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In aviation terms, a spiral is defined as when an aircraft is spinning towards the ground (like, in a really steep turn) and at the same time accelerating. As you can imagine this can be pretty dangerous, as within a matter of seconds you could potentially be at your aircraft's structural speed limit... ie: Go any faster, lose your wings. Scary stuff. This is why it is very important to learn what a spiral is, when it happens, what the early-warning symptoms are, and how to correct them quickly. You also have to know how to differentiate between a spiral and a spin (basically, you don't accelerate in a spin).

A spiral may look similar to a spin, but a quick check of your instruments should tell you that you are accelerating. After you experience a spin and a spiral in training you will see they feel quite different; they only look the same when you are looking out the window.

The spiral is in essence really only a steep turn. A very steep turn. For example, and aircraft could roll completely on its side, then you could pull back on the control column to get an even steeper turn. This is a spiral. Because the only surface providing lift is your Rudder (even that isn't producing much), you are falling quite rapidly while turning. The smaller the aircraft, the steeper the turn possible (as a general rule of thumb). A big 747 could also spiral, but it wouldn't feel the same as its turning radius is huge.

The trick is that you can get into that attitude and not realize it by doing a high speed stall, not recognize it, and smash the plane up real good. Entering a spin incorrectly on purpose may result in a spiral. Yanking the controls around too quickly while doing maneuvers may result in a spiral. Many things! Read the excellent node on stalls to see how to enter a high-speed stall.

Recovery from a spiral is simple if you realize that you are not in a spin:
• Close the throttle. This will reduce the amount of speed gained in the spiral and the recovery (as speed is your greatest enemy here).
• Roll the wings level. Most spirals will occur at odd attitudes, and the quickest way to stop the spiraling motion is to roll those wings level with the ailerons (this point is important because you use the rudder to roll out of a spin).
• Ease out of the dive. Do this by pulling back on the control column in a nice fluid motion that isn't too quick. If you pull back too quickly you might stall and enter another spiral, blackout, or just plain break the aircraft.
• Only apply power when you are sure doing so will not exceed the airspeed limits of the aircraft.
If you were to use the rudder to recover, thinking it was a spin, you would probably damage the aircraft and make the spiral even worse. Be careful!