YouTube Transcript:
Altimetría y Procedimientos de Ajuste Altimétrico

Today we will talk about the altimetry and altimeter setting procedures, these
are a set of definitions and procedures used to ensure adequate vertical separation
of the aircraft with respect to the ground and also with respect to other nearby aircraft
making use of the barometric altimeter which are the altimeters found in all
aircraft, these procedures take into account daily atmospheric pressure changes,
terrain characteristics, air traffic flow, and the ATS services provided as well as
local and regional procedures, however, before entering into this
matter. We are going to remember some things from the previous video about the altimeter, first let's remember
that the vertical distance depends on what reference  we are using, for example if we measure
the vertical distance between mean sea level and the aircraft in flight we will be measuring the altitude
if we measure the distance between the ground and the aircraft we will be measuring the height and if
we measure the distance between the ground and the mean sea level we will be measuring the elevation
it is also worth remembering how a barometric altimeter works this as its name
says it uses atmospheric pressure or static pressure to give its reading and it takes advantage of the
principle that says that at lower altitudes we have higher air pressure while as
we climb we find lower pressure, that is, if the plane detects a higher static pressure, it
means that we are flying at a lower altitude and as the aircraft starts
To climb in the atmosphere towards sectors of lower pressure, the altimeter will interpret this as
an increase in altitude, in other words an altimeter is a sensitive barometer
that interprets changes in static pressure as changes in altitude, now since this type of
altimeter uses pressure to give the altitude reading there must be a certain pressure at which
e The altimeter indicates 0 feet that is precisely what is known as the barometric reference or reference pressure
in essence this is the pressure level at which the altimeter will indicate 0 feet
, therefore it is the reference level with respect to which the altimeter will measure the altitude and give its reading
and as we had seen in the previous video about the altimeter, these are
calibrated in such a way that the default barometric reference is 29,92
inches of mercury since that is precisely the standard pressure at sea level
but since this pressure can vary the pilot can adjust another barometric reference in its
altimeter causing it to measure the altitude with respect to another pressure level, having said that
let´s see a small example, suppose that at this time the pressure at sea level is measured to be
29,72 inches of mercury, that is to say that this would be our QNH, which as we had seen
in the previous video is the pressure that identifies the mean sea level, that is, if the pilot
adjusts 29,72 as a barometric reference, he will be measuring the altitude with respect to sea level.
On the other hand, suppose that at this aerodrome, which is at a certain elevation with respect
to sea level, we measure a pressure of 27,92 inches of mercury, this would then be our QFE
that as we said in the previous video corresponds to the pressure measured at the aerodrome
this means that if the pilot adjusts 27,92 as his barometric reference he will be measuring
the height with respect to that level, now, apart from these two settings, there is a third that can be
used which is the standard setting, that is, the pressure level of 29.92 inches of mercury, which
in this particular case is below sea level this setting is known as the QNE
or the standard setting and if the pilot adjusts this in his altimeter he will be measuring something
called flight levels therefore taking this into account we have then three different barometric
references that can be used: The QNH which identifies the mean sea level
, that is, if we adjust this we would measure altitudes, the QFE that identifies the level of the aerodrome,
that is , we would be measuring heights and the QNE that identifies the standard pressure of 29 92 and
therefore we would be measuring flight levels, let's see now if in more detail what
the flight level is, the flight level or flight level is the vertical distance between
the isobar of 29 92 inches of mercury, that is, the standard and a point in the air as
we can see in this image in other words the flight level is the indication that the altimeter
gives us when adjusting the QNE, and something important to note is that the flight levels
are identified with the initials FL followed by three digits that represent hundreds of feet let's see
an example of this suppose that adjusting the QNE in our altimeter it indicates 3,000
feet according to the nomenclature this would be represented as flight level 030 or FL030
if the altimeter would indicate 15,000 feet it would be flight level 150, if it were 26,500
feet it would be flight level 265, 30,000 feet would be flight level 300 and so on,
having seen this, let's now go through some examples of situations that may arise, let's start
with the standard conditions, that is, ISA conditions, in this model the pressure at
mean sea level is 29 92 inches of mercury, that is, in this case the QNH and QNE will be the same
therefore the altitude and flight level reading will be the
same and having this setting if we are on the ground we can also read the elevation of the aerodrome
now suppose that in this aerodrome the QFE is 27 92 that is, the pressure of that aerodrome
if we adjust then this in our altimeter it will give us the height with respect to that level
now let's see what happens when we have low pressure conditions this means that at sea level the
pressure is less than 29 92 in this case it is 29 72 well then this would be our QNH
if we adjust it in our altimeter we will have the altitude and elevation reading
here therefore the QNE or standard pressure will be below sea level
which means that the flight level will be higher than the altitude and finally suppose that in
this case the QFE is 27 62 that is to say that if we adjust it in our altimeter we will have the
reading of height. In the opposite case when there is higher pressure this means that at sea
level we have a pressure higher than 29 92 in this case it will be 30 42 that is to say that this would be
our QNH when adjusting it then we would have the altitude and elevation reading, in this
particular case the QNE or standard setting is going to be above sea level
therefore in this case the flight level will be lower than the altitude and here the QFE suppose it
is 28 42 and gives us the height reading. As we can see here then the vertical position
of an aircraft in flight can be expressed in any of these terms depending on
the barometric reference that the pilot is using, therefore, to avoid confusion with the
air traffic service, certain procedures have been established to know what setting
should be used at some point. In essence these are the altimeter setting procedures
that we will see. First we must remember that the objectives of this procedures
are to ensure an adequate separation with the terrain/obstacles and with respect to other aircraft so
first let's look at the case of the separation between aircraft in this case to have an adequate
separation it is required that both aircraft use the same barometric reference let's see an example
let's say we have two aircraft that are flying in opposite directions in
this case let's say the QNH of the area is 30 06 inches of mercury so if both
aircraft use 30.06 as a barometric setting in their altimeter, both will be measuring the altitude
with respect to the same reference and therefore they will be able to have an adequate separation in
this case one is flying at 7000 and the other at 8000 so both have an
effective separation of 1000 feet now even if these aircraft are not using the current QNH as long
as they both use the same setting, the same separation will continue to be maintained for example
suppose that these two use the QNE in this area even though they are not using the QNH
of the area, they are both measuring their altitude with respect to the same reference
therefore they will have the same vertical separation between them that is to say that regardless of the
setting they are using as long as both use the same the separation will be adequate
otherwise, if one of them is using a different setting than the other, the actual separation
will be different from the apparent separation, which it is quite dangerous let's see this through
an example suppose we have the same case with a current QNH of 30.31 inches
of mercury and in this case the aircraft on the left is using the current QNH while
the aircraft on the right It is using the QNE, apparently if both aircraft report their
altitude, one would be maintaining 7000 and the other 8000, that is, they would be separated by a thousand
feet, however, since the altimeters are measuring the altitude with respect to
different references, the real separation is different in this particular case, the aircraft using
the QNE would be flying at 7,650 feet with respect to sea level, therefore in this
situation the real separation is much less than the apparent one since in this case it is 350 feet
this is the case of the separation between aircraft, let's now see the separation with respect to the ground
to have an adequate separation with respect to the ground, it is required that the aircraft use the
QNH of the area since this is the one that will allow knowing the real altitude above sea level
here we can say that the QFE could also be used, however this is little used
worldwide therefore we are going to focus on the QNH only. we should now that the elevation of
obstacles and terrain is usually published with respect to sea level as elevations
, therefore in this example suppose that this mountain has a maximum peak of 2,000 feet of
elevation, therefore a pilot who flies through this area desiring to have a separation of
1000 feet with respect to this mountain will have to fly at 3000 feet and since he has to fly at 3000
feet with respect to mean sea level he needs to calibrate the altimeter with the QNH of the area which in
this case is 30 10, otherwise, if this pilot uses a different setting than the QNH
the real separation and the apparent separation with respect to the ground will be different since he will
not know the real altitude With respect to mean sea level, which can be quite
dangerous, let's see through this example, here we have exactly the same situation:
however the aircraft in this case is using a setting of QNE when the QNH is actually 29 70
in this particular case, although the pilot reads 3000 feet on his altimeter since he is
using a different reference to the QNH, the actual separation with respect to obstacles is no
longer a thousand feet but 800 feet, therefore this may constitute a risk of collision With
the terrain, that is to say that when flying close to the terrain it is important to always use the
QNH of the area to ensure an adequate separation taking into account what we have to do
to separate ourselves with respect to other aircraft and with respect to the terrain we will evaluate two
different cases first let's see the low altitudes. when flying at low altitudes there is presence of
terrain and obstacles and we also have the presence of other nearby aircraft. To be able to separate
with respect to the ground we have to use the QNH and to separate with respect to other aircraft
all must use the same setting, it is then logical that in order to meet
these two requirements at the same time all aircraft operating at low altitudes must use the QNH
in this way they will be separated with respect to the terrain and since they all use the same
QNH they will also be separated from each other. now let's see what happens at higher altitudes in this case
there is only the presence of nearby aircraft the terrain is no longer a problem since we are
operating at a higher altitude that is to say that the only requirement is that they all use the same setting in
this way it would be quite easy to make all the aircraft operating at higher altitudes use
the standard setting i.e. 2992 in this way when using all the same setting they will be adequately
separated, having seen these two scenarios we would have something like this at low altitudes the
aircraft would use the QNH. The only problem is that the QNH does not have a constant value since
it will depend on the zone and the atmospheric pressure, therefore the pilots must be
aware of constantly updating the setting while flying. When operating at higher speeds.
altitudes the QNE would be used, which is always 29 92 therefore we would not have that problem,
the question here would be from what point do we change from QNH to QNE and vice versa, well for
this we are going to introduce the concept of transition altitude which is abbreviated with the acronym TA
this is the altitude below which all aircraft must use the QNH of the area,
that is to say that all aircraft must express their vertical position in terms of altitude. on the other
hand we also have the transition level which is abbreviated as TL or TRL, this is the level
above which all aircraft must use the QNE, that is, the standard pressure value
therefore here the aircraft must express its vertical position in terms of flight levels
and finally we have the concept of the transition layer that corresponds to the airspace
between the transition altitude and the transition level, therefore we would have
a scheme like this. An important rule regarding this transition layer is that it is not
permitted to fly in level cruise flight within this transition layer, that is to say that
we can only cross it climbing or descending but never leveled.Now keeping this in mind,
an aircraft that is climbing and is going to change from QNH to QNE must do so when crossing the transition altitude
that is, in this yellow point, therefore, within the transition layer, the aircraft
would already be using the QNE and therefore must use flight levels. on the other hand if an
aircraft is descending from an area of ​​QNE to QNH must change when crossing the transition level
that is, at this yellow point this means that the aircraft descending are going to
use the QNH in the transition layer and therefore they are going to use altitudes
now, the question that remains is how do we know what the altitude and transition level is,
well depending on the elevations of the terrain and as provided by the civil aviation authority
, a specific altitude and transition level will be published for a certain area or aerodrome and
this is normally published in the AIP of each state, that is, mainly on navigation
charts as we can see in this example, in this case this chart it is for Cali, Colombia
it specifies that the transition altitude is 18,000 feet and the transition level is
FL190 that is to say that below 18,000 feet the local QNH would be used and above flight level 190
the QNE would be used. While between 18,000 and FL190 we would have the transition layer
now in some occasions some navigation charts can specify a fixed transition
altitude but a transition level at discretion of the air traffic control
in this case air traffic control calculates the transition level to use based
on the current QNH and the published transition altitude and this is reported through the ATIS or
the VHF communications frequency now Another important procedure is that in areas where a transition altitude has
not been established or where there is no QNH information, the aircraft
in cruise flight must use the QNE and therefore use flight levels in this
case, for example we have an aircraft flying in an area where QNH is used up to a transition altitude
of 10,000 feet, however, its flight is carried out within an area where there is
no longer information on QNH or transition altitude, therefore this aircraft must use the QNE
within in this zone until he re-enters an area where QNH is used and
a transition altitude has been published.This usually happens in uncontrolled airspace in areas that are quite
extense where there is no QNH information such as, for example, jungles, deserts, oceans, etc., in
these cases, caution must be exercised by the pilot, since a
correct altitude measurement is not being used above mean sea level, but rather a flight level. Both the
altitude reading can be quite different from the real one, especially if you fly from high
pressure areas to low pressure areas in this case the isobars are going to tilt in this way on
the left side of the screen at this point A we can see that the QNH at mean sea level
is 30.40 inches of mercury then at this point B we can see that the QNH has dropped to 30 00
and at this point C we can see that the QNH is now  29 60. in this case if an aircraft
is flying from point A with a setting of 30 40 and does not update its QNH in the altimeter, it will be
progressively descending since it has an incorrect altitude reading which can lead
to separation conflicts with the terrain, this situation is expressed colloquially with this
phrase in English that specifies that flying from an area of ​​high pressure to one of lower pressure, we
must be careful with what is below us since progressively
the aircraft will be descending if we do not update the QNH, however, not only
the changes in atmospheric pressure affect the altitude reading but also the air
temperature, let's see what effect the temperature has on the isobars, here we have, for example, an
air column in standard conditions if the temperature of the air is colder than the standard,
for example in ISA-20 conditions, the isobars will be closer together, while if
the conditions are of higher temperature, they will be further apart, that is, in other
words, the temperature affects the rate at which the pressure changes with altitude and therefore will
affect the reading of the barometric altimeters what this implies is that if for example we are
flying in standard conditions with 7000 feet and the temperature is reduced our real altitude will be
reduced while if the temperature increases the real altitude will increase precisely due
to the change in the spacing of the isobars therefore as a rule of thumb, if the
temperature is low, the altitude will be lower and according to the ICAO document 8168, the true altitude
changes by 4% for every 10 degrees of increase or decrease in temperature with respect to the standard
this approximately to minus 15 degrees of temperature since from there you must
consult the specific tables for each temperature and altitude now precisely this
effect of temperature leads to a situation quite similar to the one we saw previously
with pressure in this case if we are flying from an area of ​​greater temperature at a lower
temperature the isobars will behave in this way in the area with higher temperature
the isobars will be more separated from each other, while with a lower temperature, they will
be closer to each other, therefore, although the pressure at sea level, that is, the QNH is
constant, the simple fact that the temperature changes means that the altimeter will have
a certain error in this case, if an aircraft is flying from a high temperature zone
to a lower temperature zone, it will be gradually descending, even if it keeps the QNH updated
, which can lead to a conflict with the terrain, this is expressed colloquially in English
also with this phrase that says that From high temperature areas to low temperature areas, we must
be careful with what we have below since we will be gradually descending
apart from these altimeter measurement errors with pressure and temperature there is also an error
that can occur in mountainous areas when the wind blows with high speed and it is that in the
mountains especially in the cliffs the bernouilli effect can be produced which causes
changes in the local static pressure thus an aircraft flying in this area may
experience fluctuations in the indication of the altimeter and have an incorrect indication
therefore it must increase the safety margin with respect to the ground to avoid collisions
Having seen all this, let's move on to the last topic, which is the altimeter error tolerance,
which is that barometric altimeters may have a certain acceptable margin of error
which must be verified before each flight. The exact values ​​are published in the
ICAO document 8168. However, most manufacturers and operators consider
that an adequate margin is plus or minus 75 feet with respect to the real reading, that is, if
we are on the apron of an aerodrome with an elevation of 3000 feet and we adjust the QNH
our altimeter must indicate between 2925 and 3075 feet to be considered suitable for
operations, otherwise the altimeter must be checked by an aeronautical workshop in order
to continue with routine operations. I hope this video has helped you to
understand what the altimeter setting procedures are and the different errors
to which the altitude reading is subjected. Subscribe for more content about the aeronautical world
and leave in the comments what other types of topics would like to be covered in the videos