Introduction
Glider performance depends on design, weather, wind, and other atmospheric phenomena. While the design of the glider
affects performance to a great degree, design remains fixed and known to the pilot. Other factors change and affect launch,
cruise, and landing.
Variable Performance Factors
For a specific glider flight, some factors that affect performance include density altitude, wind, and weight.
Density Altitude
In general, an increase in density altitude refers to thinner air, while a decrease in density altitude refers to thicker air.
Conditions that result in high-density altitude include high elevation, low atmospheric pressure, high temperature, high
humidity, or some combination of these factors. Lower elevation, high atmospheric pressure, low temperature, and low
humidity reduce density altitude. Since high density altitudes may exist at lower elevations on hot days, pilots should
consider density altitude based on actual conditions before a flight.
A chart provides one way to determine density altitude. [ Figure 5-1] For example, knowing field elevation of 1,600 feet
MSL with a current altimeter setting of 29.80 "Hg and temperature of 85 °F, what is the density altitude? The right side of
the chart provides an adjustment for nonstandard pressure (29.80 "Hg) and suggests adding 112 feet to the field elevation.
This step provides a current pressure altitude of 1,712 feet. The next step involves tracing a line vertically from the bottom
of the chart from the temperature of 85 °F (29.4 °C) that intercepts the diagonal 1,712-foot pressure altitude line. The
final step involves tracing a line horizontally to the left from the interception point and reading the density altitude of
approximately 3,500 feet. Under these conditions, a self-launching glider or towplane will perform as if at 3,500 feet MSL
on a standard day.
Chapter 5: Glider Performance
Density Altitude Chart
Outside air temperature
Approximate density altitude (thousand feet)
S.L.
14,000
13,000
12,000
11,000
10,000
9,000 Pressure altitude (feet)
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
-1,000
Sea level
-18° -12° -7° -1° 4° 10° 16° 21° 27° 32° 38°
0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
Standard temperature
28.0 1,824
28.1 1,727
28.2 1,630
28.3 1,533
28.4 1,436
28.5 1,340
28.6 1,244
28.7 1,148
28.8 1,053
28.9 957
29.0 863
29.1 768
29.2 673
29.3 579
29.4 485
29.5 392
29.6 298
29.7 205
29.8 112
29.9 20
29.92 0
30.0 −73
30.1 −165
30.2 −257
30.3 −348
30.4 −440
30.5 −531
30.6 −622
30.7 −712
30.8 −803
Altimeter setting
("Hg)
Pressure altitude
conversion factor
Figure 5-1. Density altitude chart.
Many performance charts use pressure altitude and temperature inputs without requiring the pilot to calculate density
altitude. However, if the pilot wants to know the density altitude, a density altitude chart or a flight computer can supply
that information.
Atmospheric Pressure
Atmospheric pressure at a given location changes from day to day. The following sample Meteorological Aerodrome Report
(METAR) indicates a local pressure of A2953, or altimeter setting of 29.53 inHg. When considering barometric pressure
only, the lower than normal pressure reading results in a higher density altitude that decreases aircraft performance. This
reduction affects takeoff and climb performance and increases the length of runway needed during landing for both the
glider and towplane. On the other hand, if barometric pressure rises, the lower density altitude improves takeoff and climb
performance, and the length of runway needed for landing decreases.
KDAL 232153Z 21006KT 7SM -RA BKN025 BKN060 OVC110 12/11 A2953 RMK AO2 PRESFR SLP995 P0005
Temperature
Temperature changes have a significant effect on density altitude. Heated air expands—the molecules move farther apart,
making the air less dense. Higher density altitude reduces glider and towplane takeoff and climb performance and increases
the length of runway required for landing.
Consider the following METAR for two airports with same altimeter setting, temperature, and dewpoint. Love Field
(KDAL) airport elevation of 487 feet versus Denver International (KDEN) at 5,431 feet.
KDAL 240453Z 21007KT 10SM CLR 25/15 A3010 RMK AO2…
KDEN 240453Z 24006KT 10SM FEW120 SCT200 25/15 A3010 RMK AO2…
The computed density altitude for Love Field is 1,774 feet; for Denver, 7,837 feet—Denver experiences almost twice
the amount of increase compared to Love Field. The effects of altitude and temperature can surprise a pilot who does not
consider the related performance issues.
Wind
Wind also affects glider performance. Headwind during launch or landing results in a shorter ground roll, while tailwind
causes a longer ground roll. [ Figure 5-2] Crosswinds during launch or landing require proper crosswind procedures or
control input to track along the runway.
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Wind
Takeoff and climb-out: no wind
Takeoff and climb-out: 25-knot headwind
Figure 5-2. Apparent wind effect on takeoff distance and climb-out angle.
Due to diminishing ground friction between the wind and the ground, wind speed often increases with altitude. This “wind
gradient” during a ground launch could result in an exceedance of the maximum launch speed. [Figure 5-3]
Wind 20 knots
Wind 10 knots
Wind gradient
knotsAirspeed 0
knotsAirspeed 0
57.5 MPH (50 knots)
Figure 5-3. A wind gradient may affect airspeed during a ground tow.
During cruising flight, headwinds reduce the groundspeed of the glider. A glider flying at 60 knots true airspeed into a
headwind of 25 knots has a groundspeed of only 35 knots. Tailwinds increase the groundspeed of the glider. A glider flying
at 60 knots true airspeed with a tailwind of 25 knots has a groundspeed of 85 knots.
Some self-launching gliders can cruise for extended periods using the engine. The self-launching glider’s fuel capacity
normally limits maximum range and duration with the engine running. Wind has no effect on flight duration but does have
a significant effect on range. During powered cruising flight, a headwind reduces range, and a tailwind increases range.
The Glider Flight Manual/Pilot’s Operating Handbook (GFM/POH) provides recommended airspeeds and power settings
to maximize range when flying in no wind, headwind, or tailwind conditions.
When flying straight and level and following a selected ground track, the pilot can point the glider into the prevailing wind
as the preferred method to correct for wind drift. The wind speed, the angle between the wind direction and the glider's
longitudinal axis, and the airspeed of the glider determine the required wind correction angle. [Figure 5-4] Crosswinds may
also have a head or tailwind component that results in a lower or higher groundspeed.
Wind at 10 knots
No wind
Heading = Track
Final glide
Crosswind from the right
Heading is crabbed upwind of track
Final glide
Track
Target
Glider Airspeed – 60 Knots
Wind – 10 Knots and 90° off the nose
Glider Ground Speed – 59 Knots
Crab angle of about 9.5°
Glider Airspeed – 60 Knots
Wind – 10 Knots and 45° off the nose
Glider Ground Speed – 52 Knots
Crab angle of about 6°
Glider Airspeed – 60 Knots
Wind – 10 Knots and 30° off the nose
Glider Ground Speed – 51 Knots
Crab angle of about 4°
Heading
Heading
Target
Crab angle
Track
Figure 5-4. Crosswind effect on final glide.
A headwind during an approach results in greater altitude loss per distance traveled. The glider descends at a constant rate,
but a lower groundspeed increases the observed approach angle. Pilots use various techniques to ensure safe touchdowns
in different wind conditions. For example, if landing in a strong headwind, the glider pilot should plan for a base leg
closer to the landing zone to allow for the steeper approach. Another technique uses delayed extension of spoilers or dive
brakes with a faster airspeed to counter the headwind component. In the case of a tailwind and an apparent lower angle
of approach, the glider pilot can use more spoiler or dive brake extension, slip, or use a combination of the two if allowed
by the GFM. In any case, the pilot generally aims for a spot past the threshold of the runway to provide a safety factor
that accounts for the effects of the wind gradient or other factors that may cause the approach to be shorter than expected.
[Figure 5-5]
34 34 34
Wind
Wind
Touchdown point
Touchdown point
Touchdown point
Landing: 25-knot headwind
Landing: no wind
Landing: 15-knot tailwind
Figure 5-5. The effect of wind on final approach and landing distance.
When approaching to land during windy and gusty conditions, a pilot normally adds half of the difference between the
steady wind and gusts to the approach speed to mitigate any variations in airspeed. Instead of holding the glider off
the ground for a low kinetic energy landing during these conditions, the pilot can land a little faster than normal. Upon
touchdown, extending the air brakes prevents the glider from becoming airborne from a gust during the landing roll.
The pilot usually expects any headwind to diminish during descent to a landing. This is called the wind gradient. If the
wind gradient is abrupt, the pilot may experience a sudden airspeed loss. After the pilot lowers the nose to compensate, it
takes a finite time interval to overcome the inertia of the glider and regain airspeed. A significant speed loss near the ground
may preclude recovering any or all the lost speed. A wind gradient on final approach may cause the glider to land short of
the point of intended touchdown therefore, a closer approach may be necessary.
The pilot landing with a tailwind has a higher groundspeed and should expect a longer landing roll. As the surface friction
slows the winds, the pilot may observe an increase in airspeed.
A strong windshear gradient can affect a glider during a steep turn on a low altitude final approach at a low airspeed. The
gradient can create different lift on the low wing and the raised wing. [Figure 5-6] The rolling force created by the gradient
can overcome the ailerons, cause a loss of control, and explains why the pilot should limit bank angle while close to the
ground and transitioning a strong wind gradient.
Wind velocity
18 mph
Wind velocity 16 mph
Wind velocity 14 mph
Wind velocity12 mph
Wind
velocity10 mph
Figure 5-6. Effect of wind velocity gradient on a glider. Stronger airflow over higher wing may cause bank to steepen.
Weight
The weight of a glider affects acceleration during launch. A glider at maximum takeoff weight takes longer to attain flying
speed. After takeoff, a tow plane with a heavy glider will climb more slowly. Increasing the weight of a powered glider
causes it to accelerate more slowly and climb more slowly. [Figure 5-7]
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Takeoff and climb-out: lightweight glider
Takeoff and climb-out: heavy glider
Figure 5-7. Effect of weight on takeoff distance and climb-out rate and angle.