Originally Posted by H Peacock
Surely if you tell your aeroplane that the temperature is hotter than the prevailing conditions it will give you more power. I'm sure everything that I have flown to date has had a worse performance getting airborne on a warm day than a colder day. A warm day is when I would want more -not less - power.
Now if you told the 'aeroplane' that it was colder than for real, then I would expect it to use/need less power.
Ah, but you're not telling the aircraft; you're telling the ENGINE.
The fact that the aircraft would like more power on a hot day is due to the fact that an engine can only generate a lower amount of power on that hot day; you can't ask it to give the same power as on a 'cold' day because it'll suffer some form of thermal or mechanical damage, through overrunning a limit. therefore by giving the engine an inaccurately high temperature indication you cause it to, in effect, throttle back to protect itself against what it thinks is the risk of high temp operations. That causes a reduction in max thrust.
Other than the engine effects, the temperature effects on the aircraft are somewhat secondary by comparison.
Flat rating, De Rating, reduced Power
The way I like to simplistically think of these are as follows:
If an engine were allowed to generate the maximum thrust its design offers, thrust would be maximum at some absurdly low minimum operating temperature, and lowest at the maximum cleared temperature. Let's say ISA-40 and ISA+40
If you left the design like that, you'd have various design cases driven by the very high thrust at ISA-40, which is of no real practical use, since most operations are in the ISA +/-15 range. So, to avoid having penalties due to an essentially useless thrust level, the engine is run at less than max capability up to some temperature, at which point we decide we NEED the max thrust the engine gives, and then we run as hard as possible above that. The engine is then said to be "flat rated" up to the temperature where we start to run 'flat out'.
A second benefit of this is that the engine is now often run at less than 'flat-out', which saves wear on the engine.
That's a FLAT RATED engine.
If now, in practice, I find I don't actually need or want even the amount of thrust that engine gives for a specific aircraft design, I could choose a different flat rated thrust level. I redefine my thrust vs temp curves, and now have a different set of engine performance from the same physical engine. In this case I have a DE-RATED engine. (If I go the other way, I could squeeze more power from the engine and in effect RE-RATE or UP-RATE the engine). In effect, derating/rerating is a way to put a different engine on the aircraft (in terms of its impact on the design) while keeping the same actual parts.
Finally, assuming I have a given engine on my aircraft, with a given flat rating, because the OEM decided they wanted 20,000lbf per engine for the design mission, but I only need 18,000lbf per engine to meet my mission-of-the-day (long runway, light aircraft, whatever). Ideally I'd like the OEM to give me exactly the right engine/airframe for each flight, but that's not practical. What IS practical is to change the engine behaviour 'on the fly' by reducing the thrust artificially for today's flight only; that's where a 'reduced thrust' takeoff comes in. It's a way to get the right match of thrust and mission for every flight, without having a hangar full of engines all at different RATINGS that I have to swap on and off of the plane every day.