DATUM An imaginary vertical plane or line from which all measurements of moment arm are taken. The datum is established by the manufacturer.

DECIDE MODEL Model developed to help pilots remember the six-step decision-making process: Detect, Estimate, Choose, Identify, Do, Evaluate.

DECK ANGLE The angle of the cart’s lower frame (from the front wheel to the rear wheels), to the landing surface.

DENSITY ALTITUDE Pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the PPC’s performance.

DEPARTURE LEG The leg of the rectangular traffic pattern that is a straight course aligned with, and leading from, the takeoff runway.

DESIGNATED PILOT EXAMINER (DPE) An individual designated by the FAA to administer practical tests to pilot applicants.

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 pendulum design is the primary contributor to directional stability, causing a PPC in flight to align with the relative wind.

DOWNWIND LEG Leg of the traffic pattern flown parallel to the landing runway, but in a direction opposite to the intended landing direction.

DPE See DESIGNATED PILOT EXAMINER.

DRAG An aerodynamic force on a body acting parallel and opposite to the relative wind. The resistance of the atmosphere to the relative motion of an aircraft. Drag opposes thrust and limits the speed of the aircraft.

DRAG COEFFICIENT (Cd) A dimensionless number used to define the amount of total drag produced by an aircraft.

DRIFT CORRECTION Correction that is applied to counter the affects of wind on an aircraft’s flight and ground track.

DUAL FLIGHT Flight time that is received and logged as training time. Dual flight time must be endorsed by a Certificated Flight Instructor.

DYNAMIC PRESSURE (q) The pressure a moving fluid would have if it were stopped. Reference 14 CFR 61.51(h).

EIS See ENGINE INFORMATION SYSTEM.

E-LSA (EXPERIMENTAL LIGHT-SPORT AIRCRAFT) An aircraft issued an experimental certificate under Title 14 of the Code of Federal Regulations (14 CFR) part 21.

EMERGENCY FREQUENCY Frequency that is used by aircraft in distress to gain ATC assistance. 121.5 MHz is an international emergency frequency guarded by Flight Service Stations and some military and civil aircraft. Reference AIM paragraph 6-3-1.

ERROR CHAIN A series of mistakes that may lead to an accident or incident. Two basic principles generally associated with the creation of an error chain are: (1) one bad decision often leads to another; and (2) as a string of bad decisions grows, it reduces the number of subsequent alternatives for continued safe flight. Aeronautical decision making is intended to break the error chain before it can cause an accident or incident.