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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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