From the C+ Maint. Manual:
Power Assurance Input Signals.
Outside Air Temperature (OAT), displayed as 1 OAT and 2 OAT on IIDS display units, is referred to as T0 in the following description. The T0 readings displayed on cockpit instrumentation are derived from a dual element probe located under the aircraft nose.
Indications from element No. 1 are displayed on the copilot IIDS display unit and supply the No. 1 DECU. Indications from element No. 2 are displayed on the pilot IIDS display unit and supply the No. 2 DECU. Since the IIDS display unit displays only the whole number with any decimal rounded up or down and the DECU receives 4 decimal places, there can be as much as a 0.5°C difference between the indicated rounded-off temperature and the exact temperature used in determining automatic target torque. This would cause a 0.5% difference between power margins obtained using automatic and manual target torque look-up. If manual target torque is being established and copilot T0 display has been rounded up 0.5°C while pilot T0 display has been rounded down 0.5°C, there can be a 1.0% power margin difference depending on which data has been entered.
Pressure Altitude (Hp), displayed as 1 PO/FEET and 2 PO/FEET on power assurance and performance pages on IIDS display units, is referred to as P0 in the following description. The P0 signal is derived from the static reference static pressure port located in the tailcone as part of the RDAU. The same signal feeds both the IIDS displays units and the DECUs. The only difference here would
be as a result of the rounding off to a whole number on the IIDS display units. The P0 readings can be verified by using the N1 test switch. This is because a comparison is made between the primary biased N1 signal from the DECU using T1 and DECU P0 and the backup biased N1 signal displayed by the IIDS using cockpit T0 and P0. The maximum allowable tolerance of 6 0.2% ensures that the T1/T0 and the DECU P0 and cockpit P0 readings are close together.
The T5 signal displayed on the IIDS display units is conformed T5 and a secondary source of T5 information, separate from the source used by the DECU. The T5 information displayed on the IIDS display units is derived by using the direct chromel/alumel signal to the RDAU from the second T5 harness in the engine and incorporating the conformation resistor values transmitted over the ARINC data bus to the IIDS. The DECU uses the primary source of T5 information from the other T5 chromel/alumel harness and also calculated conformed T5, but is completely separate from the IIDS display signal. The two values of T5 should be fairly close but may be a few degrees different
due to thermocouple wire tolerances,. To avoid any power assurance differences between automated DECU power assurance and manual reading/checks done by the pilot from his indication, the IIDS calculates the final automated T5 power assurance margin by using the pilot indication of T5 and then subtracting the DECU value of specification T5. This way there will be no difference in automated and manual T5 margin as the same source of indicated T5 is used. Roundoff error in OAT is still a factor, however.