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I come across it in many different situations e.g. "settings changed to flight detent" or "landing altitude changes at first detent". There are other scenarios which I came across in the past, but I don't remember them.

Could someone explain me all the different scenarios and what it really means?

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    $\begingroup$ Simply put, it's a tactile click stop on some lever or knob (typically some input with a continuous range), usually used to indicate that there's something special about that position. Depending on the purpose, some may even have lock-out mechanisms for protection. $\endgroup$ – aerobot Feb 12 '18 at 13:14
  • $\begingroup$ The word comes from the same root as detain, which means to hold something in place. $\endgroup$ – Robusto Feb 12 '18 at 14:37
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    $\begingroup$ @Robusto Does it? MW and OED say detent is from Latin tendere (to stretch), while detain is from Latin de + tenere (away + to hold), both via Old French. So while tendere and tenere are similar, they are not the same verb. $\endgroup$ – bishop Feb 12 '18 at 16:26
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    $\begingroup$ If you drive, you probably use detents every day. Your windshield wiper control stalk probably has detents for the different wiper speeds. If you have an automatic transmission, your transmission lever or stalk probably has detents for each setting. If you have a mouse with a scroll wheel, that wheel might be detented to have a detent for every three (or whatever) lines of scrolling. $\endgroup$ – Todd Wilcox Feb 12 '18 at 17:56
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A detent is usually a discontinuity in force at a certain position: the control likes to move into that position, and moving it away takes more than average force.

enter image description hereImage source

In the image above, the flap detents are clearly identifiable as the notches in the guide rail: in order to move the lever away from a notch, it needs to be lifted first and can only then be positioned in a different detent.

The flight control sticks often have a detent that indicates force trim position: on either side of stick travel is a preloaded spring, and the stick can only be moved once the breakout force is exceeded. The rest of the stick travel will then simply increase force proportional with the spring gradient - if released to neutral, the force will gradually decrease until the stick falls into the breakout position.

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  • $\begingroup$ Thank you very much for your answer. So in my current case, does this mean the landing altitude changes every time there is a discontinuity in force at a certain position? But then what I don't understand is : Is the landing altitude not set by the pilot (fixed --> landing altitude =runway altitude)? How can landing altitude change at first detent, 2nd detent ? $\endgroup$ – kg1913 Feb 12 '18 at 13:15
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    $\begingroup$ And note that this isn't specific to aviation -- all kinds of levers, knobs and other mechanisms have detents. $\endgroup$ – David Richerby Feb 12 '18 at 14:20
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    $\begingroup$ @kg1913 It's much more basic than that. It's a feature of the control itself - the motion of the lever wants to stop at that position and you have to provide extra force to move it out of that position. It will then stop at the next position, and so on. Think of a rotary knob with multiple stops. It "clicks" from one to the next. The impact of what the control actually does to the flight altitude or whatever is completely independent from this concept. $\endgroup$ – GalacticCowboy Feb 12 '18 at 15:21
  • $\begingroup$ @kg1913 It's a feature of the input mechanism, the effect it has depends on the function of the device that the setting is selected of. $\endgroup$ – Koyovis Feb 12 '18 at 20:44
  • $\begingroup$ Does the word "detent" also apply to when the discontinuity isn't intentional, but rather the result of (for instance) faulty prouction or wear and tear? $\endgroup$ – Arthur Feb 13 '18 at 14:34
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An example most people would be familiar with is when shifting into reverse in a car with a manual transmission; normally there is a detent you need to "push through" to get into (and prevent accidental engagement of) reverse.

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  • $\begingroup$ Good example, and welcome to Av.SE! $\endgroup$ – Ralph J Feb 12 '18 at 23:12
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    $\begingroup$ Yes, it impedes from neutral to reverse, but does not impedee between neutral and drive, between which you can switch freely; and should. Simply so you know how the car handles there, and can get it there using muscle memory in an unintended acceleration incident. $\endgroup$ – Harper Feb 13 '18 at 0:22
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A detent can be simply a palpable change in operative force.

Take a wall dimmer: it has free movement in the dimming range. At the lowest dim setting, if you continue, a significantly greater force will snap it past a detent and into "hard off", which causes a plain switch to de-energize the bulb so you can change it. At the other end is another detent to "hard on", where a plain switch bypasses the dimmer electronics entirely (so they will stop buzzing.)

E.G. if you set throttles to full military, you don't need to release a latch to get it past TO/GA, just the extra force to push past the detent.

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