Here are words from Airbus regarding the use of MCT:
According to FCOM PER-THR-MCT MAXIMUM CONTINUOUS-DEFINITION,the MCT is defined as:"It is the maximum thrust certified for continuous use. This rating should be used, at the pilot’s discretion, only when required to ensure safe flight (engine failure)."
Thus,although the use of MCT thrust is possible with no time limitation, it not recommended by Airbus in case all engines are operative for the following reasons:
Setting MCT thrust with both operating engines corresponds to an extreme use of the engines implying a reduction of the EGT margins in normal operation and could have an impact on the service lifetime of the engines.
In addition, according to some engine manufacturers, engine maintenance costs may be impacted by the frequency of maximum continuous rating use.
In addition, you can already find a more accurate definition of MCT thrust in Chapter 5 “Engine Limitations” of Getting to Grips with Aircraft Performance available on Airbus World website.
小编帮你搬过来，内容如下：
The Maximum Continuous Thrust (MCT) is the maximum thrust that can be used unlimitedly in flight. It must be selected in case of engine failure, when TOGA thrust is no longer allowed due to time limitation.
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Therefore, the use of MCT thrust lever position is only recommended in flight for failure cases such as:
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Both thrust lever angle TLA sensors are not in agreement on one engine,
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Thrust Lever Fault
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And more particularly, single engine operations
Generally speaking, MCT has been designed to cope with abnormal situations only to ensure safe flight path, and it is usually used in case of engine out. It is not aimed at being used for normal operations.
