Question on Derate takeoff's

[KevMac 0]

Hi All - I was just wantching Nick's latest video where he changed the outside temperature in the FMC (CDU) to 57 degrees celsius. However, the N1 is subsequently changed to 88.3 percent. Here is where I am confused. Hotter air is less dense, therefore the hotter the air, the more thrust you will need. I figured putting in a temperature that hot of 57c would yield a much greater N1 level, such as 104 or 105 instead of 88. Can someone shed some light on this? Thanks,Kevin M

[sosuflyer 30]

The N1 is lowered because it is replicating the power that would be available if the temperature were 57, which would be significantly less.

[hontzt 14]

It's an assumed temperature. To simplify, prior to departure, the performance calculations might determine at a particular T/O weight, air temp, runway, etc there is extra runway remaining which the airplane using max takeoff power won't be using and also which calculations show won't be used in case of a reject takeoff. This excess runway could be exchanged by using less thrust during T/O. This is where assumed temp comes in. Actual performance at current ambient temp leaves lots of room at the end of the runway. But if the temp was much greater, say, 50 C, the airplane would use all the allowable runway (worse performance on hot days). This is the purpose of assumed temp, to have the airplane perform as if it were an actual 50 degree C day, suffice it to say using up lots of runway. The assumed 50C means less thrust must be generated so the airplane uses the larger amount of runway as if it really were a 50C day. Assumed temp which is a component of T/O FLEX (Airbus terminology) is not used as a cut and dry derated T/O, but rather a tailored derate for prevailing conditions such as aircraft and runway parameters. A different assumed temp might be applied to a longer or shorter runway. A different assumed temp would be applied for a different T/O weight. Weather conditions might preclude a FLX T/O. Mechanical issues might restrict the airplane to full thrust T/O's. I hope this helps explain the concept. VR Todd

[Jack_C 1]

Hey Kevin. I know it seems a bit confusing. You didn't give me all the data he was using so I will just make up some to get my point across. Let's say he wasn't using an assumed temp takeoff therefore the N1 would be about, say, 93%. So if you look at charts, it would say something like, at your current weight 93% power is enough power to maintain the required take off and climb performance UP to a temp of 57 degrees. But heck, the actual temperature outside today is only 25 degrees. So Kevin, as you know we are going to perform better since it is colder than 57 degrees, right? So therefore how much can we pull the power back to get the EQUIVALENT performance of a 57 degree day. In the case you gave it was 88.5. Therefore a N1 of 88.5% on a 25 degree day will result in the same performance as using 93% on a 57 degree day. Does that make sense. Sorry, just trying to put it in layman's terms. Hope that helps! JackColwill

[bburkley 0]

Hi Kevin, Jet engines produce less thrust as ambient air temperature increases. Flex temp is used to reduce wear on the engines and extend their operable life. Therefore, the entire point is to reduce power as much as you can. Flex temp is the highest temperature at which the engines would produce the required thrust to takeoff given your present constraints - runway length, fuel, payload, weather conditions, and other variables. As you said, at higher ambient temperatures, the air is less dense - meaning the engines have less air to compress and produce more thrust out of. Think of it as a way of exchanging thrust (more wear and tear) for longer takeoff length (more runway). Hope that helps, Brandon Burkley

[captainklm 1]

I thought the only reason why is cause when its hotter the engine gets more stressed but there was a lot more to hot derated T/Os

[Flamin_Squirrel 71]

Hopefully someone will correct me if I'm wrong... I believe that derate and reduced thrust are not the same.Reduced thrust is as described above, for reducing engine ware. In the event of an emergency you can push the thrust levers all the way forward to get max thrust.A derate actually reduces the max thrust available, even with the thrust leavers full forward. This sounds like a strange thing to do deliberately, but I think it's used for reducing Vmcg.

[CLett737 14]

The ATM(Assumed temp method) achieves a takeoff thrust less than the maximum takeoff thrust byassuming a temperature that is higher than the actual temperature. The thrustreduction authorized by most regulatory agencies is limited to 25% below anycertified takeoff thrust rating.The primary thrust setting parameter (N1) is not considered a limitation. Takeoffspeeds consider ground and in-air minimum control speeds (VMCG and VMCA)with full takeoff thrust for the actual temperature. If conditions are encounteredduring the takeoff where additional thrust is desired, such as windshear, the crewshould not hesitate to manually advance thrust levers to maximum takeoff thrust.

[aentwis 61]

That is how I see it Flamin Squirrel... but what is your real name? I hate calling people something like Flamin Squirrel... :) Andrew

[RPRIEVER 52]

I use TOPCAT for departure calculations, and an assumed temperature is one of the options. On the takeoff page, it gives me a range of assumed temps, along with the estimated stopping distances, etc. I had heard that while a higher assumed temp prolongs engine life, it adds to the fuel costs. What parameters should be used to apply the correct assumed temp?

[jon b 2,050]

Interestingly in the event of an engine failure using an assumed temp there is no need to add extra thrust as performance would be equal or better than if you were actually taking off at the assumed temp heat and lost an engine, IE the margins are still built in. Jon Bunting