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I learned how to slip during my powered training as a way to lose altitude when too high and, of course, a way to track the runway center-line during a crosswind landing.

Quite a bit more emphasis was put on slipping during my glider training. I guess the reason for this is pretty much common sense. There is no concept of a "go-around" when landing a glider...you only get one shot. That said, I don't practice slips often enough when soaring because purposefully losing altitude in a glider is counter-productive, except when too high in the pattern.

For this reason, I've now gotten into the habit of using a slip to descend when flying powered. This allows me to practice slipping with the knowledge that I am guaranteed by virtue of having an engine to have the option of regaining that altitude. Note: I usually only do this while flying solo since slipping can be a comfort issue for passengers.

This got me to wondering...

Questions:

  • Putting aside the comfort of passengers, Is there any disadvantage to always using a slip to descend when flying powered?

  • If not, why does it seem that this method of losing altitude is not used/taught more frequently?

Update after reviewing answers

I wanted to give this some time to stew before I checked an answer. I came into this fairly (but not absolutely) certain that the short answer was no. I think the responses confirm this. I understand the need some have to "warn" against stalling, but I don't really see how this warning couldn't be applied to any maneuver and really has nothing to do with the fact that you are slipping, short of making a mistake...like skidding instead.

The conclusion I'm taking from the answers is:

  • Slipping can be uncomfortable to passengers, pretty much for the same reason that healing in a sailboat can be perceived as uncomfortable.
  • Slipping is aerodynamically inefficient. Since the objective is to lose altitude, this would appear to be a benefit, rather than a detractor.
  • That some planes, in some configurations, have restrictions on when slipping is allowed/recommended/optimal.
  • That slipping will get you down faster, thus making it preferred if getting down faster is the objective.
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  • $\begingroup$ In reference to your second bullet point, being aerodynamically inefficient isn't really a benefit where there are better alternatives (like starting down sooner for instance). If the assumption in this question is that you are already high on an approach and you need to lose altitude quickly (which it sounds like it is), then the question would be improved by explicitly stating that. $\endgroup$
    – Lnafziger
    Jun 29, 2017 at 20:09
  • $\begingroup$ @Lnafziger, the question wasn't meant to be specifically about being high on approach...it was about descending in general. I agree that slipping should not be a substitute for flying a good pattern/approach. $\endgroup$
    – bclarkreston
    Jun 29, 2017 at 21:34

4 Answers 4

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To address your questions: First, slips are perfectly reasonable for controlling descents. In some aircraft, for example, many Cessna 172s, are placarded warning against extended slips with full flaps. In the 172 case, there may be some buffeting.

Years ago, slips were emphasized more than they are today. Of course there were more primary aircraft without flaps then. So today the vast majority of primary trainers have flaps, and with time instructors have simply avoided emphasizing slips as much. Consequently, the forward slip becomes more of a saving maneuver than a routine maneuver.

Your mileage will vary. There are some instructors who put more emphasis on slips for energy management. Most instructors will emphasize them for more "urgent" maneuvers like making a field with a simulated engine out.

Addendum #1 Being the best pilot you can means constantly developing skill and adding new tools to your bag. Slips are normally covered when doing simulated engine outs. I also cover them first with pre-solo students when we cover other emergencies such as flap failure (asymmetric flap extension) or electrical failures with electric flaps. Slips are also helpful when there is a cabin fire, and help clear smoke faster (it's never really fast enough) in planes with bilateral windows.

Addendum #2 Slips have an advantage over flaps, in a temporal sense. You can slip, and then take it out, then put it back in. As much or as little as you want. Since flap changes normally mean trim changes, and in the case of the 172 there is a flap motor limiting application and removal rates, the same cannot be said for using flaps. As an example, landing during very turbulent and gusty conditions, I might only use partial flaps, and use slips for glide path management, particularly if going to some mountain top strip with all kinds of up and down drafts influencing the path. Just another thought, for someone as they consider enhancing their proficiency with slips.

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  • $\begingroup$ My understanding is that there was an older model 172 that prohibited forward slips with flaps, but that most of the placards are not outright prohibitions. I've never actually flown a 172 that has this placard or limitation so I've never seen it personally. $\endgroup$
    – Devil07
    Jun 27, 2017 at 0:15
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    $\begingroup$ I have, and I believe the C172M (1976) was the last one so placarded. It is not a prohibition, rather an advisory to avoid, as there may be some buffeting. I think that came from pilots being startled by a buffet, but the behavior is not extreme and I have never experienced any adverse effects. YMMV $\endgroup$
    – mongo
    Jun 27, 2017 at 0:22
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Putting aside the comfort of passengers, Is there any disadvantage to always using a slip to descend when flying powered?

No not really. Practically speaking it's well within the maneuvering limits of the aircraft so you are not really going to cause any problems. The only disadvantage might be a slightly higher risk of a low altitude stall due to the uncoordinated flight and presumably low speed. Apparently some aircraft can not be slipped with flaps extended

One additional thought! Some manufacturers specifically placard their aircraft that slips are not allowed with flaps extended. Not sure if that's their lawyers or their safety folks speaking, but it is a reality we must acknowledge.

If not, why does it seem that this method of losing altitude is not used/taught more frequently?

My instructor was big on it when we practiced emergency engine outs and I received plenty of training in slipping. I'll use it on occasion if I'm really high on final but most powered planes don't have nearly the glide ratio of gliders so cutting the power is more than enough to get the plane to really come down. I would say its not used as much in powered aircraft because you have plenty of control already and you don't need to over-complicate the situation.

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I wouldn't use it as a "normal" means of descent, simply because it isn't aerodynamically efficient. You are purposefully creating more drag and lowering the glide ratio of the airplane without a good reason. You end up using more fuel when you could have just pulled the power back (using less fuel) and started a descent or glide earlier.

That being said, good job practicing it in a safe, learning environment when you don't actually need it. This should pay off down the road when you do want to lower your glide ratio by slipping when high on final, etc. It's a good tool to have and should be used when appropriate.

As far as those who discuss spinning from a slip, you should always maintain sufficient airspeed (just like when you aren't slipping), but please take a look at this question about what happens when you stall during a slip: What happens in a stall during a slip?.

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    $\begingroup$ The point of making "non shallow approaches" in a plane that doesn't drop like a brick, is to make the plane more inefficient, and have it drop like a brick. Flaps help. Spoilers help. But both of them lower the glide ratio...does that make them unacceptable because they are also aerodynamically inefficient? $\endgroup$
    – mongo
    Jun 27, 2017 at 21:29
  • $\begingroup$ @mongo The question wasn't limited to approaches, and as I said it is a good tool and should be used when appropriate, however I stand by my statement that it should't be a "normal" means of descent (whether on approach or not). $\endgroup$
    – Lnafziger
    Jun 28, 2017 at 15:01
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Another disadvantage of slipping in order to lose altitude rapidly is that, at any section of the wing, the AoA is considerably increased, because the horizontal speed is reduced slightly, while the sink speed (and thus, the vertical airspeed component) becomes much higher. The combination of both factors result in an increase of the AoA, even if the attitude of the plane is kept the same, and increasing the AoA may lead to a partial or total wing stall...

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  • $\begingroup$ This doesn't make sense. Assuming the loss of horizontal speed is negligible (say, a sustained slip), you need a constant AoA to have a constant amount of lift. The higher sink rate is due to pitching down, and have the fuselage drag due to slip prevent the increase in horizontal speed usually associated with pitching down. $\endgroup$
    – Sanchises
    Oct 18, 2017 at 12:54
  • $\begingroup$ When slipping, you lose altitude because the drag becomes much higher due to the increased angle of the relative wind to the body of the plane. That higher drag slows down the plane, the lift diminishes, and the plane starts to sink with no attitude change. There's no pitching down at all in sideslipping... Hence, the horizontal component of the airspeed goes down, and the vertical component goes up, increasing the AoA. $\endgroup$
    – xxavier
    Oct 18, 2017 at 14:02
  • $\begingroup$ When sideslipping, it's advisable to dip the nose a bit, precisely in order to reduce the AoA and avoid a stall... But sideslipping is always done just with aileron and opposite rudder. The virtue of that maneuver is losing altitude rapidly without gaining speed... $\endgroup$
    – xxavier
    Oct 18, 2017 at 14:15

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