I confess to not being an expert, but engine spool-up time is specified in the certification rules:
e.g. Airbus states in "Aircraft Energy Management during Approach":
The engine certification (FAR – Part 33) ensures a time of 5 seconds or less to accelerate from 15 % to 95 % of the go-around thrust.
The aircraft certification (FAR – Part 25) ensures that the thrust achieved after 8 seconds from power application (starting from flight/approach idle) allows a minimum climb gradient of 3.2 % for go-around.
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Last edited by toffeez; 3rd Mar 2013 at 16:14.