Assumed temperature thrust

[joshuahsong]

Hi all.
I'm reading a book about turbine engine aircraft for beginners.
Having read this chapter mentioning reduced thrust takeoff (assumed temperature thrust) just wondering why the engine thrust should be reduced even the assumed OAT is higher that it actually is.
I think, as the OAT gets higher, density altitude goes up, so more thrust is required for safe takeoff.

[john_tullamarine]

Perhaps the secret behind the trick was not explained in your reading ..

(a) consider the case where actual aircraft weight is rather less than maximum permitted for the runway and day

(b) if you use rated (ie maximum) thrust you are up and away much earlier and better than you really need

(c) if follows that you could use less thrust and still be OK so far as all the safety and regulatory considerations are concerned

(d) so how might we figure out how to use less thrust ?

(e) look up the RTOW chart at increasing OAT values until you find the same weight as your actual weight (most folks use a small pad to make the sums more conservative)

(f) what this means is that, were the actual OAT on the day equal to the higher OAT for the actual weight you have, you would be able to take off satisfactorily

(g) so, if you set that thrust (to suit the higher, assumed, OAT) you will obtain a satisfactory takeoff somewhat better than the book presumes .. due to the actual OAT's being lower than the book OAT

(h) benefit is reduced maintenance and, possibly, more favourable contract considerations/costs

[OhNoCB]

The engines are rated to give rated thrust at or below a certain temperature (ie. flat rated). Above this temperature, thrust reduces because of reduced density. If you tell the engines that the temperature is higher than it actually is, a lower n1 will be commanded to attain the assumed temperature thrust.