During the start sequence, the electrical starter fitted to the engine will provide power to accelerate N1 until the mass airflow is sufficiently high to permit the introduction of fuel into the combustion liner and light off is achieved.
The additional power required to continue accelerating N1 up towards GI must be generated by the Gas Producer Turbine. The only indication of this power is the temperature produced and indicated on the TOT Gauge. Thus it is important for you to carefully monitor TOT throughout the start.
The main criteria for achieving a good start is the correct setting of the basic fuel flow schedule. The best indication that this is correctly set is the light off temperature achieved.
The figure quoted for the different RR 250 engines vary slightly, but basically lie between 760°C and 820°C and ideally should be towards the middle to the top end of the temperature range.
The start time should be between 20 and 25 seconds if an high capacity starter generator is fitted.
Temperature measurement is sensed after the second stage turbine wheel, i.e. between the Gas Producer and Power Turbines
HIGH TOT
The turbine inlet temperature at the first stage nozzle will be some 200° - 250°C above the reading on the TOT gauge. Excessively high inlet temperature will result in cracking and burning of the first stage and possibly second stage nozzles, plus blade tip rub. This could result in seizure of the first and second stage turbine wheels. In addition, a surge condition may occur with the likelihood of further damage.
High TOT results in a faster, hotter start.
LOW TOT
A low TOT indicates a weak air/fuel ratio, therefore less power is generated in the Gas Producer Turbine and the start time will be longer.
The weaker mixture will result in an elongated combustion flame reaching closer to the turbine inlet. This reduces the time that the cooling air flow has to mix with the combustion gases in order to provide an acceptable and even temperature at the turbine inlet.