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What is the phenomenon on a commercial airliner (say, a 737-800) where shortly after takeoff and before we reach 10,000 feet the engines feel like they slow down dramatically or even power off momentarily, then spin back up?

This happens fairly frequently when I'm flying and, although I'm used to it, it still takes me by surprise every time!

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Do you mean the engines immediately spool back up or after a while? –  Jan Hudec Jul 22 at 9:36
2  
I want to upvote twice to offset the pointless -1 :( –  shortstheory Jul 22 at 16:01

4 Answers 4

up vote 19 down vote accepted

There are several kinds of power reduction which could happen after takeoff:

  1. Cutback (noise reduction when the airplane is very low, after takeoff)
  2. Climb thrust

Both are set on the CDU (Control Display Unit, the screen and keyboard of the FMC, Flight Management Computer).

Since you describe a temporary and dramatical power decrease, this should be the cutback. When you hear the power increase again, it must be that the setting goes from cutback to climb thrust.

In the B737-800

The cutback option on the CDU

  • This is the CDU (Control Display Unit), where you set up the cutback. It is in the second page of TAKEOFF REF. Here is the screen before turning cutback ON.
    Cutback off

    • ACCEL HT : This is the height (above ground level) at which we will let the plane accelerate (we will lower the pitch a little bit and gradually retract flaps).
    • EO ACCEL HT: Same as above, but applies when one engine is not running (Engine Out Accelaration Height). This setting will be used in case of engine failure after takeoff.
    • REDUCTION: This is the height at which the thrust setting will go from TOGA to CLIMB. We need maximal thrust during takeoff, because it's a critical phase and it's safer. But at the same time, keeping this setting too long decreases the engines lifetime because of the high temperature. It also burns more fuel, it's noisier and less environment-friendly. That's the reason why the airplane won't climb with maximum thrust.
  • But this question is about cutback. Here is CDU again after turning cutback ON.
    Cutback on

    • CUTBACK N1: This will be the target N1 setting when the REDUCTION height will be reached. The Auto-throttle together with the EEC will arrange for maintaining this lower shaft speed (here you can see it's 84,6%). This will be a lot less noisy but will still provide enough power to climb!
    • EO ACCEL HT: Same as above
    • REDUCTION: At this height, cutback will happen and CUTBACK N1 will be the engines target N1 speed.
    • RESTORE: This is the height at which cutback is no longer needed, and climb thrust N1 setting will be the new N1 target for the engines (92.3% in this example).

Takeoff with cutback

  1. This is the takeoff roll. IAS indicator is not active yet (below 45KIAS). Ground Speed is currently 35kt.
    N1 and TO/GA are displayed on the FMA (Flight Mode Annunciator) at the top of the PFD (Primary Flight Display). On the right, you can see on the ED (Engine Display) that the green marker is indicating TO thrust (because of TO/GA mode).
    Takeoff roll

  2. Few seconds after the rotation. You can see at the bottom of the PFD the radio altimeter indicating 300ft AGL. We haven't reached the REDUCTION height, so the cutback is not active yet. The ED still indicates TO thrust and the green markers are just below 100% N1.
    After rotation

  3. Few seconds after reaching REDUCTION height. N1 is active on the FMA. Radio altimeter indicates 960ft AGL, this is above our REDUCTION height, so the cutback should be active and our engines should be a little quieter. A look at the ED and we can see that the green target N1 marker is now indicating 82.6% N1. Our engines are still rotating faster than that (98% N1), and the EEC is currently decreasing the fuel flow arriving to the combustion chambers.
    Cutback, 960AGL

  4. We are now reaching 2500ft AGL. This is below the RESTORE height of 3000ft, so the cutback is still active, as shown by the ED. The green target N1 marker is indicating 83.0% N1 and the current N1 for both engines is also 83.0% N1. In 500ft, the N1 target speed should increase again to climb thrust settings.
    Cutback, 2500AGL

  5. We've just reached the RESTORE height (above 3000ft AGL), and we can see the green markers show 92.3%. We are back to the climb thrust settings and our plane is loud again! The flaps are still down, and the pink speed marker on the PFD just moved to the top. We are going to retract the flaps and slats, and accelerate to 250KIAS. It seems that RESTORE height is also the ACCELERATION height when cutback mode has been turned on. [I'm not sure, could a B737 pilot confirm this?]
    Restore & Acceleration height

  6. The flaps are up, our speed is now 250KIAS.
    Acceleration done

Further readings

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In addition to noise reduction aspect I suspect there are human factors involved. On takeoff and the initial climb you are exposed to a continual acceleration which you become accustomed to. The reduction to climb thrust could feel like a dramatic reduction as the forward acceleration diminishes. –  Adrian Jul 22 at 11:00
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That makes a lot of sense. The plane needs high engine thrust to get off the ground but once airborne and low, it also doesn't want to disturb nearby residences, so power would be cut back to enough to maintain a climb. Once high enough where engine noise isn't a problem, climbing thrust would kick in. Sweet! –  Green Jul 22 at 13:37
    
:) I will edit my answer in several hours to add a screenshot of cutback option details in the B737 NG. –  Fox Jul 22 at 13:55
    
Thank you - that makes perfect sense and it's what I thought was the case! –  philwinkle Jul 22 at 14:42
    
Why would the engines cutback at FL 100? –  shortstheory Jul 22 at 16:02

Takeoff clearances are received from Tower ATC and contain an altitude limit. "Delta 632, cleared for takeoff runway 32L climb and maintain 3,000." Upon reaching 3.000 the pilot must level off which means he must throttle back. Then he will receive a further climb clearance from Departure Control and in most cases that clearance will be to a higher altitude. The pilot must throttle up for that.

Noise abatement procedures also affect this as well.

When at a non-busy airport ATC may clear the plane higher before he levels off, in which case the climb will be continuous.

The throttle down you notice is routine and normal.

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This would be under FAA jurisdiction, as take-off clearances in ICAO-land do not contain altitude restriction, but rather have an initial climb altitude on the SID chart. And airplanes are usually cleared higher before reaching the initial climb altitude, as it's in many cases some 4000-5000ft above airport elevation. Additionally, the OP does not really specify that a level off occured. –  SentryRaven Jul 22 at 13:24
    
"When at a non-busy airport ATC may clear the plane higher before he levels off" - and at busy airports as well (from my experience at EHAM). In my opinion initial clearances are mostly formalities related to a handover: both so controllers stay away from each other's airspace and to provide a safe situation in case the next controller doesn't issue the next clearance in time. But in practice ATC's secondary objective is to make flights as smooth as possible and providing clearance well in advance so pilots don't have to take unnecessary actions is part of standard procedure. –  CompuChip Jul 22 at 16:46

I'm sure someone will have a better answer, but I think its when the aircraft moves from a take off power setting to a climb power setting.

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There's no hurry to have an answer posted immediately so why not just wait for somebody to post the better answer? –  David Richerby Jul 22 at 9:43
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@DavidRicherby I think "to gain rep" would be a very valid answer, especially when someone is new at stackexchange. –  Volker Siegel Jul 23 at 7:43
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I have no interest in "gaining rep" actually, I answered because I thought it might help answer the question. But with the attitude people have I think it would put anyone off trying to pass any knowledge they might have. –  browners777 Jul 24 at 8:21

Because the engines in some of these aircraft are more powerful than needed. Remember, 250 airspeed rule below 10k feet. If an aircraft is cleared to "after takeoff climb and maintain 2000" then he wont need all of this power. Additionally, the aircraft isn't fighting drag from gear/flaps at this point, nor friction on the ground. And it is no longer a race to get off the runway. Why not save fuel AND avoid a stern talking to from the FAA at the same time? Once the aircraft is cleared to a higher altitude, they spool up again for the climb.

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"Because the engines in some of these aircraft are more powerful than needed." Engineers don't design planes to be overpowered. Yes, certain runways/load conditions/weather situations make the plane overpowered FOR THAT SITUATION. FAA regulations require that transport category aircraft must be able to maintain a certain climb gradient at max gross weight with one engine inoperative (OEI). If you do the math, this means a twin engine airliner will lose about 80+% of its climb ability with OEI. Single engine performance determines how powerful the engines have to be... (cont.) –  Skip Miller Aug 12 at 13:40
    
So my argument here is that no certified airliner is overpowered, but I agree that in certain situations a plane may have excess thrust for a given situation. Sorry for picking this giant nit... –  Skip Miller Aug 12 at 13:41

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