About the Flex temps in MCDU of 737

[cauc0001]

Dear Everyone,

I have an interesting question about the flex temps in MCDU of 737. If the max assumed temperature is ISA+53℃， but I input 99℃ or 70℃ as the flex temperature into MCDU, what will happen? How about the thrust will be changed? Is there any limitation automatically from the FMC?

Thank you for your answers.

[Skyjob]

I assume you mean Assumed Temperature?

For airplanes without the aspirated TAT option, OAT is entered on the N1 LIMIT page of the FMC CDU. For airplanes with the aspirated TAT option, OAT entry is not required, but if OAT is entered, the entered value is used for takeoff settings instead of the sensed value.
Then there is the Assumed Temperature (Flex equivalent) - Regulations permit the use of up to 25% takeoff thrust reduction for operation with assumed temperature reduced thrust.
Maximum assumed temperature can be found in FCOM, it is a function of OAT and pressure altitude, 73C at -1000' with OAT 55C, in the ISA range from 69 to 51 is allowed...

To find the maximum allowable assumed temperature enter the Maximum Assumed Temperature table with airport pressure altitude and OAT. Compare this temperature to that at which the airplane is performance limited as determined from available takeoff performance data. Next, enter the Maximum Takeoff %N1 table with airport pressure altitude and the lower of the two temperatures previously determined, to obtain a maximum takeoff %N1. Do not use an assumed temperature less than the minimum assumed temperature shown. Enter the %N1 Adjustment table with OAT and the difference between the assumed and actual OAT to obtain a %N1 adjustment. Subtract the %N1 adjustment from the maximum takeoff %N1 found previously to determine the assumed temperature reduced thrust %N1.
Apply %N1 adjustments as provided when applicable.

[cauc0001]

I assume you mean Assumed Temperature?

For airplanes without the aspirated TAT option, OAT is entered on the N1 LIMIT page of the FMC CDU. For airplanes with the aspirated TAT option, OAT entry is not required, but if OAT is entered, the entered value is used for takeoff settings instead of the sensed value.
Then there is the Assumed Temperature (Flex equivalent) - Regulations permit the use of up to 25% takeoff thrust reduction for operation with assumed temperature reduced thrust.
Maximum assumed temperature can be found in FCOM, it is a function of OAT and pressure altitude, 73C at -1000' with OAT 55C, in the ISA range from 69 to 51 is allowed...

To find the maximum allowable assumed temperature enter the Maximum Assumed Temperature table with airport pressure altitude and OAT. Compare this temperature to that at which the airplane is performance limited as determined from available takeoff performance data. Next, enter the Maximum Takeoff %N1 table with airport pressure altitude and the lower of the two temperatures previously determined, to obtain a maximum takeoff %N1. Do not use an assumed temperature less than the minimum assumed temperature shown. Enter the %N1 Adjustment table with OAT and the difference between the assumed and actual OAT to obtain a %N1 adjustment. Subtract the %N1 adjustment from the maximum takeoff %N1 found previously to determine the assumed temperature reduced thrust %N1.
Apply %N1 adjustments as provided when applicable.

Dear Skyjob,
Thank you for your reply. I know the limitation for the assumed temperature. I just want to know how about the logic in the FMC? If I input a very high temperature in the MCDU, is there any protection or some warming to inform the pilot the error input?
Thank you.

[Banana Joe]

Not that I can think of. That's why during performance both pilots must calculate on their own EFB separately the performance data and crosscheck together all the data before executing.

[Skyjob]

Maximum allowable entry is 70 degrees C (158 degrees F)
AFIK there is no protection against very high numbers entered in error.

[cauc0001]

Maximum allowable entry is 70 degrees C (158 degrees F)
AFIK there is no protection against very high numbers entered in error.
Thank you very much.

[sonicbum]

Hi,

from the FCOM

"Maximum allowable entry is 70 degrees C (158 degrees F). The FMC, however,
will limit the N1 to 25% takeoff reduction."

Haven't tried it yet in the aircraft (luckily) or in the SIM, but I believe the N1 You get will be limited by 25 % take off reduction regardless of the SEL/ temperature entered.