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has been caused by a distant disturbance.
18.2.4Swell Face.The side of the swell toward the
observer. The backside is the side away from the
observer. These definitions apply regardless of the
direction of swell movement.
18.2.5Primary Swell.The swell system having the
greatest height from trough to crest.
18.2.6Secondary Swells.Those swell systems of
less height than the primary swell.
18.2.7Fetch.The distance the waves have been
driven by a wind blowing in a constant direction,
without obstruction.
18.2.8Swell Period.The time interval between the
passage of two successive crests at the same spot in
the water, measured in seconds.
18.2.9Swell Velocity.The velocity with which the
swell advances with relation to a fixed reference
point, measured in knots. There is little movement of
water in the horizontal direction. Swells move
primarily in a vertical motion, similar to the motion
observed when shaking out a carpet.
18.2.10Swell 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.
18.2.11Swell 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
25feet are not common at any spot on the oceans.
Successive swells may differ considerably in height.
18.3Swells
18.3.1It is extremely dangerous to land into the wind
without regard to sea conditions. The swell system, or
systems, must be taken into consideration.
AIP
United States of America
GEN 3.6−16
15 MAR 07
Nineteenth Edition Federal Aviation Administration
FIG GEN 3.6−18
Single Swell (15 knot wind)
SWELL
DITCHING
HEADING
WIND
FIG GEN 3.6−19
Double Swell (15 knot wind)
PRIMARY
SWELL
SECONDARY
SWELL
HEADING
WIND
DITCHING
FIG GEN 3.6−20
Double Swell (30 knot wind)
PRIMARY
SWELL
SECONDARY
SWELL
WIND
DITCHING
HEADING
FIG GEN 3.6−21
(50 knot wind)
SWELL
WINDWIND
Aircraft with low landing speeds - land into the wind.
Aircraft with high landing speeds - choose compromise
heading between wind and swell.
Both - land on back side of swell.
AIP
United States of America
GEN 3.6−17
15 MAR 07
Federal Aviation Administration Nineteenth Edition
FIG GEN 3.6−22
Wind−Swell−Ditch Heading
GOOD
GOOD !!
BACK SIDE POOR!!!
FACE
GOOD
FAIR
BEST
DIRECTION OF
SWELL MOVEMENT
Landing parallel to the major swell
Landing on the face and back of swell
18.3.2In 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.
18.3.3If 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
45degrees 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.
18.3.4If the swell system is formidable, it is
considered advisable, in landplanes, to accept more
crosswind in order to avoid landing directly into the
swell.
18.3.5The 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
　
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