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 GrGLYPH<df00>GLYPH<df00>k lGLYPH<df00>ttGLYPH<df00>r GLYPH<df00>xprGLYPH<df00>ssGLYPH<df00>d by thGLYPH<df00> symbol Δ to indicatGLYPH<df00> a changGLYPH<df00> of valuGLYPH<df00>s. As an GLYPH<df00>xamplGLYPH<df00>, ΔCG indicatGLYPH<df00>s a changGLYPH<df00> (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. 

