this compensation effect, the system can use a total energy probe, which sits in undisturbed airflow ahead of the aircraft
nose or tail fin. [Figure 4-25]
ASI ALT VARIO AUDIO
4 6 8 10
24 6 8 10
Static port
Pitot tube Tubing to total energy probe
Capacity flask
Total energy probe which
regulates pressure at the
static outlet of the variometer
Figure 4-25. A total energy variometer system.
Another type of total energy system design uses a diaphragm-type compensator placed in line from the pitot tube to the
line coming from the reference chamber (thermos or capacity flask). Deflection of the diaphragm offsets the effect the
airspeed change has on pitot pressure. In effect, the diaphragm modulates pressure changes in the capacity flask and masks
stick thermals.
Netto
A Netto variometer indicates the vertical movement of the air mass, regardless of the glider’s climb or descent rate. Some
Netto variometer systems employ a calibrated capillary tube that functions as a tiny valve. Pitot pressure pushes minute
quantities of air through the valve and into the reference chamber tubing. This removes the glider’s known sink rate at
various airspeeds from the variometer indication. [Figure 4-26]
ASI ALT VARIO AUDIO
Static port
Pitot tube Capillary
Capacity flask
Figure 4-26. An example of a Netto variometer system.
Computerized (electronic) Netto variometers employ a different method to remove the glider sink rate. In this type of
system, sensors for both pitot pressure and static pressure provide airspeed information to the computer. The computer
stores the known sink rate of the glider at every airspeed. At any given airspeed, the computer removes the sink rate of the
glider, and the variometer displays the rate of ascent or descent of the air mass itself.
Electronic Flight Computers
Since nonpowered gliders lack a generator or alternator, electrical components, such as the flight computer, if installed,
draw power from the glider’s rechargeable battery or batteries. Some gliders use solar cells arrayed behind the pilot, or
on top of the instrument panel cover, to supply additional power to the electrical system during flight in sunny conditions.
The primary components of most flight computer systems include an electric variometer, a coupled GPS receiver, and
a microprocessor. The variometer measures rate of climb and descent. The GPS provides position information. The
microprocessor interprets altitude, speed, and position information. The microprocessor output aids the pilot in cross-
country decision-making by suggesting a speed to fly. Figure 4-27 shows a glider flight computer.
Figure 4-27. Glider flight computer display.
The GPS-coupled flight computer can provide the following information:
• Current position
• Previous position
• Speed and time to destination
• Distance to planned destination
• Height needed to glide to destination
• Current climb or descent rate
• The optimum airspeed to fly to the next thermal
• The optimum airspeed to fly to a location on the ground, such as the finish line in a race or the airport of intended
landing at the end of a cross-country flight
The primary benefits of the flight computer divide into two areas: navigation assistance and performance (speed)
enhancement.
Flight computers utilize the concept of a waypoint, which includes latitude, longitude, and altitude. Glider races and
cross-country glider flights frequently involve flight around a series of waypoints called turnpoints. The course may be an
out-and-return course, polygon shape, or just a series of waypoints. The glider pilot navigates from point to point, using
available lift sources to climb periodically so that flight can continue to the intended goal. The GPS-enabled flight computer
aids in navigation, summarizing flight progress, and logging completion of tasks or flight goals. When encountering strong
lift, for example, the pilot can use the flight computer to mark the location of the thermal. If rounding a nearby turnpoint,
the glider pilot might use the flight computer to return to the marked thermal for a rapid climb before continuing.
During the climb portion of the flight, the flight computer’s variometer constantly updates the achieved rate of climb.
During cruise, the GPS-coupled flight computer aids in navigating accurately to the next turnpoint. The flight computer
also suggests the optimum cruise airspeed for the glider to fly, based on the expected rate of climb in the next thermal.
During final glide to a goal, the flight computer can display glider altitude, altitude required to reach the goal, distance to
the goal, the strength of the headwind or tailwind component, and optimum airspeed to fly.
When the flight computer detects rapid climbs, it suggests higher cruise airspeeds to enhance performance. When the
computer detects a low rate of climb, it compensates for the weaker conditions by suggesting lower airspeeds. The flight
computer frees the pilot to look for other air traffic, look for sources of lift, watch the weather ahead, and plot a strategy
for the remaining portion of the flight.
As explained in the next chapter, water ballast increases the optimal speed to fly. The flight computer compensates for
water ballast carried, adjusting speed-to-fly computations according to the weight and performance of the glider. Some
flight computers require the pilot to enter data regarding the ballast load of the glider. Other flight computers automatically
compensate for the effect of water ballast by constantly measuring the performance of the glider and deducing the
operating weight of the glider from these measurements. If the wings of the glider become contaminated with bugs, glider
performance declines. The flight computer can be adjusted to account for the resulting performance degradation.
Magnetic Compass
Most gliders do not come under regulation for powered aircraft as referenced in Title 14 of the Code of Federal Regulations
(14 CFR) part 91, section 91.205, and only need to comply with regulations for “civil aircraft.” For this reason, some
gliders do not have a compass unless required per the aircraft's Type Certificate Data Sheets (TCDS).
Slip/Skid Indicators
Yaw String
A piece of yarn mounted in the free airstream and easily visible to the pilot, provides an effective slip/skid indicator.
[Figure 4-28] During coordinated flight, the yarn points straight back. During a slipping turn, the tail (unattached end) of
the yaw string offsets toward the outside of the turn. During a skidding turn, the tail of the yaw string offsets toward the
inside of the turn. A pilot having difficulty sensing and correcting uncoordinated flight can apply pressure to the rudder
pedal not aligned with the yaw string tail.
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
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
+
−
PWR
FNCN
SEL
VOL BATT
KNOTS
PUSH
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
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
+
−
PWR
FNCN
SEL
VOL BATT
KNOTS
PUSH
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
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
+
−
PWR
FNCN
SEL
VOL BATT
KNOTS
PUSH
Slip
Coordinated
Skid
Figure 4-28. Left turn condition indications of a yaw string and inclinometer.
Inclinometer
An inclinometer also provides slip/skid indication. The inclinometer responds to centrifugal force and gravity. The
inclinometer consists of a metal ball in an oil-filled, curved glass tube. When the glider flies in coordinated fashion, the ball
remains centered at the bottom of the glass tube. The inclinometer differs from the yaw string during uncoordinated flight.
The ball moves to the inside of the turn to indicate a slip and to the outside of the turn to indicate a skid. [Figure 4-28] The
phrase, “step on the ball” explains how a pilot should respond to restore coordinated flight. During a spin, the inclinometer
does not provide direction of rotation information, and pilots should not use the inclinometer for guidance.
Gyroscopic Instruments
Unpowered gliders do not usually have gyroscopic instruments, while self-launching gliders often have one or more
gyroscopic instruments on the panel. Common gyroscopic instruments include the attitude indicator, heading indicator,
and turn coordinator.
G-Meter
A panel-mounted G-meter registers positive G forces from climbs and turns, as well as negative G forces when diving
down or pushing over from a climb. During straight, unaccelerated flight in calm air, the G-meter registers a load factor of
1 G (1.0 times the force of gravity). During flight in turbulent air, the glider and pilot experience G-loads greater than or
less than 1 G when encountering updrafts or downdrafts.
Each glider type can withstand a specified maximum positive G-load and a specified maximum negative G-load. The
operating limitations, as described in the GFM/POH or on placards, are the definitive source for this information. Exceeding
the allowable limit loads may result in deformation of the glider structure. In extreme cases, exceeding permissible limit
loads may cause structural failure of the glider. The G-meter allows the pilot to monitor G-loads from moment to moment
during aerobatic flight and during flight in rough air. Most G-meters also record and display the maximum positive G-load
and the maximum negative G-load encountered during flight. The recorded maximum positive and negative G-loads can
be reset by adjusting the control knob of the G- meter. [Figure 4-29]
PUSH
4 6
-2 -4
Acceleration
G UNITS
Figure 4-29. The G-meter.
FLARM Collision Avoidance System
A mid-air collision presents a risk to all pilots, and glider pilots have additional mid-air scenarios to consider when
thermaling, cloud street flying, or ridge running. Periodically, mid-air collisions or near misses occur in club flying, during
competitions, and between gliders and towplanes after release.
FLARM systems (the name being inspired from “flight alarm”) can warn pilots of impending collisions with other
FLARM-equipped gliders and give the location of non-threatening nearby FLARM-equipped gliders. Figure 4-30 shows
the interior of an ASW 19 glider with a FLARM unit on top of the instrument panel. FLARM transmits and receives
information, models the unique flight characteristics of gliders, and stays quiet unless it detects a real threat. However, it
can only provide information about other gliders with an operating FLARM system.
Figure 4-30. FLARM unit on top of the instrument panel.
FLARM obtains its position from an internal global positioning system (GPS) and a barometric sensor and then broadcasts
this data with forecast data about the future 3D flight track. Its receiver receives signals from other FLARM devices
typically within 3-5 kilometers and processes the information received. Motion-prediction algorithms predict potential
conflicts for up to 50 other signals and warn the pilot using sound and visual means. FLARM can also store information
about static aerial obstacles, such as cables, into a database.
Why use FLARM in a glider? Conventional Airborne Collision Avoidance Systems (ACAS) would provide continuous
and unnecessary warnings about all aircraft in the vicinity. FLARM only gives selective alerts to aircraft posing a collision
risk. It consumes much less power than a transponder or ADS-B and is relatively inexpensive to buy and install. While
versions exist for use in light aircraft and helicopters, as well as gliders, the short range of the signal makes FLARM
unsuitable for avoiding collisions with fast aircraft.
While gliders are exempt from carrying transponders and ADS-B out transmitters in most, but not all airspace, the Soaring
Society of America strongly encourages pilots to install this equipment when operating near high density airspace. This
increases their visibility to other users of the National Airspace System (NAS).
Transponder Code
The Federal Aviation Administration (FAA) assigned transponder code 1202 for use by gliders not in contact with air
traffic control (ATC) as of March 7, 2012. Effective November 1, 2021 (JO 7110.66G), the FAA amended this practice to
include gliders in contact with ATC. Glider pilots operating in areas with an agreement with local ATC to use a different
code should contact the agreement sponsor for guidance.
Definitions
• SQUAWK CODE: The 4-digit code set in the transponder, such as 1202.
• IDENT or SQUAWK IDENT: A controller may direct a pilot to “ident” or “squawk ident” to verify the aircraft’s
location on the radar screen. When directed, the pilot pushes the button on the transponder marked IDENT. This
causes the target on the controller’s radar screen to change for several seconds. The pilot should not push the ident
button without direction from ATC.
• Tow planes normally squawk 1200 unless otherwise instructed by ATC.
Outside Air Temperature (OAT) Gauge
The outside air temperature gauge (OAT) mounts with the sensing element in contact with the outside air. OAT gauges
display degrees Celsius, degrees Fahrenheit, or both, and provide the glider pilot with information about freezing
temperatures which could affect water ballast or flight controls. [Figure 4-31]
-20
-40
-60
-40
-20
Figure 4-31. Outside air temperature (OAT) gauge.
When flying a glider loaded with water ballast, knowledge of the height of the freezing level can affect safety of flight.
Extended operation of a glider loaded with water ballast in below-freezing temperatures may result in frozen drain valves,
ruptured ballast tanks, and structural damage to the glider.
Chapter Summary
This chapter introduced the pitot-static system and its associated instruments including the airspeed indicator, the altimeter,
and the variometer. A glider has various V-speeds and some of these speeds correspond to specific markings on the airspeed
indicator regardless of altitude. The chapter also explains what a pilot should know about setting an altimeter, nonstandard
pressure and temperature effects on indicated altitude, and different kinds of altitude. Variometers normally indicate the
sum of the descent rate of the glider added to the lift or sink of the surrounding air mass. A compensated variometer
removes indications that result from changes in airspeed (stick thermals). A netto removes the sink rate of the glider and
provides the vertical speed of the surrounding air mass. Flight computers can integrate information from different sources,
including from memory, and provide a wealth of information to the glider pilot. Other instruments described in this chapter
include a magnetic compass, yaw string, inclinometer, gyroscopes, G-meter, and outside temperature gauge (OAT). Pilots
can use FLARM systems for short range collision avoidance, however the system can only detect other aircraft that use the
FLARM system. A transponder-equipped glider automatically reports position and altitude to ATC.