High on final? Here's how to use a forward slip to increase your descent rate without ballooning your airspeed...

Entering The Forward Slip

Here's the scenario: you realize you're high on final, but there's still time to correct your mistake before touchdown. It's time to enter a forward slip.

To enter a forward slip, first bring the power to idle, and make sure you're fully configured for landing with full flaps, if your manufacturer allows.

Using ailerons, lower the wing on the side in the direction you want to slip. If there's a crosswind, bank into the wind. Simultaneously, yaw the airplane's nose in the opposite direction by applying opposite rudder.

Step on the rudder enough so that you maintain a straight ground track toward the runway.

When you're flying a forward slip, you're cross-controlling the airplane, and creating a large amount of drag (see the red on the diagram below). You're exposing a large amount of fuselage surface area into the wind. This increases both drag and your descent rate, without a substantial gain in airspeed.

The sideslip you use for crosswind landings is the same concept, except you keep the longitudinal axis of the airplane aligned with the runway in a crosswind landing. Both maneuvers are initiated the same way, but the forward slip uses larger flight control deflections, so that you create more drag to increase your descent rate. Also, your nose won't be aligned with the centerline in a forward slip (see diagram below).

Increasing And Decreasing Descent Rate

In a forward slip, the amount of slip, and therefore the sink rate, is determined by the bank angle. The steeper the bank is, the steeper the descent (FAA). So, if you're still high, increase your bank angle and apply more opposite rudder. Once you've nearly reached the proper glidepath to the runway, remove your slip by relaxing the flight controls, and slowly increase power to your normal final approach power setting.

What's Your Slip-Limit?

In light airplanes, the steepness of forward slips and sideslips is limited by the amount of available rudder, or the rudder-limit. You may reach a point where full rudder is required to maintain heading. In other words, you could add more aileron input to steepen the slip, but you don't have enough rudder travel to maintain a constant ground track.

If this happens and you need to continue increasing your descent rate, try lowering the nose. As you lower the nose, airspeed and descent rate will both increase. The additional airspeed will give your rudder more effectiveness, allowing you to increase the slip. The opposite is true of raising the nose.

Recovering From The Forward Slip

A safe recovery altitude depends on your experience level, and any recommendations in your POH. If you're very high on glide path, and you need to maintain a forward slip on short final to hit your touchdown point, it's probably best to go-around and try again.

When you do recover from a forward slip, gently level the wings and simultaneously release rudder pressure while adjusting pitch attitude to maintain a regular glidepath. At the same time, slowly increase power to your normal final-approach power setting.

Also, be careful to gradually reduce rudder, so you don't abruptly swing the nose into line. This has a tendency to rapidly increase airspeed.

Pitot/Static Errors

According to the FAA's Airplane Flying Handbook, "because of the location of the pitot tube and static vents, airspeed indicators in some airplanes may have a considerable error when the airplane is in a slip. The pilot must be aware of this possibility and recognize a properly performed slip by the attitude of the airplane, the sound of the airflow, and the feel of the flight controls." As you expose the side of the airplane to the relative wind, you're changing the angle at which air enters the pitot tube or static ports.

The extent of these pitot/static errors are airplane specific and you may or may not notice subtle to dramatic changes in airspeed data as you enter a forward slip.

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