Hu,
I suspect you are referring to statements found in engine and aircraft manufacturers' brochures, such as "The ……. engine will produce 80000 lbf static thrust, at ISA msl, flat rated to ISA + 30 degrees C." To understand what this means we must first consider the factors that affect the thrust output of an engine.
Thrust is proportional to the mass flow of air passing through an engine. For any given engine this depends upon RPM and air density. Air density is related to static pressure and temperature, such that increasing altitudes and temperatures, decrease both density and thrust output. An engine in as-new condition, running at a given RPM, might produce 80000 lbf of static thrust at sea level in ISA conditions. But if it is used at greater altitudes or temperatures, it will produce less thrust at that RPM. This reduction in thrust will reduce the maximum mass at which the aircraft can take-off.
The obvious solution is to open up the throttles a little bit further so that the engine runs at a higher RPM, restoring its original mass flow and thrust. This option poses two problems: Firstly it means that a different power lever position is required for every possible combination of airfield altitude and temperature. The probability of every pilot getting this right for every take-off is (as they say in all the best mathematics books) vanishingly small. Some will take-off with too much thrust, while some (will fail to take-off) with too little.
The second problem is that opening the throttle a little bit more will also increase the turbine inlet temperature. While the life of a modern engine might be 20 or 30 thousand hours at normal temperatures, increasing turbine inlet temperature sufficiently will reduce this to matter of seconds! The effects of lesser levels of abuse will be less dramatic, but will still increase the probability of catastrophic failures. What is required is a system whereby the pilot puts the power levers in the same position for every take-off and the engine produces the advertised level of thrust without over-cooking the turbines.
The first part of the solution is (as JT said in his earlier post) to fit engines that are more powerful than is really required. If for example we want 80000 lbf of thrust in msl ISA conditions, we might use an engine that can actually produce 90000 lbf. We can then add a control system that senses the ambient pressure and temperature and calculates the EPR (or N1) required to give 80000 lbf thrust, and the turbine temperature that this will produce. When the power lever is set in the take-off position, the system opens the throttle while monitoring EPR (or N1) and turbine temperature. If the required EPR is achieved before the temperature limit is reached, the engine will produce the required 80000 lbf thrust. But if conditions are such that the turbine temperature limit is reached first, then acceleration will stop at a lower thrust. In the case of the brochure referred to above, the engine will give 80000 lbf of thrust at msl in all temperatures up to 45 degrees C. The extra thrust capability also offers benefits in terms of airfield altitude. With 10000 lbf of thrust is hand the engine described above could clearly still produce the required 80000 lbf at some altitude above msl, provided the temperature isn't 45 degrees C.
The engine manufacturers produce a performance chart indicating the relationship between altitude, ambient temperature and thrust. This typically has altitude on the vertical scale and temperature on the horizontal scale. In the example above it might include a box going from msl up to perhaps 8000 ft, then horizontally to 20 degrees C, then down to 45 degrees C at msl. In ambient conditions within this box, setting the power levers to take-off power will produce the same (flat rated) 80000 lbf of thrust. Outside the box (higher, hotter or both) the thrust will be reduced to avoid exceeding the limiting turbine temperature. So thrust is flat rated within the box and dependent upon ambient conditions outside the box. This aproach is equally applicable at other power settings.
This system provides the following advantages:
a. The aircraft can take-off at maximum mass in a wide range of temperature and altitude conditions.. .b. The pilot always sets the power levers to the same position for take-off.. .c. Engine life is increased because under most conditions the engines operate well below their limiting turbine temperature.