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Should the Thrust Reduction height be the same, lower or higher than the Acceleration Height? In other words, what is their "correct" sequence. I am referring specifically to the B737NG, if it makes a difference. And I am assuming no Noise Abatement procedures are in place, nor any obstacle clearance requirements. Just assume one takes off towards the open sea.

I have already read the thread What determines the "acceleration" and "thrust reduction" heights? but it did not answer my above "simpler" question.

I always thought one does not reduce thrust before cleaning up but reading some posts I am in doubt now.

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  • $\begingroup$ In my flight simulator I use 0ft acceleration height and once I reach 330 knots I gently pull up and only pull back on the throttle upon reaching 5min of TOGA time. In my experience this yields the shortest time to destination :) $\endgroup$
    – Jan
    Oct 15, 2020 at 17:34
  • $\begingroup$ On a more serious note: If you reduce thrust first you usually have to reduce the pitch angle slightly to keep speed. If you reach acceleration height you have to pitch forward again. That can be discomfortable for the pax. If you accelerate first and then reduce thrust you'll create quite a bit of noise for the people on the ground. So I think for passenger comfort and noise abatement keeping the acc and thr red heights the same is the best compromise. Depending on the airport and turns up ahead it may not be practical to accelerate at low altitude, then you increase acc height. $\endgroup$
    – Jan
    Oct 15, 2020 at 17:39

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In absence of any other constraints, both thrust reduction and acceleration should be initiated at 1000 feet above aerodrome elevation:

Flap Retraction Schedule

During training flights, 1,000 feet AFE is normally used as the acceleration height to initiate thrust reduction and flap retraction. For noise abatement considerations during line operations, thrust reduction typically occurs at approximately 1,500 feet AFE and acceleration typically occurs between 1,500 and 3,000 feet AFE, or as specified by individual airport noise abatement procedures.

(Boeing 737 NG FCTM 3.32 - Takeoff and Initial Climb)

Regarding your thought of

I always thought one does not reduce thrust before cleaning up but reading some posts I am in doubt now.

It is actually quite normal to reduce thrust before accelerating and retracting flaps. During NADP 1 (Noise Abatement Departure Procedure), which is most common around the world, thrust is reduced first (typically between 800 and 1500 feet) and acceleration is initiated later (typically at 3000 feet). For NADP 2 both occur at the same time. I don't know any procedure where thrust reduction occurs after acceleration.

Both heights can be set independently on the TAKEOFF REF page 2:

TAKEOFF REF page 2

Thrust Reduction (THR REDUCTION)

Altitude above origin airport elevation at which the autothrottle reduces from takeoff N1 to climb N1.
The default value is determined by the airline and is stored in the model/engine database. The default is displayed in small font.
Manual entries allowed on the ground. Entries must be between 800 to 9,999 feet and are displayed in large font.

Acceleration Height (ACCEL HT)

Displays acceleration hight for flap retraction.
Default value is from the airline.
Entry is optional. Value is a height from 400 to 9999 feet.

(Boeing 737 NG FCOMv2 11.40 - Flight Management, Navigation - FMC Preflight)

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  • $\begingroup$ Based on the above, when does the target speed switch from V2+20 to 230 kt and finally to 250 kt? $\endgroup$
    – Stamatis Vellis
    Oct 15, 2020 at 20:39
  • $\begingroup$ That depends on the selected pitch mode. When taking off with VNAV armed, it will engage at 400 feet AGL and then command 230 kt at acceleration height and 250 kt when the leading edge flaps are retracted. If you take off without an armed pitch mode, it depends on when you manually select a mode (and of course which mode you select). $\endgroup$
    – Bianfable
    Oct 16, 2020 at 7:11
  • $\begingroup$ "and 250 kt when the leading edge flaps are retracted." Not when speed passes the "UP" marker in the speed tape? $\endgroup$
    – Stamatis Vellis
    Oct 16, 2020 at 11:11
  • $\begingroup$ @StamatisVellis See Why does the FMC in B737 limit the climb speed to 230 knots on the VNAV mode until the leading edge devices are fully retracted? $\endgroup$
    – Bianfable
    Oct 16, 2020 at 11:29
  • $\begingroup$ "For noise abatement considerations during line operations, thrust reduction typically occurs at approximately 1,500 feet AFE and acceleration typically occurs between 1,500 and 3,000 feet AFE, or as specified by individual airport noise abatement procedures." In such a case, the commanded speed between 1,500 and 3,000 ft will stay at V2+20? $\endgroup$
    – Stamatis Vellis
    Oct 28, 2020 at 22:35

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