Militarily, sideslip can be employed as a defense tactic when being targeted by gunfire. Sideslip isn't immediately apparent to the human observer which causes them to aim at the point where the aircraft would be if it were flying straight.
Slips are especially useful when operating aircraft that have neither high-drag flaps nor spoilers (typically pre-1950s training aircraft, or in aerobatic aircraft such as the Pitts Special), or in any aircraft in which the flaps cannot be extended due to a system failure.
Two forms are employed, the forward-slip and the sideslip. Aerodynamically these are identical once established, but are entered in different manners and will create different ground tracks and headings relative to those prior to entry. Slips are particularly useful in performing a short field landing over an obstacle (such as trees, or power lines), or to avoid an obstacle (such as a single tree on the extended centerline of the runway), and may be practiced as part of emergency landing procedures. These methods are also commonly employed in flying from farmstead or rough country airstrips where approach hazards are present.
As with any low altitude maneuver it is extremely important to maintain proper airspeed in order to prevent a stall and a consequent spin (due to the "cross controlled" state). Airspeed indications may also be less accurate at high sideslip angles in some aircraft.
A forward-slip is useful when a pilot has set up for a landing approach with excessive height or must descend steeply beyond a tree line to touchdown near the start of a short runway. Assuming that the runway is properly lined up, the forward slip will allow the aircraft track to be maintained while steepening the descent without adding excessive airspeed. Since the heading is not aligned with the runway, the slip must be removed before touchdown to avoid excessive side loading on the landing gear, and if a cross wind is present an appropriate side slip may be necessary at touchdown as described below.
In the United States, student pilots are required to know how to do forward slips before embarking on their first solo flight. The logic is that in the event of an engine failure, the pilot will have to land on the first attempt and will not have a chance to go around if the aircraft is too high and/or too fast.
In the sideslip condition, the airplane's longitudinal axis remains parallel to the original flightpath, but the airplane no longer flies straight along its original track. Now, the horizontal component of lift forces the airplane to move sideways toward the low wing.
A sideslip is also one of the methods used by pilots to execute a crosswind landing. In order to land crosswind using the sideslip method, the pilot puts the airplane into a sideslip toward the wind to maintain runway centerline position while maintaining heading on the centerline with the rudder, touching one main landing gear, followed by the second main gear, and finally the nose gear (or tail gear if employed). This allows the wheels to be constantly aligned with the track, thus avoiding any side load at touchdown.
The slideslip method for cross-wind landings is not suitable for long winged and low sitting aircraft such as sailplanes, where instead a crab angle (heading into the wind) is maintained until a moment before touchdown. However glider pilots regularly use side-slips to increase their rate of descent while landing, particularly in the case of older sailplanes not equipped with high drag devices (spoilers or flaps).
There are other, specialized circumstances where slips can be useful in aviation. For example, during aerial photography, a slip can lower one side of the aircraft to allow ground photos to be taken through a side window. Pilots will also use a slip to land in icing conditions if the front windshield has been entirely iced over — by landing slightly sideways, the pilot is able to see the runway through the aircraft's side window. Slips also play a role in aerobatics and aerial combat.
In normal flight air passes over an airfoil wing section at exactly 90 degrees to the leading edge, thus all the airflow is involved with creating lift. When a side slip is initiated this angle reduces so only a component of the air flow is flowing over the wing at 90 degrees. So if less air is flowing over the wing at 90 degrees then less lift is being produced. Furthermore the airflow over the fuselage is at an angle, increasing the relative frontal area and in fuselages of rectangular cross section greatly increasing the drag, with the combined effects of reduced lift and increased drag causing the aircraft to descend very rapidly.