CDU Preflight, Boeing 738

[KristianNorway]

Dear folks

How come we enter the OAT into the FMC CDU during preflight of the Boeing 737-800?
I know one gets a reduced max thrust available if one doesn't.

Is it because the TAT-probe needs one pack functioning to give a correct readout to the ADIRU? But then again.. one would have ample time for OAT readings after startup and prior to T/O if this was the case.

[Denti]

The 737 is available with or without aspirated TAT probe. If you have the unaspirated one your TAT before takeoff will creep up within 2 or 3 minutes to 99°C (and more, but that isn't displayed) after probe heat is switched on, invalidating any data based on that. Even the aspirated probe is often considerably off the real OAT, depending on position vs. the sun and other factors.

[KristianNorway]

Thank you. A quick and to the point response that answered my question :ok: