14 CFR § 27.1045

General. For each stage of flight, the cooling tests must be conducted with the rotorcraft—

In the configuration most critical for cooling; and

Under the conditions most critical for cooling.

Temperature stabilization. For the purpose of the cooling tests, a temperature is “stabilized” when its rate of change is less than two degrees F. per minute. The following component and engine fluid temperature stabilization rules apply:

For each rotorcraft, and for each stage of flight—

The temperatures must be stabilized under the conditions from which entry is made into the stage of flight being investigated; or

If the entry condition normally does not allow temperatures to stabilize, operation through the full entry condition must be conducted before entry into the stage of flight being investigated in order to allow the temperatures to attain their natural levels at the time of entry.

For each helicopter during the takeoff stage of flight, the climb at takeoff power must be preceded by a period of hover during which the temperatures are stabilized.

Duration of test. For each stage of flight the tests must be continued until—

The temperatures stabilize or 5 minutes after the occurrence of the highest temperature recorded, as appropriate to the test condition;

That stage of flight is completed; or

An operating limitation is reached.