Figure 8-5. Vertical speed indicator (VSI).
3
4
I
0
2
2I
3
VERTICAL SPEED
THOUSAND FT PER MINUP
DOWN
Diaphragm
Direct static pressure
Calibrated leak
3. Absolute altitude—the vertical distance of an aircraft
above the terrain, or above ground level (AGL).
4. Pressure altitude—the altitude indicated when
the altimeter setting window (barometric scale) is
adjusted to 29.92 "Hg. This is the altitude above
the standard datum plane, which is a theoretical
plane where air pressure (corrected to 15 °C) equals
29.92 "Hg. Pressure altitude is used to compute density
altitude, true altitude, true airspeed (TAS), and other
performance data.
5. Density altitude—pressure altitude corrected
for variations from standard temperature. When
conditions are standard, pressure altitude and density
altitude are the same. If the temperature is above
standard, the density altitude is higher than pressure
altitude. If the temperature is below standard, the
density altitude is lower than pressure altitude. This
is an important altitude because it is directly related
to the aircraft’s performance.
A pilot must understand how the performance of the aircraft
is directly related to the density of the air. The density of
the air affects how much power a naturally aspirated engine
produces, as well as how efficient the airfoils are. If there are
fewer air molecules (lower pressure) to accelerate through
the propeller, the acceleration to rotation speed is longer
and thus produces a longer takeoff roll, which translates to
a decrease in performance.
As an example, consider an airport with a field elevation
of 5,048 feet MSL where the standard temperature is 5 °C.
Under these conditions, pressure altitude and density altitude
are the same—5,048 feet. If the temperature changes to
30 °C, the density altitude increases to 7,855 feet. This
means an aircraft would perform on takeoff as though the
field elevation were 7,855 feet at standard temperature.
Conversely, a temperature of –25 °C would result in a density
altitude of 1,232 feet. An aircraft would perform much better
under these conditions.
Instrument Check
Prior to each flight, a pilot should examine the altimeter for
proper indications in order to verify its validity. To determine
the condition of an altimeter, set the barometric scale to the
current reported altimeter setting transmitted by the local
airport traffic control tower, flight service station (FSS), or
any other reliable source, such as ATIS, AWOS, or ASOS.
The altimeter pointers should indicate the surveyed field
elevation of the airport. If the indication is off more than
75 feet from the surveyed field elevation, the instrument
should be referred to a certificated instrument repair station
for recalibration.
Vertical Speed Indicator (VSI)
The VSI, which is sometimes called a vertical velocity
indicator (VVI), indicates whether the aircraft is climbing,
descending, or in level flight. The rate of climb or descent is
indicated in feet per minute (fpm). If properly calibrated, the
VSI indicates zero in level flight. [Figure 8-5]
Principle of Operation
Although the VSI operates solely from static pressure, it is a
differential pressure instrument. It contains a diaphragm with
connecting linkage and gearing to the indicator pointer inside
an airtight case. The inside of the diaphragm is connected
directly to the static line of the pitot-static system. The area
outside the diaphragm, which is inside the instrument case,
is also connected to the static line but through a restricted
orifice (calibrated leak).
Both the diaphragm and the case receive air from the static
line at existing atmospheric pressure. The diaphragm receives
unrestricted air, while the case receives the static pressure via
the metered leak. When the aircraft is on the ground or in level
flight, the pressures inside the diaphragm and the instrument
case are equal, and the pointer is at the zero indication.
When the aircraft climbs or descends, the pressure inside
the diaphragm changes immediately, but due to the metering
action of the restricted passage, the case pressure remains
higher or lower for a short time, causing the diaphragm to
contract or expand. This causes a pressure differential that
is indicated on the instrument needle as a climb or descent.
When the pressure differential stabilizes at a definite ratio,
the needle indicates the rate of altitude change.
Figure 8-6. An IVSI incorporates accelerometers to help the
instrument immediately indicate changes in vertical speed.
3
4I
0
2
2I
3
UP
DOWN
.5
.5
Accelerometer
Inlet from static port
Calibrated leak
Figure 8-7. Airspeed indicator (ASI).
100
200
50
150
Diaphragm
Handstaff pinionStatic air line
Pitot tube
Ram air
SectorLong lever
Pitot connection
The VSI displays two different types of information:
• Trend information shows an immediate indication of
an increase or decrease in the aircraft’s rate of climb
or descent.
• Rate information shows a stabilized rate of change in
altitude.
The trend information is the direction of movement of the
VSI needle. For example, if an aircraft is maintaining level
flight and the pilot pulls back on the control yoke causing the
nose of the aircraft to pitch up, the VSI needle moves upward
to indicate a climb. If the pitch attitude is held constant,
the needle stabilizes after a short period (6–9 seconds) and
indicates the rate of climb in hundreds of fpm. The time
period from the initial change in the rate of climb, until the
VSI displays an accurate indication of the new rate, is called
the lag. Rough control technique and turbulence can extend
the lag period and cause erratic and unstable rate indications.
Some aircraft are equipped with an instantaneous vertical
speed indicator (IVSI), which incorporates accelerometers
to compensate for the lag in the typical VSI. [Figure 8-6]
Instrument Check
As part of a preflight check, proper operation of the VSI
must be established. Make sure the VSI indicates a near zero
reading prior to leaving the ramp area and again just before
takeoff. If the VSI indicates anything other than zero, that
indication can be referenced as the zero mark. Normally, if the
needle is not exactly zero, it is only slightly above or below
the zero line. After takeoff, the VSI should trend upward to
indicate a positive rate of climb and then, once a stabilized
climb is established, a rate of climb can be referenced.
Airspeed Indicator (ASI)
The ASI is a sensitive, differential pressure gauge that
measures and promptly indicates the difference between pitot
(impact/dynamic pressure) and static pressure. These two
pressures are equal when the aircraft is parked on the ground
in calm air. When the aircraft moves through the air, the
pressure on the pitot line becomes greater than the pressure
in the static lines. This difference in pressure is registered by
the airspeed pointer on the face of the instrument, which is
calibrated in miles per hour, knots (nautical miles per hour),
or both. [Figure 8-7]
The ASI is the one instrument that utilizes both the pitot,
as well as the static system. The ASI introduces the static
pressure into the airspeed case while the pitot pressure
(dynamic) is introduced into the diaphragm. The dynamic
pressure expands or contracts one side of the diaphragm,
which is attached to an indicating system. The system drives
the mechanical linkage and the airspeed needle.
Just as in altitudes, there are multiple types of airspeeds.
Pilots need to be very familiar with each type.
• Indicated airspeed (IAS)—the direct instrument
reading obtained from the ASI, uncorrected for
variations in atmospheric density, installation error,
or instrument error. Manufacturers use this airspeed
as the basis for determining aircraft performance.
Takeoff, landing, and stall speeds listed in the AFM/
POH are IAS and do not normally vary with altitude
or temperature.
Figure 8-8. Single engine airspeed indicator (ASI).
80100
120
140
T.A.S.KTS
PRESS
ALT 0 5 10 15 20
F° 120 90 60 30 0 -30
100120
40
60
80100
120
140
160
MPHMPH
AIRSPEED
KNOTS
VS0
VNE (red line)
Yellow arc
VN0
Green arc VFE
White arc
VS1
• Calibrated airspeed (CAS)—IAS corrected for
installation error and instrument error. Although
manufacturers attempt to keep airspeed errors to a
minimum, it is not possible to eliminate all errors
throughout the airspeed operating range. At certain
airspeeds and with certain flap settings, the installation
and instrument errors may total several knots. This
error is generally greatest at low airspeeds. In the
cruising and higher airspeed ranges, IAS and CAS
are approximately the same. Refer to the airspeed
calibration chart to correct for possible airspeed errors.
• True airspeed (TAS)—CAS corrected for altitude
and nonstandard temperature. Because air density
decreases with an increase in altitude, an aircraft has
to be flown faster at higher altitudes to cause the same
pressure difference between pitot impact pressure
and static pressure. Therefore, for a given CAS, TAS
increases as altitude increases; or for a given TAS,
CAS decreases as altitude increases. A pilot can find
TAS by two methods. The most accurate method is
to use a flight computer. With this method, the CAS
is corrected for temperature and pressure variation by
using the airspeed correction scale on the computer.
Extremely accurate electronic flight computers are
also available. Just enter the CAS, pressure altitude,
and temperature, and the computer calculates the TAS.
A second method, which is a rule of thumb, provides
the approximate TAS. Simply add 2 percent to the
CAS for each 1,000 feet of altitude. The TAS is the
speed that is used for flight planning and is used when
filing a flight plan.
• Groundspeed (GS)—the actual speed of the airplane
over the ground. It is TAS adjusted for wind. GS
decreases with a headwind and increases with a
tailwind.
Airspeed Indicator Markings
Aircraft weighing 12,500 pounds or less, manufactured after
1945, and certificated by the FAA are required to have ASIs
marked in accordance with a standard color-coded marking
system. This system of color-coded markings enables a pilot
to determine at a glance certain airspeed limitations that are
important to the safe operation of the aircraft. For example, if
during the execution of a maneuver, it is noted that the airspeed
needle is in the yellow arc and rapidly approaching the red
line, the immediate reaction should be to reduce airspeed.
As shown in Figure 8-8, ASIs on single-engine small aircraft
include the following standard color-coded markings:
• White arc—commonly referred to as the flap operating
range since its lower limit represents the full flap stall
speed and its upper limit provides the maximum flap
speed. Approaches and landings are usually flown at
speeds within the white arc.
• Lower limit of white arc (V S0)—the stalling speed
or the minimum steady flight speed in the landing
configuration. In small aircraft, this is the power-off
stall speed at the maximum landing weight in the
landing configuration (gear and flaps down).
• Upper limit of the white arc (V FE)—the maximum
speed with the flaps extended.
• Green arc—the normal operating range of the aircraft.
Most flying occurs within this range.
• Lower limit of green arc (V S1)—the stalling speed
or the minimum steady flight speed obtained in a
specified configuration. For most aircraft, this is the
power-off stall speed at the maximum takeoff weight
in the clean configuration (gear up, if retractable, and
flaps up).
• Upper limit of green arc (V N0)—the maximum
structural cruising speed. Do not exceed this speed
except in smooth air.
• Yellow arc—caution range. Fly within this range only
in smooth air and then only with caution.
• Red line (VNE)—never exceed speed. Operating above
this speed is prohibited since it may result in damage
or structural failure.
Other Airspeed Limitations
Some important airspeed limitations are not marked on the
face of the ASI, but are found on placards and in the AFM/
POH. These airspeeds include:
Figure 8-9. A blocked pitot tube, but clear drain hole.
Static port
Drain hole
Pitot tube
Blockage
• Design maneuvering speed (V A)—the maximum
speed at which the structural design’s limit load can
be imposed (either by gusts or full deflection of the
control surfaces) without causing structural damage.
It is important to consider weight when referencing
this speed. For example, VA may be 100 knots when
an airplane is heavily loaded, but only 90 knots when
the load is light.
• Landing gear operating speed (VLO)—the maximum
speed for extending or retracting the landing gear if
flying an aircraft with retractable landing gear.
• Landing gear extended speed (V LE)—the maximum
speed at which an aircraft can be safely flown with
the landing gear extended.
• Best angle-of-climb speed (V X)—the airspeed at
which an aircraft gains the greatest amount of altitude
in a given distance. It is used during a short-field
takeoff to clear an obstacle.
• Best rate-of-climb speed (V Y)—the airspeed that
provides the most altitude gain in a given period of time.
• Single-engine best rate-of-climb (V YSE)—the best
rate-of-climb or minimum rate-of-sink in a light
twin-engine aircraft with one engine inoperative. It is
marked on the ASI with a blue line. VYSE is commonly
referred to as “Blue Line.”
• Minimum control speed (VMC)—the minimum flight
speed at which a light, twin-engine aircraft can be
satisfactorily controlled when an engine suddenly
becomes inoperative and the remaining engine is at
takeoff power.
Instrument Check
Prior to takeoff, the ASI should read zero. However, if there
is a strong wind blowing directly into the pitot tube, the ASI
may read higher than zero. When beginning the takeoff,
make sure the airspeed is increasing at an appropriate rate.
Blockage of the Pitot-Static System
Errors almost always indicate blockage of the pitot tube, the
static port(s), or both. Blockage may be caused by moisture
(including ice), dirt, or even insects. During preflight, make
sure the pitot tube cover is removed. Then, check the pitot and
static port openings. A blocked pitot tube affects the accuracy
of the ASI, but a blockage of the static port not only affects
the ASI, but also causes errors in the altimeter and VSI.
Blocked Pitot System
The pitot system can become blocked completely or only
partially if the pitot tube drain hole remains open. If the pitot
tube becomes blocked and its associated drain hole remains
clear, ram air is no longer able to enter the pitot system. Air
already in the system vents through the drain hole, and the
remaining pressure drops to ambient (outside) air pressure.
Under these circumstances, the ASI reading decreases to
zero because the ASI senses no difference between ram and
static air pressure. The ASI no longer operates since dynamic
pressure cannot enter the pitot tube opening. Static pressure
is able to equalize on both sides since the pitot drain hole
is still open. The apparent loss of airspeed is not usually
instantaneous but happens very quickly. [Figure 8-9]
If both the pitot tube opening and the drain hole should
become clogged simultaneously, then the pressure in the pitot
tube is trapped. No change is noted on the airspeed indication
should the airspeed increase or decrease. If the static port
is unblocked and the aircraft should change altitude, then a
change is noted on the ASI. The change is not related to a
change in airspeed but a change in static pressure. The total
pressure in the pitot tube does not change due to the blockage;
however, the static pressure will change.
Because airspeed indications rely upon both static and
dynamic pressure together, the blockage of either of these
systems affects the ASI reading. Remember that the ASI has
a diaphragm in which dynamic air pressure is entered. Behind
this diaphragm is a reference pressure called static pressure
that comes from the static ports. The diaphragm pressurizes
against this static pressure and as a result changes the airspeed
indication via levers and indicators. [Figure 8-10]
For example, take an aircraft and slow it down to zero knots
at a given altitude. If the static port (providing static pressure)
and the pitot tube (providing dynamic pressure) are both
unobstructed, the following claims can be made:
1. The ASI would be zero.
2. Dynamic pressure and static pressure are equal.
3. Because both dynamic and static air pressure are equal
at zero speed with increased speed, dynamic pressure
Figure 8-10. Blocked pitot system with clear static system.
Climb
Descent
Static port
Drain hole
Pitot tube
Blockage
must include two components: static pressure and
dynamic pressure.
It can be inferred that airspeed indication must be based upon
a relationship between these two pressures, and indeed it is.
An ASI uses the static pressure as a reference pressure and
as a result, the ASI’s case is kept at this pressure behind the
diaphragm. On the other hand, the dynamic pressure through
the pitot tube is connected to a highly sensitive diaphragm
within the ASI case. Because an aircraft in zero motion
(regardless of altitude) results in a zero airspeed, the pitot tube
always provides static pressure in addition to dynamic pressure.
Therefore, the airspeed indication is the result of two
pressures: the pitot tube static and dynamic pressure within
the diaphragm as measured against the static pressure in the
ASI’s case.
If the aircraft were to descend while the pitot tube is
obstructed, the pressure in the pitot system, including the
diaphragm, would remain constant. But as the descent
is made, the static pressure would increase against the
diaphragm causing it to compress, thereby resulting in an
indication of decreased airspeed. Conversely, if the aircraft
were to climb, the static pressure would decrease allowing
the diaphragm to expand, thereby showing an indication of
greater airspeed. [Figure 8-10]
The pitot tube may become blocked during flight due to
visible moisture. Some aircraft may be equipped with pitot
heat for flight in visible moisture. Consult the AFM/POH for
specific procedures regarding the use of pitot heat.
Blocked Static System
If the static system becomes blocked but the pitot tube
remains clear, the ASI continues to operate; however, it
is inaccurate. The airspeed indicates lower than the actual
airspeed when the aircraft is operated above the altitude
where the static ports became blocked because the trapped
static pressure is higher than normal for that altitude. When
operating at a lower altitude, a faster than actual airspeed is
displayed due to the relatively low static pressure trapped
in the system.
Revisiting the ratios that were used to explain a blocked pitot
tube, the same principle applies for a blocked static port. If
the aircraft descends, the static pressure increases on the pitot
side showing an increase on the ASI. This assumes that the
aircraft does not actually increase its speed. The increase in
static pressure on the pitot side is equivalent to an increase
in dynamic pressure since the pressure cannot change on
the static side.
If an aircraft begins to climb after a static port becomes
blocked, the airspeed begins to show a decrease as the
aircraft continues to climb. This is due to the decrease in
static pressure on the pitot side, while the pressure on the
static side is held constant.
A blockage of the static system also affects the altimeter and
VSI. Trapped static pressure causes the altimeter to freeze
at the altitude where the blockage occurred. In the case of
the VSI, a blocked static system produces a continuous zero
indication. [Figure 8-11]
Some aircraft are equipped with an alternate static source in
the flight deck. In the case of a blocked static source, opening
the alternate static source introduces static pressure from the
flight deck into the system. Flight deck static pressure is lower
than outside static pressure. Check the aircraft AOM/POH for
airspeed corrections when utilizing alternate static pressure.
Figure 8-12. Primary flight display (PFD). Note that the actual location of indications vary depending on manufacturers.
XPDR 5537 IDNT LCL10:12:34
INSET PFD OBS CDI DME XPDR IDENT TMR/REF NRST ALERTS
VOR 1
270°
2
1
1
2
4300
4200
4100
3900
3900
3800
4300
3600
3500
3400
3300
3200
3100
20
80
4000
4000
130
120
110
90
80
70
1
100
9
TAS 106KT
OAT 6°C
NAV1 108.00 113.00
NAV2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _°T TRK 360°
Attitude indicator
Altimeter
Horizontal situation indicator
Vertical speed indicator (VSI)
Slip skid indicator
Airspeed indicator
Turn indicator
VOR 1
270°
270°
Slip/skid indicator
Turn rate indicator tick marks
Turn rate trend vector
Figure 8-11. Blocked static system.
30.0 29.9 29.8
Pitot tube
Blockage
Static port
Frozen altimeter
Constant zero indication on VSI
Inaccurate airspeed indications
Electronic Flight Display (EFD)
Advances in technology have brought about changes in the
instrumentation found in all types of aircraft; for example,
Electronic Flight Displays (EFDs) commonly referred to
as “glass cockpits.” EFDs include flight displays such as
primary flight displays (PFD) and multi-function displays
(MFD). This has changed not only what information is
available to a pilot, but also how the information is displayed.
In addition to the improvement in system reliability, which
increases overall safety, EFDs have decreased the overall cost
of equipping aircraft with state-of-the-art instrumentation.
Primary electronic instrumentation packages are less prone
to failure than their analogue counterparts. No longer is it
necessary for aircraft designers to create cluttered panel
layouts in order to accommodate all necessary flight
instruments. Instead, multi-panel digital flight displays
combine all flight instruments onto a single screen that is
called a primary flight display (PFD). The traditional “six
pack” of instruments is now displayed on one liquid crystal
display (LCD) screen.
Airspeed Tape
Configured similarly to traditional panel layouts, the ASI
is located on the left side of the screen and is displayed as
a vertical speed tape. As the aircraft increases in speed, the
larger numbers descend from the top of the tape. The TAS is
displayed at the bottom of the tape through the input to the air
data computer (ADC) from the outside air temperature probe.
Airspeed markings for VX, VY, and rotation speed (VR) are
displayed for pilot reference. An additional pilot-controlled
airspeed bug is available to set at any desired reference speed.
As on traditional analogue ASIs, the electronic airspeed tape
displays the color-coded ranges for the flap operating range,