CHARACTERISTICS OF WATER
A competent seaplane pilot is knowledgeable in the
characteristics of water and how they affect the sea-
plane. As a fluid, water seeks its own level, and forms
a flat, glassy surface if undisturbed. Winds, currents,
or objects traveling along its surface create waves and
movements that change the surface characteristics.
Just as airplanes encounter resistance in the form of
drag as they move through the air, seaplane hulls and
floats respond to drag forces as they move through
water. Drag varies proportionately to the square of
speed. In other words, doubling the seaplane’s speed
across the water results in four times the drag force.
Forces created when operating an airplane on water
are more complex than those created on land. For
landplanes, friction acts at specific points where the
tires meet the ground. Water forces act along the
entire length of a seaplane’s floats or hull. These
forces vary constantly depending on the pitch atti-
tude, the changing motion of the float or hull, and
action of the waves. Because floats are mounted
rigidly to the structure of the fuselage, they provide
no shock absorbing function, unlike the landing gear
of landplanes. While water may seem soft and yielding,
damaging forces and shocks can be transmitted
directly through the floats and struts to the basic
structure of the airplane.
Under calm wind conditions, the smooth water surface
presents a uniform appearance from above, somewhat
like a mirror. This situation eliminates visual refer-
ences for the pilot and can be extremely deceptive. If
waves are decaying and setting up certain patterns,
or if clouds are reflected from the water surface, the
resulting distortions can be confusing even for
experienced seaplane pilots.
DETERMINING SEA CONDITIONS
The ability to read the water’s surface is an integral
part of seaplane flying. The interaction of wind and
water determine the surface conditions, while tides and
currents affect the movement of the water itself.
Features along the shore and under the water’s surface
contribute their effects as well. With a little study, the
interplay between these factors becomes clearer.
A few simple terms describe the anatomy and charac-
teristics of waves. The top of a wave is the
crest
, and
the low valley between waves is a
trough
. The height
of waves is measured from the bottom of the trough to
the top of the crest. Naturally, the distance between two
wave crests is the wavelength. The time interval
between the passage of two successive wave crests at a
fixed point is the period of the wave.
Waves are usually caused by wind moving across
the surface of the water. As the air pushes the water,
ripples form. These ripples become waves in strong or
sustained winds; the higher the speed of the wind, or
the longer the wind acts on them, the larger the waves.
Waves can be caused by other factors, such as under-
water earthquakes, volcanic eruptions, or tidal
movement, but wind is the primary cause of most
waves. [Figure 3-1 on next page]
Calm water begins to show wave motion when the
wind reaches about two knots. At this windspeed,
patches of ripples begin to form. If the wind stops, sur-
face tension and gravity quickly damp the waves, and
the surface returns to its flat, glassy condition. If the
wind increases to four knots, the ripples become small
waves, which move in the same direction as the wind
and persist for some time after the wind stops blowing.
As windspeed increases above four knots, the water
surface becomes covered with a complicated pattern of
waves. When the wind is increasing, waves become
larger and travel faster. If the wind remains at a con-
stant speed, waves develop into a series of evenly
spaced parallel crests of the same height.
In simple waves, an object floating on the surface
shows that waves are primarily an up and down motion
of the water, rather than the water itself moving down-
wind at the speed of the waves. The floating object
describes a circle in the vertical plane, moving upward
as the crest approaches, forward and downward as the
crest passes, and backward as the trough passes. After
each wave passes, the object is at almost the same place
as before. The wind does cause floating objects to drift
slowly downwind.
While the wind is blowing and adding energy to the
water, the resulting waves are commonly referred to
as wind waves or
sea
. (Sea is also occasionally used
Ch 03.qxd 8/24/04 10:42 AM Page 3-1
to describe the combined motion of all the factors
disturbing the surface.) These waves tend to be a
chaotic mix of heights, periods, and wavelengths.
Because the wind causes the height to increase faster
than the wavelength, they often have relatively
steep, pointed crests and rounded troughs. With a
windspeed of 12 knots, the waves begin to break at
their crests and create foam.
The height of waves depends on three factors: wind-
speed, length of time the wind blows over the water,
and the distance over which the wind acts on the water.
As waves move away from the area where they were
generated (called a
fetch
), they begin to sort them-
selves by height and period, becoming regular and
evenly spaced. These waves often continue for thou-
sands of miles from where they were generated.
Swell
is the term describing waves that persist outside the
fetch or in the absence of the force that generated them.
A swell may be large or small, and does not indicate the
direction of the wind. The wake of a boat or ship is also
a swell.
Unlike wind and current, waves are not deflected much
by the rotation of the Earth, but move in the direction
in which the generating wind blows. When this wind
ceases, water friction and spreading reduce the wave
height, but the reduction takes place so slowly that a
swell persists until the waves encounter an obstruction,
such as a shore. Swell systems from many different
directions, even from different parts of the world, may
cross each other and interact. Often two or more swell
systems are visible on the surface, with a sea wave sys-
tem developing due to the current wind.
In lakes and sheltered waters, it is often easy to tell
wind direction by simply looking at the water’s sur-
face. There is usually a strip of calm water along the
upwind shore of a lake. Waves are perpendicular to the
wind direction. Windspeeds above approximately eight
knots leave wind streaks on the water, which are paral-
lel to the wind.
Land masses sculpt and channel the air as it moves over
them, changing the wind direction and speed. Wind
direction may change dramatically from one part of a
lake or bay to another, and may even blow in opposite
directions within a surprisingly short distance. Always
pay attention to the various wind indicators in the area,
especially when setting up for takeoff or landing.
Terms Used
by U.S.
Weather Service
Calm
Light Air
Light
Breeze
Gentle
Breeze
Moderate
Breeze
Fresh
Breeze
Strong
Breeze
Moderate
Gale
Velocity
m.p.h.
Less than 1
1 - 3
4 - 7
8 - 12
13 - 18
19 - 24
Estimating Velocities
on Land
Smoke rises vertically.
Smoke drifts; wind
vanes unmoved.
Wind felt on face; leaves
rustle; ordinary
vane moves by wind.
Leaves and small twigs
in constant motion;
wind extends light flag.
Dust and loose paper
raised; small branches
are moved.
Small trees begin to
sway; crested wavelets
form in inland water.
Large branches in motion;
whistling heard in telegraph
wires; umbrellas used
with difficulty.
Whole trees in motion;
inconvenience felt in
walking against the wind.
Sea like a mirror.
Ripples with the appearance
of scales are formed but
without foam crests.
Small wavelets, still short
but more pronounced; crests
have a glassy appearance
and do not break.
Large wavelets; crests begin to
break. Foam of glassy appearance.
(Perhaps scattered whitecaps.)
Small waves, becoming longer;
fairly frequent whitecaps.
Moderate waves; taking a more
pronounced long form; many
whitecaps are formed.
(Chance of some spray.)
Large waves begin to form;
white foam crests are more
extensive everywhere.
(Probably some spray.)
Sea heaps up and white foam
from breaking waves begins
to be blown in streaks along
the direction of the wind.
Estimating Velocities
on Sea
Check your glassy water
technique before water flying
under these conditions.
Ideal water flying
characteristics in
protected water.
This is considered rough
water for seaplanes and
small amphibians,
especially in open water.
This type of water condition
is for emergency only in small
aircraft in inland waters and
for the expert pilot.
Figure 3-1. The size of waves is determined by the speed of the wind.
Ch 03.qxd 8/24/04 10:42 AM Page 3-2
can subject the floats to hard pounding as they strike
consecutive wave crests. Operating on the surface in
rough conditions exposes the seaplane to forces that
can potentially cause damage or, in some cases,
overturn the seaplane. When a swell is not aligned
with the wind, the pilot must weigh the dangers
posed by the swell against limited crosswind
capability, as well as pilot experience.
On the other hand, calm, glassy water presents a different
set of challenges. Since the wind is calm, taxiing and
docking are somewhat easier, but takeoffs and landings
require special techniques. Takeoff distances may be
longer because the wings get no extra lifting help from
the wind. The floats seem to adhere more tenaciously to
the glassy water surface. When landing, the flat,
featureless surface makes it far more difficult to
gauge altitude accurately, and reflections can create
confusing optical illusions. The specific techniques
for glassy water operations are covered in Chapter
4, Seaplane Operations–Preflight and Takeoffs, and
Chapter 6, Seaplane Operations–Landing.
Tides are cause for concern when the airplane is
beached or moored in shallow water. A rising tide can
lift a beached seaplane and allow it to float out to sea if
the airplane is not properly secured. Depending on the
height of the tide and the topography of the beach, an
outgoing tide could leave a beached seaplane stranded
far from the water. [Figure 3-2]
While waves are simply an up and down undulation of
the water surface,
currents
are horizontal movements
of the water itself, such as the flow of water down-
stream in a river. Currents also exist in the oceans,
where solar heating, the Earth’s rotation, and tidal
forces cause the ocean water to circulate.
WATER EFFECTS ON OPERATIONS
Compared to operations from typical hard-surface
runways, taking off from and landing on water pres-
ents several added variables for the pilot to consider.
Waves and swell not only create a rough or uneven
surface, they also move, and their movement must be
considered in addition to the wind direction.
Likewise, currents create a situation in which the
surface itself is actually moving. The pilot may
decide to take off or land with or against the current,
depending on the wind, the speed of the current, and
the proximity of riverbanks or other obstructions.
While a landplane pilot can rely on windsocks and
indicators adjacent to the runway, a seaplane pilot
needs to be able to read wind direction and speed from
the water itself. On the other hand, the landplane pilot
may be restricted to operating in a certain direction
because of the orientation of the runway, while the sea-
plane pilot can usually choose a takeoff or landing
direction directly into the wind.
Even relatively small waves and swell can compli-
cate seaplane operations. Takeoffs on rough water
Figure 3-2. An outgoing tide can leave a seaplane far from the water. A rising tide can cause a beached seaplane to float away.
Ch 03.qxd 8/24/04 10:42 AM Page 3-3
Many of the operational differences between land-
planes and seaplanes relate to the fact that seaplanes
have no brakes. From the time a seaplane casts off,
it is usually in continuous motion due to the wind
and current, so the pilot must take deliberate action
to control this movement. Often these forces can be
used to the pilot’s advantage to help move the seaplane
as desired. Starting the engine, performing the engine
runup, and completing most pre-takeoff checks are all
accomplished while the seaplane is in motion. The
seaplane continues moving after the engine is shut
down, and this energy, along with the forces of wind
and current, is typically used to coast the seaplane to
the desired docking point.
As with land airplanes, the wind tends to make the
airplane weathervane, or yaw, until the nose points
into the wind. This tendency is usually negligible on
landplanes with tricycle landing gear, more pro-
nounced on those with conventional (tailwheel) gear,
and very evident in seaplanes. The tendency to
weathervane can usually be controlled by using the
water rudders while taxiing, but the water rudders are
typically retracted prior to takeoff. Weathervaning
can create challenges in crosswind takeoffs and
landings, as well as in docking or maneuvering in
close quarters.
SEAPLANE BASE OPERATIONS
In the United States, rules governing where seaplanes
may take off and land are generally left to state and
local governments.
Some states and cities are very liberal in the laws
regarding the operation of seaplanes on their lakes
and waterways, while other states and cities may
impose stringent restrictions. The Seaplane Pilots
Association publishes the useful Water Landing
Directory with information on seaplane facilities,
landing areas, waterway use regulations, and local
restrictions throughout the United States. Before
operating a seaplane on public waters, contact the
Parks and Wildlife Department of the state, the State
Aeronautics Department, or other authorities to
determine the local requirements. In any case, sea-
plane pilots should always avoid creating a nuisance
in any area, particularly in congested marine areas or
near swimming or boating facilities.
Established seaplane bases are shown on aeronautical
charts and are listed in the Airport/Facility Directory.
The facilities at seaplane bases vary greatly, but most
include a hard surface ramp for launching, servicing
facilities, and an area for mooring or hangaring sea-
planes. Many marinas designed for boats also provide
seaplane facilities.
Seaplanes often operate in areas with extensive recre-
ational or commercial water traffic. The movements of
faster craft, such as speedboats and jet-skis are unpre-
dictable. People towing skiers may be focusing their
attention behind the boat and fail to notice a landing
seaplane. Swimmers may be nearly invisible, often
with just their heads showing among the waves. There
is no equivalent of the airport traffic pattern to govern
boat traffic, and although right-of-way rules exist on
the water, many watercraft operators are unaware of
the limits of seaplane maneuverability and may
assume that seaplanes will always be able to maneuver
to avoid them. Many times, the seaplane itself is an
object of curiosity, drawing water traffic in the form of
interested onlookers.
When seaplane operations are conducted in bush
country, regular or emergency facilities are often lim-
ited or nonexistent. The terrain and waterways are
frequently hazardous, and any servicing becomes the
individual pilot’s responsibility. Prior to operating in
an unfamiliar area away from established seaplane
facilities, obtain the advice of FAA Accident
Prevention Counselors or experienced seaplane
pilots who are familiar with the area.