AIM
4/3/14
6−3−5
Distress and Urgency Procedures
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.
c.
In order to select a good heading when ditching
an aircraft, a basic evaluation of the sea is required.
Selection of a good ditching heading may well
minimize damage and could save your life. It can be
extremely dangerous to land into the wind without
regard to sea conditions; the swell system, or systems,
must be taken into consideration. Remember one
axiom− AVOID THE FACE OF A SWELL.
1.
In ditching parallel to the swell, it makes little
difference whether touchdown is on the top of the
crest or in the trough. It is preferable, however, to land
on the top or back side of the swell, if possible. After
determining which heading (and its reciprocal) will
parallel the swell, select the heading with the most
into the wind component.
2.
If only one swell system exists, the problem
is relatively simple−even with a high, fast system.
Unfortunately, most cases involve two or more swell
systems running in different directions. With more
than one system 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 eight feet high, and the
other three feet, plan to land parallel to the primary
system, and on the down swell of the secondary
system. If both systems are of equal height, a
compromise may be advisable−select 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.
3.
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.
4.
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
to two directions paralleling the primary system. One
direction is downwind and down the secondary swell,
and the other is into the wind and into the secondary
swell, the choice will depend on the velocity of the
wind versus the velocity and height of the secondary
swell.
d.
The simplest method of estimating the wind
direction and velocity is to examine the windstreaks
on the water. These appear as long streaks up and
down wind. Some persons may have difficulty
determining wind direction after seeing the streaks on
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
estimated by noting the appearance of the whitecaps,
foam and wind streaks.
1.
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.
2.
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 short
and rough seas. 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.
3.
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.
e.
Once preditching preparations are completed,
the pilot should turn to the ditching heading and
commence let−down. The aircraft should be flown
low over the water, and slowed down until ten knots
or so above stall. At this point, additional power
should be used to overcome the increased drag caused
by the nose up attitude. When a smooth stretch of
water appears ahead, cut power, and touchdown at the
best recommended speed as fully stalled as possible.
By cutting power when approaching a relatively
smooth area, the pilot will prevent overshooting and