Flat-rated
Flat-rated is a technical term used in aviation. Flat-rated or flat-rating designated at aircraft turbines a particular range of functional linear relationship between the engine thrust and the temperature of the surrounding air, in which the thrust no longer increases with decreasing temperature. This power limitation ensures that the specific design limits of a turbine construction are not exceeded, thus minimizing the risk of damage and unnecessarily shortening the service life of the engine.
The two main factors that limit the maximum output of a turbine are the compression and the turbine inlet temperature. As the air temperature falls, the maximum achievable thrust increases, but the output of a turbine is capped at the top by the maximum permissible compression. The thrust is not increased any further, but remains constant ( flat ). As a rule, an engine is designed so that it can deliver its maximum static thrust up to a temperature of 29 ° C. At higher outside temperatures, however, the maximum permissible turbine inlet temperature takes effect as a limiting factor.
In order to protect the engines by keeping compression and turbine inlet temperature as low as possible, the pilots use the flight performance tables or the on-board computer to determine the engine performance sufficient for a safe take-off at each take-off, which is far below the maximum thrust at low loads and low temperatures can lie ( flexible rating ).