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Altimetría y Procedimientos de Ajuste Altimétrico
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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
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