Aerodynamics of a Final Approach Slip Explained
In an online forum, a professional pilot wondered whether he might be incorrectly controlling the aircraft when he performed a slip on final approach because the airspeed always increased, not decreased as he’d been taught. Slips in transport aircraft are sometimes restricted or even prohibited, making it hard for pilots to know how to handle them when they are required.
In response to the questions posed in the forum, Aviation Performance Systems director of flight operations Clarke “Otter” McNeace reviewed a few of the airmanship basics as follows: “As you enter a slip, the nose will tend to drop on its own due to the decreased lift being generated by the wings. If airspeed is increasing during the slip, it’s because the nose is being lowered too much. As you apply full rudder and opposite aileron, adjust pitch attitude to maintain your approach speed. There is no need to increase airspeed during a slip. Some backpressure may be required to hold the nose at a constant-airspeed attitude. Aerodynamically, stall speed goes down during a slip because lift is gradually being transferred from the wings to the fuselage.
“Once you have applied full rudder (we don’t necessarily recommend full rudder be used), adjust your angle of bank to control track. At this point, most pilots believe airspeed must increase because the pitot tube is delivering an inaccurate airspeed indication. While there will be an error generated, maintaining current indicated airspeed provides adequate airspeed for the slip. Be sure to enter and exit the slip smoothly and well controlled. Do not make abrupt control inputs and you’ll accomplish safe slips for a lifetime.”