Technique

Getting crabby about slips

Originally there was to be a pro-and-con discussion in this space. Is it better to crab into a crosswind on final approach, transitioning to a slip just before touchdown, or should you just slip the aircraft into the wind during the entire approach and touchdown (what the FAA calls the wing-low technique)?

But a funny thing happened on the way to the debate. Those who favor use of the slip alone failed to show up.

Flight schools surveyed for this article now teach crabbing into the wind on final, then converting to a slip when just above the runway. So the crab-and-slip method wins by default.

For the sake of discussion, let's focus on the student who is just learning about crosswinds.

"In a go-around during the approach," one instructor said, "we don't want our students to add full power while cross-controlled, as they would when using the slip method." Makes sense.

The arguments against using the crab-and-slip method are well known. First, some feel it is more difficult for the student to learn. After all, the argument goes, when just above the ground it's a little late for students to be changing control inputs. So why do those proposing that argument think it is so easy to slip, especially for students?

To maintain a slip, students must adopt an awkward and cross- controlled attitude that varies with the wind. The idea behind a slip is that the aircraft slips sideways toward the centerline, and into the wind, at the same speed at which the wind is attempting to move the aircraft away from the centerline. The result is that the aircraft stays over the centerline as it approaches. When the wind changes, the degree of slip (bank angle and amount of rudder used) must vary. A crab is easy to enter — all it takes is a normal, coordinated turn into the wind. (In fact, beginners are sometimes taught a still easier method: Point to the centerline with the rudder and forget about the ailerons until more experience has been gained.) You don't sit there holding rudder and aileron pressure after turning to a crab angle.

Yes, the transition from a crab to a slip just above the runway takes time to learn and coordination to accomplish, but so does a slip. Once the student has practiced slips at a safe altitude, however, there should be little difficulty doing them five or 10 feet above the runway.

Had opponents of the crab-and-slip method shown up, they might say the slip or wing-low method is more stable. But what is so unstable about the crab method? Following that initial, coordinated turn, the pilot merely monitors the crab angle to see that it is doing its job and makes adjustments with a series of additional, and slight, coordinated turns. What could be more stable than that?

There are other problems with the slip. Some aircraft have restrictions against slips when fuel quantities are low, because the slipping attitude may unport a fuel tank in some aircraft, preventing fuel from entering the fuel line.

And what about the transition to heavier aircraft with longer wings? Ever see an airliner tilt on its side during the approach? The passengers obviously would think something is wrong (especially if it is a large aircraft and a wingtip strikes the ground). Passenger comfort is an important consideration. While the student's flight instructor may not be uncomfortable leaning to one side while approaching the runway, non-aviation-oriented passengers are a different story. Passengers are sure to like the crab method better than the wing-low method.

While the FAA gives equal weight to both methods, flight schools seem to agree: It's the crab-and-slip method from now on.