The roundout in a small airplane is a slow, smooth transition from a normal approach attitude to a landing attitude, gradually rounding out the flightpath to one that is parallel with, and within a very few inches above, the runway.
If you flare while the airplane is drifting or in a crab, it will contact the ground while moving sideways. This can impose extreme side loads on the landing gear, and if severe enough, may cause structural failure.
The most effective method to prevent drift in primary training airplanes is the wing-low method. This technique keeps the longitudinal axis of the airplane aligned with both the runway and the direction of motion throughout the approach and touchdown.
We encourage you to visit this topic in more depth in Chapter 8, Approaches and Landings, in the Airplane Flying Handbook, which is available online at www.FAA.gov. The direct link is: http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-4of7.pdf
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