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Pilot's Handbook of Aeronautical Knowledge
Aircraft Performance
Takeoff and Landing Performance

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Water on the Runway and Dynamic Hydroplaning
Water on the runways reduces the friction between the tires
and the ground, and can reduce braking effectiveness. The
ability to brake can be completely lost when the tires are
hydroplaning because a layer of water separates the tires from
the runway surface. This is also true of braking effectiveness
when runways are covered in ice.

When the runway is wet, the pilot may be confronted with
dynamic hydroplaning. Dynamic hydroplaning is a condition
in which the aircraft tires ride on a thin sheet of water rather
than on the runway's surface. Because hydroplaning wheels
are not touching the runway, braking and directional control
are almost nil. To help minimize dynamic hydroplaning,
some runways are grooved to help drain off water; most
runways are not.

Tire pressure is a factor in dynamic hydroplaning. Using
the simple formula in Figure 10-17, a pilot can calculate
the minimum speed, in knots, at which hydroplaning will
begin. In plain language, the minimum hydroplaning speed
is determined by multiplying the square root of the main gear
tire pressure in psi by nine. For example, if the main gear tire
pressure is at 36 psi, the aircraft would begin hydroplaning
at 54 knots.

Tire pressure.
Figure 10-17. Tire pressure.

Landing at higher than recommended touchdown speeds will
expose the aircraft to a greater potential for hydroplaning.
And once hydroplaning starts, it can continue well below the
minimum initial hydroplaning speed.

On wet runways, directional control can be maximized
by landing into the wind. Abrupt control inputs should be
avoided. When the runway is wet, anticipate braking problems
well before landing and be prepared for hydroplaning. Opt for
a suitable runway most aligned with the wind. Mechanical
braking may be ineffective, so aerodynamic braking should
be used to its fullest advantage.

Takeoff Performance
The minimum takeoff distance is of primary interest in
the operation of any aircraft because it defines the runway
requirements. The minimum takeoff distance is obtained by
taking off at some minimum safe speed that allows sufficient
margin above stall and provides satisfactory control and
initial rate of climb. Generally, the lift-off speed is some fixed
percentage of the stall speed or minimum control speed for the
aircraft in the takeoff configuration. As such, the lift-off will
be accomplished at some particular value of lift coefficient
and AOA. Depending on the aircraft characteristics, the liftoff
speed will be anywhere from 1.05 to 1.25 times the stall
speed or minimum control speed.

To obtain minimum takeoff distance at the specific lift-off
speed, the forces that act on the aircraft must provide the
maximum acceleration during the takeoff roll. The various
forces acting on the aircraft may or may not be under the
control of the pilot, and various procedures may be necessary
in certain aircraft to maintain takeoff acceleration at the
highest value.

The powerplant thrust is the principal force to provide the
acceleration and, for minimum takeoff distance, the output
thrust should be at a maximum. Lift and drag are produced
as soon as the aircraft has speed, and the values of lift and
drag depend on the AOA and dynamic pressure.

In addition to the important factors of proper procedures,
many other variables affect the takeoff performance of an
aircraft. Any item that alters the takeoff speed or acceleration
rate during the takeoff roll will affect the takeoff distance.
For example, the effect of gross weight on takeoff distance
is significant and proper consideration of this item must be
made in predicting the aircraft's takeoff distance. Increased
gross weight can be considered to produce a threefold effect
on takeoff performance:
1. Higher lift-off speed
2. Greater mass to accelerate
3. Increased retarding force (drag and ground friction)
If the gross weight increases, a greater speed is necessary to
produce the greater lift necessary to get the aircraft airborne
at the takeoff lift coefficient. As an example of the effect of
a change in gross weight, a 21 percent increase in takeoff
weight will require a 10 percent increase in lift-off speed to
support the greater weight.