Takeoff Target N1%/EPR displayed on EICAS vs. Performance vendor calculated values
 

Hello,

I am wondering if any operators have specific tolerances for variation between the aircraft's calculated target N1 or EPR value for a given takeoff thrust setting compared to the calculated N1/EPR that comes from the performance vendor or performance chart. My current operator instructs us to compare the numbers for reasonability but stops short of assigning a value beyond which the difference is unreasonable. The usual culprit in large variance are temperature discrepancies between ATIS/Metar temps used by the performance calculator compared to the temperatures being sensed by the thermometers in the aeroplane. At what point would you halt the operation over such a discrepancy? 1%? 2%? 5%? How would you attempt to remedy the situation? Force maximum thrust?

If the performance calculator has been designed and certified correctly, then the difference shouldn't be unsuitable. Have you ever seen it more than 1-2% out? The check is useful to make sure you've got the correct derate set.

If you see a significant difference between the two values, it would be a good idea to check the aircraft TAT vs. what the tower is reporting. Aircraft TAT sensors are usually heated but also aspirated so that they give accurate readings without forward airspeed. However, that aspiration can fail or the TAT sensor get obstructed with bugs or other debris which can adversely affect the TAT reading. Although commercial aircraft have 'dual' TAT sensors, they are usually in the same probe so they are not truly independent measurements (off hand, the 747 is the only Boeing aircraft I can think of that has dual TAT probes as basic).