DISCLAIMER - The best way to enter and recover from a spin is detailed in your POH (Aircraft Manual), and further more detailed information can be found in your flight training textbooks. Keep in mind that, in most cases, only flight instructors are legally allowed to enter a spin. The following information is only to be used as a guideline to the theory behind spins. I take no responsibility and yadda yadda yadda. Though a spin is the least dangerous of the dangerous maneuvers of an aircraft, it still causes many fatalities and some spins can NOT be recovered from.

In Fixed-Wing Aviation, a spin is defined as a state of flight where the nose of an aircraft is pointed roughly towards the ground, the aircraft is rolling at a constant speed in one direction, and the airspeed of the aircraft remains stable (you aren't plummeting to the ground faster and faster). To help visualize this, imagine you have a model airplane in your hands. Face the aircraft downwards, now use your Globe Trotter skills and spin the airplane on your finger, but spin it from the tip of one of the wings instead of from the center. It should look roughly like this:

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As you could observe while this model airplane is spinning on your finger, the cockpit is moving quite a bit, but the tail section of the aircraft and one wing of the aircraft aren't moving through the air too much, they are just rotating in place. The outer wing is whipping through the air quite quickly, however!

What is happening here is that the airflow over the inside wing is less than the airflow over the outer wing. The outer wing is producing a lot of lift, but the inner wing is dragging it down. When the inner wing is dragging the plane down more than it is creating lift, it is said that the wing is stalled. Keep in mind that there is still a lot of air resistance being met by the inner wing - it's just that the air isn't flowing over the wing in such a direction as to provide sufficient lift. This is why your airspeed in a spin remains constant, and does not increase no matter how much time you spend plummeting towards the ground (which is, incidentally, a speed of about 500 feet every 3 seconds, and 1000 feet per 360 degree rotation... therefore 6 seconds per rotation... in the average small aircraft in average conditions, of course).

It is incorrect to say that ailerons are ineffective during a spin; there is still sufficient airflow over the wings to allow all of the control surfaces to work. There is still SOME lift being generated by the stalled inner wing, but the lift is simply so low the plane is falling. The reason why ailerons should NOT be used to counteract a spin is because the harm it does outweighs the good, and by using the ailerons you may only tighten the spin, or worse, enter a spiral. This is because the aileron on the stalled wing, when moved, interrupts the airflow over the wing even more than it currently is, further reducing the lift the wing produces and increasing the drag. This slows the inner wing, but through momentum the outer wing starts to spin faster - tightening the spin and making it all the more uncomfortable for you.

When recovering from a spin it is very important to recognize that you are in a spin (as opposed to a Spiral or a Flat Spin), that one of your wings is stalled, and that you should NOT operate the ailerons. This is a strong urge to fight, and most people will turn the control column without even realizing they are doing so. Heck, after five or six practice runs, I still move the ailerons a little bit.

The correct way to recover from a spin is to extend full rudder opposite the direction of the spin and press forwards on the control column. This "kicks" the airplane from my first diagram above to a straight-down attitude. This should also stop the spin, as the airflow over both wings is now equalized. You may now notice that since there is no more airplane parts resisting your descent, your airspeed is rising quite quickly. Pull back on the control column to ease out of the dive, regain altitude, and bleed off airspeed.

The official "what to do" checklist would look something like this (and don't forget about my disclaimer above):
  • Reduce throttle to idle - This is done as to not pick up any more speed.
  • Retract flaps - During the pull-out phase of recovery, the high airspeed may result in damage to the flaps or the wing structure.
  • Neutralize Ailerons - As described above, use of the ailerons only makes the situation worse.
  • Apply full rudder in the opposite direction of the spin - This kicks the aircraft into a straight-down, non-rotating position.
  • Push forward on the control column - Pulling back on the column at this stage or leaving it neutral *may* result in entering a flat spin or a spiral. Pressing forward on the column ensures the aircraft exits the spin facing the ground, as to pick up airspeed and to break the stall.
  • Gain airspeed - Gain enough airspeed to ensure the aircraft is stable, is no longer spinning, and is no longer stalled.
  • Recover altitude and attitude - Straighten the aircraft out, and use your airspeed to help climb back to the altitude you entered the stall at. Do not apply power until you are sure you will not exceed the maximum structural airspeed.
Spins have no practical application in normal flight. The only reason people would practice spins are to learn the symptoms of a spin to know how to treat it, if one to occur accidentally. And, of course, for aerobatics. Spins would normally occur in situations where the aircraft is travelling slowly and while banking - most commonly during takeoff, usually resulting in fatalities since the aircraft is so low to the ground. Spins can, of course, occur at any time an aircraft is travelling slowly relative to the air around it, so knowing how to avoid them is imperitive.

The easiest way to enter a spin in a Cessna 172 is to, well, first read my disclaimer above. Then, climb at a steep angle with about 10%-20% throttle - a 30 or 40 degree climb works best. As the aircraft approaches stalling speed, press full rudder in the direction you want the spin to start and pull back on the control column simultaneously. These are MAXIMUM inputs - don't pull the control column halfway back, and don't let go of the inputs until you are ready to recover from the stall. If you change your mind halfway through and ease off on the controls, you will almost guaranteed enter a more dangerous Spiral.

Entering a spin is one of the most stomach-upsetting experiences I've ever had, and I love roller coasters and thrill rides slightly more than the average joe. When you are beginning your climb, it feels like you are sitting upright in a chair, nice and relaxed, as if you were watching TV at home. As your airspeed decreases, it feels like you are reclining backwards in a nice comfy La-Z-Boy chair. Then, suddenly, as the controls are yanked around to enter the spin, you feel like you are being pressed into your seat.. You feel like you are rising... Then suddenly the seat falls out from under you (I hope you had your seatbelt on) and twists to the side, dragging you with it. As the aircraft whips around 180 degrees it also begins twisting - kind of like a two-string kite when one of the strings snaps. The aircraft doesn't really "settle," it just sort of slams to a halt once it's facing straight down. Now being pressed into your seat as you are staring at the ground, you think "good thing that's over," until your mind realizes that you are still spinning towards the ground at a decent pace. Then you start to panic again.

I would like to bring up my disclaimer one last time. Some spins cannot be recovered from, and you will spiral straight into the ground. This may be due to the design of the aircraft, the particular attitude of your spin, or the specific balance of weight between you and your passenger.

Commonly mistaken for spins, spirals feel nearly the same but have a rapidly increasing airspeed, and flat spins are harder to recover from but are easier to distinguish. It is best to know the symptoms before you treat the problem, as if you try to use a spiral recovery on a spin, you will make matters worse!