pilots must keep track of a lot of data when they are in the cockpit of an airplane for a specific
flight configuration be it landing or economy cruising a plane speed must remain within a fairly
narrow range of values if it flies too slowly it can suffer an aerodynamic stall when there is an
insufficient lift to overcome the downward force of gravity if it flies too fast it can suffer
structural damage such as the loss of flaps since the forged speed of an airplane is equal to the
speed of the air flowing past it engineers started to look for the device to accurately measure this
initially meteorological instruments like the robinson cup anemometer were used but the
problem is that they have to be placed far from the cockpit before it can be read by the pilot
the pressure plate provided another method but still have to be placed outside the cockpit and
can only be readable at a distance from the pilot this solved a problem by creating another imagine
sitting in a plane and your pilot walks away from the cockpit during landing to measure airspeed the
huddle to solve this problem went on for years tests to use spit-up tubes to measure air speed
started in 1902 and in 1903 the accuracy of the device was determined and confirmed
using wind tunnels the pitot tube did not only give accurate measurements but it also solved
the distant reading problem because the head could be placed well away from the cockpit
and the readout could be placed in front of the pilot the principle was simple as the air speed
increases the pressure in the pitta tube increases and the difference between this pressure and the
static pressure outside the tube is the air speed to understand how the pitot static system works
we will talk about the system one instrument at a time we will clarify what the instrument displays
and how the pitot static system provides the measurements that are used for the instruments
do you ever wonder how pilots judge how high they are in the air and how altimeters use a pitastatic
system to calculate altitude an altimeter on board an aircraft is basically just a pressure
gauge even on modern aircraft like the 787 dreamliner and the a350s the altimeters are
just measuring the outside pressure and this technology is pretty much the same as the ones
airplanes back in the 50s 60s and 70s hat so as a pilot we obviously need some sense of how high we
are above the ground when we're flying and we get this information primarily from the altimeter but
you'll find in ground school that the altimeter is somewhat of a fickle instrument so fitted in
most general aviation aircraft is what's known as a sensitive altimeter and this is just an
aneroid barometer that measures the absolute pressure of ambient air and displays it in terms
of feet or meters above a selected pressure level so what you're looking at in the panel is really a
barometer but one that we've carefully painted with units of feet rather than some unit of
pressure we need to be careful here not to think of the altimeter as an altitude measuring device
but rather a pressure measuring device that we can use to approximate our altitude above sea level
so as we ascend in the atmosphere the ambient air pressure understandably decreases but
the rate at which that pressure changes this is known as the pressure lapse rate
can vary significantly depending on the weather condition during that day so when they first began
designing altimeters they needed some idealized atmosphere something with the well-defined lapse
rate that would more or less provide an accurate altitude read on the instrument
and what they came up with is now known as the icao international standard atmosphere and this
goes hand in hand with the international standard day that you're probably also familiar with
and in the standard atmosphere we have a pressure of 29.92 inches of mercury at sea level with a
temperature of 15 degrees celsius just like on a standard day and as we move up in the atmosphere
we see the pressure decrease one inch of mercury for every thousand feet and this will remain true
until we get to about five thousand feet so in a standard atmosphere one thousand feet above
sea level would have an ambient air pressure of 28.92 inches of mercury note that this is one inch
lower than it was at sea level but of course we'll never be flying on a truly standard day
and to account for this our altimeters can tune using the kohlman window and of course
we do this before every flight and periodically as we fly along and and what is happening when
you enter the altimeter setting is we're actually shifting the entire standard atmosphere vertically
you can think of this as a simple linear offset if you want so this means that 29.92 inches of
mercury is no longer the pressure at sea level but has been translated up or down
note that this has no effect on the lapse rate of the instrument however it simply shifts the
pressure level that we start lapsing from but when you think about it this is a fairly limited
method of calibrating the instrument if you can imagine the infinite variations of pressure we
can encounter as we move through the atmosphere surely you can see how the altimeter errors can
begin to compound you may have heard the mnemonic from high to low lookout below which i would say
is debatably effective but let's look exactly how the altimeter can deviate from our true altitude
if you remember back to high school chemistry at some point you probably encountered the ideal
gas law which shows the relationship between the volume temperature and pressure of an ideal gas
and it demonstrates that as a gas is cooled its pressure increases and it becomes more
dense whereas a gas that is warmed will expand and become less dense now put
yourself in the left seat of an airplane and visualize a fixed column of air beneath you
you're flying the airplane at a constant indicated altitude which means the static port is feeling a
constant pressure if that column of air below you is cooled it will contract and to keep the
pressure the same you will inadvertently descend the aircraft closer to the ground
note that the reverse is also true if you heat the column of air it will expand and
you will inadvertently climb to keep the altimeter needle in the same spot
so if the altimeter is prone to such errors and with no way of correcting the instrument in flight
why do we still use this pressure based system well the purpose of the altimeter is twofold first
it gives the pilot an approximate indication of his or her altitude in order to avoid terrain and
obstacles but secondly it provides a standardized system for us to coordinate with other aircraft
and maintain vertical separation so despite the errors induced by non-standard temperature and
pressure we can be assured that all other aircrafts will be affected in the same way
these errors lessen our ability to find our absolute altitude
but remain very effective at indicating our altitude relative to other aircraft in the area
and do know that these pressure differences are fairly minuscule except for flying in very cold
temperatures under moderate conditions you should be able to maintain an altitude above the ground
that is far greater than any error induced by non-standard temperature pressure on the altimeter
the icio has published a standard cold temperature correction table if you're curious about the
magnitude of this error so like all instruments in the cockpit the altimeter is not without its
limitations but as safe pilots we must understand these limitations to use the tool safely and
effectively relying on pressure reading alone to calculate can be dangerous since the atmosphere is
not completely uniform all over the globe because we have areas of high mountains and deep valleys
these areas have different pressure reading that can trick the altimeter to think the aircraft is
higher above the ground than it truly is because altimeters are essentially a pressure gauge it
also means that it does not really know where we are to start with the only thing it knows is that
the pressure is decreasing or increasing and it uses this reading to calculate altitude so pilots
use something called the qnh at the airport of departure since the atmospheric pressure changes
over time and from place to place you might have high pressure at your airport of departure
and low pressure on the airport of destination so pilots have to listen to the air traffic control
for the local q h value anytime an aircraft gets cleared to an altitude they will be getting the
local q and h let's say the q and h is 1 0 9 hector pascal what pilots do is to give that
value to the altimeter then the system will use the local atmospheric pressure as a reference to
calculate the correct altitude of the aircraft above sea level unh basically gives us the
pressure setting to know how high we are above the sea level with reference to the airport we are at
now why is this interesting to know is because the q and h values are different
between different airports so we must use the correct pressure setting at the airport we are
because otherwise our altimeters are not going to show where we are it will simply use the pressure
difference to calculate altitude if for example you are at jfk airport with qnh of 1015 and then
you wait for a few hours and the pressure drops by 5 hectopascals the altimeters will assume that
the airplane climbed 150 feet higher while the airplane is actually on the ground all because
the qnh dropped by 5 hectare pascals this is why pilots continuously updates the pressure setting
when either departing from an airport or descending into an airport as you might have
understood by now that since the pressure value differs between different airports and we have a
lot of aircraft taking off from these different airports flying to their various destinations
for example let's say you take off from jfk then you are flying towards california you are going
to meet other aircrafts that are flying from other airports where the pressure was different
pilots would know their vertical separation with these other aircrafts because they took off from
a different airport with a completely different q and h value and altitude reading this can easily
lead to a mid-air collision so the way that has been solved is that by making use of a standard
pressure value at transition altitude which is qnh 1013.25 hector pascal to all airplanes
flying about the transition altitude will set the same pressure because airplanes fly so high
that how high they are above the ground doesn't matter only the vertical separation with
other aircraft matters so that means that all over the world when you hear a flight level or
transitional attitude it means that all of those aircraft are flying at the standard
pressure setting of 1013 hector pascal to make sure that everyone they meet have at
least the vertical separation distance that they needed for safe flight you can support us now on
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