abruptly! The actual procedure for thrust reduction is fairly simple assuming you have the correct performance data. We use airfield specific data which gives us allowable weights for a given runway at a given temperature, wind speed and pressure. We look at the actual temperature and determine our maximum weight for take off in the current conditions and put that figure in the load sheet as the 'regulated take off weight' or RTOW!Once the loading is complete and we know what our actual take off weight is we look at the performance data and find the actual weight for the current wind. Then we look and see what temperature this weight allows us to derate to for example with a calm wind at Leeds runway 32 with an actual take off weight of 51,000kg we would enter the table in the calm wind column and go up the temperatures until we find the figure nearest to 51,000kg. Lets say this says 51,332kg is allowed at 40 Celcius, we now know that we can derate to 40 Celcius and so enter 40 in the SEL TEMP box in the PERF INIT page. Hey presto we have derated!Hope that helpsKris

If I put a higher temp into the FMC for both DF's 737 and 767 PIC, I get a higher N1%. Which is why I always insert a lower temp then the current OAT.>>>>One poster suggests he puts in 0 degrees in the FMC and then goes off into the wild blue yonder <<<<

Thanks for a really informative post, Kris!BR,Frank

"if the above statement by Kris is correct"!!!Surely, you've got to be joking???PIC is a wonderful program, and I believe DF is excellent, but when it comes to weighing the knowledge and experience of a RL 737 pilot against the characteristics of sub 50 dollar pieces of software, I know where my money is going!BR,Frank

Kris thank you for that informative post. Am I correct in assuming that it would be very difficult to enter a correct value into the PMDG without having the charts for the specific airport and runway? I would be better off using the derated values I can select underneath the SEL/OAT field?