Is it dangerous to do a skidding turn to lose altitude without increasing airspeed?

Question

I was taking some lessons in a light sport aircraft in South Africa a few years back, my instructor taught me something which I had never seen or heard of before or since. We went through an emergency scenario in which we had to make an emergency landing in a field (not really landing but just going through the motions, of course). I found a place to land and started my descent, but had to loop back around after I passed it. The instructor then told me to give full right rudder with left aileron to quickly descend without gaining airspeed.

Wouldn't this qualify as a recipe for creating a spin and possibly killing myself? Should I actually implement that skidding turn in an emergency to land if need be?

Answer 1

As Ron Beyer pointed out in a comment, your instructor was showing you a forward slip - a pretty standard maneuver for light aircraft.

A spin would involve full rudder and a stalled wing, which usually means pulling the yoke or stick back a lot. That's why overshooting the base-to-final turn can be dangerous - you likely already have a lot of rudder in and it's tempting to pull back on the yoke to try to tighten up the turn. An unexpected spin at low altitude is not recommended as recovery tends to be both abrupt and fatal.

As long as you're keeping the yoke more or less centered forward and back and your airspeed up, there should be nothing to worry about.

Answer 2

I just want to go back to the OP's concern that rudder one way and aileron the other way is related to a spin - it's my understanding that this is a cross-controlled situation, and that is something that can't be ignored, so (again, I'm saying this from an armchair not-even-a-pilot standpoint) it will influence what you have to think about doing at the same time - i.e. not getting slow or finding another way to approach stall regime.

See also the link KJP gave in a comment for the difference between a slip and a skid: "What is a skidding turn (vs slipping turn)?" - this link includes information about the opposite spin-resistance vs. not-spin-resistance of those two maneuvers.

While slipping on final straight-in may not be an easy or likely way to enter a spin, it's not a completely risk-free proposition (like many aspects of flight aren't) for various reasons and while I wouldn't personally avoid it, I'd learn more about it, talk to my instructor about my concerns, practice it with her up high, and of course read answers to similar questions to get a sense for the widespread Internet commentator feelings on the matter."

Googling "can forward slip lead to spin" yields (among others) results (which I lack reputation to post more of):

• http://www.askacfi.com/5189/forward-slip-and-classic-countercontrol-stallspin.htm

Answers and components of answers seem to run a range of things like:

• "slips like this are perfectly OK"
• "slips like this are probably perfectly OK, depending on aircraft and weather details"
• "don't cause a stall and you won't spin"
• "even stalling in a slip condition won't/shouldn't cause a spin because you're not yawing"
• "slips like this are OK but from this condition you can enter certain other conditions which aren't OK"
• "try it way up high and see"

And the FAA "airplane flying handbook" has this (and more) to say: (https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/airplane_handbook/ chapter 8, section "Intentional Slips", pages 11 and 12, which I identified from one of the comments in the above google results):

"Unlike skids, however, if an airplane in a slip is made to stall, it displays very little of the yawing tendency that causes a skidding stall to develop into a spin. The airplane in a slip may do little more than tend to roll into a wings level attitude. In fact, in some airplanes stall characteristics may even be improved."

Answer 3

Slipping an aircraft is mostly perfectly safe. I say mostly because there are exceptions:

1. Some aircraft operator's manuals explicitly forbid slipping. This might be because of the general handling or aerodynamic characteristics of the aircraft.

2. There are other reasons outside of handling why slipping is dangerous to some types of aircraft.
   For example aircraft with jet engines, especially fuselage-mounted ones are prone to engine stall when airflow to the engine is disturbed. Other reason could be unexpected results on ie. fuel feeding with sideways acceleration.

Also slipping disturbs the airflow to the pitot-static system, so speed, altitude and vertical speed indications are unreliable while slipping.
That said, as a maneuver (on an aircraft that slipping is allowed with) it is safe and normal use of flight controls.

Answer 4

"Is it dangerous to do a skidding turn to lose altitude without increasing airspeed"

This is a great question as it underscores the importance of approaching at around 1.3 times stall speed. Also, of critical importance, is your instructor did NOT request a skidding turn, what was requested was a forward slip, although I admit this may have taken me by surprise too.

Anything like this should be learned from an expert and practiced at altitude in a plane that allows for it. In a Cessna 172, approaching at 65 knots, with 20 degrees of flaps, they are a joy to do. 50 knots would be much more dangerous.

A skidding TURN is dangerous as it creates a much greater speed differential between the wings, prompting a poor soul to "hold the inside wing up" with aileron. This is the killer.

However, in a forward slip, The airplane maintains its heading (straight), creates drag with the rudder push, and controls lateral movement along the heading line with aileron. In a straight wing GA aircraft, after the rudder push, there is no difference in the wing tip speeds, just make sure you keep an eye on the airspeed.

The change in glide slope can be fairly dramatic, making this a very useful tool. Just roll out and straighten out before pulling elevator to "round out" your short final. Your instructor did not steer you wrong.