Is there a slight difference baked into the aircraft's throttle control systems or are they always pumping out the exact same thrust as each other?


If the fuel control is hydromechanical (old engines) or electronically trimmed hydromechanical (not so old engines designed in the 70s and 80s), you set each thrust lever to achieve a target value on the fan RPM (N1) indicators or Engine Pressure Ratio indicator (EPR) indicators (for a pure jet). Each engine will be set to its takeoff thrust setting, which should be the same for each since they are all the same model, but the thrust levers may or may not quite line up due to mechanical and other tolerances (there are limits).

On some modern airplanes with Full Authority Digital Engine Control, you move the levers to a detent setting, usually one for climb and one for take off, that is created by a roller running in a track inside the quadrant for each lever, that drops into a dip in the track; you can feel it click into place.

For takeoff you push them up through the climb detent to the takeoff thrust detent, so it's "click click". That position tells the FADEC computers to set takeoff power and the levers will all be perfectly lined up, the position being a function of the detents in the quadrant. Of course, if it's got an autothrottle system, the computers take care of moving the levers as well.

In any case, the thrust settings for each engine should be the same, unless the airplane had engines with different thrust ratings inboard and outboard for some reason, and which would be an unpopular feature with most airlines. I'm not aware of one.

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    $\begingroup$ Airbus and Boeing both use FADEC. Airbus has “detents”, Boeing does not. $\endgroup$ – Mike Sowsun Sep 7 '19 at 6:52
  • $\begingroup$ When the levers are in the same position are the engine control systems calibrated in some regard for equal thrust, equal fuel flow, or something else? $\endgroup$ – crasic Sep 7 '19 at 7:39
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    $\begingroup$ You rig for synchronized levers with thrust/power equal, because that's what you are generally using to set power and that's what the crew will want to see. On a turbofan, fuel flows and core rpm might vary a little bit for a given fan rpm from engine to engine depending on how worn out an engine is compared to its sister. If two engines are set to the same N1 and the thrust levers have significant split, they may snag it, depending on how fussy the capt is. One may snag it if they are split a quarter inch, another not until they are half an inch. $\endgroup$ – John K Sep 7 '19 at 14:57
  • $\begingroup$ @MikeSowsun how does Boeing do it specifically as far as where to place a lever for say climb thrust? $\endgroup$ – John K Sep 7 '19 at 15:04
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    $\begingroup$ We almost always use Autothrottle so it is done automatically. If for some reason you need to disconnect the Autothrottle, you just advance the thrust levers to get the desired thrust on the N1 gauge just like a regular aircraft. $\endgroup$ – Mike Sowsun Sep 7 '19 at 15:42

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