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waves is the number of crests counted minus one.
(A complete wave runs from crest to crest. Since
the timing starts with a crest and ends with a
crest, there is one less wave than crests.) Time
and count each swell system.
3. Obtain the swell period by dividing the time in
seconds by the number of waves. For example, 5
waves in 30 seconds equates to a swell period of
6 seconds.
4. Determine the swell velocity in knots by multiplying
the swell period by 3. In this example, 6
seconds multiplied by 3 equals 18 knots.
Resultant Wave
Wave A Wave B
Resultant Wave
Wave A Wave B
Figure 8-3.Wave interference.
Swell Period
Swell Velocity Swell Period x 3 knots
Swell Length Swell Period2 x 5 Feet
Time in Seconds
Number of Waves Counted
Figure 8-4. Rules of thumb to determine swell period,
velocity, and length.
Ch 08.qxd 8/25/04 11:27 AM Page 8-3
8-4
SELECT TOUCHDOWN AREA
On final approach, select the touchdown area by
searching for a null or smooth area in the swell system,
avoiding rough areas if possible. When doing so,
consider the conditions discussed in the following
sections.
LANDING PARALLEL TO THE SWELL
When landing on a swell system with large, widely
spaced crests more than four times the length of the
floats, the best landing heading parallels the crests and
has the most favorable headwind component. In this
situation, it makes little difference whether touchdown
is on top of the crest or in the trough.
LANDING PERPENDICULAR TO THE SWELL
If crosswind limits would be exceeded by landing parallel
to the swell, landing perpendicular to the swell
might be the only option. Landing in closely spaced
swells less than four times the length of the floats
should be considered an emergency procedure only,
since damage or loss of the seaplane can be expected.
If the distance between crests is less than half the length
of the floats, the touchdown may be smooth, since the
floats will always be supported by at least two waves,
but expect severe motion and forces as the seaplane
slows.
A downswell landing on the back of the swell is preferred.
However, strong winds may dictate landing into
the swell. To compare landing downswell with landing
into the swell, consider the following example.
Assuming a 10-second swell period, the length of the
swell is 500 feet, and it has a velocity of 30 knots or 50
feet per second. Assume the seaplane takes 890 feet and
5 seconds for its runout.
Downswell Landing—The swell is moving with the
seaplane during the landing runout, thereby increasing
the effective swell length by about 250 feet and
resulting in an effective swell length of 750 feet. If
the seaplane touches down just beyond the crest, it
finishes its runout about 140 feet beyond the next
crest. [Figure 8-5]
Landing into the Swell—During the 5 seconds of
runout, the oncoming swell moves toward the seaplane
a distance of about 250 feet, thereby shortening the
effective swell length to about 250 feet. Since the seaplane
takes 890 feet to come to rest, it would meet the
oncoming swell less than halfway through its runout
and it would probably be thrown into the air, out of
control. Avoid this landing heading if at all possible.
[Figure 8-6]
If low ceilings prevent complete sea evaluation from
the altitudes prescribed above, any open sea landing
should be considered a calculated risk, as a dangerous
but unobserved swell system may be present in the
proposed landing area. Complete the descent and
before-landing checklists prior to descending below
1,000 feet if the ceiling is low.
LANDING WITH MORE THAN ONE SWELL
SYSTEM
Open water often has two or more swell systems
running in different directions, which can present a
confusing appearance to the pilot. When the secondary
swell system is from the same direction as the
wind, the preferred direction of landing is parallel
to the primary swell with the secondary swell at
some angle. When landing parallel to the primary
swell, the two choices of heading are either upwind
and into the secondary swell, or downwind and
downswell. The heading with the greatest headwind
is preferred; however, if a pronounced secondary
swell system is present, it may be desirable to land
downswell to the secondary swell system and accept
some tailwind component. The risks associated with
landing downwind versus downswell must be carefully
considered. The choice of heading depends on
the velocity of the wind versus the velocity and the
　
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