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172 GLOSSARY
Distance Measuring Equipment (DME): An instrument used in navigation which measures
distance using the delay between the sending and receiving of a radio signal. Aircraft use this to
determine their distance from a fixed NAVAID.
Drag: The aerodynamic force (created by a fluid such as air flowing around an object) that slows
the object’s motion.
Electronic Flight Instrument System (EFIS): A flight instrument system (found in an aircraft’s
panel) with electronic displays rather than the mechanical gauges of a standard panel.
Go Around (GA): An autopilot mode that raises the nose in a wings-level attitude and calls
for lots of power in order to get back to altitude after a botched landing approach).
Glideslope (G/S): The angle at which an aircraft approaches (or needs to approach) a runway;
often used when discussing navigation by instruments. See Chapter 7 for more information.
Global Positioning System (GPS): A form of navigation using data from satellites.
Heading (HDG): An aircraft’s heading is the direction that its nose is pointing. This is also a
mode in the autopilot that lets the pilot hold a pre-defined heading, typically magnetic. A magnetic
heading is heading to the magnetic north pole, something a hair different than true north, which
is a geographic heading that will take one to the true geographic North Pole. Remember, since
the magnetic north pole is separated from the geographic north pole by a bit, true and magnetic
heading are not typically the same! They may be off by 5 or 10 degrees in the medium latitudes.
The difference between the true and magnetic north poles is called the magnetic variation.
Hold (HLD): Pressing this button will engage the autopilot in altitude hold mode. See the
Autopilots section of Chapter 7 for more information.
Horizontal Situation Indicator (HSI): This instrument is found in the panel of many aircraft
in X-Plane. It serves the same function as an OBI—that is, it indicates course deviation. See Chapter
7 for more information.
Instrument Flight Rules (IFR): The procedure for flying an aircraft based solely on the
craft’s instrument panel. Environmental conditions requiring such flight (such as the poor visibility
on a rainy day) are referred to as IFR conditions. This is contrasted with VFR conditions (those
operating under visual flight rules). In bad weather or above 18,000 feet, pilots need to fly by
Instrument Flight Rules, following their instruments and air traffic control instructions carefully
to avoid hitting the ground or other planes, or going off course and messing up the carefully laid
plans of the air traffic controller. When flying IFR, it really makes no difference whether the pilot
can see out the front of the plane or not, since he or is on a carefully mapped procedure to stay on
a safe course. Seeing out the window in this case is an unneeded luxury.
Instrument Landing System (ILS): A ground-based system for guiding approaching aircraft
into the runway via radio signals. See the Chapter 7 for more information.