Introduction
Flight instruments provide information regarding the glider’s direction, altitude, airspeed, and performance. Instruments
can consist of a basic set typically found in training aircraft or a more advanced set in a high-performance glider used for
cross-country or competition flying. Refer to the Pilot’s Handbook of Aeronautical
Knowledge (FAA-H-8083-25) for detailed descriptions of different instruments.
Instruments displaying airspeed, altitude, and vertical speed are part of the pitot-static system. Heading
instruments display magnetic direction by sensing the earth’s magnetic field. Performance instruments, using gyroscopic
principles, display the aircraft’s attitude, heading, and rate of turn. Electronic instruments using computer and global
positioning system (GPS) satellite technology provide pilots with moving map displays, electronic airspeed and altitude, air
mass conditions, and other functions relative to flight management. Examples of self-contained instruments and indicators
that are useful to the pilot include the yaw string, inclinometer, and outside air temperature (OAT) gauge.
This chapter describes basic glider instruments and systems. Pilots flying gliders with advanced electronic instruments
should consult the manufacturer’s documentation for a complete description of operation and seek instruction as needed.
Pitot-Static Instruments
The pitot-static system uses two different air pressure measurements:
1. Static ports transport ambient atmospheric pressure to instruments through tubing.
2. The pitot tube transports ambient air pressure plus any ram air pressure resulting from forward motion to instruments
through tubing.
Impact & Static Pressure Lines
Impact or ram-air pressure from the forward motion of the glider increases air pressure in the pitot tube, which mounts
on either the nose or vertical stabilizer to allow uninterrupted exposure to the oncoming airflow. Glider static ports often
mount either on the vertical stabilizer or the side of the fuselage. [Figure 4-1]
Pitot tube
Pitot tube
Static port
Figure 4-1. A pitot tube often mounts in the glider’ s nose or the vertical fin, with the forward-facing open end exposed directly to the
oncoming airflow.
Chapter 4: Flight Instruments
As airspeed increases, pressure builds in the pitot tube and in the connected expandable diaphragm. The pressure rises
in the pitot tube and diaphragm until it reaches a state of equilibrium preventing any further rise in pressure. Conversely,
pressure in the system decreases as airspeed decreases since air can also flow out of the system. [Figure 4-2]
Speed: 0 knots
No oncoming airflow
Diaphragm pressure: 1013 mb
sea level atmosphere
Speed: 30 knots
+2 mb due to oncoming airflow
Speed: 60 knots
+6 mb due to oncoming airflow
Pitot tube
Diaphragm
Figure 4-2. Pressure inside the diaphragm as a function of airspeed.
The static pressure (pressure of the still air) comes from the movement of air in and out of the static ports and tubing.
Gliders using flush mounted static sources often have two vents, one on each side of the fuselage. This compensates for
variation of static pressure due to changes in glider attitude and air turbulence.
Pilots check the openings of both the pitot tube and the static port(s) during the preflight inspection to ensure obstructions
do not block the free flow of air. A certificated mechanic should clean any blockage. Blowing into these openings could
damage flight instruments.
Airspeed Indicator
The airspeed indicator measures the difference between the pitot pressure and static pressure, and displays this difference
as the indicated airspeed (IAS) of the glider. [ Figure 4-3] Color-coded arcs depict airspeed ranges for different phases of
flight. The upper and lower limits of the arcs correspond to defined airspeeds. Figure 4-4 shows the internal structure of
an airspeed indicator.
Figure 4-3. Airspeed indicator.
Static pressure inletTotal (pitot) pressure inlet
Display needle Linkages and gearing:
Connect the diaphragm
capsule to the display needle.
Enclosure:
Airtight except for the static pressure inlet. It has
a glass front through which the display needle
can be viewed.
Diaphragm capsule:
A capsule with elastic properties that is
airtight, except for its connection to the
total pressure inlet. Like a balloon, it
expands as the pressure inside it
increases above the pressure outside it.
Figure 4-4. Anatomy of the airspeed indicator.
As shown in Figure 4-4 above, the airspeed indicator contains a diaphragm that expands or contracts in response to the
difference between pitot and static pressure. This diaphragm movement drives the needle (airspeed needle pointer) on
the face of the instrument. When pitot pressure equals static pressure, the indicator reads zero. As pitot pressure becomes
progressively greater than static pressure, the needle moves and points to the corresponding airspeed. [Figure 4-5]
10 0 90
Atmospheric
pressure
1,013 mb
Atmospheric
pressure
1,013 mb
Atmospheric
pressure
1,013 mb
10 0 90
Ram air pressure
1,019 mb
Speed: 0 knot
Speed: 60 knots
In a stationary aircraft, the air throughout
the airspeed indicator has equalized with
local atmospheric pressure. The diaphragm
is collapsed like a deflated balloon,
and the needle is at zero.
As airspeed increases, the pressure
inside the diaphragm rises above local
atmospheric pressure due to the force
exerted down the pitot tube by the
oncoming airflow. The diaphragm inflates
like a balloon, moving the linkages and
rotating the needle.
Figure 4-5. Airspeed indicator operation.
The Effects of Altitude on the Airspeed Indicator
The airspeed indicator displays dynamic pressure (pitot pressure minus static pressure) calibrated to an airspeed at standard
sea-level pressure. Due to lower air density as altitude increases, the buildup of pressure in the diaphragm decreases and the
indication becomes lower than at sea level. The error becomes greater as altitude increases. [Figure 4-6]
Airspeed indicator of an
aircraft flying at 60 knots
at 10,000 ft
Airspeed indicator of an
aircraft flying at 60 knots
at sea level
Airspeed indicator of an
aircraft flying at 60 knots
at 20,000 ft
Figure 4-6. Effects of altitude on the airspeed indicator.
Types of Airspeed
Pilots work with various airspeed numbers including indicated airspeed (IAS), calibrated airspeed (CAS), and true airspeed
(TAS). [Figure 4-7]
Normal static source
Airspeed Calibration
FLAPS UP
KIAS
KCAS
60 80 100 120
59 78 98 119
A Indicated airspeed: 100 knots
B Calibrated airspeed: 98 knots
knotsTrue airspeed 0
knotsTrue airspeed 8
A Indicated airspeed: 100 knots
B Calibrated airspeed: 98 knots
RAM AIR
15,000 feet MSL
RAM AIR
Sea level
STANDARD CONDITIONS
Figure 4-7. Three types of airspeed.
Indicated Airspeed (IAS)
The pilot reads the IAS directly from the airspeed indicator, uncorrected for installation error or instrument error. Although
air density affects indicated airspeed, a particular glider in steady wings-level flight at a fixed weight stalls at a specific
dynamic pressure (indicated airspeed) regardless of the air density. [Figure 4-8]
Altitude
center vertically
1,000 feet
10,000 feet
20,000 feet
30,000 feet
40,000 feet
True airspeed
(TAS)
41 knots
48 knots
56 knots
64 knots
72 knots
IAS at which stall occurs in
steady wings-level flight
40 knots
40 knots
40 knots
40 knots
40 knots
Figure 4-8. Indicated stall airspeeds and true airspeeds at various altitudes.
Calibrated Airspeed (CAS)
CAS corrects IAS for installation and instrument errors. Significant errors may occur at low airspeeds. At cruising and
higher airspeed ranges, IAS and CAS differences become small.
Pilots should refer to the airspeed calibration chart to correct for possible airspeed errors because airspeeds, such as those
found on the color-coded face of the airspeed indicator, on placards, or in the Glider Flight Manual or Pilot’s Operating
Handbook (GFM/POH), usually reflect CAS. Some manufacturers use IAS rather than CAS to denote the airspeeds.
Dirt, dust, ice, or snow collecting at the pitot tube opening may obstruct air passage and prevent correct indications.
Degradation due to age and vibration may also affect the sensitivity of the diaphragm. Therefore, airspeed indicators
should undergo periodic calibration.
True Airspeed (TAS)
TAS is the actual speed at which the aircraft moves through the air. In still air, TAS equals the actual speed over the ground.
However, the airspeed indicator only indicates TAS under standard atmospheric conditions at sea level (29.92 inches of
mercury ("Hg) and 15°C). At a constant IAS, TAS increases as the glider climbs because air density decreases with an
increase of altitude.
The airspeed indicator provides a convenient means to manage most flight parameters since it indicates dynamic pressure.
If the airspeed indicator did display TAS, the pilot would need to use a quick reference card to look up the stall speed,
best L/D speed, and minimum sink speed for the current altitude while trying to fly. Fortunately, the pilot only needs to
remember one set of numbers for most parameters. For example, Figure 4-8 illustrates that for each glider, the pilot need
only remember one stall speed for all altitudes.
A pilot can determine TAS by two methods. The first and more accurate method involves using a flight computer. In
this method, the pilot corrects the CAS for temperature and pressure variation using the airspeed correction scale on the
computer. The second method approximates TAS by increasing the current IAS by 2 percent for every 1,000 feet above
sea level.
Airspeed Indicator Markings
Gliders manufactured after 1945 have airspeed indicators that conform to a standard color-coded marking system.
[Figure 4-3] This system enables the pilot to determine important airspeed information quickly. For example, the green
arc represents the normal operating range, while the yellow arc represents the caution range. A pilot who notes an airspeed
needle in the yellow arc and rapidly approaching the red line while maneuvering should immediately take corrective action
to reduce the airspeed. In this case, the pilot should use smooth control pressure at high airspeeds to avoid unsafe stress on
the glider structure.
A description of the standard markings on an airspeed indicator follows:
• The white arc—flap operating range.
• The lower limit of the white arc—stalling speed in the landing configuration.
• The top of the white arc—maximum speed for use of full flaps. If flaps are operated at higher airspeeds, severe strain
or structural failure could result.
• The lower limit of the green arc—stalling speed with the wing flaps and landing gear retracted.
• The upper limit of the green arc—maximum structural cruising speed. This is the maximum speed for normal
operations.
• The yellow arc—caution range. The pilot should avoid speeds within this area unless in smooth air.
• The red line—never-exceed speed. This is the maximum speed at which the glider can be operated in smooth air.
This pilot should never intentionally exceed this speed.
Effect of Altitude on VNE
The never-exceed speed (VNE) decreases with increased altitude due to the possibility of flutter at higher true airspeeds.
At high altitudes maintaining a speed at or below the red line may exceed actual VNE. Since the decrease in VNE varies by
model, the flight manual may include a table, such as the one shown in Figure 4-9, that documents the decrease in VNE with
altitude. At high true airspeeds during a rapid descent, the glider structure could suddenly flutter and break apart. Glider
manufacturers test for flutter and adherence to VNE speeds published in the flight manual for the specific make and model
should prevent it.
Up to 6,500
10,000
13,000
16,500
Altitude (in feet) VNE (IAS in knots)
Figure 4-9. IAS corresponding to VNE decreases with altitude.
Other Airspeed Limitations
Placards in the view of the pilot may display other important airspeed limitations not marked on the face of the airspeed
indicator. [Figure 4-10]
• Maneuvering speed (V~A)—– a structural design
• airspeed used in determining the strength requirements for the glider and its control surfaces. The structural design
requirements do not cover multiple inputs in one axis or control inputs in more than one axis at a time at any speed,
even below V~A. If encountering rough air or severe turbulence during flight, the pilot should reduce airspeed to
maneuvering speed or less to prevent exceeding structural limits. Maneuvering speed varies with the weight of
the glider and does not appear as a specific airspeed on the airspeed indicator. For gliders with a published rough
airspeed limitation (V~B), the pilot should keep below that speed in rough air to accommodate maximum gust
intensity.
• Landing gear operating speed (V~LO)—maximum speed for extending or retracting the landing gear if using a
glider equipped with retractable landing gear.
• Minimum sink speed—airspeed that results in the least amount of altitude loss over a given time, or
• which maximizes the altitude gain when taking advantage of rising air.
• Best glide speed—airspeed that results in the least amount of altitude loss over a given distance, not considering the
effects of wind.
• Maximum aerotow or ground launch speed—maximum airspeed for tow without exceeding design specifications.
OR
OR
Valid when lower or side hook is installed:
Maximum winch-launching speed
Maximum aerotowing speed
Maximum maneuvering speed
Valid when front hook only is installed:
Maximum aerotowing speed
Maximum maneuvering speed
65 KIAS
81 KIAS
81 KIAS
81 KIAS
81 KIAS
Maximum winch-launching speed
Maximum aerotowing speed
Maximum maneuvering speed
Maximum aerotowing speed
Maximum maneuvering speed
120 km/hr IAS
150 km/hr IAS
150 km/hr IAS
150 km/hr IAS
150 km/hr IAS
Figure 4-10. Sample speed limitation placards placed in a glider and in view of the pilot.
Altimeter
The altimeter measures the static air pressure of the surrounding air. Tubing connects the altimeter static pressure inlet to
the static port holes located on the side of the glider. [Figure 4-11]
Static vents
Altimeter
Figure 4-11. Static vents and altimeter plumbing.
If using the local altimeter setting (the local pressure corrected to sea level), the altimeter indicates the glider's current
height above mean sea level (MSL). [ Figure 4-12] Subtracting the ground elevation from current MSL altitude gives the
height above ground.
30.029.929.8
I00 FEET
CALIBRATED
TO
20,000 FEET
ALT
Feet in 10,000s pointer
Feet in 100s pointer
Feet in 1,000s pointer
Altitude indication scale
Altimeter setting adjustment knob
Altimeter setting
The barometric pressure can be
changed approximately 10 feet for each
.01 "Hg to compensate for changes
in atmospheric pressure. Increasing the
altimeter setting causes the indicated
altitude to increase, while decreasing
the altimeter setting causes the
indicated altitude to decrease.
Figure 4-12. Altimeter.
The weight of a column of air above a given location creates atmospheric pressure. At sea level, an overlying column of air
exerts a force equivalent to 14.7 pounds per square inch, 1013.2 mb, or 29.92 inches of mercury under standard conditions.
At a higher altitude, the shorter overlying column of air weighs less and exerts less pressure. Therefore, atmospheric pressure
decreases with altitude. At 18,000 feet, atmospheric pressure drops to approximately half that at sea level. [Figure 4-13]
Space
0 feet 1,013 mb
18,000 feet 505 mb
30,000 feet 300 mb
Figure 4-13. Atmospheric pressure and altitude.
Principles of Operation
The altimeter works like an aneroid barometer that measures atmospheric pressure at the current elevation except that the
altimeter indicates pressure in feet. The altimeter indicates changes in altitude during a climb or descent as atmospheric
pressure changes. Figure 4-14 and Figure 4-15 illustrate how the altimeter functions. Some altimeters have one pointer
while others have more.
9 0
9 0
Atmospheric pressure at sea level
1,013 mb
Atmospheric pressure at 3,500 feet
Decrease in pressure has caused aneroid capsule
to expand and rotate the needle.
890 mb
The altimeter’s static pressure inlet must be
exposed to air that is at local atmospheric
pressure.
The pressure of the air inside the altimeter’s
casing equalizes to local atmospheric
pressure via the static pressure inlet.
Atmospheric pressure decreases with
altitude.
As atmospheric pressure decreases, the
aneroid capsule expands, moving the
linkages and rotating the display needle(s).
Figure 4-14. How the altimeter functions.