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18.2.10 Swell Direction. The direction from which a swell is moving. This direction is not necessarily the result of the wind present at the scene. The swell encountered may be moving into or across the local wind. Swells, once set in motion, tend to maintain their original direction for as long as they continue in deep water, regardless of changes in wind direction.
18.2.11 Swell Height. The height between crest and trough, measured in feet. The vast majority of ocean swells are lower than 12 to 15 feet, and swells over 25 feet are not common at any spot on the oceans. Successive swells may differ considerably in height.
18.3 Swells
18.3.1 It is extremely dangerous to land into the wind without regard to sea conditions. The swell system, or systems, must be taken into consideration.

GEN 3.6.16 AIP 10 MAR 11 United States of America
FIG GEN 3.6.18 FIG GEN 3.6.20
Single Swell (15 knot wind) Double Swell (30 knot wind)

FIG GEN 3.6.21
(50 knot wind)
FIG GEN 3.6.19
Double Swell (15 knot wind)

Twentieth Edition Federal Aviation Administration
FIG GEN 3.6.22
Wind.Swell.Ditch Heading

GOOD Landing parallel to the major swell

Landing on the face and back of swell
18.3.2 In ditching parallel to the swell, it makes little difference whether touchdown is on top of the crest or in the trough. It is preferable, if possible, to land on the top or back side of the swell. After determining which heading (and its reciprocal) will parallel the swell, select the heading with the most into the wind component.
18.3.3 If only one swell system exists, the problem is relatively simple . even with a high, fast system. Unfortunately, most cases involve two or more systems running in different directions. With many systems present, the sea presents a confused appearance. One of the most difficult situations occurs when two swell systems are at right angles. For example, if one system is 8 feet high, and the other 3 feet, a landing parallel to the primary system, and down swell on the secondary system is indicated. If both systems are of equal height, a compromise may be advisable . selecting an intermediate heading at 45 degrees down swell to both systems. When landing down a secondary swell, attempt to touch down on the back side, not on the face of the swell. Remember one axiom . AVOID THE FACE OF A SWELL.
18.3.4 If the swell system is formidable, it is considered advisable, in landplanes, to accept more crosswind in order to avoid landing directly into the swell.
18.3.5 The secondary swell system is often from the same direction as the wind. Here, the landing may be made parallel to the primary system, with the wind and secondary system at an angle. There is a choice of two headings paralleling the primary system. One heading is downwind and down the secondary swell; and the other is into the wind and into the secondary swell. The choice of heading will depend on the velocity of the wind versus the velocity and height of the secondary swell.
18.4 Wind
18.4.1 The simplest method of estimating the wind direction and velocity is to examine the wind streaks on the water. These appear as long streaks up and down wind. Some persons may have difficulty determining wind direction after seeing the streaks in the water. Whitecaps fall forward with the wind but are overrun by the waves thus producing the illusion that the foam is sliding backward. Knowing this, and by observing the direction of the streaks, the wind direction is easily determined. Wind velocity can be accurately estimated by noting the appearance of the whitecaps, foam and wind streaks.

18.5 Preditching Preparation
18.5.1 A successful aircraft ditching is dependent on three primary factors. In order of importance they are:
18.5.1.1 Sea conditions and wind.
18.5.1.2 Type of aircraft.
18.5.1.3 Skill and technique of pilot.
18.5.2 The behavior of the aircraft on making contact with the water will vary within wide limits according to the state of the sea. If landed parallel to a single swell system, the behavior of the aircraft may approximate that to be expected on a smooth sea. If landed into a heavy swell or into a confused sea, the deceleration forces may be extremely great . resulting in breaking up of the aircraft. Within certain limits, the pilot is able to minimize these forces by proper sea evaluation and selection of ditching heading.
18.5.3 When on final approach the pilot should look ahead and observe the surface of the sea. There may be shadows and whitecaps . signs of large seas. Shadows and whitecaps close together indicate that the seas are short and rough. Touchdown in these areas is to be avoided. Select and touchdown in any area (only about 500 feet is needed) where the shadows and whitecaps are not so numerous.
18.5.4 Touchdown should be at the lowest speed and rate of descent which permit safe handling and optimum nose up attitude on impact. Once first impact has been made there is often little the pilot can do to control a landplane.
　
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