PERFORMANCE CONSIDERATIONS
FOR TAKEOFF, CLIMB, CRUISE, AND
LANDING
Since many pilots are accustomed to a certain level of
performance from a specific make and model of land
airplane, the changes in performance when that same
airplane is equipped with floats can lead to trouble for
a careless or complacent pilot. Floats weigh somewhat
more than the wheeled landing gear they replace, but
floats are designed to produce aerodynamic lift to off-
set some of the weight penalty. Generating lift
inevitably creates induced drag, which imposes a small
reduction in overall performance. By far the greatest
impact on performance comes from the parasitic drag
of the floats.
TAKEOFF
In a landplane, takeoff distance increases with addi-
tional takeoff weight for two reasons: it takes longer
for the engine and propeller to accelerate the greater
mass to lift-off speed, and the lift-off speed itself is
higher because the wings must move faster to produce
the extra lift required. For seaplanes, there are two
more factors, both due to water drag. As seaplane
weight increases, the floats sink deeper into the water,
creating more drag during initial acceleration. As with
the landplane, the seaplane must also accelerate to a
higher airspeed to generate more lift, but the seaplane
must overcome significantly more water drag force as
speed increases. This extra drag reduces the rate of
acceleration and results in a longer takeoff run.
Naturally, the location of the additional weight within
the seaplane affects center of gravity (CG) location.
Because of the way the floats respond to weight, the
CG location affects the seaplane’s handling charac-
teristics on the water. If the CG is too far aft, it may
be impossible to put the seaplane on the step. If the
CG is located to one side of the centerline, one float
will be pushed deeper into the water, resulting in
more water drag on that side. Be sure to balance the
fuel load between left and right wing tanks, and pay
attention to how baggage or cargo is secured, so that
the weight is distributed somewhat evenly from side
to side. [Figure 5-1]
The importance to weight and balance of pumping out
the float compartments should be obvious. Water
weighs 8.34 pounds per gallon, or a little over 62
pounds per cubic foot. Performance decreases when-
ever the wings and engine have to lift and carry useless
water in a float compartment. Even a relatively small
amount of water in one of the front or rear float com-
partments could place the airplane well outside of CG
limits and seriously affect stability and control.
Naturally, water also moves around in response to
changes in attitude, and the sloshing of water in the
floats can create substantial CG changes as the sea-
plane is brought onto the step or rotated into a climb
attitude.
Some pilots use float compartments near the CG to
stow iced fish or game from hunting expeditions. It is
imperative to adhere to the manufacturer’s weight and
balance limitations and to include the weight and
moment of float compartment contents in weight and
balance calculations.
Density altitude is a very important factor in seaplane
takeoff performance. High altitudes, high tempera-
tures, high humidity, and even low barometric pressure
can combine to rob the engine and propeller of thrust
and the wings of lift. Seaplane pilots are encouraged
to occasionally simulate high density altitude by
using a reduced power setting for takeoff. This exer-
cise should only be attempted where there is plenty
of water area, as the takeoff run will be much longer.
An experienced seaplane instructor can assist with
choosing an appropriate power setting and demon-
strating proper technique.
Unbalanced Fuel Load
Figure 5-1. The location of the CG can affect seaplane
handling.
Ch 05.qxd 8/25/04 10:43 AM Page 5-1
CLIMB AND CRUISE
When comparing the performance of an airplane with
wheels to the same airplane equipped with floats, the
drag and weight penalty of the floats usually results in
a reduced climb rate for any given weight. Likewise,
cruise speeds will usually be a little lower for a partic-
ular power setting. This in turn means increased fuel
consumption and reduced range. Unless the airplane
was originally configured as a seaplane, the perform-
ance and flight planning information for a landplane
converted to floats will typically be found in the
Supplements section rather than the Performance sec-
tion of the Airplane Flight Manual (AFM) or Pilot’s
Operating Handbook (POH).
In addition to working within the limits of the sea-
plane’s range, the pilot planning a cross-country flight
must also consider the relative scarcity of refueling
facilities for seaplanes. Amphibians have access to land
airports, of course, but seaplanes without wheels need
to find water landing facilities that also sell aviation
fuel. While planning the trip, it is wise to call ahead to
verify that the facilities have fuel and will be open at
the intended arrival times. The Seaplane Pilots
Association publishes a Water Landing Directory that
is very helpful in planning cross-country flights.
In flight, the seaplane handles very much like the cor-
responding landplane. On many floatplanes, the floats
decrease directional stability to some extent. The floats
typically have more vertical surface area ahead of the
airplane’s CG than behind it. If the floats remain
aligned with the airflow, this causes no problems, but if
the airplane begins to yaw or skid, this vertical area acts
somewhat like a large control surface that tends to
increase the yaw, making the skid worse. [Figure 5-2]
Additional vertical surface well behind the CG can
counteract the yaw force created by the front of the
floats, so many floatplanes have an auxiliary fin
attached to the bottom of the tail, or small vertical sur-
faces added to the horizontal stabilizer. [Figure 5-3]
LANDING
Landplane pilots are trained to stay on the lookout for
good places to land in an emergency, and to be able to
plan a glide to a safe touchdown should the engine(s)
fail. An airplane equipped with floats will usually have
a steeper power-off glide than the same airplane with
wheels. This means a higher rate of descent and a
diminished glide range in the event of an engine fail-
ure, so the pilot should keep this in mind when spotting
potential landing areas during cruising flight.
Seaplanes often permit more options in the event of an
unplanned landing, since land can be used as well as
water. While a water landing may seem like the only
choice for a non-amphibious seaplane, a smooth
landing on grass, dirt, or even a hard-surface runway
usually causes very little damage to the floats or hull,
and may frequently be the safer alternative.
Figure 5-2. The side area of the floats can decrease direc-
tional stability.
Figure 5-3. Vertical surfaces added to the tail help restore
directional stability.
Ch 05.qxd 8/25/04 10:43 AM Page 5-2
The most extreme pitch force logically results from a
sudden engine failure, when the full thrust of the
engine and its associated downward pitching force are
suddenly removed. Forward thrust is replaced by the
drag of a windmilling propeller, which adds a new
upward pitching force. Since the seaplane is already
trimmed with a considerable elevator force to coun-
teract the downward pitch force of the engine, the
nose pitches up abruptly. If this scenario occurs just
after takeoff, when the engine has been producing
maximum power, airspeed is low, and there is little
altitude, the pilot must react instantly to overpower
the upward pitching forces and push the nose down to
avoid a stall.
The reversal of typical pitch forces also comes into
play if porpoising should begin during a takeoff. As
discussed in Chapter 4, Seaplane Operations -
Preflight and Takeoffs, porpoising usually occurs
when the planing angle is held too low by the pilot,
forcing the front portion of the floats to drag until a
wave builds up and travels back along the float. The
same thing can happen with the hull of a flying boat,
and the nose-down force of a high thrust line can make
porpoising more likely. Once porpoising develops, the
standard solution is to reduce power and let the air-
plane settle back into the water. But if power is
reduced too quickly in a seaplane with a high-mounted
engine, the sudden upward pitching force can combine
with the porpoising to throw the seaplane into the air
with inadequate airspeed for flight, decreasing thrust,
and inadequate altitude for recovery.
Depending on how far the engine is from the airplane’s
CG, the mass of the engine can have detrimental
effects on roll stability. Some seaplanes have the
engine mounted within the upper fuselage, while oth-
FLIGHT CHARACTERISTICS OF
SEAPLANES WITH HIGH THRUST LINES
Many of the most common flying boat designs have
the engine and propeller mounted well above the air-
frame’s CG. This results in some unique handling
characteristics. The piloting techniques necessary to
fly these airplanes safely are not intuitive and must be
learned. Any pilot transitioning to such an airplane is
strongly urged to obtain additional training specific to
that model of seaplane.
Designing a seaplane with the engine and propeller
high above the water offers some important advan-
tages. The propeller is out of the spray during takeoffs
and landings, and more of the fuselage volume can be
used for passengers and cargo. The pilot usually sits
well forward of the wing, and enjoys an excellent view
in almost every direction.
Pilots who fly typical light twins are familiar with what
happens when one engine is producing power and the
other is not. The airplane tends to yaw toward the dead
engine. This happens because the thrust line is located
some distance from the airplane’s CG. In some
respects, this situation is similar to the single-engine
seaplane with a high thrust line, except that the sea-
plane flies on one engine all the time. When power is
applied, the thrust tends to pitch the nose down, and as
power is reduced, the nose tends to rise. [Figure 5-4]
This is exactly the opposite of what most pilots are
accustomed to. In typical airplanes, including most
floatplanes, applying power raises the nose and initi-
ates a climb.
Naturally the magnitude of these pitch forces is pro-
portional to how quickly power is applied or reduced.
Figure 5-4. Pitching forces in seaplanes with a high thrust line.
Increasing Thrust
Decreasing Thrust
Ch 05.qxd 8/25/04 10:43 AM Page 5-3
ers have engines mounted on a pylon well above the
main fuselage. If it is far from the CG, the engine can
act like a weight at the end of a lever, and once started
in motion it tends to continue in motion. Imagine
balancing a hammer upright with the handle on the
palm of the hand. [Figure 5-5]
Finally, seaplanes with high-mounted engines may
have unusual spin characteristics and recovery tech-
niques. These factors reinforce the point that pilots
need to obtain thorough training from a qualified
instructor in order to operate this type of seaplane
safely.
MULTIENGINE SEAPLANES
A rating to fly single-engine seaplanes does not entitle
a pilot to fly seaplanes with two or more engines. The
addition of a multiengine sea rating to a pilot
certificate requires considerable additional training.
Dealing with engine failures and issues of asymmetri-
cal thrust are important aspects in the operation of
multiengine seaplanes.
Figure 5-5. Roll instability with a high-mounted engine.