Glossary
Quick definitions for common weather, navigation, IFR, and proficiency terms used throughout the site.
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Showing all 228 terms.
1-2-3 Rule
The common Part 91 alternate-filing rule that generally requires no alternate if, from one hour before to one hour after ETA, the destination forecast is at least 2,000-foot ceilings and 3 statute miles visibility.
3P Model
A decision-making model meaning Perceive, Process, and Perform, used to recognize change and respond before risk escalates.
Absolute Altitude
The aircraft’s actual height above the terrain directly below it, commonly expressed as AGL.
ADF
Automatic Direction Finder. Aircraft equipment that points to an NDB station and is used for NDB navigation and approaches.
Adiabatic Lapse Rate
The rate at which a parcel of air changes temperature as it rises or descends without exchanging heat with its surroundings.
Advection Fog
Fog formed when moist air moves over a colder surface and cools to its dew point.
AGL
Above Ground Level. Altitude measured vertically from the surface directly below the aircraft.
Aircraft Category
The approach-speed category, such as Category A through E, used to determine which instrument approach minima and circling protections apply to an aircraft.
AIRMET
An advisory for weather that may affect all aircraft but is especially significant to light aircraft, including hazards such as icing, turbulence, and IFR conditions.
AIRMET Zulu
An icing advisory that forecasts areas of moderate icing and provides freezing-level information.
Altimeter Setting
The pressure value placed in the altimeter’s Kollsman window so indicated altitude reflects approximate altitude above mean sea level.
ANDS
A mnemonic for Northern Hemisphere compass acceleration errors: Accelerate North, Decelerate South on east or west headings.
Angle of Attack
The angle between the wing chord line and the relative wind; a primary factor controlling lift and stall behavior.
APR Mode
Approach mode on an autopilot or flight director. It arms or captures published approach lateral guidance and, when available and properly configured, the associated glideslope or glidepath.
APV
Approach with Vertical Guidance. An instrument approach that provides lateral and approved vertical guidance to a decision altitude but is not classified as a precision approach.
ARTCC
Air Route Traffic Control Center. An en route ATC facility that manages IFR aircraft between terminal areas across a large region.
ASOS/AWOS
Automated weather observing systems that report conditions such as wind, visibility, temperature, altimeter setting, and sometimes density altitude.
ASR
Airport Surveillance Radar approach. A nonprecision radar approach with azimuth guidance from ATC but no vertical guidance.
ATIS
Automatic Terminal Information Service. A recorded airport weather and operational information broadcast used before arrival or departure.
Attenuation
A radar limitation in which strong precipitation weakens the radar energy enough to hide or understate weather behind the strongest return.
Automation Management
The disciplined use and monitoring of autopilot, flight director, and avionics modes so automation reduces workload instead of causing mode confusion.
AVEF
A memory aid for lost-communications route selection under IFR: Assigned, Vectored, Expected, Filed.
Bernoulli's Principle
A lift theory concept stating that increased airflow speed is associated with lower pressure, often discussed alongside angle of attack and Newtonian explanations of lift.
Boundary Layer
The lowest part of the atmosphere directly influenced by the surface, also called the planetary boundary layer.
Braking Action
A report or assessment of runway braking effectiveness, especially important for IFR arrivals when runway contamination or recent precipitation reduces stopping margin.
Calibrated Altitude
Indicated altitude corrected for instrument and position error.
CAPE
Convective Available Potential Energy. A measure of how much buoyant energy is available for rising air parcels and therefore how supportive the atmosphere may be for convection.
CAT
Clear-Air Turbulence. Turbulence that occurs outside visible convective clouds, often near the tropopause or jet stream.
CDI
Course Deviation Indicator. A navigation display showing how far the aircraft is from the selected course.
CDI Scaling
The sensitivity of the course deviation indicator as the navigation phase changes, such as en route, terminal, and approach, making the same needle movement represent smaller lateral error near the runway.
Ceiling
The height above the earth’s surface of the lowest cloud layer reported as broken, overcast, or obscured.
Center Weather Advisory (CWA)
A short-term advisory issued by a Center Weather Service Unit to highlight regional aviation weather hazards that may affect IFR operations near an ARTCC area.
CIA
Cross-check, Interpret, Adjust. The three-step fundamental loop for instrument flight described in the FAA Instrument Flying Handbook. Cross-check is the disciplined scan of the instruments; Interpret is reading what each instrument is telling you; Adjust is making the correct control or power input in response. The cycle repeats continuously throughout every phase of instrument flight.
CIN
Convective Inhibition. The energy barrier that must be overcome before a parcel can rise freely into convection, often described as the atmospheric cap strength.
Circling Approach
An instrument approach maneuver used to position the aircraft visually for landing on a runway not aligned with the straight-in final approach course.
Cirrostratus
A high, thin veil-like cloud layer made largely of ice crystals that often produces halos and may signal an approaching frontal system.
Clear Ice
A smooth, dense ice accretion formed when larger supercooled droplets spread before freezing.
Clearance Limit
The point to which an aircraft is granted an ATC clearance, often the destination airport but sometimes a fix or holding point instead.
Climb Gradient
The rate of climb required over distance, usually stated in feet per nautical mile, to satisfy obstacle clearance or published IFR departure requirements. Convert to fpm using: ROC (fpm) = Gradient (ft/nm) × GS (kt) ÷ 60.
Compass Correction Card
A placard near the magnetic compass listing the corrected headings to steer because of aircraft-specific deviation errors.
Compass Deviation
Compass error caused by magnetic influences within the aircraft, corrected by compensation and noted on the compass correction card.
Conditional Instability
An atmospheric state that is stable for unsaturated air but unstable after the air becomes saturated.
Cone of Confusion
An area directly over a VOR where signal geometry becomes unreliable and CDI or flag indications may fluctuate.
Contact Approach
A pilot-requested IFR approach clearance that allows descent and navigation visually clear of clouds with at least 1 statute mile flight visibility.
Coriolis Force
The apparent force caused by Earth’s rotation that deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
CRAFT
A common IFR clearance-copy framework meaning Clearance limit, Route, Altitude, Frequency, and Transponder.
Crossing Restriction
An altitude or speed requirement that must be met at a specified fix, commonly used on STARs, approaches, and ATC-assigned terminal routing.
Crosswind Component
The portion of the wind acting at a right angle to the runway or aircraft path.
DA
Decision Altitude. The altitude on a precision or vertically guided approach where the pilot must decide to continue visually or execute the missed approach.
DALR
Dry Adiabatic Lapse Rate. The rate at which an unsaturated parcel cools as it rises, about 3°C per 1,000 feet.
DECIDE Model
A structured aeronautical decision-making model used to detect problems, estimate their significance, choose an outcome, identify actions, do the action, and evaluate the result.
Decision Height (DH)
The height above touchdown zone elevation used as the decision point on certain approaches, operationally paired with DA when the chart or system references height instead of altitude.
Delay Vectors
ATC vectors requested or issued to create time and space for sequencing, approach briefing, reprogramming, or workload management before the next phase of flight.
Density Altitude
Pressure altitude corrected for non-standard temperature; the altitude the aircraft effectively experiences for performance purposes.
Direct Entry
A holding entry in which the aircraft crosses the fix and turns directly into the normal racetrack pattern on the holding side.
Direct-To
A navigator function that creates direct routing to a selected fix, often bypassing intermediate legs or procedure structure if used carelessly during IFR operations.
DME
Distance Measuring Equipment. A radio-navigation system that gives slant-range distance to a ground station and is often used to identify fixes, step-downs, or the missed approach point.
Drag
The aerodynamic force opposing an aircraft’s motion through the air.
EFC
Expected Further Clearance time. The time ATC expects to issue additional clearance, especially important during holding and lost-communications planning.
ELR
Environmental Lapse Rate. The actual rate at which surrounding atmospheric temperature changes with altitude.
Exosphere
The outermost atmospheric layer where particles are extremely sparse and the atmosphere gradually transitions into space.
FIKI
Flight Into Known Icing. Refers to aircraft certified and equipped for operation in known icing conditions within approved limits.
Final Approach Fix (FAF)
The fix where the final approach segment begins and where descent on the final segment typically starts on an instrument approach.
Flight Level
A pressure-altitude-based altitude reference used above the transition altitude with the altimeter set to 29.92 inches of mercury.
Fly-by Waypoint
A waypoint the RNAV system anticipates and turns before crossing so the aircraft can roll smoothly onto the next leg.
Fly-over Waypoint
A waypoint the RNAV system requires the aircraft to cross before initiating the next turn or leg.
FMA
Flight Mode Annunciator. The avionics display area that shows which lateral, vertical, and automation modes are actually active or armed.
Fog
A surface-based visibility restriction caused by suspended water droplets, often reducing visibility below VFR or approach minima.
Freezing Drizzle
Small supercooled droplets that freeze on contact, often creating rapid structural icing.
Freezing Level
The altitude where the atmospheric temperature reaches 0°C, often used to assess icing risk and precipitation type changes.
Freezing Rain
Rain that falls through shallow subfreezing air and freezes on impact, creating a severe large-droplet icing hazard.
G-AIRMET
Graphical AIRMET. A time-and-space depiction of advisory aviation hazards such as icing, turbulence, IFR conditions, and mountain obscuration.
GBAS
Ground Based Augmentation System. An airport-based correction system that supports GLS precision-like approaches.
Geostrophic Balance
A condition aloft where the pressure-gradient force and Coriolis force balance so wind flows nearly parallel to isobars.
Glidepath
The computed vertical descent path on procedures such as LPV or LNAV/VNAV. It serves a similar operational role to a glideslope but is generated differently and should be briefed according to the minima line being used.
Glideslope
The vertical guidance component of an ILS that provides a descent path to the runway.
GLS
GBAS Landing System. A precision-like approach using GBAS-corrected satellite guidance for lateral and vertical path information.
GPSS
GPS Steering. A roll-steering output that lets the autopilot follow navigator turn anticipation and leg geometry more smoothly than basic heading or course intercept logic alone.
GRABCARD
Memory aid for the IFR minimum equipment required by 14 CFR § 91.205(d): Generator/alternator, Radios (comm and nav), Altimeter (sensitive, adjustable), Ball (slip-skid indicator), Clock (hours, minutes, seconds), Attitude indicator (gyroscopic), Rate of turn indicator, Directional gyro (heading indicator). All eight must be installed and operational before flight under IFR.
Graphical Forecasts for Aviation (GFA)
A broad-area aviation weather planning product that displays forecast clouds, hazards, visibility, winds, freezing levels, and related route-scale weather information.
GTG
Graphical Turbulence Guidance. A model-derived aviation turbulence product used to estimate the location and intensity of forecast turbulence layers.
GUMPS
A pre-landing checklist mnemonic: Gas (fuel selector on fullest tank), Undercarriage (gear down), Mixture (rich), Propeller (set for landing), Switches/Safety (as required). Commonly used in complex aircraft as a final approach flow check.
Heading Indicator
A gyroscopic heading instrument that provides stable short-term heading information and is periodically aligned to the magnetic compass.
Hearback
The controller side of the readback-hearback loop, where ATC listens to a pilot readback to confirm the clearance or instruction was understood correctly.
Hearback Loop
The communication-check process in which a pilot reads back an ATC instruction and the controller listens to confirm that it was understood correctly.
HILPT
Hold in Lieu of Procedure Turn. A published holding pattern used as part of an instrument approach course-reversal procedure.
Holding Clearance
An ATC clearance that specifies the holding fix, course, direction, leg length if needed, altitude, and often an expected further clearance time.
Holding Pattern
A racetrack-shaped IFR maneuver used to delay an aircraft or position it within protected airspace while awaiting further clearance.
HSI
Horizontal Situation Indicator. A navigation display combining heading information with course guidance.
IA
Indicated Altitude. The altitude shown on the altimeter when it is set to the current local altimeter setting.
Ice Pellets
Frozen precipitation, also called sleet, that indicates a layered temperature structure with melting aloft and refreezing below, often a warning sign for icing risk nearby.
IFR
Instrument Flight Rules. Regulations and procedures for operating primarily by reference to instruments, typically in reduced visibility or cloud.
ILS
Instrument Landing System. A precision approach that provides localizer lateral guidance and glideslope vertical guidance.
IMSAFE
A personal readiness checklist covering Illness, Medication, Stress, Alcohol, Fatigue, and Emotion/Eating.
Indicated Altitude
The altitude read directly from the altimeter with the current local altimeter setting applied.
Initial Approach Fix (IAF)
The fix where an instrument approach begins from the en route or arrival environment and where a published transition may join the procedure.
Intermediate Fix (IF)
A fix between the initial and final approach segments that helps organize the aircraft onto the final approach path.
Inversion
A layer in the atmosphere where temperature increases with altitude instead of decreasing, creating strong stability.
ISA
International Standard Atmosphere. The standard temperature and pressure model used for altimetry and performance reference.
LAMP
Localized Aviation MOS Program. The NOAA-supported aviation MOS guidance referenced in FAA weather material, providing hourly airport-specific forecast guidance.
Lapse Rate
The rate at which temperature changes with altitude.
LCL
Lifting Condensation Level. The altitude where a rising air parcel becomes saturated and cloud formation begins.
Lift
The aerodynamic force that acts perpendicular to the relative wind and opposes weight in flight.
LLWS
Low-Level Wind Shear. A sharp change in wind speed or direction close to the surface, especially important during takeoff, approach, and go-around.
Localizer
A lateral guidance signal aligned with an ILS runway, or a localizer-only approach when used without glideslope guidance.
Localizer Back Course
A nonprecision approach that uses the back side of a localizer signal for lateral guidance. It requires careful course-sense and avionics-mode briefing because the display and autopilot logic can differ by installation.
LP
Localizer Performance minima on an RNAV approach that provide enhanced lateral guidance but no approved vertical guidance.
LPV
Localizer Performance with Vertical Guidance. A WAAS-based RNAV approach providing precise lateral guidance and an approved vertical path to a DA.
MAA
Maximum Authorized Altitude. The highest altitude at which adequate navigation signal coverage is assured for a published airway segment or route structure.
Magnetic Anomaly
A localized disturbance in the earth's magnetic field caused by underground iron or volcanic mineral deposits, which can cause the magnetic compass to deflect unexpectedly while flying overhead. Significant anomalies may be noted on aeronautical charts.
Magnetic Compass
A direction-sensing instrument that aligns with the earth's magnetic field and indicates magnetic heading.
Magnetic Dip
The downward slant of the earth's magnetic field toward the magnetic poles, responsible for compass turning and acceleration errors.
Magnetic Variation
The angular difference between true north and magnetic north at a given location, used to convert true directions to magnetic directions.
MALR
Moist Adiabatic Lapse Rate. The rate at which a saturated air parcel cools as it rises, less than the dry adiabatic lapse rate because latent heat is released.
MCA
Minimum Crossing Altitude. The lowest altitude at which an aircraft may cross a specified fix while still meeting obstacle-clearance and signal-reception requirements beyond that fix.
MDA
Minimum Descent Altitude. The lowest altitude to which descent is authorized on a nonprecision approach unless required visual references are in sight.
MEA
Minimum Enroute Altitude. The lowest published altitude between fixes that provides both acceptable navigation signal coverage and required obstacle clearance along the route segment.
METAR
A routine aviation weather observation reporting current conditions such as wind, visibility, ceiling, temperature, dew point, and altimeter setting at an airport.
MIA
Minimum IFR Altitude. The lowest altitude authorized by ATC in a specific area when vectoring or providing IFR service, based on obstacle and terrain clearance criteria.
Missed Approach Hold
The published holding pattern or holding fix associated with a missed approach, where the aircraft stabilizes while awaiting further clearance or re-briefing the next plan.
Missed Approach Point (MAP)
The point on an instrument approach where the pilot must execute the missed approach if the required visual references are not in sight and landing cannot be continued safely.
Mist
A visibility restriction caused by very small water droplets, reported in METARs as BR and usually less severe than fog.
Mixed Ice
An ice accretion combining the roughness of rime ice with the density of clear ice.
MOCA
Minimum Obstruction Clearance Altitude. The lowest published altitude between fixes that provides obstacle clearance and guarantees navigation signal coverage only within 22 NM of a VOR in the normal service volume.
Mode Confusion
A cockpit error in which the pilot believes the autopilot or flight director is in one mode while the system is actually flying another.
MOS
Model Output Statistics. An automated forecast product that refines raw weather model output with local statistical relationships to estimate airport weather such as ceiling, visibility, and wind.
Mountain Obscuration
A condition in which terrain is hidden by clouds, precipitation, fog, or other visibility restrictions, creating a significant IFR terrain hazard.
MRA
Minimum Reception Altitude. The lowest altitude at which an aircraft can receive off-course navigation guidance needed to identify a specific fix.
MSA
Minimum Safe Altitude. A charted emergency altitude, usually within 25 NM of a facility or waypoint, that provides at least 1,000 feet of obstacle clearance (2,000 feet in designated mountainous areas).
MVA
Minimum Vectoring Altitude. The lowest altitude at which ATC may vector IFR aircraft in a radar environment while maintaining required obstacle clearance.
N-DRUMS
Acronym for the six mandatory IFR reports a pilot must make without being asked (AIM 5-3-3): Non-radar reporting points (when not in radar contact), Deviations from any ATC clearance, Requests for report (when ATC has specified one), Unforecast weather, Malfunctions of navigational/approach/communication equipment, and Safety of flight risks. D, U, M, and S apply in all environments; N applies outside radar coverage.
National Airspace System
The integrated U.S. aviation system of airspace, airports, navaids, routes, ATC facilities, procedures, and rules that supports both VFR and IFR operations.
NDB
Non-Directional Beacon. A ground-based radio aid used with an ADF for bearing guidance and older nonprecision approaches.
Nonprecision Approach
An instrument approach that provides lateral guidance only and normally uses an MDA rather than a decision altitude.
NTSB Part 830
49 CFR Part 830. NTSB rules for aircraft accident and specified incident notification/reporting, including events that require immediate notification under § 830.5 and potential written reporting under § 830.15.
OBS Mode
A GPS navigator mode that suspends normal waypoint sequencing and lets the pilot select and hold a desired inbound or outbound course to a fix.
Obstacle Departure Procedure (ODP)
A published IFR departure procedure designed primarily to provide terrain and obstacle clearance after takeoff.
ODP
Obstacle Departure Procedure. A published IFR departure procedure focused on terrain and obstacle clearance after takeoff, especially when ATC vectors or a SID are not expected immediately.
Operational Risk Management
A structured process for identifying hazards, assessing their seriousness, reducing exposure, and monitoring risk through the flight.
OROCA
Off-Route Obstruction Clearance Altitude. A grid-based IFR planning altitude shown on en route charts that provides obstacle clearance but does not guarantee navigation signal coverage or ATC surveillance.
Outflow Boundary
The leading edge of cooler air spreading outward from thunderstorms or strong showers, often producing wind shifts, turbulence, and new convective development away from the main core.
PAR
Precision Approach Radar. A controller-talkdown approach providing both azimuth and elevation guidance to decision altitude.
Parallel Entry
A holding entry in which the aircraft initially flies outbound on the non-holding side roughly parallel to the inbound course before turning back to intercept inbound.
PAVE
A preflight risk-assessment model covering Pilot, Aircraft, enVironment, and External pressures.
Personal Minimums
A pilot’s self-imposed operational limits for weather, runway, aircraft, and readiness that are more conservative than regulatory minimums.
PGF
Pressure-Gradient Force. The force that pushes air from higher pressure toward lower pressure and starts wind moving.
PIREP
Pilot Report. An in-flight report of actual weather such as turbulence, icing, cloud tops, or visibility.
Planetary Boundary Layer
The same as the boundary layer: the lowest part of the atmosphere directly influenced by the earth’s surface.
Preferred Route
A routing published or commonly used by ATC to organize traffic flow efficiently between certain airports or airspace regions.
Pressure Altitude
Altitude above the standard datum plane, found by setting the altimeter to 29.92 inches of mercury or by calculation from indicated altitude and altimeter setting.
Procedure Turn
A published course-reversal maneuver used to align the aircraft with the inbound approach course while remaining inside protected airspace. It is flown only when the chart and clearance require or permit it.
Prog Chart
A prognostic chart showing the forecast position of fronts, pressure systems, precipitation areas, and other broad weather features over time.
Q-Route
A high-altitude RNAV route used in the IFR system to provide structured point-to-point routing above the low-altitude airway environment.
QNH
The altimeter setting adjusted to mean sea level pressure so the altimeter indicates approximate altitude above mean sea level.
Quadrantal Deviation
A component of compass deviation caused by soft-iron induced magnetism whose magnitude varies with the aircraft's orientation relative to the earth's field. It peaks on intercardinal headings and is corrected during compass swinging.
Radiation Fog
Fog that forms at night as the ground cools by radiation and chills the surface air to its dew point.
Rate of Climb (ROC)
The vertical speed of an aircraft climbing, measured in feet per minute. To find the required ROC for a published climb gradient: ROC (fpm) = Gradient (ft/nm) × Ground Speed (kt) ÷ 60.
Rate of Descent (ROD)
The vertical speed of an aircraft descending, measured in feet per minute. For a standard 3° descent path, the rule of thumb is: ROD (fpm) ≈ Ground Speed (kt) × 5.
Readback
A pilot’s spoken repetition of an ATC clearance or instruction so errors can be detected before they become flight-path or surface-movement errors.
Receiver Autonomous Integrity Monitoring (RAIM)
A GPS integrity-monitoring function that verifies the satellite solution is reliable enough for the current phase of non-WAAS IFR navigation or approach use.
Release
An ATC authorization allowing an IFR departure to be launched, often used at non-towered airports or when a release window or traffic constraint applies.
Rime Ice
A rough, opaque ice deposit formed when small supercooled droplets freeze quickly on impact.
RMI
Radio Magnetic Indicator. A bearing display that points toward a selected radio navigation source relative to aircraft heading.
RNP AR
Required Navigation Performance Authorization Required. A specialized RNAV procedure needing specific aircraft, crew, and operator approval.
Runway Environment
The runway or runway-related visual cues required for continuing below DA or MDA, such as threshold, lights, markings, or other approved visual references.
RVR
Runway Visual Range. A measured value, typically in feet (for example, RVR 2400), describing how far a pilot can see down the runway surface environment.
RVR Reporting Points
The touchdown, midpoint, and rollout runway segments where separate RVR values may be reported to describe visibility along different parts of the runway.
RVV
Runway Visual Value. A visibility value estimated by a human observer rather than automated RVR sensors, commonly seen at airports without full RVR equipment.
Scalloping
Small irregular CDI fluctuations on a VOR caused by signal reflections or terrain effects.
SDF
Simplified Directional Facility. A localizer-type system with wider course guidance than a localizer, used for some nonprecision approaches.
Semicircular Deviation
A component of compass deviation caused by hard-iron permanent magnetism in the airframe that produces a roughly constant error on a given heading direction. Addressed during compass swinging.
SID
Standard Instrument Departure. A published IFR departure procedure used to organize traffic and connect the departure airport to the en route structure.
SIGMET
A weather advisory for significant hazards such as severe icing, severe turbulence, dust storms, or thunderstorms.
Skew-T
A thermodynamic chart, also called a skew-T log-P, used to analyze temperature, dew point, winds, stability, cloud layers, and freezing-level structure through the atmosphere.
Stable Air
Air that resists vertical movement, often associated with layered clouds, smooth conditions, and widespread lower ceilings or reduced visibility.
Stall Speed
The minimum speed at which the wing can produce enough lift for the existing load factor and configuration before stalling.
Standard Alternate Minimums
The default legal minima used when filing an airport as an alternate unless nonstandard alternate minimums are published for that airport or procedure.
Standard Instrument Departure (SID)
A published IFR departure procedure used to organize traffic flow and connect the airport environment to the en route system.
Standard Rate Turn
A turn rate of 3 degrees per second, completing a full 360° in two minutes. Required bank angle increases with airspeed: approximately TAS (kt) ÷ 10 + 7 degrees. At a fixed bank angle, turn radius grows with speed and turn rate decreases; to maintain standard rate at higher speeds, more bank is required. Bank is limited to 30° in instrument procedures, so at high speeds the aircraft turns slightly slower than standard rate.
Standard Terminal Arrival Route (STAR)
A published IFR arrival procedure used to organize descent and traffic flow from the en route structure into terminal airspace.
STAR
Standard Terminal Arrival Route. A published IFR arrival procedure that connects the en route system to terminal airspace and organizes descent, routing, and often crossing restrictions.
Station Passage
The moment an aircraft passes over or abeam a VOR or similar facility, often indicated by CDI fluctuation and TO/FROM reversal.
Step-Down Fix
A fix on a nonprecision approach that authorizes descent to a lower published altitude after the fix is crossed.
Subsidence Inversion
An inversion formed as sinking air under a high-pressure system compresses and warms aloft.
Supercooled Liquid Water
Liquid water droplets that remain unfrozen below 0°C and can freeze rapidly on aircraft surfaces.
Suspend Mode
A navigator state that intentionally stops automatic waypoint sequencing until the pilot confirms the next leg or missed-approach transition.
T-Route
A low-altitude RNAV route used in the IFR system to provide structured routing without relying only on ground-based Victor airways.
TAF
Terminal Aerodrome Forecast. A coded forecast of expected airport weather conditions over a defined time period.
Tailplane Stall
A stall of the horizontal stabilizer, often associated with tail icing and sometimes triggered by flap extension or speed changes.
Takeoff Minimums
Weather or procedural limits considered before IFR departure; under Part 91 these are often more about obstacle clearance, aircraft performance, and operational judgment than a simple universal takeoff visibility number.
Task Saturation
A condition in which the number or timing of cockpit tasks exceeds the pilot's available mental capacity, increasing the risk of IFR errors and loss of prioritization.
Teardrop Entry
A holding entry in which the aircraft turns outbound at an offset heading on the holding side before returning inbound to the fix.
TEC Route
Tower En Route Control route. A commonly used IFR route structure for shorter trips, especially in busy terminal areas, that keeps aircraft within terminal control airspace rather than high-altitude center routing.
Thermosphere
The atmospheric layer above the mesosphere that includes the ionosphere and affects long-range radio propagation and satellite operations.
Threshold Crossing Height (TCH)
The charted height of the theoretical glidepath above the runway threshold, used to describe how an approach path crosses the threshold area.
Thrust
The forward-acting force produced by the propulsion system that opposes drag.
TOGA
Takeoff/Go-Around mode or switch. A control that commands the flight director or autopilot into a takeoff or missed-approach pitch mode, often with associated lateral-mode changes.
Top of Descent (TOD)
The point along the route where descent should begin to arrive at a lower altitude on a planned profile. For a 3° path the rule of thumb is: TOD distance (nm) = Altitude to Lose (ft) ÷ 300.
Top Strip
The top section of an instrument approach chart containing the procedure title, frequencies, and notes that often control equipment, legality, or operational limitations.
Touchdown Zone (TDZ)
The marked runway area just beyond the threshold where landing rollout is expected to begin and where many runway markings and lighting references are concentrated.
Touchdown Zone Elevation (TDZE)
The highest runway elevation in the touchdown zone, used on approach charts because runway surface elevation can vary noticeably from threshold to midpoint.
Transition
The charted or cleared segment that connects one IFR phase or procedure to another, such as an en route route to a STAR, or a SID to the filed route.
Tropopause
The boundary between the troposphere and stratosphere where lapse rate changes and jet-stream turbulence often concentrates.
Troposphere
The lowest major atmospheric layer where nearly all day-to-day aviation weather occurs.
True Altitude
The aircraft’s actual height above mean sea level.
UNOS
A mnemonic for Northern Hemisphere compass turning errors: Undershoot North, Overshoot South when rolling out from turns.
Unstable Air
Air that tends to keep rising once lifted, often associated with convective clouds, thermals, and turbulence.
Unusual Attitude
An aircraft attitude not normally required for instrument flight, such as a nose-high or nose-low upset. Recovery is based on attitude instrument interpretation, control priority, and avoiding overcontrol.
Upslope Fog
Fog formed when moist air is forced upslope and cools to its dew point.
Vectors-to-Final
A navigator setup or ATC join method that bypasses full transition sequencing and arms the final approach course directly from radar vectors.
VFR
Visual Flight Rules. Regulations and procedures for operating primarily by outside visual reference in weather conditions that meet required visibility and cloud-clearance criteria.
Victor Airway
A low-altitude airway in the U.S. defined primarily by VOR radials and used for IFR en route navigation.
Visual Approach
An IFR clearance to continue to an airport visually when weather and traffic conditions permit.
Void Time
A time included in some IFR departure clearances after which the clearance is no longer valid if the aircraft has not departed.
VOR
VHF Omnidirectional Range. A ground-based radio navigation system that provides magnetic bearing information to and from a station.
VOT
VOR Test Facility. A signal used to verify VOR receiver accuracy before IFR flight.
VV
Vertical Visibility. The measured vertical distance a pilot can see into an obscured sky, reported in METARs when ceiling layers are not defined.
WAAS
Wide Area Augmentation System. A satellite-based enhancement that improves GPS accuracy and integrity for aviation navigation and approaches.
Weight
The force of gravity acting on the aircraft, opposing lift in flight.
Wet Snow
Snowflakes that have partially melted, usually indicating temperatures near the freezing level.
Wind Shear
A sudden change in wind speed or direction over a short distance, either vertically or horizontally.