9 0
Static pressure inlet
Display needle
Linkages and gearing:
Connect the diaphragm capsule
to the display needle(s)Aneroid capsule:
A sealed copper and beryllium alloy
capsule from which the air has been
removed. It is springy and designed to
compress as the pressure around it
increases, and expand as it decreases.
Enclosure:
Airtight except for the static pressure inlet, it has a glass
front through which display needle(s) can be viewed
Figure 4-15. Inside the altimeter.
The markings on the dial of a typical altimeter include numerals arranged clockwise from 0 to 9 inclusive, as shown in
Figure 4-12. When the surrounding pressure changes, the expansion or contraction of the aneroid element moves the hands
through a gear train. The hands sweep the calibrated dial to indicate altitude. In the altimeter with three hands shown in
Figure 4-12, the thinnest hand with an end shaped like a triangle indicates altitude in tens of thousands of feet; the shortest
pointed hand indicates thousands of feet; and the long pointed hand indicates hundreds of feet, subdivided into 20-foot
increments.
Types of Altitude
Altitude corresponds to a vertical distance above some point or level used as a reference. Altitude measured from different
reference levels serves different purposes. [Figure 4-16]
Standard datum plane
Sea level
True altitude
Pressure altitude
AGL
Terrain
Absolute Altitude
Figure 4-16. Types of altitude.
Glider pilots should understand the following altitudes:
• Indicated altitude—altitude read directly from the altimeter. During preflight, the pilot should set the altimeter to the
current local altimeter setting. If the indicated altitude deviates from the known field elevation by ±75 feet or more,
the pilot should not fly and refer the altimeter to an appropriately rated repair station for evaluation and correction.
The pilot can use indicated altitude for terrain and obstacle clearance. However, true altitude and indicated altitude
may differ depending on pressure and temperature conditions.
• True altitude—the vertical distance of the glider above sea level in standard atmospheric conditions (known as MSL
often expressed in this manner: 10,900 feet MSL, 5,280 feet MSL, or 940 feet MSL). Airport, terrain, and obstacle
elevations found on aeronautical charts are expressed as MSL (true altitudes).
• Pressure altitude—altitude indicated with the altimeter setting adjusted to 29.92. The pressure altitude corresponds
to the height above the standard datum plane, a theoretical plane where air pressure equals 29.92 inHg. Pilots use
pressure altitude for computer solutions to determine density altitude, true altitude, and TAS, etc. When flying in
class A airspace, pilots set the altimeter to 29.92.
• Density altitude —pressure altitude corrected for nonstandard temperature variations. In standard conditions,
pressure altitude equals density altitude. In temperatures above standard, the density exceeds pressure altitude. With
temperatures below standard, the density altitude is less than pressure altitude. The density altitude determines the
glider's performance and affects the power output of a tow plane or self-launching glider.
• Absolute altitude—vertical distance above the terrain, above ground level (AGL). The pilot can use absolute altitude
to estimate gliding distance over terrain without benefit of lift.
Effect of Nonstandard Pressure
On a flight made from a high-pressure area to a low-pressure area without adjusting the altimeter, the glider descends if the
pilot maintains a given indicated altitude. When flying from a low-pressure area to a high-pressure area without adjusting
the altimeter, the glider climbs if the pilot maintains a given indicated altitude. The pilot adjusts for this phenomenon
by setting the altimeter. A correctly set altimeter provides an appropriate amount of vertical separation between aircraft
at different cruising altitudes and can help prevent mid-air collisions. It also allows a more accurate absolute altitude
computation, which gives a glider pilot the ability to determine gliding distance more precisely.
Setting the Altimeter
To adjust the altimeter for nonstandard pressure, the pilot sets the pressure scale in the altimeter window (Kollsman
window) to the given local altimeter setting or to the field elevation. Altimeter settings correspond to station pressure
reduced to sea level, expressed in inches of mercury.
A reporting station takes an hourly measurement of the atmospheric pressure and corrects this value to sea-level pressure.
These altimeter settings reflect height above sea level only near the reporting station. When flying below 18,000 feet MSL,
the pilot should re-adjust the altimeter as the flight progresses from one station to the next. When flying at or above 18,000
feet MSL, the pilot sets the altimeter to 29.92.
When flying over high mountainous terrain, certain atmospheric conditions can cause the altimeter to indicate an altitude
of 1,000 feet or more above the true altitude. For this reason, the pilot should fly with a margin of increased altitude—not
only for possible altimeter error, but also for downdrafts, which may occur if encountering high winds.
A cross-country flight from TSA Gliderport, Midlothian, Texas, to Winston Airport, Snyder, Texas, via Stephens County
Airport, Breckenridge, Texas, illustrates the use of altimeter settings. Before launch from TSA Gliderport, the pilot receives
the current local altimeter setting of 29.85 and adjusts the altimeter to this value. The indication varies slightly from the
known airport elevation of 660 feet due to a slight altimeter calibration error.
When over Stephens County Airport, the pilot receives a current area altimeter setting of 29.94 and applies this setting to
the altimeter. Before entering the traffic pattern at Winston Airport, the pilot receives a new altimeter setting of 29.69 from
the Automated Weather Observing System (AWOS). If the pilot desires to enter the traffic pattern at approximately 1,000
feet above the terrain, and if the field elevation of Winston Airport is 2,430 feet MSL, the pilot should use an indicated
altitude of 3,400 feet.
2,430 feet + 1,000 feet = 3,430 feet, rounded to 3,400 feet
For illustration, assume a distraction caused the pilot to neglect the adjustment for the Winston Airport altimeter setting
and to continue using the Stephens County Airport setting of 29.94. The pattern entry would occur approximately 250 feet
below the Winston Airport’s traffic pattern altitude of 3,400 feet and the altimeter would indicate approximately 2,680 feet
upon landing or 250 feet higher than the field elevation.
Actual altimeter setting = 29.94
Correct altimeter setting = 29.69
Difference = .25
One inch of pressure is equal to approximately 1,000 feet of altitude.
.25 × 1,000 feet = 250 feet
In this scenario, the pilot, although low, might fly a successful visual approach angle to the landing zone. The pilot should
adjust the visual angle to the landing zone to compensate for the lower altitude. However, risk of an accident increases
due to the incorrect pattern entry altitude. For example, the glider might strike an obstacle in the flight path if not seen by
the pilot. If the pilot does not reset the altimeter, the following memory aid illustrates what can happen “From a high to a
low—look out below.”
Effect of Nonstandard Temperature
Variations in air temperature also affect the altimeter. On a warm day, air weighs less per unit volume than on a cold day.
For example, the pressure level at which the altimeter indicates 10,000 feet occurs at a higher altitude on a warm day than
under standard conditions. On a cold day, the 10,000-foot indication moves lower. The adjustment made by the pilot to
compensate for nonstandard pressure does not compensate for nonstandard temperature. If considering terrain or obstacle
clearance during the selection of a cruising true altitude, particularly at higher altitudes, the pilot should consider this
effect. Colder than standard temperature places the glider closer to the ground for a given true altitude, and the pilot should
use a higher altitude to provide adequate terrain clearance. [Figure 4-17]
True altitude
Indicated altitude
4,000
3,000
3,500
Feet (MSL)
30.130.029.9
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
30.130.029.9
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
30.130.029.9
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
30.130.029.9
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
30.130.029.9
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
30.130.029.9
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
CO
ALTIMETE
TING
30.00
inches Hg
CO
ALTIMETE
TING
30.00
inches Hg
CO
ALTIMETE
TING
30.00
inches Hg
Standard
Colder
Warmer
Aircraft altimeter setting Aircraft altimeter setting Aircraft altimeter setting
Figure 4-17. Nonstandard pressure and temperature.
Variometer
Variometer instruments measure the vertical ascent or descent of the local air mass and glider combined and display that
information as vertical speed. The variometer can be considered a simple flow meter measuring air flowing between an
outside reference (static or total energy port) and an internal reference flask. The variometer depends upon the pressure
lapse rate in the atmosphere to derive information about rate of climb or rate of descent. A non-electric variometer uses
a separate insulated tank (thermos or capacity flask) as a reference chamber to increase sensitivity and accuracy of the
instrument. The tubing runs from the reference chamber through the variometer instrument to an outside static port in an
uncompensated variometer. [Figure 4-18 and 4-19]
10 0 90
Capacity flask Static vents
Static pressure inlet
Variometer
Static port
Figure 4-18. Uncompensated variometer plumbing.
Static pressure inlet
Linkages and gearing:
Connects the diaphragm
capsule to the display needle.
Capacity flask:
A rigid, thermally insulated flask (similar
to an insulated beverage container).
Capillary hole:
A small hole in the diaphragm capsule
designed to allow the pressure inside the
capacity flask to slowly equalize (within a
few seconds) with the atmospheric pressure
air in the main body of the instrument.
Diaphragm capsule:
A capsule with elastic properties. Like a plastic
water bottle, it expands if the pressure inside is
greater than the pressure outside. It compresses if
the pressure outside it is greater than the pressure
inside it.
0.5 liters (approx.)
Vertical speed:
Indicated in knots of vertical
climb and descent.
Figure 4-19. A variometer diaphragm anatomy.
Pressure differences between the air inside the variometer/reference chamber system and the air outside of the system
tend to equalize as air flows from high-pressure areas to low-pressure areas. When pressure inside the reference chamber
exceeds the pressure outside, air flows out of the reference chamber through the mechanical variometer to the outside
environment, and the variometer indicates a climb. When air pressure outside the reference chamber exceeds the pressure
inside, air flows through the variometer and into the reference chamber until pressure equalizes. In this case, the variometer
needle indicates conditions that force the glider to lose height, Figures 4-20 and 4-21 illustrate how the variometer works
in level flight and while the glider ascends. In addition, Figure 4-22 illustrates certain flight maneuvers that cause the
variometer to display changes in altitude.
900 mb
Local atmospheric pressure
at 3,000 feet
900 mb
900 mb
900 mb
Altitude 3,000 feet level flight
In level flight, the pressures throughout
the variometer and capacity flask
equalize to the local atmospheric
pressure at the flight altitude via the
static vents.
There is no difference between
the pressure inside and the pressure
outside the diaphragm capsule.
The capsule is neither compressed nor
inflated, and the display needle points
to zero.
Vertical speed indicated in knots
Figure 4-20. Uncompensated variometer in level flight.
894 mb
Local atmospheric pressure
at 3,500 feet
Pressure falling from
900 mb to 884 mb
as altitude increases
892 mb
894 mb
892 mb
Altitude 3,500 during climb from 3,000 feet to 4,000 feet
As a glider climbs, the atmospheric
pressure around the glider falls.
The pressure inside the variometer’s
case matches this pressure drop
almost instantaneously.
The pressure of the air inside
the capacity flask takes several
seconds to catch up because it must
vent through the small capillary hole.
During a climb, the pressure inside the
capacity flask and diaphragm
capsule is slightly higher than the
pressure inside the variometer’s case.
The diaphragm capsule expands,
rotating the display needle to show
an increasing altitude.
Figure 4-21. Uncompensated variometer in a climb.
1 The natural sink of the glider
3 Changes in altitude caused by the glider slowing down or speeding up.
(e.g., if the pilot pulls back on the stick, the glider slows down and gains
altitude as speed is converted into height.)
2 External up (lift) or down (sink)
drafts cause the glider to gain or
lose altitude (e.g., thermals)
A glider pilot does not actually want a variometer to display
this type of altitude change because every time the pilot
slows down, that pilot will think the glider is in a strong thermal.
Stick pulled back
3,120 ft
50 knots
3,000 ft
80 knots
Figure 4-22. Flight maneuvers that display altitude changes on an uncompensated variometer.
Electric-powered variometers offer several advantages over the non-electric variety. These advantages include more rapid
response rates and separate audible signals for climb and descent.
Some electric variometers use special sensors. As air flows into or out of the reference chamber, it cools sensors in a circuit
and alters the electrical resistance measured by the system. The resulting change in resistance corresponds to the rate of
climb or descent. The system displays that information on the variometer.
Many electric variometers provide audible tones, or beeps, that indicate the rate of climb or rate of descent of the glider.
Pilots using an audio variometer can listen for the rate of climb or descent, which allows more time to focus attention
outside the aircraft. [Figure 4-23]
30.0
29.929.8
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
OFF
GO
NAV
NET AVG
PULL
PUSH
HW
DIST
ALT
PUSH
4 6
-2 -4
Acceleration
G UNITS
PUSH
N 30 60 E 120 150
STEER
FOR
S 210 240 W 300 330
STEER
RADIO
FOR
ON ON
120 80
knots
knotswinter
50 60 70 80 90 100
knots SALTO
4 6 8 10
24 6 8 10
knotswinter
50 60 70 80 90 100
knots SALTO
4 6 8 10
24 6 8 10
Non-electricElectric
Current climb rate Climb rate during last circle
Figure 4-23. When an electric variometer is mounted to the glider, installation of a non-electric variometer can provide a backup.
In the past, some variometers had a rotatable rim speed scale called a MacCready ring. This scale indicates the optimum
airspeed to fly when traveling between thermals for maximum cross-country performance. During the glide between
thermals, the pilot sets the index arrow to the rate of climb expected in the next thermal. On the speed ring, the variometer
needle points to the optimum speed to fly between thermals. If the pilot expects a low rate of climb, the instrument selects
a lower optimum speed between thermals. When the pilot expects strong lift at the next thermal, the instrument suggests
a faster optimum cruise airspeed. [ Figure 4-24] A MacCready ring has single pilot values, values with a passenger, and
may also adjust for ballast since more weight results in higher gliding speeds. Electronic instrument displays have become
more common than Macready rings.
Figure 4-24. The MacCready ring.
Pilot induced climbs and dives result in changes in airspeed and affect an uncompensated variometer by causing changes in
pressure altitude. In still air, when the pilot initiates a dive, the variometer indicates a descent. When the glider pilot pulls
out of the dive and initiates a rapid climb, the variometer indicates an ascent. A glider with an uncompensated variometer
gives an accurate indication of rising and descending air only if the pilot maintains a constant airspeed.
Total Energy System
A variometer with a total energy system senses changes in airspeed and tends to cancel out the resulting climb and dive
indications (stick thermals). This gives a glider pilot an indication of rising or descending air despite changes in airspeed.
A popular type of total energy system consists of a small venturi, a pair of holes, or simply a slot on the back side of a small
vertical tube mounted in the air stream and connected to the static outlet of the variometer. When airspeed increases during
a dive, more suction from the venturi offsets the increased pressure at the static outlet of the variometer. Similarly, when
airspeed decreases during a climb, reduced suction from the venturi offsets the pressure reduction at the static outlet of the
variometer. The net effect reduces climb and dive indications caused by airspeed changes. To maximize the precision of