Slips: Forward Or Side, What’s The Difference And Why Care?

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This article’s title would’ve been killer material in the third grade, and since pilots never grow up, let’s consider what is barely whispered in polite aviation society: Slips. Forward or side, the terms are casually interchanged while illustrating uncoordinated maneuvers with ambiguous hand gestures that compound uncertainty. So, after decades of teaching them, I’m determined to attempt to learn the difference.

For legitimacy I’ll reference the FAA’s Airplane Flying Handbook (AFH, FAA-8083-3B), the grandest of government manuals with a hint of 1930s British whimsy. Although, I’d suggest renaming it the Aeroplane Flying Handbook. Not as dull as the AIM or as incomprehensible as FARs, the AFH is an unsung tome, moldering in public domain.

A slip is an uncoordinated turn, that may or may not be turning and should be intentional. It’s not complicated but intimidates those who believe the slip/skid indicator’s coordination ball must remain welded to the base of the inclinometer’s curved tube, lest all onboard die from inevitable stall/spin embarrassment. Except without slips, landings in even the mildest crosswind will stink (ignore Ercoupes). More important, slips add to the full measure of aeronautical satisfaction, an unreachable goal, constantly sought. We’ve previously covered slips in crosswinds, but nothing succeeds in education like repetition.

Most single-engine airplanes land smoothly in crosswinds with aileron input into the wind plus appropriate rudder to keep the spinner pointed down the centerline and the tires on their rims. That intentional, and necessary, cross-controlling is a slip. But what flavor, side or forward? And what’s the difference?

They’re the same thing. End of lesson. Class dismissed.

Except they’re not so back in your seats. Both forward and side slips involve aileron input and opposite rudder. The coordination ball slips to the lowered wing. In a skid—the slip’s demented roommate—the ball skids opposite to the turn, like SUVs skidding off freeway exit ramps, when drivers put blind faith in cruise control. Intentional slips are good; skids, though, should be avoided at low altitude but can be fun and enlightening higher up.

Boilerplate disclaimer: Review spin procedures before experimenting with skids and discontinue use if crash results. Consult your instructor if skids last more than three hours.

To correct for a slip, apply less aileron and/or “step on the ball” with rudder. The wayward sphere returns to center for coordinated flight. Release aileron and rudder input entirely, and most airplanes with positive static directional stability will sigh in relief and seek relatively Zen-centric flight. In leaky old airplanes like mine, wind noise increases when entering a slip and decreases on exit. Those sounds are from the relative wind no longer flowing smoothly about the fuselage but, instead, smacking its side, which increases drag that’s desirable for steeper descent. Slipping in a climb, glide, or cruise is sloppy pilot technique and inhibits performance. Antidote: Keep the ball centered.

Side slips are what all good pilots of good airplanes do when correcting for crosswinds in landing. Forward slips are what good pilots of great airplanes do to lose altitude, generally after power is reduced, and with the glidepath still too high, the pilot wonders, “Now what?” Yes, flaps steepen descent, but many airplanes lack flaps, don’t need them; think 7GCAA Citabria (Adventure). A forward slip steepens descent enough to clear the trees and keep the runway aiming point from sliding too far behind.

Forward slips with flaps deployed are legit, depending on what the POH advises. Once in position to land, the pilot gracefully transitions from forward to side slip by raising the lowered wing with aileron and simultaneously adjusting rudder to align the longitudinal axis with the runway. As with applying top English in billiards, success comes with experimentation. Unlike billiards, whisky should not be involved.

Lacking a crosswind, no side slip may be needed. If you can’t remember whether you’re side or forward slipping, don’t sweat it. Terminology is irrelevant, mere labels created by humans who insist upon cataloging everything, making skills testable rather than learnable. The airplane doesn’t care. Although, DPEs might. Think about them as two forms of the same thing: Slips for descent (forward, as in “I’m determined to go forward with this approach”) or slips for touchdown (side, as in “the wheel on the upwind side touches first”).

Forward slips might feel awkward when first employed with any gusto. The airplane falls catawampus from the sky as the instructor insanely calls like the Titanic captain for “More rudder!” Properly executed, you won’t die but, again, check the POH. It is possible—and likely—to “run out of rudder,” a term pilot-loungers love to use while waving our hands. The AFH prefers “practical slip limit.”

In a slip, the more you bank, the steeper you descend. Forward slips are usually steepest, but at some point, you reach that limit with rudder hammered to the stop while opposite aileron is still available. You’ve run out of rudder. Gad-zooks! If you add more aileron the airplane turns, probably not what you wanted. You can lower the nose a bit to gain airspeed, which increases rudder effectiveness, allowing you to slip even more dramatically.

But, as Orvil Newton’s First Law of Unintended Consequences states, “for every solution there’s an opposite and annoying problem.” If you abruptly release the slip to land, the airplane quits flying sideways, becomes less draggy and floats in ground effect past the orange dot that the Oshkosh tower told you to hit. Plan accordingly, knowing that when you throw your Cub into a hairy forward slip on final the airport community will remark how cool you look but snicker like third graders when an unruly slip exposes your deficiencies.

Forward and side slips are vexing terms, but one could go a whole flying life without keeping the terminology straight—I have—provided you understand and enjoy their benefits. Next semester, the Aeroplane Flying Handbook will unmuddy the stalactite vs. stalagmite, counterintuitive waters of Movement vs. non-Movement areas. 

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30 COMMENTS

  1. I’ve noted very few pilots employ slips for altitude loss, preferring to circle or zig-zag about in coordinated flight when distance and required altitude loss don’t jibe. I, of course, never make such a miscalculation, but when I do the slip is my go-to tool when a semi-dive and pullout won’t work. At least as long a any passengers who may be aboard are up to it, because an all-out nose down slip can be disconcerting even to pilots.

    • Sideslips are extremely common when flying gliders and are SOOo easy. you just have to note your pitch attitude before you start the slip, and maintain it, then when you come out you are at the same airspeed you went in at! Easy!!

  2. When being flight tested at one point early in my career, the inspector pulled the power for a forced approach.
    When I was a bit high for my chosen landing spot, I used a slip to reduce the height.
    My thinking was/is, the a slip can be got rid of quickly; flaps not so quickly.
    I used the slip to correct the profile, then when the landing spot was assured; flaps to bring the speed back.
    The Inspector was very complimentary, pointing out that most pilots are not confident enough to slip properly.
    I sometimes fly a 1943 Tiger Moth which has no flaps; slips are the way to go.
    Even Jets will respond nicely to a slip when required; ask me how I know!
    (At one point, my airline would throw in a power loss at 10000 feet on final in the Sim.
    Slips were a great way to correct the profile)
    Don’t know how to slip? Go out and learn.

  3. Ercoupe? If both pilot and passenger stick their arms out the window, it’ll come down like a brick. no slip necessary (or possible).
    The B777-200 is certified to land in a 55kt crosswind, in a CRAB! The certification film is on Youtube.

  4. Having soloed in the late ‘50s in a Knocker, and thenT-Carts, Luscombes, and even 3 different Aeronca C3s, and such having remained my choice of flying machines for most of my flying career, slips were just part of the go up and come back down experience. However, when doing them at my local airport in later years in my Chief, the locals and especially the new “professional pilot program” graduates, now building hours instructing, they all assumed I was crashing, or out of control, or just being “careless and reckless”. Nothing but a dangerous maneuver to them, one which they had never done themselves. I might add, with my Chief I could actual dump altitude on final faster than using a slip by just nose over and adding about 10 – 15 mph (I don’t talk knots in my flying machines, too depressing). The drag on the Air Knocker is so high naturally that as soon as I began leveling off, that extra speed dissipated almost immediately. But then, not as much fun and not the air show that the observers kinda enjoyed, even if I was being reckless.
    One more who cares thing about slips and my Chief. It will not stall in a slip. (I didn’t say skid. I said slip. A skidding stall will let you go inverted quickly in my machine). But in a slip, you can ease the yoke back, back, back until it is against the stops and all that will happen is, yep, run out of rudder, but also just some mild shudder and the plane, nose high, slowly turns in the direction of the slip. Don’t personally know if that works in the other small planes. Try it and see. Of course way up at altitude until you know the answer. But for me, it assured me that a slow slipping turn on final is not going to result in a stall/spin into the ground, at least not from that.

  5. Thanks Paul,
    Great writing. Great subject. As an instructor for over half a century I’ve long since decided a slip is a slip is a slip. Seemed like side vs forward added unnecessary confusion to something that is actually simple. That said: concur with slip vs skid. I’ll stir the stick with this at the next hangar flying event at the home drome .

  6. “A slip is an uncoordinated turn, that may or may not be turning…” Hmmm. I think I get the meaning, but feel like I hit a logical speed bump. But now I know it’s the forward slip that can save the too high approach at the thrilling risk of a quick snap and premature death. This will help with the subsequent paperwork.
    -JK

  7. For those of us who burn fuel out of one wing tank or the other but not both simultaneously, it’s helpful to take stock of which tank is selected prior to the slip. Slipping is great but slipping unported, not so much. And if you’re really good you’ll do some advanced planning so as to arrive at your slip with more fuel in your downwind tank, preferably to slip upwind, making the downwind tank the high tank which in turn makes the upwind tank the low tank, if you get my drift.:) Paul you made it sounds so easy. Are you sure we’re ready for the movement and non-movement areas lesson?

  8. I have a question: can you slip an airplane with swept wings, such as modern airliners?
    I have never heard of anyone slipping an airliner and just wondered if it is possible to do so
    safely.

    • The answer is yes, though swept wing slips are generally performed just before touchdown to prevent exceeding side load limits on the gear. Are there slip limits? Probably, because just as you imply the fuselage would seem to block airflow over the downwind wing. I’d have to find my old books and dig a bit – it just makes sense.

      • Usually crabbed (yawed with wings level to avoid catching a tip – or a pod depending on roll attitude), taken out just before touchdown. There are videos on the Internet, many may be at the old airport in Hong Kong.

        Note that swept wing airplanes lose even more lift on one wing because yaw angle changes airfoil effective profile/

        Now, if you want trickier – with an engine out, thrust asymmetry. (Ignore whining of 8-engine types with an engine out. :-o)

        There’s probably a book, with a title like Flying The Big Jets.

    • They obviously can as it’s used for crosswind landing. Running out of rudder shouldn’t happen too soon as twinjets need quite some ruder to deal with engine-out situations. I suspect there might be issues with the engine inlet airflow if you overdo it.

  9. I belong to the “no difference” school; once I learned to put the airplane in my aimed-for touchdown spot. As a control tower operator at FWA, people in the tower with me could probably see little red hearts fluttering upward from my head when I saw little airplanes fly a close-in downwind and turn off the runway with no wasted space behind them, occupying the runway for maybe 30 seconds max. My greatest piloting occurred in 1964, close-in downwind past the departure end of my 8000′ landing runway, 22, at 800′ AGL. The controller on LC was also a pilot-friend (most of them were) and transmitted to my RONLY J-3 (I had phoned in ahead for NORDO landing info): “Mac, If you can make it from there, cleared to land RWY 4.” I did, and turned off at midfield with minimum landing roll. I was astonished, myself, at being able to do that, at the same time never doubting that I would; observing the “clear the runway in 30 seconds” rule. I was “one” with that airplane.

  10. Paul, you said that lowering the nose will enable an even more extreme slip. Are you sure? That has not at all been my experience. (That said, I’m rather careful about slowing down; if I don’t have enough slip at a normal approach speed, going around seems like a better option than slowing down, especially given the unreliability of ASI readings in a slip.)

  11. I mostly flew gliders, then a few motorgliders and some ultralights.

    Slipping was absolutely normal when I learned flying, it is a bit out of fashion nowadays but still teached for soaring in Germany.

    One thing that I never understood, but could prove while flying myself: I could not stall an aircraft while being in a stable forward slip. I tried this on a few gliders and except some vibration, maybe a bit of porpoising and a lot of acoustic racket nothing happened. Why is this the case?

    Another two slipping anecdotes:
    1.If you slip an ASW19b glider hard, the rudder will stall and it will get sucked into its limit stop. It requires a little bit of push on the opposing pedal to get it out of the stall. Nothing serious and it is even noted in the flight manual. But still interesting.

    2. If you slip an ancient Ka8b glider hard to the left, the canopy will open a little bit. It opens to the right and is kept shut by a latch on the left cockpit wall that is secured by a spring. In a slip the canopy will create enough lift to compress this spring and the canopy will open up about 5mm to 10mm. Suddenly it is very noisy and windy in the cockpit. That was a very interesting experience.

  12. I just tested my slip technique this afternoon in my CallAir Cadet. Slips are slips, pretty much as you say, Paul. The difference is terminology and purpose; in my not particularly humble but honest opinion, the orientation of the longitudinal axis of the airplane relative to the runway. If the longitudinal axis is displaced from the runway alignment, it’s a forward slip, used to increase descent rate/angle. If the longitudinal axis is lined up with the runway, the relative wind is from the low-wing side and is therefore a side slip, used to keep my taildragger wheels rolling in the right direction, hopefully without drift. I don’t spend a lot of time thinking about them, they’re just something I do.

  13. I fly a slip-happy Luscombe and, of course, enjoyed Paul’s article. However, in reference to the remark about Ercoupes landing in crosswinds, 50 years ago I saw a Cessna 195 taxi the length of our local grass strip somewhat sideways. The pilot was giving another pilot an orientation ride, and when they returned, I asked him how in the world an airplane could do such a thing. He told me that his airplane was equipped with something called a “crosswind” landing gear and that he could actually land in a crab up to 15 degrees, kind of like an Ercoupe. Pretty cool. Again, enjoyed the article and the comments.

    • Many years ago, as a teen, was working at the TOA FBO. I was pushing a C195 across the ram and one of the wheels dropped down into the wash rack water faucet hole. The wheel went sideways. I was in fear. I had broken this custome’rs wheel almost off his airplane. I confessed in the office. They came out and lifted the gear out of the hole. All was well. Just my first introduction to the C195 crosswind gear.

  14. As usual, great article…and for once a clear explanation that a slip is a slip…forward or side. I demonstrated a turning slip to a mission aviator trainee one day in our straight tail 172 with full Horton kit. Once established on base, I did a turning slip to touchdown. He had never seen that, asking for an example of why one would do such a thing. I relied, I would do this if my windshield became obstructed for a variety of reasons, or if I was dealing with an engine fire. This is another way to lose altitude quickly, with no speed increase, keeping runway in sight when dealing with high trees on the extended runway centerline common in many unimproved strips. Many times there are no or less obstructions on an angle to the runway center line. He just nodded and said he wanted some instructional practice which I gladly provided.

    About five years later, he emailed me a thank you for saving his life. He had an oil line to the prop governor rupture on an Arrow he and a friend were flying during a very cold winter flight. His windshield became a frozen goo at about 7,000 ft over somewhere in Tennessee. His flying buddy that day was an instructor who did not know what to do when he lost all forward visibility. Our mission aviator took the controls and did a long circling, slipping, dead stick approach to an airport below to an uneventful, quiet landing. He said it worked great using an open storm window in the side window to keep the runway in sight.

    While unporting of fuel can be a problem on some airplanes, including the Bonanza we own, usually the unporting happens in a prolonged slip lasting 30 seconds or more. Short slips can be done safely even in a Bo. As others have shared, once the proper pitch angle is established with proper speed going into the slip, turning or otherwise, if maintained throughout the slip, the speed will be the same when the slip is ended.

    Nice to see where a slip is a slip can very useful. Thanks for a wonderfully well written and useful article!

  15. Great article, Paul. An instructor introduced me to slips by announcing “Today we’ll be flying in the Bizarro World, where everything is opposite.” He explained that either way, a side slip or a forward slip, the airplane was doing the same thing. The difference between them was all in where you were looking to see your direction of travel. In a side slip you looked forward, out the windscreen, and in a forward slip you looked out the side window. And with that, he said “Welcome to the Bizarro World.”

  16. I grew up in an ag flying business with both parents commercially rated pilots. Dad was former USAAF pilot and USAF civilian IP in T-6G in 50s. Both my brother and I learned to fly in a J-3 Cub on a 2000′ grass strip in the 70s. Slips were part of our training and became second nature. To this day, we both routinely use slips in a variety of airplanes, especially as many of the local fields we fly from have tall trees on one or both ends. Also because they are just fun to do. A Cub, Stearman and Pitts are all outstanding “slippers” and great tool for making plane do what you want it to do and go where you want it to go…a phrase I heard repeatedly from my dad who had 10,000 hours in Stearmans!

  17. The FAA inspector, during my pre-test CFI practical phone meeting, admonished me to know the difference between forward and side slips. I still think they are the same thing and the way they are used accounts for the difference.

  18. It sounds like ya’ll know something about slips. It might be worth noting that slips are a required manuever to be covered before solo.
    I have found that a slip can be a little disconcerting to an new flyer.

    A good exercise to give them some confidence in slip execution is what I call a “full rudder slip to a point”.

    It is started at a safe altitude and demo’d by the instructor who slowly pushes a rudder to its stop while maintaining directional control to the point with the aileron which can be done with many aircraft, but not all.

    Once the rudder is at the stop the new flyer is asked to take over holding the controls (rudder first and then aileron) in the manuever for a few moments and then slowly releasing them back to coordinated flight. Once they have gained some confidence in application of the controls for a slip, they are ready to slip in the opposite direction on their own.

    Slips done for a purpose such as losing altitude or for a cross wind don’t have to be full rudder slips of course.

    Another exercise is to alternate slips with a crab toward a hypothetical runway where a good crosswind situation exists and a crab and side slip can be demo’d and alternated first by the instructor and then the new flyer during a long practice descent. Don’t forget the occasional power application to avoid shock cooling, if needed! Of course if you get to the point in an actual side slip approach where the wind is strong enough that full rudder doesn’t do the job, you need to go to Plan B.

    I think Wolfgang Langewiesche would agree that being able to slip well is more important than knowing the difference between a side and forward slip.