Your airplane flight and altitude.

[Parvaz]

A common question I hear from passengers while flying is “How high
are we?” It seems like an easy question to answer. The questioner,
looking out the window, wants to know how high above the scenery they
are. In aviation that altitude is referred to height AGL; Above
Ground Level. If you are in level flight and you are flying 1,000’
over a high mountain peak you are at 1,000’ AGL (above ground level).
Let’s move forward in time a little bit; your jet has remained in
level flight but is now over the Mediterranean Sea. You have not
changed altitude at all, but now how high are you? Looking out the
window down to the waves, far below, you are certainly not at 1,000’-
what changed?

When we speak about a mountain peak, Mont Blanc on the Italian French
border for example, we say that it is 4,808 meters or 15,774 feet
high. But what is that height number referenced from? Is Mont Blanc
4,808 meters above the Post Office, House of Parliament, what? Sea
level is the reference point. Mont Blanc is 4,808 meters (15,774’)
above sea level.

A baseline level of the ocean (sea) is used. This reference point is
called MSL; Mean Sea Level. Mean sea level (MSL) is average height of
the sea for all stages of the tide over a 19-year period, usually
determined from hourly height readings. Most of the time your pilots
are using the aircraft’s height above sea level as the reference
point. When air traffic control assigns an altitude to fly, it is a
MSL altitude they are assigning. But how do we measure the altitude
the jet is above this level of the sea, when the nearest ocean may be
over a thousand miles away?

The altimeter is the primary instrument your pilots use to determine
altitude. At thousands of airports each day hourly weather
measurements are taken. The pressure of the air is among the weather
items measured, that information is processed into what is called an
altimeter setting. An “altimeter setting” is the same numbers you
sometimes hear on the evening news weather report. For example the
commentator may say something like “The barometric pressure is rising
(the trend) at 29.98 HG (1015.24 Mb)”. Your pilot takes the official
altimeter setting and sets those numbers in the altimeter by turning a
mechanical knob. The altimeter then displays an altitude above sea
level.

For example envision a fictional airport located up on a plateau
4,000’ above sea level. Your airplane is parked at the jetway. The
local altimeter setting is 29.98. When 29.98 is set in the
altimeter(s) the altimeter(s) will read 4,000’ MSL. The jet is at the
airport and the airport is 4,000’ feet above sea level so the
altimeter correctly reads 4,000’. Looking out your window you are
certainly not 4,000’ above the ramp.

Once you takeoff and are on your way air traffic control assigns an
altitude of 10,000’. Your jet climbs to and levels off at 10,000’
above sea level (MSL). However you are flying over a plateau that is
4,000’ above sea level. So you are 6,000’ above the plateau below,
looking out your window you will be 6,000’ AGL. At the same time the
jet is at 10,000’ MSL and 6,000’ AGL. If your jet flies over the
ocean it will be at 10,000’ MSL and 10,000’ AGL.

As we continue our flight the jet is assigned a climb to 35,000’
MSL. At and above 18,000’ altitudes are referred to as “flight
levels”. Consequently 35,000’ MSL becomes FL350; “flight level three-
five-zero”. And above FL180 (18,000’ MSL) a standard four digit
number is entered as the altimeter setting. That keeps the potential
confusion down and keeps pilots from constantly having to find and set
new altimeter settings as the jet moves along at better than 8 miles a
minute.

Radar altitude is a height above the surface below that is generated
by an instrument installed in some aircraft. This instrument emits a
signal directed below the aircraft that measures the distance from the
aircraft to the terrain below. Its range is usually less than 3,000’
and is used during some instrument approaches when the visibility at
the airport is poor due to weather. The number generated by the radar
altimeter is the same as AGL. However radar altitude is not used
during the enroute portion of your flight.

You should have several more terms to add to your knowledge base and
repertoire regarding your commercial flight. MSL; mean sea level, it
is the height measured above the ocean. MSL is the most common type
of altitude used while in flight. AGL; above ground level, is the
distance above the underlying terrain that the airplane is flying
over. This distance changes with every change in the altitude of the
underlying terrain. Radar altitude is a measurement taken from the
aircraft to the terrain directly below the aircraft. Radar altitude
is not used during the enroute portion of your flight (commercial
aviation). It is used during some instrument approaches by some
crews. Flight levels are altitudes at and above 18,000 MSL, and are
spoken by individual numbers. FL180 is correctly pronounced as flight
level one-eight-zero.

So the next time you are in flight looking out the window and the
pilot comes over the public address system and tells you that you are
flying at 35,000 thousand feet you will know that the airplane is at
FL350 (flight level three-five-zero) and 35,000’ feet above sea level
and probably not 35,000’ above the terrain below. That height would
be AGL. If you are told that the mountain peak is at 14,000’, and you
are at FL350 then you know that you are 21,000’ above the peak.

[pavas]

An interesting and surprising fact is that if the world was the size
of an apple, the altitudes we fly at would not take us higher than the
skin of that apple.