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.  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 N$_{3}$ high pressure compressor, all of  which rotate on separate shafts at different speeds.  Timing.  The application of muscular coordination at the  proper instant to make flight, and all maneuvers incident  thereto, a constant smooth process.  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.  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.  Troposphere.  The layer of the atmosphere extending from  the surface to a height of 20,000 to 60,000 feet depending  on latitude.  Torque meter.  An indicator used on some large reciprocating  engines or on turboprop engines to indicate the amount of  torque the engine is producing.  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.  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.  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.  Total drag.  The sum of the parasite and induced drag.  Touchdown zone lights.  Two rows of transverse light bars  disposed symmetrically about the runway centerline in the  runway touchdown zone.  T-tail.  An aircraft with the horizontal stabilizer mounted on  the top of the vertical stabilizer, forming a T.  Track.  The actual path made over the ground in flight.  Turbine blades.  The portion of the turbine assembly that  absorbs the energy of the expanding gases and converts it  into rotational energy.  Turbine outlet temperature (TOT).  The temperature of  the gases as they exit the turbine section.  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.  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.  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.  Turbocharger.  An air compressor driven by exhaust gases,  which increases the pressure of the air going into the engine  through the carburetor or fuel injection system.  G-17