convair880mfan wrote:So my question is this. If a, say, 50,000 pound thrust engine is derated to say, 45,000 pounds of thrust for takeoff, can a pilot get the full 50,000 pounds of thrust in a pinch or is it mechanically impossible? Or if mechanically possible, is it inadvisable or dangerous?
Starlionblue wrote:As discussed above, there are thrust limitations in the installation itself. These cannot be overridden by the pilots. We simply get up to the maximum "installed".
We typically use reduced thrust (that is, less than the "installed" max thrust) to reduce engine wear. Mostly this is in the form of flex/assumed-temp thrust. There is an important difference between the operational use of fixed derate thrust and flex/assumed-temp thrust.
Both fixed derate and flex/assumed-temp are reduced thrust settings. However, in general, derate is used on contaminated runways where VMC in an engine-out scenario is limiting. Therefore with a fixed derate, TOGA is not permitted below a certain speed (F speed on Airbus) because going to TOGA may exceed rudder authority. However, with flex/assumed-temp , TOGA may be selected at any time.
There is one exception to the "no TOGA" rule with derate, which is a windshear event. In that case, go TOGA because not doing so is more risky than the chance of an engine failure at that precise time.
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