Specific fuel consumption. Number of pounds of fuel consumed in 1 hour to produce 1 HP.
Speed brakes. A control system that extends from the airplane structure into the airstream to produce drag and slow the airplane.
Speed instability. A condition in the region of reverse command where a disturbance that causes the airspeed to decrease causes total
drag to increase, which in turn, causes the airspeed to decrease further.
Speed sense. The ability to sense instantly and react to any reasonable variation of airspeed.
Speed. The distance traveled in a given time.
Spin. An aggravated stall that results in what is termed an “autorotation” wherein the airplane follows a downward corkscrew path. As
the airplane rotates around the vertical axis, the rising wing is less stalled than the descending wing creating a rolling, yawing, and
pitching motion.
Spiral instability. A condition that exists when the static directional stability of the airplane is very strong as compared to the effect of
its dihedral in maintaining lateral equilibrium.
Spiraling slipstream. The slipstream of a propeller-driven airplane rotates around the airplane. This slipstream strikes the left side of
the vertical fin, causing the airplane to yaw slightly. Vertical stabilizer offset is sometimes used by aircraft designers to counteract this
tendency.
Split shaft turbine engine. See free power turbine engine.
Spoilers. High-drag devices that can be raised into the air flowing over an airfoil, reducing lift and increasing drag. Spoilers are used
for roll control on some aircraft. Deploying spoilers on both wings at the same time allows the aircraft to descend without gaining
speed. Spoilers are also used to shorten the ground roll after landing.
Spool. A shaft in a turbine engine which drives one or more compressors with the power derived from one or more turbines.
Stabilator. A single-piece horizontal tail surface on an airplane that pivots around a central hinge point. A stabilator serves the
purposes of both the horizontal stabilizer and the elevator.
Stability. The inherent quality of an airplane to correct for conditions that may disturb its equilibrium, and to return or to continue on
the original flightpath. It is primarily an airplane design characteristic.
Stabilized approach. A landing approach in which the pilot establishes and maintains a constant angle glidepath towards a
predetermined point on the landing runway. It is based on the pilot’s judgment of certain visual cues, and depends on the maintenance
of a constant final descent airspeed and configuration.
Stall strips. A spoiler attached to the inboard leading edge of some wings to cause the center section of the wing to stall before the
tips. This assures lateral control throughout the stall.
Stall. A rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical angle of
attack. A stall can occur at any pitch attitude or airspeed.
Standard atmosphere. At sea level, the standard atmosphere consists of a barometric pressure of 29.92 inches of mercury ("Hg) or
1013.2 millibars, and a temperature of 15 °C (59 °F). Pressure and temperature normally decrease as altitude increases. The s tandard
lapse rate in the lower atmosphere for each 1,000 feet of altitude is approximately 1 "Hg and 2 °C (3.5 °F). For example, the standard
pressure and temperature at 3,000 feet mean sea level (MSL) is 26.92 "Hg (29.92 – 3) and 9 °C (15 °C – 6 °C).
Standard day. See standard atmosphere.
Standard empty weight (GAMA). This weight consists of the airframe, engines, and all items of operating equipment that have fixed
locations and are permanently installed in the airplane; including fixed ballast, hydraulic fluid, unusable fuel, and full engine oil.
Standard weights. These have been established for numerous items involved in weight and balance computations. These weights
should not be used if actual weights are available.
Standard-rate turn. A turn at the rate of 3º per second which enables the airplane to complete a 360º turn in 2 minutes.
Starter/generator. A combined unit used on turbine engines. The device acts as a starter for rotating the engine, and after running,
internal circuits are shifted to convert the device into a generator.
Static stability. The initial tendency an aircraft displays when disturbed from a state of equilibrium.
Station. A location in the airplane that is identified by a number designating its distance in inches from the datum. The datum is,
therefore, identified as station zero. An item located at station +50 would have an arm of 50 inches.
Stick puller. A device that applies aft pressure on the control column when the airplane is approaching the maximum operating speed.
Stick pusher. A device that applies an abrupt and large forward force on the control column when the airplane is nearing an angle of
attack where a stall could occur.
Stick shaker. An artificial stall warning device that vibrates the control column.
Stress risers. A scratch, groove, rivet hole, forging defect, or other structural discontinuity that causes a concentration of stress.
Subsonic. Speed below the speed of sound.
Supercharger. An engine- or exhaust-driven air compressor used to provide additional pressure to the induction air so the engine can
produce additional power.
Supersonic. Speed above the speed of sound.
Supplemental Type Certificate (STC). A certificate authorizing an alteration to an airframe, engine, or component that has been
granted an approved type certificate.
Swept-wing. A wing planform in which the tips of the wing are farther back than the wing root.
Tailwheel aircraft. See conventional landing gear.
Takeoff roll (ground roll). The total distance required for an aircraft to become airborne.
Target reverser. A thrust reverser in a jet engine in which clamshell doors swivel from the stowed position at the engine tailpipe to
block all of the outflow and redirect some component of the thrust forward.
Taxiway lights. Omnidirectional lights that outline the edges of the taxiway and are blue in color.
Taxiway turnoff lights. Flush lights which emit a steady green color.
Tetrahedron. A large, triangular-shaped, kite-like object installed near the runway. Tetrahedrons are mounted on a pivot and are free
to swing with the wind to show the pilot the direction of the wind as an aid in takeoffs and landings.
Throttle. The valve in a carburetor or fuel control unit that determines the amount of fuel-air mixture that is fed to the engine.
Thrust line. An imaginary line passing through the center of the propeller hub, perpendicular to the plane of the propeller rotation.
Thrust reversers. Devices which redirect the flow of jet exhaust to reverse the direction of thrust.
Thrust. The force which imparts a change in the velocity of a mass. This force is measured in pounds but has no element of time or
rate. The term, thrust required, is generally associated with jet engines. A forward force which propels the airplane through the air.
Timing. The application of muscular coordination at the proper instant to make flight, and all maneuvers incident thereto, a constant
smooth process.
Tire cord. Woven metal wire laminated into the tire to provide extra strength. A tire showing any cord must be replaced prior to any
further flight.
Torque meter. An indicator used on some large reciprocating engines or on turboprop engines to indicate the amount of torque the
engine is producing.
Torque sensor. See torque meter.
Torque. 1. A resistance to turning or twisting. 2. Forces that produce a twisting or rotating motion. 3. In an airplane, the tendency of
the aircraft to turn (roll) in the opposite direction of rotation of the engine and propeller.
Total drag. The sum of the parasite and induced drag.
Total energy error. An energy error where the total amount of mechanical energy is not correct. The airplane has too much or too
little total energy relative to the intended altitude-speed profile. When this error occurs, the pilot will observe that altitude and airspeed
deviate in the same direction (e.g., higher and faster than desired; or lower and slower than desired). An example would be an airplane
on final approach that is above the desired glide slope and at a faster airspeed than desired.
Total mechanical energy. Sum of the energy in altitude (potential energy) and the energy in airspeed (kinetic energy).
Touchdown zone lights. Two rows of transverse light bars disposed symmetrically about the runway centerline in the runway
touchdown zone.
Track. The actual path made over the ground in flight.
Trailing edge. The portion of the airfoil where the airflow over the upper surface rejoins the lower surface airflow.
Transition liner. The portion of the combustor that directs the gases into the turbine plenum.
Transonic. At the speed of sound.
Transponder. The airborne portion of the secondary surveillance radar system. The transponder emits a reply when queried by a radar
facility.
Tricycle gear. Landing gear employing a third wheel located on the nose of the aircraft.
Trim tab. A small auxiliary hinged portion of a movable control surface that can be adjusted during flight to a position resulting in a
balance of control forces.
Triple spool engine. Usually a turbofan engine design where the fan is the N 1 compressor, followed by the N 2 intermediate
compressor, and the N3 high pressure compressor, all of which rotate on separate shafts at different speeds.
Tropopause. The boundary layer between the troposphere and the mesosphere which acts as a lid to confine most of the water vapor,
and the associated weather, to the troposphere.
Troposphere. The layer of the atmosphere extending from the surface to a height of 20,000 to 60,000 feet depending on latitude.
True airspeed (TAS). Calibrated airspeed corrected for altitude and nonstandard temperature. Because air density decreases with an
increase in altitude, an airplane 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 calibrated airspeed, true airspeed increases as altitude increases; or for a given true
airspeed, calibrated airspeed decreases as altitude increases.
True altitude. The vertical distance of the airplane above sea level—the actual altitude. It is often expressed as feet above mean sea
level (MSL). Airport, terrain, and obstacle elevations on aeronautical charts are true altitudes.
T-tail. An aircraft with the horizontal stabilizer mounted on the top of the vertical stabilizer, forming a T.
Turbine blades. The portion of th e turbine assembly th at absorbs th e energy of th e expanding gases and converts it into rotational
energy.
Turbine outlet temperature (TOT). The temperature of the gases as they exit the turbine section.
Turbine plenum. The portion of the combustor where the gases are collected to be evenly distributed to the turbine blades.
Turbine rotors. The portion of the turbine assembly that mounts to the shaft and holds the turbine blades in place.
Turbine section. The section of the engine that converts high pressure high temperature gas into rotational energy.
Turbocharger. An air compressor driven by exhaus t gases, which increas es the pressure of the air going into th e engine through the
carburetor or fuel injection system.
Turbofan engine. A turbojet engine in which additional propulsive thrust is gained by extending a portion of the compressor or
turbine blades outside the inner engine case. The extended blades propel bypass air along the engine axis but between the inner and
outer casing. The air is not combusted but does provide additional thrust.
Turbojet engine. A j et engine incorporating a turbine-driven air compressor to tak e in and compress air for th e combustion of fuel,
the gases of combustion being used both to rotate the turbine and create a thrust producing jet.
Turboprop eng ine. A turbin e engin e th at driv es a propeller through a reduction gearing arrangemen t. Mos t of th e energy in the
exhaust gases is converted into torque, rather than its acceleration being used to propel the aircraft.
Turbulence. An occurrence in which a flow of fluid is unsteady.
Turn coordinator. A rate gyro th at senses both ro ll and yaw du e to th e gimbal being canted. Has largely replaced th e turn-and-slip
indicator in modern aircraft.
Turn-and-slip indicator. A flight instrument consisting of a rate gyro to indicate the rate of yaw and a curved glass inclinometer to
indicate the relationship between gravity and centrifugal force. The turn-and-slip indicator indicates the relationship between angle of
bank and rate of yaw. Also called a turn-and-bank indicator.
Turning error. One of the errors inherent in a magnetic compass caused by the dip compensating weight. It shows up only on turns to
or from northerly headings in the Northern Hemisphere and southerly headings in the Southern Hemisphere. Turning error causes the
compass to lead turns to the north or south and lag turns away from the north or south.
Ultimate load factor. In stress analysis, the load that causes physical breakdown in an aircraft or aircraft component during a strength
test, or the load that according to computations, should cause such a breakdown.
Unfeathering accumulator. Tanks that hold oil under pressure which can be used to unfeather a propeller.
UNICOM. A nongovernment air/ground radio communication station which may provid e airport information at public use airports
where there is no tower or FSS.
Unusable fuel. Fuel that cannot be consumed by the engine. This fuel is considered part of the empty weight of the aircraft.
Useful loa d. Th e weigh t of th e pilo t, copilo t, passengers, baggage, usab le fuel, and drainab le o il. I t is th e b asic empty weight
subtracted from the maximum allowable gross weight. This term applies to general aviation aircraft only.
Utility category. An airplane that has a seating configuration, excluding pilo t seats, of nin e or less, a maximum certificated takeoff
weight of 12,500 pounds or less, and intended for limited acrobatic operation.
V1. Critical engine failure speed or takeoff decision speed. It is the speed at which the pilot is to continue the takeoff in the event of an
engine failure or other serious emergency. At speeds less than V 1, it is considered safer to stop the aircraft within the accelerate-stop
distance. It is also the minimum speed in the takeoff, following a failure of the critical engine at V EF, at which the pilot can continue
the takeoff and achieve the required height above the takeoff surface within the takeoff distance.
V2. takeoff safety speed, or a referenced airspeed obtained after lift-off at which the required one engine-inoperative climb
performance can be achieved.
VA. The design maneuvering speed. This is the “rough air” speed and the maximum speed for abrupt maneuvers. If during flight,
rough air or severe turbulence is encountered, reduce the airspeed to maneuvering speed or less to minimize stress on the airplane
structure. It is important to consider weight when referencing this speed. For example, V A may be 100 knots when an airplane is
heavily loaded, but only 90 knots when the load is light.
Vapor lock. A condition in which air enters the fuel system and it may be difficult, or impossible, to restart the engine. Vapor lock
may occur as a result of running a fuel tank completely dry, allowing air to enter the fuel system. On fuel-injected engines, the fuel
may become so hot it vaporizes in the fuel line, not allowing fuel to reach the cylinders.
V-bars. The flight director displays on the attitude indicator that provide control guidance to the pilot.
Vector. A force vector is a graphic representation of a force and shows both the magnitude and direction of the force.
Velocity. The speed or rate of movement in a certain direction.
Vertical axis. An imaginary line passing vertically through the center of gravity of an aircraf t. The vertical axis is called the z-axis or
the yaw axis.
Vertical card compass. A magnetic compass that consists of an azimuth on a vertical card, resembling a heading indicator with a
fixed miniature airplane to accurately present the heading of the aircraft. The design uses eddy current damping to minimize lead and
lag during turns.
Vertical speed indicator (VSI). An instrument that uses static pressure to display a rate of climb or descen t in feet per minute. The
VSI can also sometimes be called a vertical velocity indicator (VVI).
Vertical stability. Stability about an aircraft’s vertical axis. Also called yawing or directional stability.
VFE. The maximum speed with the flaps extended. The upper limit of the white arc.
VFO. The maximum speed that the flaps can be extended or retracted.
VFR Terminal Area Charts (1:250,000). Depict Class B airspace which provides for the control or segregation of all the aircraft
within the Class B airspace. The chart depicts topographic information and aeronautical information which includes visual and radio
aids to navigation, airports, controlled airspace, restricted areas, obstructions, and related data.
V-G diagram. A chart that relates velocity to load factor. It is valid only for a specific weight, configuration, and altitude and shows
the maximum amount of positive or negative lift the airplane is capable of generating at a given speed. Also shows the safe load factor
limits and the load factor that the aircraft can sustain at various speeds.
Visual approa ch slo pe indicato r (VASI). The mos t common visu al glidepath system in use. Th e VA SI provid es obstruction
clearance within 10° of the extended runway centerline, and to 4 nautical miles (NM) from the runway threshold.
Visual Flight Rules (VFR). Code of Federal Regulations that govern the procedures for conducting flight under visual conditions.
VLE. Landing gear extended speed. The maximum speed at which an airplane can be safely flown with the landing gear extended.
VLO. Landing gear operating speed. The maximum speed for extending or retracting the landing gear if using an airplane equipped
with retractable landing gear.
VLOF. Lift-off speed. The speed at which the aircraft departs the runway during takeoff.
VMC. Minimum control airspeed. This is the minimum flight speed at which a twin-engine airplane can be satisfactorily controlled
when an engine suddenly becomes inoperative and the remaining engine is at takeoff power.
VMD. Minimum drag speed.
VMO. Maximum operating speed expressed in knots.
VNE. Never-exceed speed. Operating above this speed is prohibited since it may result in damage or structural failure. The red line on
the airspeed indicator.
VNO. Maximum structural cruising speed. Do not exceed this speed except in smooth air. The upper limit of the green arc.
VP. Minimum dynamic hydroplaning speed. The minimum speed required to start dynamic hydroplaning.
VR. Rotation speed. The speed that the pilot begins rotating the aircraft prior to lift-off.
VS0. Stalling speed or the minimum steady flight speed in the landing configuration. In small airplanes, this is the power-off stall
speed at the maximum landing weight in the landing configuration (gear and flaps down). The lower limit of the white arc.
VS1. Stalling speed or the minimum steady flight speed obtained in a specified configuration. For most airplanes, this is the power-off
stall speed at the maximum takeoff weight in the clean configuration (gear up, if retractable, and flaps up). The lower limit of the green
arc.
V-speeds. Designated speeds for a specific flight condition.
VSSE. Safe, intentional one-engine inoperative speed. The minimum speed to intentionally render the critical engine inoperative.
V-tail. A design which utilizes two slanted tail surfaces to perform th e same functions as the surfaces of a conventional elevator an d
rudder configuration. The fixed surfaces act as both horizontal and vertical stabilizers.
VX. Best angle-of-climb speed. The airspeed at which an airplane gains the greatest amount of altitude in a given distance. It is used
during a short-field takeoff to clear an obstacle.
VXSE. Best angle of climb speed with one engine inoperative. The airspeed at which an airplane gains the greatest amount of altitude
in a given distance in a light, twin-engine airplane following an engine failure.
VY. Best rate-of-climb speed. This airspeed provides the most altitude gain in a given period of time.
VYSE. Best rate-of-climb speed with one engine inoperative. This airspeed provides the most altitude gain in a given period of time in a
light, twin-engine airplane following an engine failure.
Wake turbulence. Wingtip vortices that are created when an airplane generates lift. When an airplane generates lift, air spills over the
wingtips from the high pressure areas below the wings to the low pressure areas above them. This flow causes rapidly rotating
whirlpools of air called wingtip vortices or wake turbulence.
Waste gate. A controllab le valv e in th e tailpip e of an aircraf t reciprocating engin e equipped with a turbocharger. Th e valv e is
controlled to vary the amount of exhaust gases forced through the turbocharger turbine.
Weathervane. The tendency of the aircraft to turn into the relative wind.
Weight and balance. The aircraft is said to b e in weigh t and balance when th e gross weigh t of th e aircraft is under th e max gross
weight, and the center of gravity is within limits and will remain in limits for the duration of the flight.
Weight. A measure of the heaviness of an object. The force by which a body is attracted toward the center of the earth (or another
celestial body) by gravity. Weight is equal to the mass of the body times the local value of gravitational acceleration. One of the four
main forces acting on an aircraft. Equivalent to the actual weight of the aircraft. It acts downward through the aircraft’s center of
gravity toward the center of the earth. Weight opposes lift.
Wheelbarrowing. A condition caused when forward yoke or stick pressure during takeoff or landing causes the aircraft to ride on the
nose-wheel alone.
Wind correction angle. Correction applied to the course to establish a heading so that track will coincide with course.
Wind directio n indicato rs. Indicators th at includ e a wind sock, wind tee, or tetrahedron. Visu al reference w ill determin e wind
direction and runway in use.
Wind shear. A sudden, drastic shift in wind speed, direction, or both that may occur in the horizontal or vertical plane.
Windmilling. When the air moving through a propeller creates the rotational energy.
Windsock. A truncated cloth cone open at both ends and mounted on a freewheeling pivot that indicates the direction from which the
wind is blowing.
Wing area. The total surface of th e wing (squar e feet), which includ es control surfaces and may includ e wing area covered by the
fuselage (main body of the airplane), and engine nacelles.
Wing span. The maximum distance from wingtip to wingtip.
Wing twist. A design feature incorporated into some wings to improve aileron control effectiveness at high angles of attack during an
approach to a stall.
Wing. Airfoil attached to each side of the fuselage and are the main lifting surfaces that support the airplane in flight.
Wingtip vortices. The rapidly rotating air that spills over an airplane’s wings during flight. The intensity of the turbulence depends on
the airplane’s weight, speed, and configuration. It is also referred to as wake turbulence. Vortices from heavy aircraft may be
extremely hazardous to small aircraft.
Yaw string. A string on the nose or windshield of an aircraft in view of the pilot that indicates any slipping or skidding of the aircraft.
Yaw. Rotation about the vertical axis of an aircraft.
Zero fuel weight. The weight of the aircraft to include all useful load except fuel.
Zero sideslip. A maneuver in a twin-engine airplane with one engine inoperative that involves a small amount of bank and slightly
uncoordinated flight to align the fuselage with the direction of travel and minimize drag.
Zero thrust (simulated feather). An engine configuration with a low power setting that simulates a propeller feathered condition.