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G.4. OTHER AVIATION TERMS 171
G.4 Other Aviation Terms
Above Ground Level (AGL): When holding an altitude requested by air traffic control, a
pilot will hold an altitude AMSL (above mean sea level). This lets the pilot stay at a constant level
while flying. In order to avoid a horrific and instant death, however, pilots should be aware of their
altitude AGL (above ground level) as well! The altimeter in the aircraft works on air pressure, so it
measures the altitude above sea level, so awareness of minimum allowable altitudes in one’s region
is always needed in order to stay at least that high. The radio altimeter measures the height above
the ground (AGL). Most planes, however, do not have these installed. This is increasingly okay,
though, because in theory a pilot can follow the en route and approach charts, which list safe MSL
altitudes, and more and more planes have moving maps that clearly show the terrain elevation, so
pilots can be sure that their elevation is safe.
Airspeed indicator (ASI): The ASI is driven by the pressure of the air impacting a little tube
on the nose or wing of the plane. More pressure means the craft is moving faster. See the discussion
in the “Indicated airspeed (IAS)” entry below.
Air Traffic Control (ATC): The body governing aircraft operations in a given airspace.
Altitude: An aircraft’s altitude is its height above sea level. This is typically displayed on the
aircraft’s altimeter, which is driven by air pressure.
Automatic Direction-Finder (ADF): This is the old-style navigation device that just points
a needle at a transmitter on the ground. These are not used too often any more because modern
navigation involves staying on a pre-defined course (a line), not just taking any random routing to
get to a pre-defined point, like an ADF typically provides. Additionally, with GPS navigation, the
whole idea of going to pre-defined points (like picking up bread-crumbs to find one’s way home)
is thankfully disappearing. The GPS will take pilots all the way to where they want to go in a
straight line, not a zig-zaggy one like would be achieved in flying from one navigation transmitter
to another, wasting fuel with an indirect routing simply because of the locations people chose to
plant navigation transmitters fifty years ago.
Back Course (BC): This is the part of the ILS that goes beyond the touch-down zone. Read
all about it in Chapter 7, Navigation, Autopilots, and Flying on Instruments.
Course Deviation Indicator (CDI): This instrument (part of the OBI or HSI) displays which
direction the aircraft needs to turn in order to intercept the VOR course. This is discussed in
Chapter 7, Navigation, Autopilots, and Flying on Instruments.
Density altitude: As the temperature of the air increases, its density decreases. The barometric
pressure can vary based on a number of other factors, too, so at sea level on a hot, low-pressure
day, the density of the air may be the same as standard air density at 10,000 feet up in the air!
This is a 10,000 foot density altitude. This means there is less air for the engines, less air for the
propeller, and less air for the wings. All of this adds up to say that it will take the aircraft longer
to get off the ground.