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I'd like to rephrase the question:

Often I forward-slip on final right right rudder. During this time, I use the ailerons to keep the aircraft headed down the center-line; the aircraft's bank varies slightly from left wing down, to right wing down.

At some point during this procedure, I have right rudder in and a slight right bank. I believe this is a skid; Is this something I should worry about? I would very much like to avoid skidding at all times, especially on final!

Previous question, for reference:

Assuming there is a crosswind and the pilot executes a crabbed approach, during the flare the pilot is going to have to use the rudders to avoid a side-load. My understanding is that this will result in an uncoordinated flying condition and is prone to stall-spin.

However, this (the flare) does not seem to be much of an issue among pilots. What am I missing? How is this any different than a skid?

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    $\begingroup$ Spinning needs high angle of attack, which in turn needs a high sink rate. If the runway alignment is done a few feet above the ground, there is simply not enough altitude left to build up a high sink rate. The danger is to do it too early, so the aircraft is airborne for more than a few seconds after the rudder command and builds up side speed. $\endgroup$ – Peter Kämpf Jun 6 '15 at 10:17
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    $\begingroup$ @Peter: Just prior to touchdown, there is often a high angle of attack. $\endgroup$ – David Jun 7 '15 at 4:16
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    $\begingroup$ Not that high. Not even close. $\endgroup$ – Peter Kämpf Jun 7 '15 at 6:41
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Crosswind landings are always uncoordinated. I'm not familiar with big jets, but I believe they simply kick the rudder to align the nose with the runway and let the momentum carry it in the right direction until it touches down, but I might be mistaken here.

In a smaller aircraft (like a piston single) you're taught to drop a wing into the wind and effectively land in a side slip.

This is all perfectly safe, the dangerous situation you're talking about is a skidding turn, where you apply too much bottom rudder, letting the 'inner' wing slow down to the point of stalling. This only applies to an actual turn however, if you're flying straight, coordinated or not, there's still more or less the same airflow over the wings. You could however experience other aerodynamic effects where fuselage and airfoils effectively perturb the air for other airfoils and might cause a wing drop or pitch change, but it depends on the properties of the aircraft.

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Basically it's not an issue because we don't get down to a speed where we need to worry too much about a spin. It takes more than uncoordinated flight to induce a spin. You have to have a stall first. You don't stall as long as there's adequate airflow over the wings. As long as you don't get below ${V_S}_0$ (with flaps extended) on both wings, even if you're cross controlled, you're okay.

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  • $\begingroup$ I'd argue that you are at a speed where you may stall. Right before touchdown, you are almost stalling. $\endgroup$ – David Jun 7 '15 at 4:12
  • $\begingroup$ Well, the point is, you stay above stall speed. The idea is to get the plane just above the surface without stalling, and then match the reducing airspeed to back pressure on the yoke so that the stall occurs right as the wheels touch down. So you never want to be so close to stalling speed that you have to worry about being cross controlled. If you're that slow, you need to go around. $\endgroup$ – Calphool Jun 12 '15 at 23:22
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Slipping during a descent is not particularly likely to cause a stall because both wings are are sufficiently unloaded that they're not likely to exceed their critical angle of attack. Forward slips to a landing are, in fact, a very useful skill to have, especially for pilots flying very light singles (possibly without constant speed props or even flaps) and one that can allow pilots much more flexibility during landings and engine-out emergencies.

You do need to be careful not to flare too soon, as that might cause your airspeed to bleed off to the point were you will either begin to sink too rapidly and touch down hard or, in a really bad outcome, actually induce a stall and run the risk of losing control of the aircraft. It's probably worth noting that this is a concern for normal landings as well.

In addition to that, even if you were to stall the aircraft while in a slip, the aircraft will snap towards the outer, raised wing, reducing the angle of bank (most modern light trainers will generally just return to a more or less wings-level attitude) and making for a much easier (and less harrowing) recovery. A stall during a skid, by comparison, will snap the aircraft towards the inside, lowered wing, and put it into the incipient phases of a spin (and will usually do so in a far more violent and surprising manner than you might be expecting, even if you've spun that aircraft before). This is a much more unpleasant experience (even in normally benign aircraft) and requires prompt and correct action in order to successfully recover.

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  • $\begingroup$ In the original question, I'd argue that it's a skid, not a slip. Or at least, too close to call. $\endgroup$ – David Jun 7 '15 at 4:15
  • $\begingroup$ It is very much a slip. The realignment with the runway is actually by banking the aircraft; once the aircraft has been lined up, is opposite rudder being applied in order to keep the aircraft in a wing-low condition, without allowing it to turn further. Large airliners do usually make this correction with the rudder, but they don't usually land in a slipped attitude, so all they're aiming to do is correct the aircraft's alignment before touchdown; even that is too brief to be properly called a skid. $\endgroup$ – habu Jun 7 '15 at 9:43
  • $\begingroup$ habu@ Thanks. What if, in the process of aligning the airplane with the centerline, the airplane rolls the other way past level. All of a sudden, you'd be in a skid, right? $\endgroup$ – David Jun 8 '15 at 0:51
  • $\begingroup$ @DavidHinkes - in a properly performed slip, the alignment is performed through the use of ailerons and banking; (opposite) rudder is used simply to keep the turn from continuing and to keep the aircraft traveling in a straight line and aligned with the runway. I recommend the FAA Airplane Flying Handbook, page 8-10 for more details faa.gov/regulations_policies/handbooks_manuals/aircraft/… $\endgroup$ – habu Jun 8 '15 at 9:31

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