One of the major differences between
the operation of
a seaplane and that of a landplane is the method of manoeuvring
the aircraft on the surface. The landplane will usually
remain motionless with the engine idling, particularly
with the brakes applied, but a seaplane, since it is freefloating,
will invariably move in some direction, depending
upon the forces exerted by wind, water currents, propeller
thrust, and inertia. Because a seaplane has no
brakes, it is important that the pilot be familiar with the
existing wind and water conditions, effectively plan the
course of action, and mentally stay "ahead" of the aircraft.
There are three positions or attitudes in which a seaplane
can be moved about on the water: the "idling"
position, the "ploughing" position, and the "planing"
or "on the step" position.
When taxying with the engine idling or
at a low RPM,
the seaplane will remain in what is considered a displacement
condition similar to being at rest on the water. This is the "idling" position. The recommended taxy speed is usually below 6 or 7 knots so that the propeller
will not pick up water spray which causes serious erosion
of the propeller blades. In calm or light wind
conditions, the elevator control should be held full back to
raise the seaplane's nose and further reduce the possibility
of water spray on the propeller, and to improve overall manoeuvrability of the seaplane. This is particularly true if
it is equipped with water rudders because more rudder area
is kept in the water. Since seaplanes have no brakes, it is
especially important to taxy at this slow speed in congested
or confined areas because inertia forces build up rapidly,
making the seaplane vulnerable to serious damage even in
When the power is increased
idling, the seaplane will usually assume a nose-up or
"ploughing" position. Most seaplane experts
do not recommend the ploughing position for taxiing, except
in rough water when it would be desirable to raise the propeller
clear of the spray, or when turning the seaplane
downwind during strong wind conditions. To attain this
position, full power should be applied and the elevator
control held in the full aft position. Seaplanes that have a
high thrust line will tend to nose down upon application of
power, in which case it is imperative that the elevator control
be held in the full aft position. The "ploughing" position
is brought about by the combination of the propeller slipstream
striking the elevator and the hydrodynamic force of
water exerted on the underside of the float's or hull's bow.
After the planing position is attained, the power should be
reduced to maintain the proper speed.
If the water conditions are favourable and there is a long
distance to travel, the seaplane may be taxied at high speed
"on the step." This position is reached by
accelerating the seaplane to the degree that it passes
through the ploughing phase until the floats or hull are literally
riding on the water in a level position.
Basically, the planing or step
position is best attained
by holding the elevator control full aft and advancing the
throttle to full power. As the seaplane accelerates it will
then gradually assume a nose-high pitch attitude, raising
the bow of the float or hull and causing the weight of the
seaplane to be transferred toward the aft portion of the
float or hull. At the time the seaplane attains its highest
pitch attitude, back pressure should be gradually relaxed,
causing the weight to be transferred from the aft portion of
the float or hull onto the step area. This can be compared
to a speedboat's occupants moving forward in the boat to
aid in attaining a planing attitude. In the seaplane we do
essentially the same thing by use of aerodynamics (elevators).
As a result of aerodynamic and hydrodynamic lifting,
the seaplane is raised high in the water, allowing the
floats or hull to ride on top of rather than in the water.
The entire process of planing a seaplane is similar to
that of water skiing.
The skier cannot make the transition
from a submerged condition to that of being supported on
the surface of the water unless a sufficiently high speed is
attained and maintained.
As further acceleration takes place the flight controls
become more responsive just as in the landplane and elevator
deflection must be reducing in order to hold the
required planing/pitch attitude. This of course is accomplished
by further relaxing back pressure, increasing forward
pressure, or using forward elevator trim depending
on the aircraft flight characteristics.
Throughout the acceleration, the transfer of weight and
the hydrodynamic lifting of the float or hull may be seen
from the cockpit. When the seaplane is taxiing slowly, the
water line is quite high on the floats or hull as compared to
"on the step". At slow taxi speeds a small
wake is created close to the bow of the float or hull and
moves outward at a very shallow angle.
commences, the wake starts to move from the bow aft
toward the step area and the wake now turns into an outward
spray pattern. As speed and lifting action increase,
the spray pattern continues to move aft toward the step
position and increases in intensity, i.e., slow speed spray
may be approximately one foot outboard compared to
about a 20-foot outboard spray at higher speed on the step
position. Some seaplane pilots use the spray pattern as an
additional visual reference in aiding them in determining
when the seaplane has accelerated sufficiently to start easing
it over onto the step.
After the planing position has been attained, proper
control pressures must be used to control the proper pitch
attitude/trim angle. Usually this will be maintained with
slight back pressure.
As for the amount of pressure to be
held, the beginner will find a very "thin line" between easing
off back pressure too much or too little. It can perhaps
best be described as finding the "slippery spot" on the float
or hull. Too much back pressure, acceleration rate
decreases. Not enough back pressure or too much forward
pressure also decreases acceleration rate. So that fine line
or "slippery spot" is that position between not enough or
too much back pressure.
If one does not want to take off and just wants to continue
to taxi on the step, a reduction in power is initiated at
approximately the time the seaplane is eased over onto the
step. Power requirements to maintain the proper speed
with wind, load and current action will vary. More power
will be required taxiing into the wind or an up-current or
with a heavy load. However, 65 to 70 percent of maximum
power can be used as a starting point.
From either the ploughing or on-the-step position, if
power is reduced to idle, the seaplane will decelerate quite
rapidly and eventually assume the displacement or idle
Care must be taken to use proper flight control
pressures during the deceleration phase because weight is
now being transferred toward the bow and drag is increasing;
hence, some aircraft have a nose-over tendency. This
is of course controllable by proper use of the elevator controls.
If water rudders have the proper
amount of movement,
a seaplane can be turned within a radius less than the span
of the wing during calm conditions or a light breeze. It will
be found that water rudders are usually more effective at
slow speeds because they are acting in comparatively
undisturbed water. At high speeds, however, the stern of
the float churns the adjacent water, thereby causing the
water rudder to become less efficient.
because of the high speed, the water's impact on the rudders
may tend to force them to swing up or retract.
Particular attention should always be given to the risks
involved in making turns in a strong wind or at high
speeds. Any seaplane will tend to weathervane into a
strong wind if the controls are not positioned to prevent it.
In a single-engine seaplane rudder should be applied as
necessary to control the turn while aileron is held into the
wind. On twin-engine seaplanes this tendency can be overcome
through the use of differential power - using a higher
setting on the upwind side.
The rate at which the seaplane
turns when it weathervanes is directly proportional to the
speed of the crosswind. When taxiing downwind or crosswind,
the seaplane will swing into the wind as soon as the
flight controls are neutralized or power is reduced.
During a high speed taxiing turn, centrifugal forces
tends to tip the seaplane toward the outside of the turn (see
Figure 4). Simultaneously, when turning from a downwind
heading to an upwind heading, the wind force striking the
fuselage and the under side of the wing increases the tendency
for the seaplane to lean to the outside of the turn. If
an abrupt turn is made, the combination of these two
forces may be sufficient to tip the seaplane to the extent
that the outside wing drags in the water, and may even tip
the seaplane onto its back (see Figure 5). Obviously, the
further the seaplane tips, the greater will be the effect of
wind, since more wing area on the windward side is
exposed to the wind force.
Figure 4: Effect of wind and centrifugal force
When making a turn into the wind from
condition, the air-rudder may be neutralized and the seaplane
allowed to weathervane into the wind. If taxiing
directly downwind, a turn into the wind may be started by
deflecting the air rudder in the same direction that the turn
is desired. As soon as the seaplane begins to turn, the rudder
should be neutralized; if the wind is strong, some
opposite rudder may be needed during the turn. The
amount of opposite rudder depends upon the rate at which
the seaplane turns. The greater the amount of opposite rudder,
the slower the rate of turn. Normally, the power
should be reduced to idle when the turn begins because
with the power on the left turning tendency of the seaplane
may become excessive. Short bursts of power are best for
turning in a small radius, but sustained excessive power
causes a buildup of speed and a larger turning radius.
Figure 5: Effect of wind
The seaplane tends to use its centre
(COB) as a pivot point wherever it may be. Centre of
buoyancy moves laterally, as well as forward and aft. Each
object in water has a point of centre of buoyancy. In a
twin-float installation the effects of wind, power and flight
controls are shared by the two floats and the average COB
is free to move significantly.
The centre of buoyancy (COB), or average point of
support, moves aft when the seaplane is placed in a noseup
or ploughing position (see Figure 3). This position
exposes to the wind a considerable amount of float and
fuselage side area forward of the centre of buoyancy.
Therefore, when taxiing crosswind in this position, many
seaplanes will show a tendency to turn downwind because
of the wind force on the exposed area of the float and the
fuselage. For this reason it is sometimes helpful to place
the seaplane in a nose-up position when turning downwind,
particularly if the wind velocity is high. Under high
wind conditions, the throttle may be used as a turning
device by increasing power to cause a nose-up position
when turning downwind, and decreasing power to allow
the seaplane to weathervane into the wind.
Occasions often arise when it is
advisable to move the
seaplane backward or to one side because wind or water
conditions, or limited space make it impractical to attempt
a turn (see Figure 6). In this situation, particularly if there
is a significant wind, the seaplane can be "sailed" into a
space which to an inexperienced pilot might seem
extremely cramped. Even if the wind is calm and the space
is inadequate for making a normal turn, a paddle (which
should be part of every seaplane's equipment) may be used
to propel the seaplane or to turn the nose in the desired
In light wind conditions with the
engine idling or shut
down, the seaplane will naturally weathervane into the
wind and then sail in the direction the tail is pointed (see
Figure 7). With a stronger wind and a slight amount of
power, the seaplane will usually sail downwind toward the
side in which the nose is pointed. Rudder and aileron can
be deflected to create drag on the appropriate side to control
the direction of movement. Positioning the controls
for the desired direction of motion in light or strong winds
is illustrated in Figure 7. Lowering the wing flaps and
opening the cabin doors will increase the air resistance and
thus add to the effect of the wind; however, the effect of
the air rudder may be reduced in this configuration.
water rudders have little or no effect in controlling direction
while sailing, they should be lifted
With the engine shut down, most flying boats will sail
backward and toward the side to which the nose is pointed,
much as a sailboat tacks, regardless of wind velocity
because the hull does not provide as much keel (side area)
as do floats in proportion to the side of the seaplane. To
sail directly backward in a seaplane having a hull, the controls
should be released and the wind allowed to steer the
Sailing is an essential part of seaplane operation. Since
each type of seaplane has its own minor peculiarities,
depending on the design of the floats or hull, it should be
practiced until thorough familiarization with that particular
type is gained.
During initial seaplane training, sailing should be practiced
in large bodies of water such as lakes or bays, but
sufficiently close to a prominent object in order to evaluate
performance. Where there are strong tides or a rapidly
flowing current, such as in rivers, care must be taken in
observing the relative effect of both the wind and the water
current. Often the force of the current will be equal to or
greater than the force of the wind..
Figure 7: Sailing procedures
Before taxiing into a confined area,
the effect of wind
and the current should be considered carefully. Otherwise,
the seaplane may be carried into obstructions with resulting
damage to the wings, tail surfaces, floats, hull, or other
parts of the seaplane. Generally, with a seaplane of average
size and power at idle, a water current of 5 knots will
more than offset a wind velocity of 25 knots. This means
that the seaplane will move against the wind.
When operating multiengine seaplanes, differential
power can be used to aid in steering the seaplane along a
Porpoising in a seaplane is much like
the antics of a
dolphin - a rhythmic pitching and heaving while in the
water. Porpoising is a dynamic instability of the seaplane
and may occur when the seaplane is moving across the
water while on the step during takeoff or landing. It occurs
when the angle between the float or hull, and the water
surface exceeds the upper or lower limit of the seaplane's
pitch angle. Improper use of the elevator, resulting in
attaining too high or too low a pitch (trim angle) sets off a
cyclic oscillation which steadily increases in amplitude
unless the proper trim angle or pitch attitude is re-established.
A seaplane will travel smoothly across the water while
on the step, so long as the floats or hull remain within a
moderately tolerant range of trim angles.
If the trim angle
is held too low during planing, water pressure in the form
of a small crest or wall is built up under the bow or forward
part of the floats or hull. As the seaplane's forward
speed is increased to a certain point, the bow of the floats
or hull will no longer remain behind this crest and is
abruptly forced upward as the seaplane rides over the
crest. As the crest passes the step and on to the stern or aft
portion of the floats or hull, the bow abruptly drops into a
low position. This again builds a crest or wall of water in
front of the bow resulting in another oscillation. Each
oscillation becomes increasingly severe, and if not corrected
will cause the seaplane to nose into the water,
resulting in extensive damage or possible capsizing.
can also cause a premature lift off with an
extremely high angle of attack, resulting in a stall or being
in the area of reverse command and unable to climb over
Porpoising will occur during the takeoff run if the trim
angle is not properly controlled with proper elevator pressure
just after passing through the "hump" speed, or when
the highest trim angle before the planing attitude is
attained; that is, if up-elevator is held too long and the
angle reaches the upper limits.
On the other hand, if the seaplane is nosed down too
sharply, the lower trim range can be entered and will also
result in porpoising.
Usually, porpoising does not start
until a degree or two after the seaplane has passed into the
critical trim angle range, and does not cease until a degree
or two after the seaplane has passed out of the critical
If porpoising does occur, it can be stopped by applying
timely back pressure on the elevator control to prevent the
bow of the floats or hull from digging into the water. The
back pressure must be applied and maintained until porpoising
is damped. If porpoising is not damped by the time
the second oscillations occurs, it is recommended that the
power be reduced to idle and elevator control held firmly
back so the seaplane will settle onto the water with no further
The correct trim angle for takeoff, planing and landing
applicable to each type of seaplane must be learned by the
pilot and practiced until there is no doubt as to the proper
angles for the various manoeuvres.
The upper and lower trim angles are established by the
design of the aircraft; however, changing the seaplane's
gross weight, wing flap position, and centre of gravity
location will also change these limits. Increased weight
increases the displacement of the floats or hull and raises
the lower limit considerably. Extending the wing flaps frequently trims
the seaplane to the lower limit at lower
speeds, and may lower the upper limit at high speeds. A
forward centre of gravity increases the possibility of high
angle porpoising especially during landing.
Skipping is a form of instability
which may occur
when landing with excessive speed at a nose-up trim
angle. This nose-up attitude places the seaplane at the
upper trim limit of stability, and causes the seaplane to
enter a cyclic oscillation when touching the water, resulting
in the seaplane skipping across the surface. This action
may be compared to "skipping" flat stones across the
Skipping can also occur by crossing a boat wake while
fast taxiing on the step or during a takeoff. Sometimes the
new seaplane pilot will confuse a skip with a porpoise.
Pilot's body feelings can quickly determine whether a skip
or porpoise has been encountered. A skip will give the
body vertical "G" forces, similar to bouncing a landplane.
The porpoise is a rocking chair type forward and aft
Correction for skipping is made by first increasing
back pressure on the elevator control and adding sufficient
power to prevent the floats from contacting the water.
Then pressure on the elevator must be adjusted to attain
the proper trim angle and the power gradually reduced to
allow the seaplane to settle gently onto the water.
Skipping will not continue increasing its oscillations,
as in porpoising, because of the lack of forward thrust with