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My instructor asked me this question during my lesson and I couldn't come up with any answers. He asked why do I need to look at the indicated airspeed rather than ground speed when in takeoff roll or as soon as we touchdown?

It would be great if you could give me the reference to back it up.

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    $\begingroup$ Comments are to be used to request clarifications on the question. If you have comments about other comments, please use our main chat room $\endgroup$ – Federico Mar 19 at 8:37
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Because wings work on air moving past them, not ground moving below them.

Heck, in a 35 knot headwind, the Antonov-2 could be rolling backwards at 2 knots and still take off!

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    $\begingroup$ @mast My explanation is far simpler. Which seems to be warranted here. $\endgroup$ – Harper Mar 17 at 16:54
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    $\begingroup$ Agreed, this is why carriers head into the wind. More than one aircraft landed with negative ground speed during WW1. $\endgroup$ – mckenzm Mar 18 at 1:10
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    $\begingroup$ Other answers are based on the same ideas, but this one puts it most directly. $\endgroup$ – David K Mar 18 at 5:08
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    $\begingroup$ Sometimes the best answers are also the simplest. $\endgroup$ – brhans Mar 19 at 2:43
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    $\begingroup$ in a 35 knot headwind, the Antonov-2 could be rolling backwards at 2 knots and still take off! <humor>you sure that's a plane and not a motorized kite?</humor> +1 BTW. $\endgroup$ – Stelios Adamantidis Mar 19 at 9:41
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Because what determines the amount of lift generated is the indicated airspeed, not the ground speed. As usual, it is always easier to think about an extreme case. If you have an aircraft with VR (speed at rotation for takeoff) of 90 knots, and there is an 80 knots head wind, in theory it will rotate with ground speed of 10 knots even though the indicated airspeed will be 90 knots.

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    $\begingroup$ @Notts90 True, but it seems like OP is a pilot or studying to become one. Let's hope they know what Vr is $\endgroup$ – DeepSpace Mar 17 at 12:13
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    $\begingroup$ I don't remember offhand what every V speed means, and this site isn't exclusively for pilots anyway, so why not tell everyone what Vr is? Save some google searches. $\endgroup$ – Xen2050 Mar 17 at 16:07
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    $\begingroup$ For those interested: Vr is defined as the speed at which the rotation of the aircraft should be initiated to takeoff attitude. $\endgroup$ – Mast Mar 17 at 16:48
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    $\begingroup$ @DeepSpace While OP may be training to be a pilot, let's try to remember that SE is for everyone, and is likely to involve others viewing this question later who may not be training as a pilot. For that reason it's usually worth avoiding jargon or adding a quick explanation $\endgroup$ – Jon Story Mar 18 at 2:07
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    $\begingroup$ See also, the infamous "Airplane on a treadmill" meme $\endgroup$ – crasic Mar 18 at 8:03
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Airspeed is always important as it determines lift which keeps you in the air. At the moment of take-off or landing, airspeed is critical because it is the point at which you transition to or from flight; you need to be going fast enough but not too fast so that the transition is positive without being abrupt or overly stressful on aircraft components. This is why there are specified take-off and approach speeds.

Ground speed matters in navigation because it determines flight time which affects fuel required to get from A to B.

Ground speed is a consideration for take-off and landing (ideally kept to a minimum), which is why it's always preferable to do so into the wind, but ultimately it is airspeed that matters and ground speed is what it is at the moment of take-off or landing.

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    $\begingroup$ +1 for mentioning why both are important $\endgroup$ – MPW Mar 18 at 14:27
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    $\begingroup$ It should be noted that groundspeed is also important during takeoff... if you're rejecting the takeoff. $\endgroup$ – Sean Mar 19 at 3:12
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The Wikipedia article on indicated airspeed has a good description. An airspeed indicator is actually more of a "dynamic-pressure" meter, with the dynamic pressure converted to airspeed.

Dynamic pressure is $q=\frac{1}{2}\rho V^2$ where $\rho$ is density and $V$ is airspeed.

The Wikipedia article on the lift coefficient explains that lift is proportional to dynamic pressure, the area of the wings, and the lift coefficient, which in simplified terms can be considered a function of the angle of attack.

$L=\frac{1}{2}\rho V^2 S C_L(\alpha) = qS C_L(\alpha) $

The takeoff speed is the speed at which you will have enough lift to get the airplane off the ground at the angle of attack that the airplane will have post-rotation. So, for a given aircraft at a given weight, wing area, post-rotation angle-of-attack, and lift curve, you will be able to take off at a particular dynamic pressure.

In fact, all the aerodynamic forces on the aircraft are proportional to the dynamic pressure. So that's why stall speed, never-exceed-speed (above which the aerodynamic forces could start damaging the aircraft structure), etc. are all given in indicated airspeed - because it's a proxy for dynamic pressure.

True airspeed is the speed of the aircraft relative to the air mass it's flying through and can be calculated from indicated airspeed by correcting for density and temperature. Your ground speed is then the true airspeed added to the wind speed.

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The IAS includes factors such as Wind Component (Tail, Head or Cross), Pressure and Temperature. All these have an influence on your takeoff. The takeoff Speed Vr is calculated as Indicated Airspeed. Imagine you have a Headwind Component of +50. You Groundspeed would be way lower than your Airspeed. What counts is the amount of air that flows over the wing in order to takeoff, not the speed relative to the ground.

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It is important to note that along with speeds such as v_rotate, predicated on IAS, there are critical speeds on takeoff that are essentially, solely or predominately functions of ground speed. Refusal speed is one. Maximum brake energy speed is another.

V1 is a speed for which ground speed is a critical component, since it is a function of runway remaining and whether it'd be possible to stop, or to be able to continue the takeoff with a lost engine.

The reason we don't use ground speed in determining when we've passed them has to do with what is available to the pilot in the cockpit to look at and safely make decisions with during the takeoff.

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  • $\begingroup$ before we have to remove this answer for unfriendliness, could you consider rephrasing? $\endgroup$ – Federico Mar 20 at 13:19
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    $\begingroup$ You probably meant something like, "It's because pilots aren't computers." However, I recommend looking up how, for example, $V_R$ is defined and wrt to what parameters. (It has to do with lift.) $\endgroup$ – ymb1 Mar 20 at 13:24
  • $\begingroup$ Updated to make more friendly. $\endgroup$ – MikeY Mar 20 at 13:51

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