Datum plane: A plane from which angular or linear measurements are reckoned. Also called reference plane.

Design gross weight: The gross weight at take-off that an aircraft, rocket, etc., is expected to have, used in design calculations.

Dropsonde: A radiosonde equipped with a parachute, dropped from an aircraft to transmit measurements of atmospheric conditions as it descends.

Drogue: A funnel-shaped part at the end of the hose of a tanker aircraft, used in air refuelling to drag the hose out and stabilise it and to receive the probe of the receiving aircraft.

Earth Axis: Any one of a set of mutually perpendicular reference axes established with the upright axis (the Z-axis) pointing to the centre of the earth, used in describing the position or performance of an aircraft or other body in flight.

Ejection capsule: In an aircraft or manned spacecraft, a detachable compartment serving as a cockpit or cabin, which may be ejected as a unit and parachuted to the ground.

Flight attitude: The attitude of an aircraft, rocket, etc., in flight; specifically, the attitude of an aircraft with respect to the relative wind.

Flight Mach number: A free-stream Mach number measured in flight as distinguished from one measured in a wind tunnel.

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