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On our little Saab, we have a cooling down period of 2 minutes just as described above. According to Saab or GE who makes the engines, this is to 1) cool down the engine and 2) distribute the heat evenly in the nacelle.
After we shut the engine down, it is suggested that we either start the engines within 2 minutes or after 10-12 minutes. After the engine has stopped rotating, the heat transfers to the upper part of the nacelle, expanding the upper part of the rotor/spool. This causes the whole rotor with turbine and compressor assemblies to bow ever so slightly wich in turn causes them to rub against the casing. With time, this wear will degrade the engine performance.
/LnS
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