Method of starting a gas turbine engine including a cooling phase

A method of starting a gas turbine engine having a rotor comprising at least a shaft mounted compressor and turbine, with a casing surrounding the rotor, the method comprising:

• initiating a first acceleration phase where the rotational speed of the rotor is increased toward a combustion speed to force air through the gas turbine engine;
• sensing vibrations during the first acceleration phase while the rotational speed of the rotor is equal to or less than 500 rpm;
• sensing a first bowed rotor condition during the first acceleration phase;
• initiating a first bowed-rotor cooling phase during the first acceleration phase, where the rotational speed of the rotor is maintained below a first bowed-rotor threshold speed until a first non-bowed rotor condition is satisfied, wherein the air forced through the gas turbine engine cools the rotor;
• sensing the first non-bowed rotor condition;
• thereafter, initiating a second acceleration phase, wherein the rotor speed is increased from at least 500 rpm toward the combustion speed;
• sensing a second bowed rotor condition during the second acceleration phase;
• initiating a second bowed-rotor cooling phase during the second acceleration phase, where the rotational speed of the rotor is decelerated below a second bowed-rotor threshold speed and above the first bowed-rotor threshold speed until a second non-bowed rotor condition is satisfied, wherein the air forced through the gas turbine engine cools the rotor;
• sensing the second non-bowed rotor condition;
• and
• thereafter, initiating a combustion phase, where upon reaching the combustion speed during the combustion phase fuel is supplied to the gas turbine engine and ignition is turned on.