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AIM

10/12/17

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