Figure 11-16. An aircraft’s performance during takeoff depends greatly on the runway surface.
Figure 11-15. Takeoff distance chart.
The gradient or slope of the runway is the amount of change
in runway height over the length of the runway. The gradient
is expressed as a percentage, such as a 3 percent gradient. This
means that for every 100 feet of runway length, the runway
height changes by 3 feet. A positive gradient indicates the
runway height increases, and a negative gradient indicates the
runway decreases in height. An upsloping runway impedes
acceleration and results in a longer ground run during takeoff.
However, landing on an upsloping runway typically reduces
the landing roll. A downsloping runway aids in acceleration
on takeoff resulting in shorter takeoff distances. The opposite
is true when landing, as landing on a downsloping runway
increases landing distances. Runway slope information is
contained in the Chart Supplement U.S. (formerly Airport/
Facility Directory). [Figure 11-17]
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.
Figure 11-18. Tire pressure.
9 x Tire pressure (in psi)
9 x 6 = 54 knots
36 = 6
Minimum dynamic hydroplaning speed (rounded off) =
Figure 11-17. Chart Supplement U.S. (formerly Airport/Facility Directory) information.
Airport name
Runway slope and direction of slope
Runway surface
Runway
Tire pressure is a factor in dynamic hydroplaning. Using
the simple formula in Figure 11-18, a pilot can calculate
the minimum speed, in knots, at which hydroplaning begins.
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.
Landing at higher than recommended touchdown speeds
exposes 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 ROC. 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 is
accomplished at some particular value of lift coefficient and
AOA. Depending on the aircraft characteristics, the lift-off
speed is 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.
Figure 11-19. Effect of wind on takeoff and landing.
Percent increase
in takeoff or
landing distance
Percent decrease
in takeoff or
landing distance
Ratio of wind
velocity to takeoff
or landing speed
Headwind
Tailwind
Reference line
80
70
60
50
40
30
20
10
30% 20 % 10 %
10% 20 % 30 %
10
20
30
40
50
60
Ratio of wind
velocity to takeoff
or landing speed
As discussed in Chapter 6, engine pressure ratio (EPR) is the
ratio between exhaust pressure (jet blast) and inlet (static)
pressure on a turbo jet or turbo fan engine. An EPR gauge
tells the pilot how much power the engines are generating.
The higher the EPR, the higher the engine thrust. EPR is
used to avoid over-boosting an engine and to set takeoff and
go around power if needed. This information is important to
know before taking off as it helps determine the performance
of the aircraft.
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 affects 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 requires a 10 percent increase in lift-off speed to
support the greater weight.
A change in gross weight changes the net accelerating force
and changes the mass that is being accelerated. If the aircraft
has a relatively high thrust-to-weight ratio, the change in the
net accelerating force is slight and the principal effect on
acceleration is due to the change in mass.
For example, a 10 percent increase in takeoff gross weight
would cause:
• A 5 percent increase in takeoff velocity
• At least a 9 percent decrease in rate of acceleration
• At least a 21 percent increase in takeoff distance
With ISA conditions, increasing the takeoff weight of the
average Cessna 182 from 2,400 pounds to 2,700 pounds (11
percent increase) results in an increased takeoff distance from
440 feet to 575 feet (23 percent increase).
For the aircraft with a high thrust-to-weight ratio, the increase
in takeoff distance might be approximately 21 to 22 percent,
but for the aircraft with a relatively low thrust-to-weight
ratio, the increase in takeoff distance would be approximately
25 to 30 percent. Such a powerful effect requires proper
consideration of gross weight in predicting takeoff distance.
The effect of wind on takeoff distance is large, and proper
consideration must also be provided when predicting takeoff
distance. The effect of a headwind is to allow the aircraft to
reach the lift-off speed at a lower groundspeed, while the
effect of a tailwind is to require the aircraft to achieve a
greater groundspeed to attain the lift-off speed.
A headwind that is 10 percent of the takeoff airspeed reduces
the takeoff distance approximately 19 percent. However, a
tailwind that is 10 percent of the takeoff airspeed increases
the takeoff distance approximately 21 percent. In the case
where the headwind speed is 50 percent of the takeoff speed,
the takeoff distance would be approximately 25 percent of
the zero wind takeoff distance (75 percent reduction).
The effect of wind on landing distance is identical to its
effect on takeoff distance. Figure 11-19 illustrates the general
effect of wind by the percent change in takeoff or landing
distance as a function of the ratio of wind velocity to takeoff
or landing speed.
The effect of proper takeoff speed is especially important
when runway lengths and takeoff distances are critical. The
takeoff speeds specified in the AFM/POH are generally
the minimum safe speeds at which the aircraft can become
airborne. Any attempt to take off below the recommended
speed means that the aircraft could stall, be difficult to
control, or have a very low initial ROC. In some cases, an
excessive AOA may not allow the aircraft to climb out of
ground effect. On the other hand, an excessive airspeed at
takeoff may improve the initial ROC and “feel” of the aircraft
but produces an undesirable increase in takeoff distance.
Assuming that the acceleration is essentially unaffected, the
takeoff distance varies with the square of the takeoff velocity.
Thus, ten percent excess airspeed would increase the takeoff
distance 21 percent. In most critical takeoff conditions, such
an increase in takeoff distance would be prohibitive, and the
pilot must adhere to the recommended takeoff speeds.
The effect of pressure altitude and ambient temperature
is to define the density altitude and its effect on takeoff
performance. While subsequent corrections are appropriate
for the effect of temperature on certain items of powerplant
performance, density altitude defines specific effects on
takeoff performance. An increase in density altitude can
produce a twofold effect on takeoff performance:
1. Greater takeoff speed
2. Decreased thrust and reduced net accelerating force
If an aircraft of given weight and configuration is operated at
greater heights above standard sea level, the aircraft requires
the same dynamic pressure to become airborne at the takeoff
lift coefficient. Thus, the aircraft at altitude takes off at the
same indicated airspeed (IAS) as at sea level, but because of
the reduced air density, the TAS is greater.
The effect of density altitude on powerplant thrust depends
much on the type of powerplant. An increase in altitude
above standard sea level brings an immediate decrease in
power output for the unsupercharged reciprocating engine.
However, an increase in altitude above standard sea level does
not cause a decrease in power output for the supercharged
reciprocating engine until the altitude exceeds the critical
operating altitude. For those powerplants that experience
a decay in thrust with an increase in altitude, the effect
on the net accelerating force and acceleration rate can be
approximated by assuming a direct variation with density.
Actually, this assumed variation would closely approximate
the effect on aircraft with high thrust-to-weight ratios.
Proper accounting of pressure altitude and temperature is
mandatory for accurate prediction of takeoff roll distance.
The most critical conditions of takeoff performance are the
result of some combination of high gross weight, altitude,
temperature, and unfavorable wind. In all cases, the pilot
must make an accurate prediction of takeoff distance from
the performance data of the AFM/POH, regardless of the
runway available, and strive for a polished, professional
takeoff procedure.
In the prediction of takeoff distance from the AFM/POH
data, the following primary considerations must be given:
• Pressure altitude and temperature—to define the effect
of density altitude on distance
• Gross weight—a large effect on distance
• Wind—a large effect due to the wind or wind
component along the runway
• Runway slope and condition—the effect of an incline
and retarding effect of factors such as snow or ice
Landing Performance
In many cases, the landing distance of an aircraft defines the
runway requirements for flight operations. The minimum
landing distance is obtained by landing at some minimum safe
speed, that allows sufficient margin above stall and provides
satisfactory control and capability for a go-around. Generally,
the landing speed is some fixed percentage of the stall speed
or minimum control speed for the aircraft in the landing
configuration. As such, the landing is accomplished at some
particular value of lift coefficient and AOA. The exact values
depend on the aircraft characteristics but, once defined, the
values are independent of weight, altitude, and wind.
To obtain minimum landing distance at the specified landing
speed, the forces that act on the aircraft must provide
maximum deceleration during the landing roll. The forces
acting on the aircraft during the landing roll may require
various procedures to maintain landing deceleration at the
peak value.
A distinction should be made between the procedures for
minimum landing distance and an ordinary landing roll
with considerable excess runway available. Minimum
landing distance is obtained by creating a continuous peak
deceleration of the aircraft; that is, extensive use of the brakes
for maximum deceleration. On the other hand, an ordinary
landing roll with considerable excess runway may allow
extensive use of aerodynamic drag to minimize wear and tear
on the tires and brakes. If aerodynamic drag is sufficient to
cause deceleration, it can be used in deference to the brakes
in the early stages of the landing roll (i.e., brakes and tires
suffer from continuous hard use, but aircraft aerodynamic
drag is free and does not wear out with use). The use of
aerodynamic drag is applicable only for deceleration to 60
or 70 percent of the touchdown speed. At speeds less than
60 to 70 percent of the touchdown speed, aerodynamic drag
is so slight as to be of little use, and braking must be utilized
to produce continued deceleration. Since the objective during
the landing roll is to decelerate, the powerplant thrust should
be the smallest possible positive value (or largest possible
negative value in the case of thrust reversers).
In addition to the important factors of proper procedures,
many other variables affect the landing performance. Any
item that alters the landing speed or deceleration rate during
the landing roll affects the landing distance.
The effect of gross weight on landing distance is one of the
principal items determining the landing distance. One effect
of an increased gross weight is that a greater speed is required
to support the aircraft at the landing AOA and lift coefficient.
For an example of the effect of a change in gross weight, a
21 percent increase in landing weight requires a ten percent
increase in landing speed to support the greater weight.
When minimum landing distances are considered, braking
friction forces predominate during the landing roll and, for
the majority of aircraft configurations, braking friction is the
main source of deceleration.
The minimum landing distance varies in direct proportion
to the gross weight. For example, a ten percent increase in
gross weight at landing would cause a:
• Five percent increase in landing velocity
• Ten percent increase in landing distance
A contingency of this is the relationship between weight and
braking friction force.
The effect of wind on landing distance is large and deserves
proper consideration when predicting landing distance. Since
the aircraft lands at a particular airspeed independent of the
wind, the principal effect of wind on landing distance is
the change in the groundspeed at which the aircraft touches
down. The effect of wind on deceleration during the landing
is identical to the effect on acceleration during the takeoff.
The effect of pressure altitude and ambient temperature is to
define density altitude and its effect on landing performance.
An increase in density altitude increases the landing speed
but does not alter the net retarding force. Thus, the aircraft
at altitude lands at the same IAS as at sea level but, because
of the reduced density, the TAS is greater. Since the aircraft
lands at altitude with the same weight and dynamic pressure,
the drag and braking friction throughout the landing roll have
the same values as at sea level. As long as the condition is
within the capability of the brakes, the net retarding force
is unchanged, and the deceleration is the same as with the
landing at sea level. Since an increase in altitude does not
alter deceleration, the effect of density altitude on landing
distance is due to the greater TAS.
The minimum landing distance at 5,000 feet is 16 percent
greater than the minimum landing distance at sea level. The
approximate increase in landing distance with altitude is
approximately three and one-half percent for each 1,000 feet
of altitude. Proper accounting of density altitude is necessary
to accurately predict landing distance.
The effect of proper landing speed is important when runway
lengths and landing distances are critical. The landing speeds
specified in the AFM/POH are generally the minimum safe
speeds at which the aircraft can be landed. Any attempt to
land at below the specified speed may mean that the aircraft
may stall, be difficult to control, or develop high rates of
descent. On the other hand, an excessive speed at landing may
improve the controllability slightly (especially in crosswinds)
but causes an undesirable increase in landing distance.
A ten percent excess landing speed causes at least a 21
percent increase in landing distance. The excess speed
places a greater working load on the brakes because of the
additional kinetic energy to be dissipated. Also, the additional
speed causes increased drag and lift in the normal ground
attitude, and the increased lift reduces the normal force on
the braking surfaces. The deceleration during this range of
speed immediately after touchdown may suffer, and it is more
probable for a tire to be blown out from braking at this point.
The most critical conditions of landing performance are
combinations of high gross weight, high density altitude,
and unfavorable wind. These conditions produce the greatest
required landing distances and critical levels of energy
dissipation on the brakes. In all cases, it is necessary to
make an accurate prediction of minimum landing distance to
compare with the available runway. A polished, professional
landing procedure is necessary because the landing phase of
flight accounts for more pilot-caused aircraft accidents than
any other single phase of flight.
In the prediction of minimum landing distance from the
AFM/POH data, the following considerations must be given:
• Pressure altitude and temperature—to define the effect
of density altitude
• Gross weight—which defines the CAS for landing
• Wind—a large effect due to wind or wind component
along the runway
• Runway slope and condition—relatively small
correction for ordinary values of runway slope, but a
significant effect of snow, ice, or soft ground
A tail wind of ten knots increases the landing distance by
about 21 percent. An increase of landing speed by ten percent
increases the landing distance by 20 percent. Hydroplaning
makes braking ineffective until a decrease of speed that can
be determined by using Figure 11-18.
For instance, a pilot is downwind for runway 18, and the
tower asks if runway 27 could be accepted. There is a light
rain and the winds are out of the east at ten knots. The pilot
accepts because he or she is approaching the extended
centerline of runway 27. The turn is tight and the pilot must
descend (dive) to get to runway 27. After becoming aligned
with the runway and at 50 feet AGL, the pilot is already 1,000
feet down the 3,500 feet runway. The airspeed is still high
by about ten percent (should be at 70 knots and is at about
80 knots). The wind of ten knots is blowing from behind.
First, the airspeed being high by about ten percent (80 knots
versus 70 knots), as presented in the performance chapter,
results in a 20 percent increase in the landing distance.
In performance planning, the pilot determined that at 70
knots the distance would be 1,600 feet. However, now it
is increased by 20 percent and the required distance is now
1,920 feet.
The newly revised landing distance of 1,920 feet is also
affected by the wind. In looking at Figure 11-19, the affect
of the wind is an additional 20 percent for every ten miles
per hour (mph) in wind. This is computed not on the original
estimate but on the estimate based upon the increased
airspeed. Now the landing distance is increased by another
320 feet for a total requirement of 2,240 feet to land the
airplane after reaching 50 feet AGL.
That is the original estimate of 1,600 under planned conditions
plus the additional 640 feet for excess speed and the tailwind.
Given the pilot overshot the threshhold by 1,000 feet, the
total length required is 3,240 on a 3,500 foot runway; 260
feet to spare. But this is in a perfect environment. Most pilots
become fearful as the end of the runway is facing them just
ahead. A typical pilot reaction is to brake—and brake hard.
Because the aircraft does not have antilock braking features
like a car, the brakes lock, and the aircraft hydroplanes on
the wet surface of the runway until decreasing to a speed of
about 54 knots (the square root of the tire pressure (√36) ×
9). Braking is ineffective when hydroplaning.
The 260 feet that a pilot might feel is left over has long since
evaporated as the aircraft hydroplaned the first 300–500 feet
when the brakes locked. This is an example of a true story,
but one which only changes from year to year because of new
participants and aircraft with different N-numbers.
In this example, the pilot actually made many bad decisions.
Bad decisions, when combined, have a synergy greater
than the individual errors. Therefore, the corrective
actions become larger and larger until correction is almost
impossible. Aeronautical decision-making is discussed more
fully in Chapter 2, Aeronautical Decision-Making (ADM).
Performance Speeds
True airspeed (TAS)—the speed of the aircraft in relation to
the air mass in which it is flying.
Indicated airspeed (IAS)—the speed of the aircraft as
observed on the ASI. It is the airspeed without correction for
indicator, position (or installation), or compressibility errors.
Calibrated airspeed (CAS)—the ASI reading corrected for
position (or installation) and instrument errors. (CAS is
equal to TAS at sea level in standard atmosphere.) The color
coding for various design speeds marked on ASIs may be
IAS or CAS.
Equivalent airspeed (EAS)—the ASI reading corrected
for position (or installation), for instrument error, and for
adiabatic compressible flow for the particular altitude. (EAS
is equal to CAS at sea level in standard atmosphere.)
VS0—the calibrated power-off stalling speed or the minimum
steady flight speed at which the aircraft is controllable in the
landing configuration.
VS1—the calibrated power-off stalling speed or the minimum
steady flight speed at which the aircraft is controllable in a
specified configuration.
VY—the speed at which the aircraft obtains the maximum
increase in altitude per unit of time. This best ROC speed
normally decreases slightly with altitude.
VX—the speed at which the aircraft obtains the highest
altitude in a given horizontal distance. This best AOC speed
normally increases slightly with altitude.
VLE—the maximum speed at which the aircraft can be safely
flown with the landing gear extended. This is a problem
involving stability and controllability.
VLO—the maximum speed at which the landing gear can
be safely extended or retracted. This is a problem involving
the air loads imposed on the operating mechanism during
extension or retraction of the gear.
VFE—the highest speed permissible with the wing flaps in a
prescribed extended position. This is because of the air loads
imposed on the structure of the flaps.
VA—the calibrated design maneuvering airspeed. This is
the maximum speed at which the limit load can be imposed
(either by gusts or full deflection of the control surfaces)
without causing structural damage. Operating at or below