G-9
Critical altitude. The maximum altitude under standard
atmospheric conditions at which a turbocharged engine can
produce its rated horsepower.
Critical angle of attack. The angle of attack at which a
wing stalls regardless of airspeed, flight attitude, or weight.
Critical areas. Areas where disturbances to the ILS localizer
and glideslope courses may occur when surface vehicles or
aircraft operate near the localizer or glideslope antennas.
CRM. See crew resource management.
Cross-check. The first fundamental skill of instrument flight,
also known as “scan,” the continuous and logical observation
of instruments for attitude and performance information.
Cruise clearance. An ATC clearance issued to allow a
pilot to conduct flight at any altitude from the minimum
IFR altitude up to and including the altitude specified in the
clearance. Also authorizes a pilot to proceed to and make an
approach at the destination airport.
Current induction. An electrical current being induced into,
or generated in, any conductor that is crossed by lines of flux
from any magnet.
D
DA. See decision altitude.
Datum (Reference Datum). An imaginary vertical plane
or line from which all measurements of arm are taken. The
datum is established by the manufacturer. Once the datum
has been selected, all moment arms and the location of CG
range are measured from this point.
D.C. Direct current.
Dark adaptation. Physical and chemical adjustments of the
eye that make vision possible in relative darkness.
Dead reckoning. Navigation of an airplane solely by means
of computations based on airspeed, course, heading, wind
direction and speed, groundspeed, and elapsed time.
Deceleration error. A magnetic compass error that occurs
when the aircraft decelerates while flying on an easterly
or westerly heading, causing the compass card to rotate
toward South.
Decision altitude (DA). A specified altitude in the precision
approach, charted in feet MSL, at which a missed approach
must be initiated if the required visual reference to continue
the approach has not been established.
Decision height (DH). A specified altitude in the precision
approach, charted in height above threshold elevation,
at which a decision must be made either to continue the
approach or to execute a missed approach.
Deice. The act of removing ice accumulation from an
aircraft structure.
Delta. A Greek letter expressed by the symbol Δ to indicate
a change of values. As an example, ΔCG indicates a change
(or movement) of the CG.
Density altitude. Pressure altitude corrected for nonstandard
temperature. Density altitude is used in computing the
performance of an aircraft and its engines.
Departure procedure (DP). Preplanned IFR ATC departure,
published for pilot use, in textual and graphic format.
Deposition. The direct transformation of a gas to a solid
state, in which the liquid state is bypassed. Some sources use
sublimation to describe this process instead of deposition.
Detonation. The sudden release of heat energy from fuel in
an aircraft engine caused by the fuel-air mixture reaching
its critical pressure and temperature. Detonation occurs as
a violent explosion rather than a smooth burning process.
Deviation. A magnetic compass error caused by local
magnetic fields within the aircraft. Deviation error is different
on each heading.
Dew. Moisture that has condensed from water vapor. Usually
found on cooler objects near the ground, such as grass, as
the near-surface layer of air cools faster than the layers of
air above it.
Dewpoint. The temperature at which air reaches a state where
it can hold no more water.
DGPS. Differential global positioning system.
DH. See decision height.
G-10
Differential ailerons. Control surface rigged such that the
aileron moving up moves a greater distance than the aileron
moving down. The up aileron produces extra parasite drag
to compensate for the additional induced drag caused by
the down aileron. This balancing of the drag forces helps
minimize adverse yaw.
Differential Global Positioning System (DGPS). A system
that improves the accuracy of Global Navigation Satellite
Systems (GNSS) by measuring changes in variables to
provide satellite positioning corrections.
Differential pressure. A difference between two pressures.
The measurement of airspeed is an example of the use of
differential pressure.
Dihedral. The positive acute angle between the lateral
axis of an airplane and a line through the center of a wing
or horizontal stabilizer. Dihedral contributes to the lateral
stability of an airplane.
Diluter-demand oxygen system. An oxygen system that
delivers oxygen mixed or diluted with air in order to maintain
a constant oxygen partial pressure as the altitude changes.
Direct indication. The true and instantaneous reflection of
aircraft pitch-and-bank attitude by the miniature aircraft,
relative to the horizon bar of the attitude indicator.
Direct User Access Terminal System (DUATS). A system
that provides current FAA weather and flight plan filing
services to certified civil pilots, via personal computer,
modem, or telephone access to the system. Pilots can request
specific types of weather briefings and other pertinent data
for planned flights.
Directional stability. Stability about the vertical axis of an
aircraft, whereby an aircraft tends to return, on its own, to
flight aligned with the relative wind when disturbed from that
equilibrium state. The vertical tail is the primary contributor
to directional stability, causing an airplane in flight to align
with the relative wind.
Distance circle. See reference circle.
Distance measuring equipment (DME). A pulse-type
electronic navigation system that shows the pilot, by an
instrument-panel indication, the number of nautical miles
between the aircraft and a ground station or waypoint.
DME. See distance measuring equipment.
DME arc. A flight track that is a constant distance from the
station or waypoint.
DOD. Department of Defense.
Doghouse. A turn-and-slip indicator dial mark in the shape
of a doghouse.
Domestic Reduced Vertical Separation Minimum
(DRVSM). Additional flight levels between FL 290 and FL
410 to provide operational, traffic, and airspace efficiency.
Double gimbal. A type of mount used for the gyro in an
attitude instrument. The axes of the two gimbals are at right
angles to the spin axis of the gyro, allowing free motion in
two planes around the gyro.
DP. See departure procedure.
Drag. The net aerodynamic force parallel to the relative
wind, usually the sum of two components: induced drag
and parasite drag.
Drag curve. The curve created when plotting induced drag
and parasite drag.
Drift angle. Angle between heading and track.
DRVSM. See Domestic Reduced Vertical Separation
Minimum.
DUATS. See direct user access terminal system.
Duplex. Transmitting on one frequency and receiving on a
separate frequency.
Dutch roll. A combination of rolling and yawing oscillations
that normally occurs when the dihedral effects of an aircraft
are more powerful than the directional stability. Usually
dynamically stable but objectionable in an airplane because
of the oscillatory nature.
Dynamic hydroplaning. A condition that exists when
landing on a surface with standing water deeper than the
tread depth of the tires. When the brakes are applied, there is
a possibility that the brake will lock up and the tire will ride
on the surface of the water, much like a water ski. When the
tires are hydroplaning, directional control and braking action
are virtually impossible. An effective anti-skid system can
minimize the effects of hydroplaning.
G-11
Dynamic stability. The property of an aircraft that causes
it, when disturbed from straight-and-level flight, to develop
forces or moments that restore the original condition of
straight and level.
E
Eddy currents. Current induced in a metal cup or disc when
it is crossed by lines of flux from a moving magnet.
Eddy current damping. The decreased amplitude of
oscillations by the interaction of magnetic fields. In the case
of a vertical card magnetic compass, flux from the oscillating
permanent magnet produces eddy currents in a damping disk
or cup. The magnetic flux produced by the eddy currents
opposes the flux from the permanent magnet and decreases
the oscillations.
EFC. See expect-further-clearance.
EFD. See electronic flight display.
EGT. See exhaust gas temperature.
Electronic flight display (EFD). For the purpose of
standardization, any flight instrument display that uses LCD or
other image-producing system (cathode ray tube (CRT), etc.)
Elevator. The horizontal, movable primary control surface in
the tail section, or empennage, of an airplane. The elevator is
hinged to the trailing edge of the fixed horizontal stabilizer.
Elevator illusion. The sensation of being in a climb or
descent, caused by the kind of abrupt vertical accelerations
that result from up- or downdrafts.
Emergency. A distress or urgent condition.
Empennage. The section of the airplane that consists of the
vertical stabilizer, the horizontal stabilizer, and the associated
control surfaces.
Emphasis error. The result of giving too much attention
to a particular instrument during the cross-check, instead of
relying on a combination of instruments necessary for attitude
and performance information.
Empty-field myopia. Induced nearsightedness that is
associated with flying at night, in instrument meteorological
conditions and/or reduced visibility. With nothing to focus
on, the eyes automatically focus on a point just slightly ahead
of the airplane.
EM wave. Electromagnetic wave.
Encoding altimeter. A special type of pressure altimeter
used to send a signal to the air traffic controller on the ground,
showing the pressure altitude the aircraft is flying.
Engine pressure ratio (EPR). The ratio of turbine discharge
pressure divided by compressor inlet pressure, which is used
as an indication of the amount of thrust being developed by
a turbine engine.
En route facilities ring. Depicted in the plan view of IAP
charts, a circle which designates NAVAIDs, fixes, and
intersections that are part of the en route low altitude airway
structure.
En route high-altitude charts. Aeronautical charts for en
route instrument navigation at or above 18,000 feet MSL.
En route low-altitude charts. Aeronautical charts for en
route IFR navigation below 18,000 feet MSL.
EPR. See engine pressure ratio.
Equilibrium. A condition that exists within a body when the
sum of the moments of all of the forces acting on the body
is equal to zero. In aerodynamics, equilibrium is when all
opposing forces acting on an aircraft are balanced (steady,
unaccelerated flight conditions).
Equivalent airspeed. Airspeed equivalent to CAS in standard
atmosphere at sea level. As the airspeed and pressure altitude
increase, the CAS becomes higher than it should be, and a
correction for compression must be subtracted from the CAS.
Evaporation. The transformation of a liquid to a gaseous
state, such as the change of water to water vapor.
Exhaust gas temperature (EGT). The temperature of the
exhaust gases as they leave the cylinders of a reciprocating
engine or the turbine section of a turbine engine.
Expect-further-clearance (EFC). The time a pilot can
expect to receive clearance beyond a clearance limit.
Explosive decompression. A change in cabin pressure faster
than the lungs can decompress. Lung damage is possible.
F
FA. See area forecast.
FAA. Federal Aviation Administration.
FAF. See final approach fix.
G-12
False horizon. Inaccurate visual information for aligning the
aircraft, caused by various natural and geometric formations
that disorient the pilot from the actual horizon.
FDI. See flight director indicator.
Federal airways. Class E airspace areas that extend upward
from 1,200 feet to, but not including, 18,000 feet MSL, unless
otherwise specified.
Feeder facilities. Used by ATC to direct aircraft to
intervening fixes between the en route structure and the
initial approach fix.
Final approach. Part of an instrument approach procedure in
which alignment and descent for landing are accomplished.
Final approach fix (FAF). The fix from which the IFR
final approach to an airport is executed, and which identifies
the beginning of the final approach segment. An FAF is
designated on government charts by a Maltese cross symbol
for nonprecision approaches, and a lightning bolt symbol for
precision approaches.
Fixating. Staring at a single instrument, thereby interrupting
the cross-check process.
Fixed-pitch propellers. Propellers with fixed blade angles.
Fixed-pitch propellers are designed as climb propellers,
cruise propellers, or standard propellers.
Fixed slot. A fixed, nozzle shaped opening near the leading
edge of a wing that ducts air onto the top surface of the wing.
Its purpose is to increase lift at higher angles of attack.
FL. See flight level.
Flameout. A condition in the operation of a gas turbine
engine in which the fire in the engine goes out due to either
too much or too little fuel sprayed into the combustors.
Flaps. Hinged portion of the trailing edge between the
ailerons and fuselage. In some aircraft ailerons and flaps are
interconnected to produce full-span “flaperons.” In either
case, flaps change the lift and drag on the wing.
Floor load limit. The maximum weight the floor can sustain
per square inch/foot as provided by the manufacturer.
Flight configurations. Adjusting the aircraft control surfaces
(including flaps and landing gear) in a manner that will
achieve a specified attitude.
Flight director indicator (FDI). One of the major
components of a flight director system, it provides steering
commands that the pilot (or the autopilot, if coupled) follows.
Flight level (FL). A measure of altitude (in hundreds of feet)
used by aircraft flying above 18,000 feet with the altimeter
set at 29.92 "Hg.
Flight management system (FMS). Provides pilot and crew
with highly accurate and automatic long-range navigation
capability, blending available inputs from long- and short-
range sensors.
Flight path. The line, course, or track along which an aircraft
is flying or is intended to be flown.
Flight patterns. Basic maneuvers, flown by reference to the
instruments rather than outside visual cues, for the purpose
of practicing basic attitude flying. The patterns simulate
maneuvers encountered on instrument flights such as holding
patterns, procedure turns, and approaches.
Flight strips. Paper strips containing instrument flight
information, used by ATC when processing flight plans.
FMS. See flight management system.
FOD. See foreign object damage.
Fog. Cloud consisting of numerous minute water droplets
and based at the surface; droplets are small enough to be
suspended in the earth’s atmosphere indefinitely. (Unlike
drizzle, it does not fall to the surface. Fog differs from a
cloud only in that a cloud is not based at the surface, and
is distinguished from haze by its wetness and gray color.)
Force (F). The energy applied to an object that attempts to
cause the object to change its direction, speed, or motion.
In aerodynamics, it is expressed as F, T (thrust), L (lift), W
(weight), or D (drag), usually in pounds.
Foreign object damage (FOD). Damage to a gas turbine
engine caused by some object being sucked into the engine
while it is running. Debris from runways or taxiways can
cause foreign object damage during ground operations, and
the ingestion of ice and birds can cause FOD in flight.
Form drag. The drag created because of the shape of a
component or the aircraft.
G-13
Frise-type aileron. Aileron having the nose portion
projecting ahead of the hinge line. When the trailing edge
of the aileron moves up, the nose projects below the wing’s
lower surface and produces some parasite drag, decreasing
the amount of adverse yaw.
Front. The boundary between two different air masses.
Frost. Ice crystal deposits formed by sublimation when
temperature and dewpoint are below freezing.
Fuel load. The expendable part of the load of the airplane.
It includes only usable fuel, not fuel required to fill the lines
or that which remains trapped in the tank sumps.
Fundamental skills. Pilot skills of instrument cross-check,
instrument interpretation, and aircraft control.
Fuselage. The section of the airplane that consists of the
cabin and/or cockpit, containing seats for the occupants and
the controls for the airplane.
G
GAMA. General Aviation Manufacturers Association.
Gimbal ring. A type of support that allows an object, such
as a gyroscope, to remain in an upright condition when its
base is tilted.
Glideslope (GS). Part of the ILS that projects a radio beam
upward at an angle of approximately 3° from the approach
end of an instrument runway. The glideslope provides
vertical guidance to aircraft on the final approach course for
the aircraft to follow when making an ILS approach along
the localizer path.
Glideslope intercept altitude. The minimum altitude of an
intermediate approach segment prescribed for a precision
approach that ensures obstacle clearance.
Global landing system (GLS). An instrument approach with
lateral and vertical guidance with integrity limits (similar to
barometric vertical navigation (BARO VNAV).
Global navigation satellite system (GNSS). Satellite
navigation system that provides autonomous geospatial
positioning with global coverage. It allows small electronic
receivers to determine their location (longitude, latitude, and
altitude) to within a few meters using time signals transmitted
along a line of sight by radio from satellites.
Global positioning system (GPS). Navigation system
that uses satellite rather than ground-based transmitters for
location information.
GLS. See global landing system.
GNSS. See global navigation satellite system.
Goniometer. As used in radio frequency (RF) antenna
systems, a direction-sensing device consisting of two fixed
loops of wire oriented 90° from each other, which separately
sense received signal strength and send those signals to two
rotors (also oriented 90°) in the sealed direction-indicating
instrument. The rotors are attached to the direction-indicating
needle of the instrument and rotated by a small motor until
minimum magnetic field is sensed near the rotors.
GPS. See global positioning system.
GPS Approach Overlay Program. An authorization for
pilots to use GPS avionics under IFR for flying designated
existing nonprecision instrument approach procedures, with
the exception of LOC, LDA, and SDF procedures.
GPWS. See ground proximity warning system.
Graveyard spiral. The illusion of the cessation of a turn while
still in a prolonged, coordinated, constant rate turn, which
can lead a disoriented pilot to a loss of control of the aircraft.
Great circle route. The shortest distance across the surface
of a sphere (the Earth) between two points on the surface.
Ground adjustable trim tab. Non-movable metal trim tab
on a control surface. Bent in one direction or another while
on the ground to apply trim forces to the control surface.
Ground effect. The condition of slightly increased air pressure
below an airplane wing or helicopter rotor system that increases
the amount of lift produced. It exists within approximately one
wing span or one rotor diameter from the ground. It results
from a reduction in upwash, downwash, and wingtip vortices,
and provides a corresponding decrease in induced drag.
Ground proximity warning system (GPWS). A system
designed to determine an aircraft’s clearance above the Earth
and provides limited predictability about aircraft position
relative to rising terrain.
G-14
Groundspeed. Speed over the ground, either closing speed to
the station or waypoint, or speed over the ground in whatever
direction the aircraft is going at the moment, depending upon
the navigation system used.
GS. See glideslope.
GWPS. See ground proximity warning system.
Gyroscopic precession. An inherent quality of rotating bodies,
which causes an applied force to be manifested 90° in the
direction of rotation from the point where the force is applied.
H
HAA. See height above airport.
HAL. See height above landing.
HAT. See height above touchdown elevation.
Hazardous attitudes. Five aeronautical decision-making
attitudes that may contribute to poor pilot judgment: anti-
authority, impulsivity, invulnerability, machismo, and
resignation.
Hazardous Inflight Weather Advisory Service (HIWAS).
An en route FSS service providing continuously updated
automated of hazardous weather within 150 nautical miles of
selected VORs, available only in the conterminous 48 states.
Head-up display (HUD). A special type of flight viewing
screen that allows the pilot to watch the flight instruments
and other data while looking through the windshield of the
aircraft for other traffic, the approach lights, or the runway.
Heading. The direction in which the nose of the aircraft is
pointing during flight.
Heading indicator. An instrument which senses airplane
movement and displays heading based on a 360° azimuth,
with the final zero omitted. The heading indicator, also called
a directional gyro (DG), is fundamentally a mechanical
instrument designed to facilitate the use of the magnetic
compass. The heading indicator is not affected by the forces
that make the magnetic compass difficult to interpret.
Headwork. Required to accomplish a conscious, rational
thought process when making decisions. Good decision-
making involves risk identification and assessment,
information processing, and problem solving.
Height above airport (HAA). The height of the MDA above
the published airport elevation.
Height above landing (HAL). The height above a designated
helicopter landing area used for helicopter instrument
approach procedures.
Height above touchdown elevation (HAT). The DA/DH or
MDA above the highest runway elevation in the touchdown
zone (first 3,000 feet of the runway).
HF. High frequency.
Hg. Abbreviation for mercury, from the Latin hydrargyrum.
High performance aircraft. An aircraft with an engine of
more than 200 horsepower.
Histotoxic hypoxia. The inability of cells to effectively use
oxygen. Plenty of oxygen is being transported to the cells
that need it, but they are unable to use it.
HIWAS. See Hazardous Inflight Weather Advisory Service.
Holding. A predetermined maneuver that keeps aircraft
within a specified airspace while awaiting further clearance
from ATC.
Holding pattern. A racetrack pattern, involving two turns
and two legs, used to keep an aircraft within a prescribed
airspace with respect to a geographic fix. A standard pattern
uses right turns; nonstandard patterns use left turns.
Homing. Flying the aircraft on any heading required to keep
the needle pointing to the 0° relative bearing position.
Horizontal situation indicator (HSI). A flight navigation
instrument that combines the heading indicator with a CDI,
in order to provide the pilot with better situational awareness
of location with respect to the courseline.
Horsepower. The term, originated by inventor James Watt,
means the amount of work a horse could do in one second.
One horsepower equals 550 foot-pounds per second, or
33,000 foot-pounds per minute.
Hot start. In gas turbine engines, a start which occurs with
normal engine rotation, but exhaust temperature exceeds
prescribed limits. This is usually caused by an excessively
rich mixture in the combustor. The fuel to the engine must
be terminated immediately to prevent engine damage.
HSI. See horizontal situation indicator.
HUD. See head-up display.
G-15
Human factors. A multidisciplinary field encompassing the
behavioral and social sciences, engineering, and physiology,
to consider the variables that influence individual and
crew performance for the purpose of optimizing human
performance and reducing errors.
Hung start. In gas turbine engines, a condition of normal
light off but with rpm remaining at some low value rather than
increasing to the normal idle rpm. This is often the result of
insufficient power to the engine from the starter. In the event
of a hung start, the engine should be shut down.
Hydroplaning. A condition that exists when landing on a
surface with standing water deeper than the tread depth of
the tires. When the brakes are applied, there is a possibility
that the brake will lock up and the tire will ride on the
surface of the water, much like a water ski. When the tires
are hydroplaning, directional control and braking action
are virtually impossible. An effective anti-skid system can
minimize the effects of hydroplaning.
Hypemic hypoxia. A type of hypoxia that is a result of
oxygen deficiency in the blood, rather than a lack of inhaled
oxygen. It can be caused by a variety of factors. Hypemic
means “not enough blood.”
Hyperventilation. Occurs when an individual is experiencing
emotional stress, fright, or pain, and the breathing rate and
depth increase, although the carbon dioxide level in the
blood is already at a reduced level. The result is an excessive
loss of carbon dioxide from the body, which can lead to
unconsciousness due to the respiratory system’s overriding
mechanism to regain control of breathing.
Hypoxia. A state of oxygen deficiency in the body sufficient
to impair functions of the brain and other organs.
Hypoxic hypoxia. This type of hypoxia is a result of
insufficient oxygen available to the lungs. A decrease of
oxygen molecules at sufficient pressure can lead to hypoxic
hypoxia.
I
IAF. See initial approach fix.
IAP. See instrument approach procedures.
IAS. See indicated airspeed.
ICAO. See International Civil Aviation Organization.
Ident. Air Traffic Control request for a pilot to push
the button on the transponder to identify return on the
controller’s scope.
IFR. See instrument flight rules.
ILS. See instrument landing system.
ILS categories. Categories of instrument approach
procedures allowed at airports equipped with the following
types of instrument landing systems:
ILS Category I: Provides for approach to a height
above touchdown of not less than 200 feet, and with
runway visual range of not less than 1,800 feet.
ILS Category II: Provides for approach to a height
above touchdown of not less than 100 feet and with
runway visual range of not less than 1,200 feet.
ILS Category IIIA: Provides for approach without
a decision height minimum and with runway visual
range of not less than 700 feet.
ILS Category IIIB: Provides for approach without
a decision height minimum and with runway visual
range of not less than 150 feet.
ILS Category IIIC: Provides for approach without a
decision height minimum and without runway visual
range minimum.
IMC. See instrument meteorological conditions.
Inclinometer. An instrument consisting of a curved glass
tube, housing a glass ball, and damped with a fluid similar
to kerosene. It may be used to indicate inclination, as a level,
or, as used in the turn indicators, to show the relationship
between gravity and centrifugal force in a turn.
Indicated airspeed (IAS). Shown on the dial of the
instrument airspeed indicator on an aircraft. Indicated
airspeed (IAS) is the airspeed indicator reading uncorrected
for instrument, position, and other errors. Indicated airspeed
means the speed of an aircraft as shown on its pitot static
airspeed indicator calibrated to reflect standard atmosphere
adiabatic compressible flow at sea level uncorrected for
airspeed system errors. Calibrated airspeed (CAS) is IAS
corrected for instrument errors, position error (due to
incorrect pressure at the static port) and installation errors.
Indicated altitude. The altitude read directly from the
altimeter (uncorrected) when it is set to the current altimeter
setting.