Pivotal altitude.  A specific altitude at which, when an  airplane turns at a given groundspeed, a projecting of the  sighting reference line to a selected point on the ground will  appear to pivot on that point.  Pneumatic systems.  The power system in an aircraft used  for operating such items as landing gear, brakes, and wing  flaps with compressed air as the operating fluid.  Porpoising.  Oscillating around the lateral axis of the aircraft  during landing.  Position lights.  Lights on an aircraft consisting of a red light  on the left wing, a green light on the right wing, and a white  light on the tail. CFRs require that these lights be displayed  in flight from sunset to sunrise.  Power distribution bus.  See bus bar.  Positive static stability.  The initial tendency to return to a  state of equilibrium when disturbed from that state.  Power lever.  The cockpit lever connected to the fuel control  unit for scheduling fuel flow to the combustion chambers of  a turbine engine.  Power.  Implies work rate or units of work per unit of time,  and as such, it is a function of the speed at which the force is  developed. The term “power required” is generally associated  with reciprocating engines.  Powerplant.  A complete engine and propeller combination  with accessories.  Practical slip limit.  The maximum slip an aircraft is capable  of performing due to rudder travel limits.  Precession.  The tilting or turning of a gyro in response  to deflective forces causing slow drifting and erroneous  indications in gyroscopic instruments.  Preignition.  Ignition occurring in the cylinder before the time  of normal ignition. Preignition is often caused by a local hot  spot in the combustion chamber igniting the fuel/air mixture.  Pressure altitude.  The altitude indicated when the altimeter  setting window (barometric scale) is adjusted to 29.92. This  is the altitude above the standard datum plane, which is a  theoretical plane where air pressure (corrected to 15 ºC)  equals 29.92 "Hg. Pressure altitude is used to compute density  altitude, true altitude, true airspeed, and other performance  data.  Profile drag.  The total of the skin friction drag and form  drag for a two-dimensional airfoil section.  Propeller blade angle.  The angle between the propeller  chord and the propeller plane of rotation.  Propeller lever.  The control on a free power turbine  turboprop that controls propeller speed and the selection for  propeller feathering.  Propeller slipstream.  The volume of air accelerated behind  a propeller producing thrust.  Propeller synchronization.  A condition in which all of the  propellers have their pitch automatically adjusted to maintain  a constant rpm among all of the engines of a multiengine  aircraft.  Propeller.  A device for propelling an aircraft that,  when rotated, produces by its action on the air, a thrust  approximately perpendicular to its plane of rotation. It  includes the control components normally supplied by its  manufacturer.  Ramp weight.  The total weight of the aircraft while on the  ramp. It differs from takeoff weight by the weight of the  fuel that will be consumed in taxiing to the point of takeoff.  Rate of turn.  The rate in degrees/second of a turn.  Reciprocating engine.  An engine that converts the heat  energy from burning fuel into the reciprocating movement of  the pistons. This movement is converted into a rotary motion  by the connecting rods and crankshaft.  R Reduction gear.  The gear arrangement in an aircraft engine  that allows the engine to turn at a faster speed than the  propeller.  Region of reverse command.  Flight regime in which flight  at a higher airspeed requires a lower power setting and a  lower airspeed requires a higher power setting in order to  maintain altitude.  Relative wind.  The direction of the airflow with respect to  the wing. If a wing moves forward horizontally, the relative  wind moves backward horizontally. Relative wind is parallel  to and opposite the flightpath of the airplane.  Registration certificate.  A State and Federal certificate that  documents aircraft ownership.  G-13