Altimeter Setting Procedures & Altimetry

today we will talk about altimetry and altimeter setting procedures these are a set of definitions and procedures used to ensure adequate vertical separation from terrain and other aircraft using barometric altimeters which are the altimeters that we find on all aircraft these procedures take into account daily changes in atmospheric pressure terrain characteristics air traffic flow and ats services provided as well as all applicable local or regional procedures however before going into detail on this topic let's recall some of the definitions we have seen in previous videos let's start with the different ways we have to express the vertical position of an aircraft in first place if we measure the vertical distance between the mean sea level and an aircraft in flight we will be measuring altitude if we measure the vertical distance between a point on the ground and an aircraft in flight we will be measuring height and finally if we measure the vertical distance between the mean sea level and a point on the ground we will be measuring elevation now the concept of flight level also refers to a certain vertical distance however we will deal with it later we must also remember how a barometric altimeter works this type of altimeter uses static air pressure to give its reading it takes advantage of the fact that atmospheric pressure decreases with altitude this means that at low altitudes there's a higher pressure while at higher altitudes the pressure decreases gradually so if the altimeter senses a high static pressure it will be interpreted as a low altitude and as the aircraft climbs and the static pressure decreases the altimeter will interpret it as an altitude increase in other words an altimeter is a very sensitive barometer that interprets static pressure changes as altitude changes now since this type of altimeter uses pressure to give its reading there must be a certain pressure level at which the altimeter reads zero feet that pressure level is known as the barometric reference as mentioned before this is the pressure level at which the altimeter will indicate zero feet and therefore it is also the level in relation to which the altitude will be measured and as we said in the previous video about the altimeter it is calibrated by default to use a barometric reference of inches of mercury or 1013 hectopascals it is used as the default barometric reference since it is the standard pressure at sea level under isa conditions however since in practice sea level pressure varies constantly the pilot can adjust the altimeter to use another barometric reference through the colesman window causing the altimeter to measure the altitude in relation to the desired pressure level with this being said let's see an example suppose that under certain conditions the pressure at sea level is 2972 inches of mercury so in other words this would be our qnh since as we saw in the previous video the qnh is the pressure level that identifies the mean sea level this means that if the pilot adjusts the altimeter with 2972 as the barometric reference it will measure the altitude in relation to mean sea level on the other hand let's suppose that this airport that is at a certain elevation above mean sea level has a pressure of 27 92 inches of mercury in this case this would be the qfe which is the pressure level that identifies the airport level this means that if the pilot adjusts the altimeter with 27.92 as the barometric reference it will measure the height above the airport now apart from these two settings there is another one known as qne or standard setting which always represents the pressure level of 2992 inches of mercury or 1013 hectopascals which in this particular case is below mean sea level if the pilot adjusts the altimeter with the qne it will indicate the current flight level with this in mind let's see a little summary there are three main barometric references that are used in altimetry first we have the qnh which is the pressure level that identifies the mean sea level and therefore the altimeter will indicate altitudes then we have the qfe which is the pressure level that identifies a certain airport elevation and therefore the altimeter will indicate heights and finally there is the qne which identifies the standard pressure level of 2992 inches of mercury or 1013 hectopascals and therefore the altimeter will indicate flight levels with this being said let's look more in detail at the concept of flight level the flight level is the vertical distance between the standard pressure level of 2992 inches of mercury or 1013 hectopascals and an aircraft in flight in other words the altimeter will indicate the current flight level as long as the qne is set as the barometric reference something important to mention is that flight levels are identified with the abbreviation fl followed by three digits which represent hundreds of feet let's see an example suppose that with the qne set as barometric reference the altimeter indicates three thousand feet in this case this would be represented as flight level zero three zero abbreviated as fl zero three zero in this order of ideas if the altimeter reads fifteen thousand feet it would represent flight level one five zero if it indicates twenty six thousand five hundred feet it would be flight level two six five if it indicates thirty thousand feet it would be flight level three zero zero and so on now let's go through some practical examples of situations that may arise let's begin with standard conditions in this case the sea level pressure is 2992 inches of mercury which means that qnh and qne will be equal and therefore the altitude and flight level will be the same and using this setting while on the ground the altimeter will read elevation now suppose that at this particular airport the qfe is 2792 inches of mercury if the pilot sets this as barometric reference the altimeter will read the height of the aircraft above that airport let's now see what happens in a lower than standard pressure condition this means that the sea level pressure is lower than 2992 inches of mercury let's suppose it is 2972 in this example this would be then the qnh if we adjust it as barometric reference the altimeter will read the altitude of the aircraft in flight or the airport elevation if the aircraft is on the ground in this situation the qne or standard pressure level will be below mean sea level if we adjust it as barometric reference the altimeter will indicate the flight level which as we can see is higher than the altitude finally let's suppose that the qfe at this airport is 2762 inches of mercury if it is set as barometric reference the altimeter will indicate the height above that airport on the other hand if there is a higher than standard pressure condition the sea level pressure will be higher than 29.92 inches of mercury let's say it is 30 42 inches of mercury this would be then the qnh if we adjust it in the altimeter it will read the altitude or elevation above sea level in this case the qne or standard pressure level will be above sea level if it is adjusted in the altimeter it will read the flight level which as we can see is lower than the altitude and finally if the qfe is for example 2842 it will allow the altimeter to read height above the airport now the vertical position of an aircraft and flight can be expressed in any of these terms depending on the barometric reference used so in order to avoid misunderstandings between aircraft and air traffic control certain standard procedures have been established to know what barometric reference to use in a certain moment in essence these are the altimeter setting procedures first of all we must say that the main objectives of these procedures are to ensure inadequate vertical separation from terrain and other aircraft in flight so let's first look at the separation between aircraft in this case in order to have an adequate separation between aircraft it is required that both of them use the same barometric reference let's see an example suppose there are two aircraft flying in opposite directions in this case let's say the qnh of the area is 306 inches of mercury so if both aircraft use 306 as their barometric reference they will be measuring their altitude in relation to the same pressure level and therefore they can ensure an adequate vertical separation in this case one of them is flying at 7000 feet and the other at 8 000 feet so the actual separation is 1000 feet now something important to note is that even if these aircraft are not using the qnh as long as both of them use the same altimeter setting the vertical separation will be ensured for example let's say that in this case both aircraft use qne instead of the local qnh despite they are not measuring the correct altitude both are using the same reference therefore they will have the same vertical separation in conclusion regardless of the altimeter setting being used as long as both aircraft use the same the vertical separation will be ensured otherwise if one of them is using a different altimeter setting the apparent separation will be different from the actual separation which is quite dangerous let's see this through an example suppose we have the same situation here and the local qnh is 31 inches of mercury the aircraft on the left is using the local qnh of 3031 but the aircraft on the right is using qne instead of qnh apparently if both aircraft report their altitude their separation would be 1000 feet however since both altimeters are measuring the altitude in relation to different pressure levels the actual separation is different in this particular case the aircraft using qne would be flying at an actual altitude of 7650 feet despite the indication of its altimeter of 7000 feet therefore in this situation the actual separation is only 350 feet which represents a risk of collision let's now move on to the other case the separation with the terrain and obstacles in order to ensure inadequate separation with terrain and obstacles it is necessary to use the qnh since it allows the pilot to know the real altitude above sea level actually the qfe of a certain airport could also be used for this purpose however it is little used worldwide since it leads to confusion and mistakes under certain conditions so even though it is used in some countries we will focus on the use of qnh with this in mind most charts and procedures show the elevation of terrain and obstacles in relation to sea level for example here the top of this mountain has an elevation of 2000 feet above sea level therefore a pilot who is planning to fly through this area with a separation of 1000 feet would have to fly at an altitude of 3000 feet using the local qnh which in this case is 310 inches of mercury if otherwise this aircraft uses an altimeter setting other than the qnh the apparent separation with terrain will be different from the actual separation which can be really dangerous let's look at this example here we have the same situation but now the local qnh is 2970 and the altimeter of the aircraft is set with the qne here although the pilot is reading 3000 feet on the altimeter that is not the real altitude of the aircraft since it is using a barometric reference which does not correspond to the sea level pressure in this particular case the actual separation with terrain is only 800 feet in conclusion while flying close to the terrain and obstacles it is important to always use the updated qnh of the area in order to ensure an adequate terrain clearance now that we know the requirements that we need to comply with in order to have an adequate separation with other aircraft and terrain let's evaluate two different cases the first one is an aircraft flying at low altitudes in this case at low altitudes there will be obstacles and terrain as well as other nearby aircraft flying now as previously mentioned in order to have an adequate separation with terrain the altimeter must be set with the local qnh and in order to ensure inadequate separation with other aircraft all of them must use the same altimeter setting it is then logical that to comply with both requirements all aircraft operating at low altitudes must use the local qnh this way they will be separated from terrain and as all of them are using the same qnh they will also be separated from each other with this being said let's see what happens when flying at higher altitudes in this case there is no terrain or obstacles so it is not longer a problem there are only nearby aircraft flying around so it is only necessary to ensure adequate vertical separation between aircraft and to do so the only requirement is that all of them use the same altimeter setting the altimeter setting to be used at high altitudes does not necessarily have to be the qnh since as we just said the terrain is no longer a problem so in order to establish a standard setting to be used at high altitudes the only setting that remains constant regardless of the conditions is the q and e since it is always 2992 inches of mercury or 1013 hectopascals in other words the setting to be used by all aircraft operating at high altitudes will be the q and e and as all aircraft are using the same setting the separation is assured now having seen these two scenarios we obtain something like this at low altitudes all aircraft must use qnh however the problem with this is that the qnh does not have a fixed value it changes depending on the conditions of the different areas so in this case the pilot has to update the qnh setting on a regular basis depending on the information provided by the atc on the other hand aircraft operating at high altitudes will use qne which is always the same 2992 inches of mercury or 1013 hectopascals the question now would be where the change from qnh to qne and vice versa should be made well to answer that question we have to introduce a few concepts the first one is the transition altitude abbreviated as ta this is the altitude below which all aircraft must use the local qnh and therefore express their vertical position in terms of altitudes then we have the transition level abbreviated as tl or trl this is the level above which all aircraft must use q and e and therefore express their vertical position in terms of flight levels finally we have the concept of transition layer which is the air space between the transition altitude and the transition level something important to note regarding this transition layer is that it is not allowed to fly leveled within this layer the aircrafts operating here must be climbing or descending keeping this in mind all aircraft climbing through the transition layer must change from qnh to qne when crossing the transition altitude therefore within the transition layer these aircrafts must express their vertical position in terms of flight levels since they are using now qne on the other hand all aircraft descending through the transition layer must change from qne to qnh when crossing the transition level therefore within the transition layer these aircrafts must express their vertical position in terms of altitudes since they are using now qnh you might be wondering by now how do i know what is the transition altitude and transition level for a certain area well depending on terrain elevations and as provided by the civil aviation authority a transition altitude and level will be published for a certain area or airport these values can be found in the charts of each airport or in the aeronautical information publication of each country for example we can see in this chart for kali colombia that the transition altitude is eighteen thousand feet and the transition level is flight level one niner zero in other words in this case below eighteen thousand feet all aircraft must use local qnh while above flight level one niner zero all aircraft must use qne here the transition layer is between eighteen thousand feet and flight level one niner zero now in some cases the transition altitude may have a fixed value but the transition level is established by the atc in these cases the air traffic control calculates the appropriate transition level to be used based on the current qnh and the published transition altitude this transition level is reported through the itis or a communications vhf frequency now another important procedure is that in areas where a transition altitude or level is not published or when there is no qnh information all aircraft must use the qne and express the vertical position in terms of flight levels in this example this aircraft departed using the local qnh from an area where the published transition altitude is 10 000 feet but then it enters an area where there is not a published transition altitude or level in this case the pilot must set the qne and use flight levels until entering an area with a published transition altitude and where qnh information is provided usually this occurs in uncontrolled airspace in remote areas where there is no qnh information such as for example jungles deserts oceans etc in this cases the pilot must exercise extra caution since there's no way to assure an adequate terrain and obstacle clearance while using qne this is especially important when flying from high pressure areas to low pressure areas in this case the isoburs will be inclined like this on the left side we can see that in the point a the qnh is 30 40 inches of mercury then in the point b it dropped to 30 0 0 and in this point see we can see that the qnh is now 29.60 under these conditions an aircraft flying from point a to point c would depart with a qnh setting of 30-40 however if the pilot does not update the qnh in a long time while flying to the low pressure area the aircraft will be gradually descending this happens because the aircraft is no longer using the correct qnh which can lead to reduced terrain separation or even a crash this situation is colloquially expressed in the phrase from high to low look out below however not only pressure changes affect the altimeter reading but also temperature changes let's see how does the air temperature affect the altitude indication here we have a column of air under standard conditions if the temperature is colder than standard for example in iso minus 20 conditions the isoburs will be closer together while if the temperature is warmer than standard for example an isoplus 20 the isoburs will be further apart this means in other words that air temperature affects the rate at which the pressure reduces with altitude and therefore it will affect the reading of barometric altimeters what this implies is that if we are flying for example at seven thousand feet under standard conditions and the temperature reduces the true altitude will be lower than seven thousand feet while if the temperature increases the true altitude will be higher than 7000 feet this happens precisely because of the change in the spacing of the isoburs with the temperature according to the icao document 8168 the true altitude changes by 4 percent per each 10 degrees of iso deviation this rule of thumb is useful down to minus 15 degrees celsius with colder temperatures it is necessary to consult the following table published in the document now the effect of temperature leads to a situation quite similar to the one we saw with the pressure in this case if an aircraft is flying from a warmer area to a colder area the isoburs will be arranged in this way as we can see in the warmer area the isoburs are farther apart while in the colder area they are closer together this means that even if the qnh remains constant an aircraft flying to a colder area will gradually descend this can lead to a reduced terrain and obstacle clearance or even a crash this is colloquially expressed in the phrase from hot to cold look out below apart from these measurement errors of the altimeter with pressure and temperature changes there's another one to take into account it occurs in mountainous areas when the wind blows at high speeds this produces local static pressure fluctuations due to the bernoulli effect this means that an aircraft flying in these areas may present erroneous altitude indications and therefore it is necessary to increase the altitude safety margin with respect to the terrain having seen all these let's move on to the altimeter error tolerance the thing is that barometric altimeters may have an acceptable margin of error which should be checked before each flight the exact values are published on the icao document 8168 however most manufacturers and operators consider that an adequate margin is plus or minus 75 feet this means that if for example an aircraft is on the ground at an airport with an elevation of 3000 feet when adjusting the current qnh the altimeter should read between 2925 feet and 3075 feet to be considered suitable for air operations otherwise the altimeter must be checked and recalibrated by a certified aeronautical workshop i hope the information presented in this video was useful if so don't forget to share like subscribe and leave a comment down below thanks for watching [Music] [Music]

Channel: Aviation Theory

Views: 92,107

Keywords: Aviation Theory, Aviation, Airplanes, Instruments, Navigation, Aerodynamics, ATPL, CPL, PPL, Tutorial, Explanation, Pilot, Aircrafts, Knowledge, Altimeter, Altimetry, Altimeter Setting Procedures, QNH, QNE, QFE, Altitude, Height, Flight Level, Elevation, Temperature

Id: 9VEVUVS7fbg

Length: 24min 15sec (1455 seconds)

Published: Wed Apr 14 2021