Forward slip to a landing.

3 Answers

1. 
   

   
   

   Skyfox on Jun 22, 2016

   On page 8-10 of the AFH in the section on Intentional Slips:

   “Intentional slips, however, are used to dissipate altitude without increasing airspeed, and/or to adjust airplane ground track during a crosswind. Intentional slips are especially useful in forced landings, and in situations where obstacles must be cleared during approaches to confined areas. A slip can also be used as an emergency means of rapidly reducing airspeed in situations where wing flaps are inoperative or not installed.”

   Basically, when you’re on final and you’re too high, and you want to get back down to the approach path without nosing it down and picking up a bunch of speed, you can do a forward slip (yaw to the left while banking to the right, or vice versa) to greatly increase the drag on the aircraft and make it lose altitude much more quickly without picking up that airspeed. Relaxing the controls and returning to a normal flight attitude will continue the approach like normal once you get to your desired glide path. Similarly, when there’s a strong crosswind, on low approach you can transition from a crab angle into the wind to a side slip that will compensate for the wind with bank while yawing to keep the longitudinal axis of the airplane aligned with the runway and path of travel. While the big iron aircraft often land with a crab angle, for small aircraft that’s hard on the landing gear and it’s much better to touch down on one wheel so the combination of aileron and rudder can keep things lined up right without wind drift to the side.

   Also keep in mind that some aircraft have warnings in the POH and/or placards that state that forward slips are not approved while the flaps are in a fully extended (and sometimes partially extended) position.

   Timing for a forward slip isn’t so much a matter of time but of altitude; when you feel comfortable with the altitude you’ve gotten down to, stop the forward slip and continue as normal. If you’re still having to do a forward slip on low approach as you’re coming up over the threshold, it may be better to do a go around and try again. For a side slip into the wind, your timing (largely based on gut feeling as you feel is comfortable) is just about the time you’re coming up over the threshold when you transition from a level crab angle to a banked side slip.

   I’d like to provide more references but I’m at work right now and only have access to the AFH and PHAK in PDF files.

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2. 
   

   
   

   Skyfox on Jun 23, 2016

   Now that I’m home and can check my books, here’s a little bit more info and references. From “The Pilot’s Manual 1: Flight Training” by Trevor Thom (2nd edition, 1993) in the chapter about no-flap landings:

   “If you are too high on approach, reduce power and lower the nose slightly–if the power is already at idle, consider a forward slip to increase the rate of descent and steepen the approach path. Recover smoothly from the forward slip and stay aligned with the runway centerline before touching down.”

   And from the same book in the chapter about slips:

   “The forward slip is mainly used to steepen the glide without gaining airspeed, and is generally used in airplanes not fitted with flaps. The forward slip is an especially valuable maneuver when you find yourself too high on an approach to land, and flaps are not available to steepen the descent. if you tried to steepen the descent by diving, the airspeed might increase unacceptably–slipping avoids this by virtue of the greatly increased drag.
   …
   The forward slip steepens the descent by presenting the wing-down side of the airplane to the airflow, significantly increasing drag. The lift/drag ratio is decreased, which causes the rate of descent to increase and the flight path to steepen. The greater the bank angle and amount of top rudder used, the greater the slip and the steeper the descent.
   The power is usually at idle for a forward slip since the aim is to lose altitude quickly, and it would be contradictory to fly a forward slip maneuver using power at the same time.
   …
   If you are going to proceed with a landing, then the forward slip must be removed prior to landing, both to align the longitudinal axis with the runway direction and to decrease the higher rate of descent. Some altitude is required to do this; your flight instructor will advise you on the altitude above the ground at which you should cease the forward slip.”

   Something else I forgot to mention in my first answer is that the pitot and/or static systems can be affected by the changes to airflow and relative pressures against them, showing incorrect readings on all your pitot/static instruments. Also forgot to mention, when the airplane is in a slip it is banked, reducing the vertical component of lift and therefore allowing weight to take over more.

   For another reference, look at the “Flight Training Handbook” (AC 61-21A). It’s an old advisory circular publication but you should still be able to find it in print somewhere. It pretty much describes the same information outlined above.

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3. 
   

   
   

   Mark Kolber on Jun 24, 2016

   The bottom line is that the direction of the slip – into the wind or away – is not really addressed in official training material. That’s in part because, aerodynamically, it doesn’t make any difference and so is largely a matter of pilot technique preference. My preference is based on two factors.

   One is the strength of the wind. That’s based on the same consideration of transitioning to a crosswind slip at the end of the forward slip but it’s also a matter of helping to maintain the proper ground track in a strong crosswind.

   But that’s secondary most of the time to me. If you’ve done the forward slip properly, there should be plenty of time to transition to a crosswind slip from either side. Primary is my view out the window. I want a good view of the runway.

   If I slip nose left on final, I have to look diagonally across the airplane to the right, maybe even past a passenger and all the way out the right side window. OTOH, if I slip nose right, I have a much better view of the runway. That’s much more important to me than which way the wind is blowing.

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