One of my favourite tech questions,
You will notice the reduced ITT limits are for "TakeOff" the reason is for certification requirements, the rules say if an engine flames out on takeoff it must be capable of being restarted. This does not mean you must restart the engine only that it must be capable of being restarted..
During a flame out in cruise flight with very low ambient temperatures the core of the engine will lock a condition known as thermal core lock. This is because the casing around the turbines cool faster than the blades themselves, On the 125B engine this will occur (core locked) in the period between 4 and 20 minutes after flame out, depending on the actual ITT and the ambient temperature.
Due to certification requirements the reduced ITT limits for takeoff means that the engine in this range will not core lock and therefore is capable of being restated, note that the takeoff ends at the end of the 3rd segment.
Hope that answers your question.
Further to that, how do you know and engine is thermal core locked. This will be displayed as an NH of 0 percent, generally an unlocked engine will spool at around 20 percent NH at around 160 knots, therefore ignition on, fuel in and light off. Any attempt to restart the engine with use of the stater motor whilst the engine is core locked (0 percent NH) the stater motor drive shaft will sheer off, this is a deliberate weak point to prevent engine core damage in an attempt to start the engine whilst it is locked.
Be aware once the engine is core locked it may remain that way for up to 20 minutes.
Last edited by Xeptu; 22nd June 2012 at 10:24.