The manufacturer’s thrust
The thrust an engine will give you is not the same from one day to the next. Temperature moves it, pressure altitude moves it, and neither of those holds still. Which leaves an awkward question for anyone who has to plan a takeoff: this morning, on this airfield, is the engine's full rated thrust actually available or is it not?
That is not a thing to be judged by feel, because everything downstream rests on the answer — the runway length you need, the gradients you must make, the speeds you will fly. So the manufacturers took the question away from the crew and answered it once, in the certification, in a way that holds for every day the airplane will ever fly.
Seeing how takes a moment with what actually limits a jet engine, because it is not thrust at all. What the engine has instead is two ceilings it may never cross.
The first is pressure. Air is squeezed hard on its way through the compressor, and the chamber it is squeezed into can only take so much before its integrity is in doubt. There is a highest pressure the engine may build, and it is a structural fact about the machine, set by the manufacturer.
The second is temperature. Burning fuel in that air makes it very hot, and the turbine blades standing in the exhaust can only survive so much. There is a highest temperature the engine may reach, and it is a fact about what those blades are made of, set by the manufacturer as well.
The thrust will be limited by whichever of those two ceilings the engine meets first, and which one it meets first depends on the day. Let's see how that works.
Flat rating: where the two ceilings cross
The manufacturer guarantees that the engine will be able to deliver its maximum rated thrust up to a maximum temperature. That temperature has a name, and it is called the flat-rate temperature. It commonly sits around fifteen degrees above a standard day, ISA+15, near thirty degrees Celsius at sea level.
Start there, at the flat-rate temperature itself, and the engine is doing everything at once. Highest rotation speed, highest compression, and an exhaust as hot as it will ever be asked to run. Both ceilings are met in the same moment. The engine is delivering its maximum rated thrust. That is what makes this particular temperature a corner rather than an arbitrary choice.
Now make the air colder. It is denser, so at that same rotation speed the pressure in the chamber would climb past what the chamber is allowed to take. But it cannot go above its pressure limit. The engine reduces its rotation until the pressure sits on its ceiling again, and the thrust comes out exactly as before, because the pressure it is working across has not changed. Make it colder still, and it reduces its rotation further, but the thrust still stays at its maximum rated value. So we say that colder than the flat-rate temperature the engine is pressure-limited: sitting on its pressure ceiling, delivering its maximum rated thrust, and turning more slowly to do it the colder the day gets. The engine could make a great deal more thrust than that — the air is dense and cold and there for the taking — but it cannot go over its pressure limit to get at it.
Which is where the name comes from. Plot that thrust against temperature and the cold half of the picture is a flat line.
There is a consequence of this you can see. If rated thrust needs fewer turns of the fan in dense air and more of them in mild air, then the rotation speed that corresponds to rated thrust is not the same reading from one day to the next. What the setting stands for is fixed. The setting itself drifts with the temperature, which is why the same rated thrust is not always the same N1.
And notice what the cold is not buying. The engine could make more thrust down there, but it can't. What the cold gives is not push but margin, because the turbine is running far below its temperature ceiling, and that reserve of unused heat grows the colder it gets.
Now go the other way, warmer than the flat-rate temperature. The air is thin, so reaching the rated thrust would mean working it much harder, and working it harder would mean running the turbine hotter than the blades can bear. This time the temperature ceiling arrives first. To protect the turbine the engine must back its compressor down, and here there is nothing held in reserve to give back. The thrust itself falls. Above the flat-rate temperature the engine is temperature-limited, and every further degree of heat takes thrust away. The engine cannot give its maximum rated thrust.
So the picture is a flat plateau with a corner in it, and past that corner a line sloping away downward. Colder than the corner, a constant promised thrust, held there by pressure. Warmer than it, a real decline of thrust, forced by heat. It is the same corner you met in the cruise chapters when the ceiling came down on a hot day: the engine losing its ability to deliver, not choosing to hold back.
What does flat rating actually mean?
That the manufacturer promises the engine will deliver its maximum rated thrust, held constant across a range of temperatures, up to the published flat-rate temperature. Below that temperature the dense air would drive the compressor past the pressure it is allowed to build, so the engine winds itself back and delivers the rated figure with the turbine running cool: capable of more, prevented from it. Above that temperature the thin air means the turbine would have to run too hot to reach the rating at all, so the thrust available falls off as the day warms. The reduction techniques in this chapter live entirely in the first region. They spend margin that was already sitting there.