The ancient and honorable art of slipping an aircraft to achieve a desired flight path is an excellent tool to keep in one’s kit-bag of aviation tricks. Once employed regularly to adjust a landing approach, slipping has become an infrequently used maneuver, chiefly because modern airplanes have been fitted with effective wing flaps that can be used to add drag during landing. The side-slip remains necessary for cross-wind correction, and a forward slip to landing has been preserved for the Private Pilot practical exam. Often, however, pilots do not truly understand the maneuver.
Origin And Purpose
Open cockpit biplanes were (and are) generally flown from the rearmost of the tandem seating positions, located near or behind the lower wing root. Unfortunately, the pilot’s view during landing was somewhat obstructed by the assemblage of wings, struts and wires out in front, so it was common practice to side-slip during the landing approach, using rudder to hold the nose to one side while using opposite aileron to lower a wing and keep the rudder from producing a skidding turn. This allowed the pilot to see obstructions in the approach path and adjust his aiming spot. Then, the pilot would kick the rudder bar back into neutral and level the wings during the flare, maintaining runway alignment by watching the edges of the landing track (runways were typically wide and multitudinous at “flying fields” of the day).
Biplanes didn’t need flaps to steepen their landing approach, because they were amply supplied with drag and readily descended with power off. However, as more-slippery monoplanes came into vogue, “air brakes” were developed. We refer to them as “flaps” these days. They were a godsend because they could be lowered to bring the aircraft down at a steeper angle and reduce landing distance. Pilots who had been trained in biplanes, or in Mr. Piper’s early Cubs that retained the biplane’s aft-seating for PIC, still knew how to slip, even when graduating to flap-equipped airplanes.
Paved runways eventually limited one’s choice of landing directions, so crosswinds became more of an issue, so slips were used to neutralize the inevitable crosswind drift. This required the use of a side-slip, touching down on the upwind tire first. Steerable and lockable tailwheels replaced the old tailskids, and better brakes were developed to aid ground control. As tricycle landing gear supplanted the old “conventional” gear arrangement, pilots began to forget how to use a slip for landing approaches and touchdowns, largely because they didn’t need to use them as much as they once did.
Slips, Separate And Distinct
There are two types of slips, each defined by its purpose and outcome, even though both require similar control inputs. A forward slip is used to steepen the landing approach, when wing flaps have already been lowered and aren’t producing sufficient effect or when flaps aren’t available for some reason. A side slip, on the other hand, is required when a crosswind threatens to push the aircraft off the runway or, at the very least, the landing gear is going to suffer side load from the wind’s drift effect.
Both types of slip require opposing rudder and aileron inputs, an unnatural act normally punishable by a tongue-lashing from the instructor. In this case, however, cross-control is a good thing, because we are purposely flying in an uncoordinated manner to achieve a proper outcome. Modern airplane designs, chiefly tricycle gear types, are not provided with a great amount of rudder effectiveness—compared with tailwheel airplanes, that is, because the innate stability of a nosewheel configuration doesn’t require as much yaw control. By limiting rudder capability, designers are able to avoid much of the risk of inadvertent spin entry.
Thus, rudder effectiveness will be the limiting factor in how much slip can be achieved, rather than the aileron’s ability to counter the rudder input. You will run out of rudder long before you reach the aileron’s limit.
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How It’s Done
Looking first at the forward slip, let’s assume that we are on final approach with some excess altitude that threatens our ability to land in the available runway space. We can go around to make an extended traffic pattern on the next try or add more flaps, if they are available. Or we can try the venerable forward slip, which might save the day.
Since we’re already aligned with the runway, we need to make a coordinated turn of about 20 or 30 degrees of heading change, pointing the nose to one side of the runway, then we’ll hold that heading with rudder while inputting aileron, opposite to the rudder. The airplane will then track along a line offset from the aircraft’s longitudinal axis, FORWARD toward the runway as before, hence the name “forward slip.” You may be looking at the runway out of the side window, but the airplane is heading directly toward it.
Because the airplane is flying sideways through the air, presenting the side of the fuselage to the airstream, considerable drag is produced, and the descent angle thereby increases. Reassure any passengers that this is a normal, desired mode of flight, as they may consider why they are being thrown against the side of the cabin. If not descending steeply enough, add more rudder and the requisite aileron to counter it. As we said, you’ll often be pressing the rudder against the limit stop. As you approach the ground, or reach a normal glidepath, smoothly release rudder to let the nose swing around to the runway heading and level the wings for the touchdown.
This is obviously a time to beware of stalling the airplane, as you are flying cross-controlled close to the ground, with only landing-approach airspeed. Maintain the nose attitude that was working before you entered the slip, and take care not to increase angle of attack. If the airplane has a single static port, the airspeed indicator will be affected by the slip; slipping toward the static source adds ram air pressure into the opening, lowering the indicated airspeed, while slipping the other way raises IAS. Keep the nose down and expect the airspeed to come back to normal as you recover from the slip. Airplanes with dual, connected static ports will not be affected by a slip.
Unlike having flaps deployed, using a forward slip to increase drag means you won’t have the benefit of aerodynamic braking after straightening out for the landing. The airplane will be scooting along in ground effect until it decides to touch down, so you may have to brake aggressively during the rollout.
Does it matter which way you perform the forward slip, left wing down or right wing down? Not really; most pilots, seated on the left side of the cockpit, will push the nose right and slip toward the lowered left wing, for better visibility. And if making a left-hand turn to final, it’s natural to detect the need for a slip and begin the maneuver while still pointed to the right of the runway. The airplane doesn’t care which way you slip it. There is a possibility of unporting a fuel tank’s outlet in a prolonged slip, should you be foolish enough to be landing with minimal fuel in the tank.
And Then There’s The Side Slip…
Rather than slipping the airplane to lose excess altitude, dropping down over obstructions in the approach path, it’s also necessary to exercise your crossed-controls skill during a crosswind landing. This is a true side slip, making the airplane move sideways across the runway in exactly the same amount that the crosswind component is moving it in the opposite direction, thereby negating the wind’s effect and rolling the landing gear exactly along the runway direction, nose pointed at the centerline. The upwind tire will touch first, the downwind tire will be held off by aileron input for a second until it squeaks down, and the rollout continues with active use of the controls until the tiedowns are reached.
It is inevitable, of course, to avoid the “my way is better” contention revolving around landing in a crosswind. One side insists that there’s no need to put a wing down at all, that straightening up from a crabbed approach with a boot-full of downwind rudder will take care of the touchdown quite nicely. And the more artistic purists argue in favor of grinding down final with opposing roll and yaw inputs, adhering to the teaching of the respected masters teaching the Wun Weng Lo method.
Truth is, for those seeking true enlightenment, there’s more than one way to fly an airplane, and all of them are right. It is well to consider both methods of countering wind drift during landing, recognizing the advantages and limitations of each one. Some airplanes can’t be slipped aggressively as they land, because low-hanging flaps or engine nacelles limit the bank available. For them, approaching in a crab and executing a well-timed yaw maneuver to get the tires aligned with the runway is the preferred method. Airplanes possessing considerable mass will have some stored inertia that will keep them moving down the centerline until the touchdown takes place.
Light crosswinds can be adequately handled with a stomp on the rudder during the flare. But as the crosswind component cranks up, there will come a time when a side slip will be needed. No two windy-day situations are alike; the technique to be used depends on the aircraft’s capability, the amount of displacement between wind direction and runway heading, the wind’s speed and gustiness, and the adroitness of the pilot. Be ready to take a wave-off and reconsider your method as you plan another attempt.
At what point do you enter the side slip? It’s a matter of personal preference; some pilots like to approach in a crab, using aggressive downwind rudder at the last second to straighten the airplane out for the touchdown. Even so, opposite aileron will be needed during the de-crabbing maneuver to prevent the upwind wing from rising due to yaw/roll coupling in most airplanes.
If the crosswind is strong, I like to set up the side slip while still a quarter-mile out on final, so I can gauge the wind’s gustiness and see how much rudder I have remaining to counter the wind as airspeed slows during the flare and touchdown. In this case, the nose remains pointed down the runway, the wing being lowered into the wind while rudder counters the aileron input. Be ready to adjust the side slip if the wind is variable. As always, the rudder’s ability to generate yaw input will be the limiting factor; if you’re still drifting downwind across the runway with full rudder pressed in, go around and try your luck again, or go elsewhere to land.
Knowing how to slip your aircraft is a vital skill that you’ll want to practice when the opportunity presents itself. Consider yourself one of those airmail pilots of yesteryear, leaning over the cockpit coaming to slip expertly into a postal pickup airfield, your white silk scarf flapping in the SLIPstream.