PPRuNe Forums - View Single Post - Starting engine with fan rotating backwards
Old 22nd Mar 2020, 23:43
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CV880
 
Join Date: Nov 2004
Location: North America
Age: 79
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Re Bristol Britannia, yes, held props in my apprenticeship years (long ago) but being a free turbine engine it was the airflow through the engine as the starter wound up the core that caused the prop to start to rotate.
Many years later when handling BA747-136's out in the Far East I was delighted one day to see BA's newest 747 and the first with JT9D-7A engines, taxi up to the gate. On departure I had just called "N1 Rotation" to the crew during the first engine start when the fan came to a shuddering halt. I wondered "what the ???" but decided to wait until light off before saying anything. As soon as there was a belch of heat out the back the fan suddenly started rotating again and quickly accelerated to normal speed. Only 1 engine did this. A few days later a newsletter from BA Engineering arrived saying this sometimes happened with new -7A engines and was caused by the LP turbine case cooling and shrinking on to the LP turbine blades as the engine motored on the starter bringing N1 to a halt. Light off immediately expanded the case and freed N1. Phew, made the right call that time but never experienced it again!

Early 747's with JT9's were sometimes difficult to start with a tailwind and on occasions an aircraft had to be positioned more or less into wind to get a temperamental engine to start. The tailwind tended to cause a hot or hung start.
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