Insights

Forward slip, no slip, sideslip

Years ago, during my FAA practical test for a commercial glider rating, the final task was an accuracy landing, which I was asked to start at an excessively high altitude. That mandated a forward slip. I was high on left base, and the windsock indicated a right crosswind at that location. I immediately applied full right rudder, banked to the left in order to control my ground track, and delayed my turn to final so that I could make a steep bank turn and generate a high descent rate. The examiner went berserk and yelled that I had to forward slip into the wind (bank into the wind during the forward slip). Malarky!

Yes, based on the windsock, it appeared that a sideslip into the wind would be required for the landing, but no one knows the exact wind situation on final approach. That was no time to argue, so I rolled into a right-wing-down forward slip after I turned onto final approach, and because of the decreased descent performance during that transition, I had to continue the forward slip down to the flare point, where I transitioned to the sideslip for landing, which enabled me to pass the checkride. Properly trained pilots know that an aggressive forward slip close to the ground should be avoided.

I was recently reminded of that situation when I heard a designated pilot examiner (for airplanes) tell a private pilot applicant exactly what the glider examiner had told me. When I questioned the examiner, he could give me no explanation for his reasoning other than, "You must slip into the wind for landing." Of course, but that does not apply to a forward slip.

Here's a fact regarding flying, slipping, and the wind. Air movements wander like water in a creek. Consequently, you should always roll out on final approach, wings level, parallel with the runway centerline, and check for drift in order to determine what the wind is doing at your point in space. Then, if needed, you establish a crab angle and track the runway extended centerline until it's time to establish the sideslip for landing.

Any number of situations can occur. You may have no drift on final with the windsock showing a direct crosswind. You may have drift with the windsock showing no crosswind. You may have a crosswind during the initial flare only to have it stop just prior to touchdown and vice versa. With respect to wind, you must expect the unexpected and be ready to immediately correct for actual conditions.

What you experience is what you have. No other source--ATIS, tower reports, or windsock--reveals your exact situation. That is an undeniable fact.

If you must forward slip to lose altitude, remember this: Never transition directly from a forward slip to a sideslip unless you are forced to carry the forward slip down to the flare point like I did on that checkride. You should momentarily terminate the forward slip in a wings-level attitude, fuselage parallel to the runway; evaluate wind drift; and then establish the proper sideslip if one is required. Forward slip, no slip, sideslip. That's the sequence.

If you must forward slip while on base leg, lower the wing that's pointed toward the runway and use opposite rudder--left wing down with right rudder when on left base; right wing down with left rudder when on right base. Now you're set up for the turn to final if you must continue the forward slip through that turn.

The limiting factor for all slips is rudder authority. A maximum-performance forward slip requires full rudder displacement and whatever bank angle is necessary to control ground track. At that point, the only way to further increase descent rate is to decrease pitch attitude, but be certain to immediately restore the normal pitch attitude when you terminate the slip so that airspeed does not increase excessively.

A maximum-performance sideslip occurs when a strong crosswind requires full rudder input while banking in order to keep the airplane parallel with the runway centerline. Warning! That's an extremely dangerous situation. Go around and divert to a suitable airport. That crosswind would far exceed the airplane's demonstrated crosswind component as listed in the pilot's operating handbook.

Remember to use the two-step correction rule during a crosswind landing. When drift occurs, apply a bank correction that you know is excessive. The instant that correction starts to work, remove half of the initial input and continually use rudder to keep the airplane parallel with the runway centerline.

If you follow the proper procedure for slipping, one thing is certain: It's impossible to slip up.

Ralph Butcher, a retired United Airlines captain, is the chief flight instructor at a California flight school. He has been flying since 1959 and has 25,000 hours in fixed- and rotary-wing aircraft. Visit his Web site.