today we will talk about the operation of the airspeed indicator as well as the speeds used in aviation with their corresponding definitions first of all we will divide the airspeed indicator topic into two videos in this first part we will focus on how an airspeed indicator works what are the errors and corrections that must be applied as well as the different definitions of the speeds used in aviation in the next video or second part we will see the color-coded markings of the indicator how would the instrument react to a pitted tube blockage as well as a static port blockage so with this being said let's get started with how an airspeed indicator works the airspeed indicator is one of the six basic flight instruments and as we said in the previous video this instrument is linked to the pitted static system which means that its reading depends on the measurement of the air pressure let's see the schematic of the pitted static system as we can see the air speed indicator is the only instrument that is actually connected to both the pivot tube and the static port the pivot tube provides total pressure while the static port provides static pressure or in other words atmospheric pressure in order to understand why the air speed indicator needs these two types of pressure we must first review their definitions as we said in the previous video about the pitted static system static pressure is the same as the atmospheric pressure and it is exerted equally in all directions regardless if the object is moving or is still on the other hand the dynamic pressure is the pressure produced by the air when an object moves through it and it is proportional to the speed and air density so in other words the faster we move the higher the dynamic pressure and vice versa finally the total pressure corresponds to the sum of static and dynamic pressure this means that when measuring the pressure of a jet of air we will obtain the total pressure as a result now the airspeed indicator abbreviated as asi is an instrument that indicates the speed of the aircraft relative to the air and as we know the air speed is directly proportional to the dynamic pressure so if for example an aircraft flies at 80 knots it will experience less dynamic pressure than if it were flying at 120 knots with this in mind what the instrument does is that it takes advantage of this principle to measure the dynamic pressure and then give an airspeed reading based on it we can say in other words that the airspeed indicator is actually a dynamic pressure gauge that is calibrated to indicate air speed this way if the aircraft experiences a high dynamic pressure this will be interpreted as a high speed and vice versa having seen this it is clear that the air speed indicator needs the information of dynamic pressure in order to indicate airspeed the problem is that the pitted tube provides total pressure not only dynamic pressure now as we said previously the total pressure equals to dynamic pressure plus static pressure so if we want only the dynamic pressure we have to take the total pressure and subtract the static pressure which is provided by the static port this is why the instrument needs information from both the pivot tube and the static port so basically what the instrument does is that it subtracts the static pressure to the total pressure thus obtaining the dynamic pressure as a result with which it can now give an airspeed reading if we look inside the instrument we will see the following here we have the static port and pivot tube connections however the difference is that the static port is directly connected to the inside of the case while the pivot tube is connected to a diaphragm which in turn is connected to gears which allow the needle to move in the dial so this would work the following way the static port allows the whole case to be filled with static pressure while the pitted tube causes the diaphragm to fill with total pressure this means that the total pressure inside the diaphragm tries to expand it while the static pressure in the case tries to contract that diaphragm now as we will remember the total pressure is static pressure plus dynamic pressure so we can express it as a combination of these two pressures this results in that part of the pressure inside the diaphragm is dynamic and part is static and as the static pressure inside the diaphragm is the same as outside of it they can be cancelled out now there's only dynamic pressure which means that the expansion or contraction of the diaphragm will depend only on dynamic pressure so for example if dynamic pressure increases the diaphragm will expand making the gears move the needle to indicate a higher speed on the other hand if dynamic pressure reduces this will cause the diaphragm to contract making the gears move the needle once again but now to indicate a lower speed now there's something that we must keep in mind and it is that the instrument is not totally perfect which means that the airspeed indication has some errors that's the reason why we have different speed terms and definitions in aviation each of these speeds is corrected by certain errors so let's see each of them in more detail let's start with the simplest of all the indicated airspeed abbreviated as is this is basically as its name suggests the speed indicated on the instrument and it is not corrected for any type of error this speed can be expressed either in knots or in miles per hour depending on the instrument design for example here we have one which is indicating 70 knots and here we have another that is indicating 70 miles per hour in many cases the instrument includes both speed scales as we can see in this example here the outer scale corresponds to miles per hour while the inner scale corresponds to knots having already seen what is the indicated airspeed let's see some errors that are produced the first is the instrument error this is produced due to manufacturing imperfections and wear of the gears inside the instrument the second type of error is the position error that we already discuss more in detail in the video about the pitted static system this is produced due to changes of flaps and gear configuration changes of angle of attack and some other maneuvers this error affects directly the pressure measured by both the pivot tube and the static port the combination of these two errors leads us to the next speed term the calibrated air speed the calibrated air speed abbreviated as cas corresponds to the indicated airspeed corrected for the instrument and position errors in other words if we take the airspeed indicated on the instrument and we correct it for these errors we will obtain the calibrated airspeed now the question is how is the calibrated airspeed determined well in order to do so the manufacturers design special tables of airspeed calibration which are based on certain configurations and speeds and they are published in the aircraft manual in this example this is an airspeed calibration table for a cessna 172. now let's suppose for example that we want to know what would be the calibrated airspeed if we fly with an indicated airspeed of 100 knots with the flaps extended to 10 degrees in this case we have to refer to the row of flaps 10 degrees and we can see that we have both the indicated and calibrated air speeds here as a side note when speeds are expressed in knots it is common to include a k before the speed term now in the table we just have to look for 100 knots of indicated airspeed and then read the corresponding calibrated air speed of 98 knots as we can see the difference between the indicated and calibrated air speeds is quite small therefore in practice it is common to assume that the indicated and calibrated air speed are roughly equal let's now move on to the next type of error which is known as compressibility error in the design of the pitted static system the air is assumed to behave as an ideal gas and one of the characteristics of an ideal gas is that it cannot be compressed however the reality is that when flying at high speeds the air actually compresses thus generating certain errors in the measurement of air speed normally the compressibility effects start to become significant above 300 knots therefore this error is almost nil in small and slow aircraft but if we fly at high speeds we must then add a compensation for the air compressibility factor resulting in the following speed term the equivalent airspeed the equivalent air speed abbreviated as eas is the calibrated air speed corrected for compressibility errors so in other words if we take the calibrated air speed and we apply the compressibility errors correction we get the equivalent airspeed remember that this compressibility error correction is only carried out when flying at high speeds and the truth is that calculating this correction manually can be a bit complex that's why normally this correction is automatically calculated in real time by the avionics computers of the aircraft so the pilot doesn't have to worry about this correction having seen this let's continue with the other type of error the density error as previously mentioned the dynamic pressure depends not only on the speed of the aircraft but also on the density of the air in other words the indication of the air speed indicator will not only depend on the speed of the aircraft relative to the air but also on the density let's see this by means of an example here we have two pitted tubes from different aircraft in this case the tube at the top will move in air with lower density which means that the air particles will be farther apart while the tube at the bottom will move in air with higher density which means that the air particles will be closer together in this case we can observe that the tube at the top as it moves in less dense air it captures less air particles which means that it experiences a lower dynamic pressure and therefore the air speed indicator will show a lower indicated air speed on the other hand the tube at the bottom moving in denser air captures a larger amount of air particles which means that it experiences a higher dynamic pressure and therefore the airspeed indicator will show a higher indicated airspeed so we can see that although both tubes are moving at the same speed through the air the simple fact of moving in different densities result in different indicated air speeds that's why the air speed reading needs to be corrected by density error which gives as a result the true airspeed the true air speed abbreviated as tas is the air speed corrected by density error and as we know the density of air depends mainly on temperature and static pressure we can also say that this is the actual speed of the aircraft in relation to the air since we've already applied all necessary corrections so in order to know how to apply this density correction it is necessary to first understand how density behaves in the atmosphere in this case the higher the altitude the lower the static pressure which also means less air density this means that if we take two samples of air one at sea level and the other at 10 000 feet we will have less density at 10 000 feet and since there is less density an aircraft flying at that altitude will experience a lower dynamic pressure therefore the airspeed indication will not correspond to the true airspeed something important to note here is that the instrument is calibrated to indicate the true airspeed only at sea level under standard conditions so in other words the airspeed indicator assumes that the density is always that of sea level and with standard temperature this means that at sea level the indicated air speed and the true air speed will be roughly equal however as we climb and the density starts reducing then this relationship will change let's see an example here we have a column of air from 0 to 10 000 feet and we can see how density changes with altitude in this case we are going to assume that the conditions are standard that is a sea level pressure of 29 92 inches of mercury and a temperature of 15 degrees celsius with this in mind let's suppose that an aircraft flies at a very low altitude close to sea level and its airspeed indicator shows 100 knots since near the sea level the indicated and true air speeds are roughly equal the true air speed will also be 100 knots however if the aircraft flies higher in lower density air and its indicator also shows 100 knots that would mean that the aircraft must be flying at a higher true airspeed in this case 120 knots in this way the true airspeed continues to increase as we climb with a constant indicated airspeed the question is how do we calculate this true airspeed well there are two main methods the most precise is by means of a flight computer either manual or electronic but we are not going to get in detail with it the other method is by means of an approximate formula and it is based on the rule of thumb which says that the true air speed increases by two percent for each 1000 feet of altitude increase so knowing this we can apply a simple rule of three to determine the percentile increase of the true air speed at a certain altitude let's see these two methods by means of an example suppose we need to determine the true air speed based on an indicated air speed of 110 knots and under the following conditions a pressure altitude of 10 000 feet and an outside air temperature of 5 degrees celsius let's first use the formula so if we know that for every 1000 feet of altitude increase the true air speed increases by 2 how much will it increase in 10 000 feet if we do the math the result is a 20 increase with this information we could do two things we can multiply 110 by 0.20 giving as a result the 20 of 110 which is 22 knots and then add those 22 knots to the initial indicated air speed of 110 knots obtaining a true air speed of 132 knots the other way which is a lot simpler is to multiply directly 110 by 1.20 which is the same as obtaining the 120 percent of 110 giving as a result 132 knots of true airspeed either of these procedures can be performed and the result will be the same let's see now what the result would be in case of using a manual flight computer first we have to align the air temperature of 5 degrees celsius with the pressure altitude of 10 000 feet on this inner scale then we look for 110 in the middle scale and finally we observe that it corresponds to approximately 130 in the outer scale this means that the resulting true air speed is 130 knots now apart from these two methods there is one more that can be used while flying and it is by means of the true airspeed scale of the airspeed indicator as we can see some instruments include a sliding true airspeed scale incorporated so let's see how it's used we must first use this knob in the bottom right corner to adjust the current pressure altitude with the outside air temperature on the scale of the top just as we did with the flight computer once this is done the scale below will be adjusted to indicate the true air speed for any given indicated air speed within the scale for example this particular instrument shows an indicated air speed of 175 miles per hour and then on the true airspeed scale we can read approximately 202 miles per hour so in this way the pilot can determine the true airspeed in real time easily now we will look at the last speed term the ground speed the ground speed abbreviated as gs is the actual speed of the aircraft in relation to the ground and it corresponds to the true airspeed corrected by wind first of all let's see how the wind affects the speed of an aircraft when an aircraft experiences a tailwind it will literally be pushed by the wind in other words its speed in relation to the ground will increase on the other hand if the aircraft experiences a headwind it will be slowed down by the wind in other words its speed in relation to the ground will decrease that's why in order to determine the ground speed we must apply the wind correction first we must see what happens when there's no wind in this case the true air speed and the ground speed will be equal let's see an example here this aircraft is flying in calm air which means that there's no wind if this aircraft flies at 100 knots of true airspeed this means that it is flying at 100 knots in relation to the air mass and since the air mass is not moving the speed in relation to the ground will also be 100 knots now if we experience a headwind the ground speed will be less than the true airspeed let's see why here the aircraft is flying with a headwind of 20 knots this means that the air mass is moving against the aircraft at 20 knots here if the aircraft is moving at 100 knots in relation to the air mass we will observe that its speed in relation to the ground will be 20 knots lower so the ground speed will be 80 knots the opposite happens when there is a tailwind in this case the speed in relation to the ground will be higher than the speed in relation to the air for example here we have an aircraft that is flying with a tailwind of 20 knots here the aircraft travels through the air mass at 100 knots while at the same time the air mass is moving in the same direction at 20 knots this means that the aircraft is moving at 120 knots in relation to the ground with this we have already seen all the speed terms and definitions let's see a little summary if we take the indicated airspeed and we correct it for instrument and position errors we will get the calibrated air speed if then we apply compressibility corrections we will get the equivalent airspeed if then we apply density correction we will get the true airspeed and finally if we apply wind correction we obtain the ground speed now some aircraft perform this calculations automatically in real time by means of an air data computer however if the aircraft is not equipped with this avionics computer the pilot will have to do this calculations manually in that case something important to note is that in practice it is normally assumed that the indicated airspeed is roughly equal to the calibrated air speed and apart from that if we fly at low speeds below 300 knots we will have little influence of compressibility effects so we can say that the calibrated air speed is roughly the same as the equivalent airspeed so with this assumptions we can say that if we take the indicated airspeed and we correct it for density error we will obtain the true airspeed and then if we correct it for wind we will obtain the ground speed this simplified procedure is the one that is normally performed in day-to-day operations now another important thing to mention is that it is important to understand the definition of each of these speeds since each one is used for different purposes let's look at some practical applications of these speeds we know that the indicated airspeed depends directly on the dynamic pressure therefore it is useful to stablish structural speed limitations take-off and landing speeds stall speeds and some performance-related speeds such as the best glide speed vx and vy on the other hand the true airspeed is the actual speed of the aircraft in relation to the air and therefore it is used in the atc flight plan the calculation of the mach speed endurance calculations wind correction angle determination and general flight planning aspects finally the ground speed is the actual speed of the aircraft in relation to the ground and therefore it is used for calculating flight time fuel consumption climb and descent parameters and some other general flight planning data i hope the information presented in this video has been useful if so don't forget to share like subscribe and leave a comment down below thanks for watching [Music] you