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Home > Flying the Plane > Can you explain the assumed temperature take off?
Can you explain the assumed temperature take off?
Flying - Flying the Plane
Wednesday, 06 December 2006 08:27

Hi Sir!

With utmost respect, to a captain, now retired - many thanks for sharing your knowledge with us.

I have a question.

Can you explain the assumed temperature take off for jet engines as entered in the FMC?

Since engines produce more thrust at lower temperatures, it seems logical that when one enters a lower temperature, the engine would be "derated" to a lower thrust setting.

I think what I have found on the web works the other way round. I am very confused!

Johan,
The Netherlands.

Hi Johan,

I have not really retired from flying yet. I have merely switched over from flying the Boeing 777 to the Airbus A320. Yes, I still love flying and will share my knowledge with you all when my time permits :-)!

The idea of assumed temperature take off could be quite confusing unless you are already flying a jet plane. Taking off with maximum power all the times is bad for the engines and hence, airplanes manufacturers have recommended that pilots apply this take off technique whenever possible.

For instance, on a Boeing 777, if I were to take off with a full load on a 12-hour journey (approximately 287,000 kg), I would require all the available thrust (maximum power) from each engine (90,300 pounds). On the other hand, with a shorter 6-hour flight where my take off weight is 215,000 kg, I would not need all the available thrust to take off.

So, what do I do? I would fool the engines into thinking that the outside air temperature is much higher than it actually is by entering an assumed temperature into the computer (FMC). In this way, the FMC would command around 80 % power only for the take off. This reduces the take-off thrust to a lower power required for the safe take-off.

How does it work? Generally, all jet engines are guaranteed to provide the specified thrust at full "throttle" position at a given temperature (ISA + 15 or 30C). Anything above this temperature, they will give less thrust because the air becomes less dense. So, by entering an assumed temperature of say, 50 degrees onto the FMC, the computer will now think, ?Ah, the temperature is much higher now, I only need to provide a lower power setting (N1) when the captain sets the thrust levers to take off position!?

How do I know the assumed temperature? Well, this is obtained from the take off tables placed inside the cockpit. The pilot then refer to the actual take off weight of the day and note down the hottest outside air temperature at which the take-off could be safely performed. This hottest temperate is known as the "assumed temperature" (we also call it the FLEX temperature on the Airbus A320)

How is an assumed temperature take-off different from a "derated" take off? A de-rated engine is a semi-permanent engine fix used to reduce the maximum thrust available. A temporary form of de-rating is known as a "T/O de-rate"

The T/O de-rate (TO-1 & TO-2) can be up to 10 - 20% of the full power. On the Boeing 777, if the take off weight were 245,000 kg and above, full take off thrust would be used. Below this weight, we can elect to do a derated T0-1, and at the same time, apply an assumed temperature for the take off - effectively reducing the power required to about 65 % of full power!

Yes, the main purpose of doing so is to save the engine life by reducing the wear and tear and consequently, save money for the airlines.

Now, back to the confusing part; remember, the normal range of assumed temperatures is usually from 30 to 55C. The maximum temperature (55C) entered into the FMC gives the maximum amount of thrust reduction allowable (25%) and the minimum (30C) is the least power reduction. So, by entering a lower temperature, it would not further "derate" the engine but merely reduce the take off thrust "slightly" This is different from the theory, as you have said, that a jet engine produces more thrust at lower outside air temperatures.

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Comments (6)

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Thanks, But I'm still confused!
Hi Johan,

I was confused like you but I think that I have may have am explaination of sorts.
It is based on the fact that the max thrust rating for an engine is calculated at a given temperature, say 30 degrees C. As the temperature increases, the maximum thrust decreases.
The FMC calculates the theoretical maximum thrust based on the assumed temp provided by the pilot. This is the desired thrust required for T/O.
However, because the actual temperature (OAT) is lower, the desired thrust will be achieved at a lower N1. The FMC compensates by reducing the N1 to acheive the desired thrust.

This is why when you enter a higher assumed temp into the FMC, the thrust is less.

Hope this helps.
Ben , 16 May, 2009
very nice expl.
Captain Lim, thank you sir for the excellent expl.
it helped me alooooot
smilies/wink.gif
max , 31 Oct, 2009
...
I would have thought the computer would be able to integrate the relevant data to perform a reduced thrust takeoff by itself without having the potential human factors error of the pilot entering a higher than allowed assumed temp on the CDU and... well you know what would happen then!
James , 10 May, 2012
temperature
but how do you know the temp you are going to incert in the fmc
daniel , 23 Jun, 2015
Assume Temperature
hi Johan,

lets try this out. Note that Max thrust is inversely proportional to Temperature.
This is in addition to what the Captain explained.
I hope this helps.

Regards,
Chidi
Ohadoma Chidiebere , 29 Jun, 2015
FS9 Pilot
Excellent!
boballende , 25 Mar, 2016

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