Kris is correct.Don S.

It would actually be just as difficult to choose one of the fixed derated values without having the performance data for those derated thrust levels.As I explained in my first response in this thread, you can sort of use the takeoff performance data PMDG has provided to determine the assumed temperature that you can use. But, because the amount of data that PMDG has provided is very limited, it doesn't cover too many situations.Note that you don't actually need the data to be specifically tailored to each airport and runway. You just need coverage of the takeoff operating envelope (e.g, pressure altitude, temperature, wind, slope, etc.). You can then compute the takeoff performance for any specific airport and runway from these data. That's what the airlines do.Don S.

It looks like the only airport specific charts you need is for weights, correct? All we would need are some more detailed performance charts from PMDG?editet again after reading his post some more. :)

I guess I always assumed if it was hotter you needed more power. The explanation of the hotter temp needing a lower N1 makes sense for reasons of overheating. I am off to make a flight with derated N1 the proper way. Thanks for straightening me out.

MikkelBoeing provide in the QRH all manner of charts to help you out with egards to performance and in answer to the questions raised by my post NO you don't need airfield specific data in order to get any performance figures but it does make life much easier and far more precise.In order to do all of these things properly you would need either a full QRH and the training in how to use the various enclosed tables or airfield specific performance tables and again the relevant training. Performance can be a pain in the a$$ but the airfield specific data makes life far easier as many of the factorisations you would need to make when using the QRH tables are already done, eg slope, obstacles etc. The best / safest way to operate the PMDG 737 is to operate at max chat all the time as with the data provided you cannot make an accurat calculation of the allowable derate. Yes you could go CLB 1 or CLB 2 but to be honest I like having more thrust than I need rather than less. In reality if we didn't have the correct data for an airfield we simply would use the QRH data and in most cases go ful thrust as well just to make doubly sure.Hope that helpsKris

Frank,this topic is somewhat confusing at the beginning! Let me try to explain it:you are correct, an engine will produce less thrust at a higher temperature. Now then, by entering a higher assumed temperature, the FMC will check its internal database what thrust level would be achieved if it actually was that hot. The computed N1 data will be the equivalent of that thrust, just at your current temperature.So why is it done so difficult? The performance data includes diagrams that tell you how hot it may be for you to be able to depart. You can then "fool" the FMC and tell it, it is actually that warm. The FMC will then tell you what power level is required to achieve that "hotter" power thrust setting. And that computed N1 will be lower than your actual max N1.Hope that helped!Regards,Mark

Great thanks Kris, plus I guess I don't have to worry about the engine wear or overheating it anyways...still, good post and here's to hoping that we get closer to reality with every new version of fs and PMDG. :)

Got it, Mark. Thank you and everyone else for a very interesting thread. :-)Best Regards,Frank

HiKris has explained it nicely above.By inputting a higher assumed temp, therefore reducing the the max power the engine will produce, you lengthen your take roll, that why all these charts are required.For example given a certain set of conditions : temperature, wind, TOW, pressure etc. and taking off from an 8000ft runway You may calculate that at full power you can take off using 4500ft of runway. That's well and good, but needlessly strains the engines, as there is another 3500ft of runway you didn't use. By using the charts (if available), you may come up with an assumed temp setting that will enable you to use say 90% N1 power and you will become airborne using 6500ft of runway. Obviously there are many limitations, but this simplifies the idea.Essentially I have saved the engines and made better use of the available runway to become airborne.Now if only I had all those charts............ :)Graham

Well, I have just recently installed the PMDG 737-600/700 on my PC. Runs great even on MS2002. A grat sim.I've run through a few times loading a route into the FMC and EVERY time when I insert a LOWER temp then the OAT, I get a lower N1% indicated.Can anyone explain that?

In all the PMDGs 737 series as well as the PSS series aircrafts, you get maximum thrust generally when the Selected Thrust Temp is ISA+15, which at sea level is 30C.For instance, at sea level, any OAT or selected temp above and below 30C will give you a lower than maximum possible thrust. This info has been incorporated in "Tomaflex" Takeoff Performance Calculator. It takes all this into account and outputs a proper reduced takeoff when necessary, or the maximum thrust when required. Often, maximum thrust is achieved by selecting Select Thrust Temp to ISA+15.For more information on Tomaflex, visit www.tomaflex.netRegards,Jana KankesanDeveloper of Tomaflexwww.tomaflex.net

Interesting program you have there..Best Wishes,[h4]Randy J. Smith[/h4]http://www.rawbw.com/~bdoolin/shinault/southparkcartmad.gif[h3]PMDG 747![/h3]Caution! Not a real pilot, but do play one on TV ;-)ASUS KV8 DLX | AMD 3200 64 | 1 GIG PC 3200 DDR | GIGABYTE 5700 ULTRA | ViewSonic VP192b 19" |

G'dayYou don't actualy need a program to determine a rudimentary reduced thrut temp for the NG if you don't mind playing in the charts.All you need to do is determine the maximum temperature that you can take off from your selected runway given your known takeoff weight.IE look at the tables using your known weight and go through and find the absolute max temperature at which you can lift your known weight.Plug this temp into the FMC as your assumed temp.Do not operate with a reduced thrust output ie 20000Lb thrust or all the calcs go out.This value is normally company specific and in some cases hard coded by the company and not changable so just leave it at max thrust and get all your thrust reduction by assumed temp.Works well but its more fiddly than RTOW charts.Darren Howie

ServusQ. little off topic and concerning real world aviation:former jet and turboprop engines offered the opportunity to compensate perfomance loss due to ISA+ temp by water/methanol injection (wet take-off)Pilots decided for dry or wet and it had extra t/o-perfomance charts.Anyone here to give a brief when this took an end?Any of the early turbofans with water/meth injection?Any powerplants in today commercial aviation still operated 'wet'?G

GunterI previously flew the F27-500/600 which used Rolls ROyce Dart Engines, these used the watermeth injection you speak which gained us about 100shp for the take off roll. This engine was fitted to all manner of aircraft at the time some examples being the HS 748 (RAF name was the Andover), the Vanguard etc. Nowadays I think aircraft / engine manufacturers have erred away from such systems for 2 main reasons, firstly the complexity of the piping and activation systems which need to be incorporated into the design and secondly the extra weight incurred by both the delivery system and also the actual carriage of the water meth itself. Instead designers have opted to put gratuitously over powered engines on aircraft and then give the operating crew / airline the option to derate the engines at will. Doing it this way has several benefits not the least of which being that constant derating of the engine can and does increase the engines life span and thus reduces maintenance costs! Another method of increasing power is to use the same engine and uprate it. The B737-3/4/500 had the same engine with various levels of power rating. The 737-300's I fly have CFM56-B1 engines (20,000lbs thrust) which have been modiied to operate as CFM56-B2 engines (22,000lbs thrust). Apparently this is done by changing some seals, modifying the fuel control system and a few other tweaks which I am sure are far more complex than I make it sound. The up and down of this is that we have 4,000lbs more thrust from the same engines! A prime example of this is the fact that the CFM56 fitted to the average 737 is between 18.5 and 24K thrust. The A340-200/300 also flies with CFM56 engines but these are rated at 33.5K thrust. The main difference here being that the fan is significantly larger but it is the same engine core. This method of power increase is being used also in the RR Trent series fitted to the new A380! Hope that helps KrisPS Water meth also incresed the chances of melting the engine which isn't good for ones career!PPS The Harrier uses a water only injection system to cool the turbines when in the hover!