The Title is my question. Because I think that if one uses a very high Assumed Temperature, the subsequent Max CLB N1 maybe higher that the TO N1.


1 Answer 1


Possibly, but not necessarily.

The climb thrust can be selected on the FMC N1 Limit page. The choice is between CLB, CLB-1 and CLB-2. The climb thrust is then reduced according to the selection:

FMC N1 Limit page

7 - Climb (CLB)
Push – selects full rated climb thrust limit. [...]

8 - Reduced Climb (CLB–1 and CLB–2)
Push – selects the associated reduced thrust climb mode.
CLB–1 provides a climb limit reduced by 3% N1 (approximately 10% thrust).
CLB–2 provides a climb limit reduced by 6% N1 (approximately 20% thrust).
Deletion results in the selection of CLB thrust.
Manual selection of a climb thrust rating overrides the automatic selection.

(Boeing 737 NG FCOMv2 11.40.47 - Flight Management, Navigation - FMC Preflight)

When a takeoff thrust derating or an assumed temperature is entered, the FMC automatically selects a corresponding climb derating. And yes, the climb derating is computed such that it is below or equal to the takeoff thrust:

Derated Thrust Climb

Two fixed climb thrust derates can be selected on the N1 LIMIT page. CLB–1 provides a climb limit reduced by 3% N1 (approximately 10% thrust). CLB–2 provides a climb limit reduced by 6% N1 (approximately 20% thrust). The reduced climb setting gradually increases to full rated climb thrust by 15,000 feet.

In cruise, the thrust reference automatically changes to CRZ. The reference can be manually selected on the N1 LIMIT page.

Use of an assumed temperature reduced thrust takeoff or takeoff derate affects the FMCs climb derate computation. If a reduced thrust takeoff has been specified on the TAKEOFF REF page, the FMC will re-compute CLB-1 and CLB-2 values as required to avoid a climb N1 value greater than the reduced thrust takeoff N1 value.

(Boeing 737 NG FCOMv2 11.32.4 - Flight Management, Navigation - Flight Management Computer; emphasis mine)

However, if the pilots manually select CLB after the automatic selection of a derated climb thrust, then the climb thrust can actually increase over the TO thrust.

  • $\begingroup$ And higher than FL150 it by default reverts to CLB, regardless of being in CLB1 or CLB2? $\endgroup$ Commented Nov 14, 2020 at 19:36
  • 2
    $\begingroup$ @StamatisVellis - That's correct. The CLB-1 and 2 schedules match up with CLB at 15,000' and above. $\endgroup$
    – Ralph J
    Commented Nov 15, 2020 at 0:39
  • $\begingroup$ GE calls this process of eliminating the derate 'fast taper' when it happens around 14,000 ft and 'slow taper' when it merges around 27,000 ft. This is one of many options that the Operator chooses and freezes. Infact the %derate is another Operator choice. $\endgroup$
    – skipper44
    Commented Nov 17, 2020 at 16:41

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