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I know jet-powered airplanes very rarely use 100% available power for takeoff, and most often reduce power to increase engine life. How does a pilot set thrust to an exact level?

The airplanes I'm referring to are common commercial jets such as A320/B737 types.

If they'd set it manually with thrust levers, there are some complications: there are no placards on them. If you try setting revs via the displays, revs and temperatures do not change instantaneously, and looking at the indicators and adjusting the lever would be a dangerous distraction during take-off (imagine: the engines roaring, the brakes are applied and tires are rubbing against the asphalt; or the plane is rolling, and the pilot is looking at the gauges).

On the other hand, if a limit is set via FMS, how does it know when to add more power if it's urgently needed?

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    $\begingroup$ Most aircraft have a 'TO/GA' (Take Off/Go Around) button on the throttles, which overrides any autothrottle limits and delivers maximum thrust. $\endgroup$ – Someone Somewhere Jul 24 '16 at 12:47
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    $\begingroup$ @SomeoneSomewhere, Boeings have a button. On Airbus, moving the thrust levers all the way forward works that way. $\endgroup$ – Jan Hudec Jul 24 '16 at 15:28
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    $\begingroup$ Just as a matter of historical info, in 747-100/200 aircraft it was the flight engineer who, when the call was given by the flying pilot, set the takeoff power to the appropriate N1 with the thrust levers. A really smooth f.e. could do it without any jockeying of the thrust levers. If more power was urgently needed, max power would be called and the f.e. would set that. $\endgroup$ – Terry Jul 24 '16 at 19:46
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The EMB-145 used a combination of FADEC settings and the thrust levers to select the desired thrust and to signify the need for additional thrust.

The thrust levers had a detent that selected "100%" thrust for the given mode the FADEC is operating in. If you pushed the thrust levers past the detent to "max" this would give you extra thrust. The mode the FADEC is operating in is selected by a button in the cockpit that cycles through the available modes.

For takeoff power the modes were "ALT T/O", "T/O" and "E-T/O" which gave you 90%, 100% and 107% thrust for takeoff when the thrust levers were in the detent. If advanced past the detent this would put the mode into a reserve mode and depending on model would give you up to 110% or 117% thrust.

Setting the exact amount of thrust needed was up to the engine FADEC units, our job was just to tell it what thrust mode it was in and it scaled the output of the thrust levels to that mode. Flying a 90% thrust takeoff, a normal rated climb and then reducing to cruise levels only required initially setting the thrust levers into the detent and adjusting the FADEC mode.

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    $\begingroup$ I think this most accurately answers the question of "how do the pilots set the thrust?": select a mode, move the levers to the detent, let the computer do the rest. $\endgroup$ – FreeMan Jul 25 '16 at 12:24
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Usually power output for a jet engine is gauged from its low pressure turbine speed. In turbofans, this corresponds to the fan speed, displayed as N1. In general, the derated takeoff thrust will be selected based on performance calculations done prior to flight, taking into account weight, runway available, climb performance and ambient atmospherics, which will determine the appropriate N1 speed to set during the takeoff roll.

There exist two types of derated thrust operations: fixed and variable. Fixed derate is a pre-set derated thrust by the engine OEM. Variable derate offers the flight crew the ability to select the derated thrust from a range of values.

For more information, consult AC 25-13 on this.

So also is this presentation from Boeing, which includes how thes factor are loaded into to the FMS, which then computes a corresponding N1 value and displays this on EICAS, as does this presentation for Airbus.

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    $\begingroup$ I don't think this quite answers the question. He's not asking how the derated thrust target is identified, he's asking how exactly it is selected on takeoff. Do the pilots just keep advancing the throttles until N1 reaches the desired amount? You should incorporate Simon's comment in your answer to make it complete, in my view. $\endgroup$ – Ben Jul 24 '16 at 11:33
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    $\begingroup$ Interesting description of the principles. If you keep this good material and just add in introduction that the thrust levers are just in the TO/GA position regardless of the actual thrust that will be delivered by the engines (if this is the case), then this will be a good answer (as @Ben commented). $\endgroup$ – mins Jul 24 '16 at 12:35
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    $\begingroup$ @mins, on Airbus, they are not in the TO/GA position. TO/GA on Airbus always means maximum possible thrust. There is another detent, labelled MCT/FLX, that selects the derated thrust. On some models, the CLB detent can also be configured for derated climb. Above the CLB detent, the levers are never placed in arbitrary position, only in one of the detents. $\endgroup$ – Jan Hudec Jul 24 '16 at 15:45
  • $\begingroup$ @JanHudec You should add a link such as this one to your comment $\endgroup$ – Manu H Jul 24 '16 at 17:26
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caseys answer nicely describes the procedure for the Embraer 145. Since you specifically mentioned the Boeing 737 and Airbus A320, here is how it works for these aircraft:

Boeing 737

All Boeing aircraft have autothrottles with servo motors which move the thrust levers in the cockpit. This system is also used to set takeoff thrust:

  • During the preflight, the desired derating is selected in the CDU (see left image below).
  • When starting the takeoff roll, the thrust levers are advanced to about 40% N1.
  • After the engines are stabilized, the TOGA button on the thrust lever (see right images below) is pressed and the autothrottle becomes active. The servo motors then move the thrust levers to the desired position.

B737 N1 Limit and TOGA button

Airbus A320

Airbus aircraft do not move the thrust levers with autothrottle engaged. The thrust levers usually remain in the CL (climb) detent during flight and the autothrottle sets the desired thrust directly.

The takoff procedure starts similar to the 737, until the engines have stabilized. Then, the thrust levers are moved into the FLX/MCT (flex/max. continous) detent (see image below). This will set the derated thrust. For maximum takeoff thrust, the thrust levers can be placed into the TOGA detent.

A320 Thrust Levers

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