¿Cómo funciona un Altímetro? — Transcript

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1. 0:08Today we are going to talk about the altimeter
2. 0:10The altimeter is one of the six basic flight instruments
3. 0:14And it is connected to the pitot-static system
4. 0:18Which we have already seen in previous videos
5. 0:21Here we have a schematic of a common pitot-static system
6. 0:25As we can see here, the altimeter is only connected to the static port
7. 0:30So it only receives static pressure (atmospheric pressure)
8. 0:35Now, before we continue, we must say that there are different types of altimeters
9. 0:41Depending on their operation principle
10. 0:44First of all, we have the barometric altimeter
11. 0:47Which uses air pressure to measure the altitude
12. 0:52Then, there´s also the radioaltimeter (radaraltimeter)
13. 0:56Which is included in some airplanes
14. 0:59However it´s use is limited to some instrument precision approach
15. 1:05And finally, there´s the GPS altimeter (GNSS altimeter)
16. 1:10Which uses satellites to determine the altitude
17. 1:14In this video we are going to focus on the most used altimeter in almost all operations
18. 1:19Which is the barometric altimeter
19. 1:21First of all, let´s see what a barometric altimeter is
20. 1:25This is an instrument which measures the vertical distance between a reference level and the aircraft
21. 1:31The reference level is the point at which the altimeter would read 0ft
22. 1:36And then, it will measure how high or low is the aircraft in relation with that reference level
23. 1:43That vertical distance may be expressed in terms of feet or meters
24. 1:47Even though, most altimeters are calibrated in feet
25. 1:51At this point we might be wondering, why do we say "vertical distance" instead of altitude or height
26. 1:58Well, that´s because that vertical distance has different names depending on the reference level we are using
27. 2:06Let´s see an example of this
28. 2:09If we measure the vertical distance between the mean sea level and the aircraft
29. 2:14We will be measuring the altitude
30. 2:17If we measure the vertical distance between the ground level and the aircraft
31. 2:21We will be measuring the height
32. 2:24And if we measure the vertical distance between the mean sea level and the airport ground level
33. 2:29We will be measuring the elevation
34. 2:32The term "flight level" also refers to a vertical distance
35. 2:38However we are not going to talk about it until the next video
36. 2:43So far, let´s just use this 3 basic concepts of altitude, height and elevation
37. 2:51With that being said, let´s now see how does a barometric altimeter work
38. 2:56As it´s name suggests, this instrument uses the atmospheric pressure to give it´s reading
39. 3:04It takes adventage of the fact that the pressure decreases with altitude
40. 3:11Therefore, if we measure the air pressure at low altitudes we will get a higher value
41. 3:19While if we measure it at higher altitudes, we will obtain a lower pressure value
42. 3:24So this means that if the altimeter senses a high pressure value, it will interpret it as a low altitude
43. 3:33But as the aircraft starts climbing to a lower pressure areas
44. 3:38The altimeter will interpret the lower pressure values as a higher altitude
45. 3:43In summary, the altimeter interprets the changes in static pressure as changes in altitude
46. 3:49This is why the altimeter needs to be connected to the static port
47. 3:57Now, originally, the altimeter is calibrated to measure the altitude above the mean sea level under standard conditions
48. 4:05That is it´s default configuration
49. 4:09As we saw in the video about the ISA atmosphere
50. 4:13The standard pressure at sea level is 29,92 InHg
51. 4:18Or 1013 hPa
52. 4:21In this video we will only use InHg, however the concept is still the same
53. 4:27In this case, if the altimeter senses a pressure of 29,92InHg
54. 4:32It will indicate 0ft
55. 4:34Since it is the reference level (sea level)
56. 4:39And as the aircraft climbs to a lower pressure area
57. 4:43The altimeter will interpret this as an increase in altitude
58. 4:47Let´s suppose that it climbs to a point where the static pressure is now 25,14 InHg
59. 4:55Well, in this case, what the altimeter does is compare the new static pressure sensed with 29,92 InHg
60. 5:03Which is the reference
61. 5:05So, based on that pressure difference, the altimeter indecates a certain altitude
62. 5:12But let´s see what is actually happening inside the case of the instrument
63. 5:18Well...as we know, the altimeter case is connected to the static port
64. 5:22Through which the static pressure gets into it
65. 5:26Inside the instrument we find an aneroid capsule, which is sealed with a pressure inside of 29,92 InHg
66. 5:34The standard reference pressure
67. 5:37This aneroid capsule is connected to a series of gears which move the needles of the instrument
68. 5:44If the aircraft is at sea level, the static pressure in the case will be equal to the pressure inside the capsule
69. 5:52So in other words, it will be equal to the reference pressure
70. 5:56In this situation, the aneroid capsule is completely compressed
71. 5:59And therefore, the altimeter reads 0ft.
72. 6:03Now, if the aircraft starts to climb
73. 6:06The static pressure will decrease
74. 6:09Causing a higher pressure inside the capsule compared to the instrument case
75. 6:13Which will cause the capsule to expand and move the gears
76. 6:18Which in turn, make the needles move
77. 6:22Now, if the aircraft starts to descend
78. 6:26The static pressure will increase again
79. 6:29Which will cause the capsule to compress, moving the gears and indicating a lower altitude
80. 6:37So far, we have used 29,92 InHg as the reference pressure
81. 6:44This is known as "barometric reference" or "reference pressure"
82. 6:49Which is the pressure level where the altimeter reads 0ft.
83. 6:54And therefore, it will indicate the vertical distance from that level
84. 7:00By default, this barometric reference is 29,92 InHg
85. 7:07However, the pilot can actually calibrate the altimeter to use antother barometric reference
86. 7:14The question here is: why does the pilot need to change that barometric reference?
87. 7:20Well, let´s see an example
88. 7:23As we already know, the real altitude is the vertical distcance from the mean sea level
89. 7:30And under standard conditions, the pressure at sea level is 29,92 InHg
90. 7:37And it decreases with altitude at a rate of aprox. 1 InHg per 1000ft
91. 7:45and theoretically, below sea level, the pressure would increase
92. 7:50this horizontal lines that represents pressure levels are known as isobars
93. 7:56and they will be important for this explanation
94. 7:59In standard conditions, as our altimeter by default is calibrated with 29,92 InHg as barometric reference
95. 8:06it will indicate the vertical distance between the aircraft and the 29,92 isobar
96. 8:12and as 29,92 is the pressure level that identifies the sea level, the altimeter will be reading the true altitude
97. 8:19The thing is that, we don´t always have standard pressure conditions
98. 8:23So 29,92 doesn´t always identifies the mean sea level
99. 8:28The pressure at sea level can be higher or lower than 29,92
100. 8:33Let´s see what happens in those cases
101. 8:36First, let´s have a look to a higher than standard pressure situation
102. 8:39In this case, the isobars will move upwards
103. 8:45Causing 29,92 to no longer represent sea level
104. 8:49And now, the new pressure level that identifies sea level is 30,40 InHg
105. 8:56In this situation, if our altimeter is still set to 29,92 it will read a wrong altitude
106. 9:03Because it will be measuring the vertical distance between the aircraft and the 29,92 isobar
107. 9:10Instead of measuring it from the actual sea level pressure
108. 9:14The solution here is to readjust the altimeter so it uses 30,40 as barometric reference instead of 29,92
109. 9:21This way, it will measure the real altitude
110. 9:26The actual pressure value at sea level is known as QNH
111. 9:31Therefore, it is important for the pilot to get updated QNH information of the area
112. 9:37In order to calibrate the altimeter correctly
113. 9:43This is the procedure in case of higher than normal pressure
114. 9:47Let´s have a look to a lower than standard pressure condition
115. 9:51In this situation, the isobars will move downwards
116. 9:55Causing 29,92 to no longer represent sea level
117. 10:00In this case, the new pressure level that identifies sea level is 29,70
118. 10:06So if the altimeter is still set to 29,92
119. 10:09it will indicate a wrong altitude
120. 10:13Therefore, the pilot will need to recalibrate the altimeter to use 29,70 as barometric reference instead of 29,92
121. 10:23So we can say that the current QNH here is 29,70
122. 10:29So far, we have seen some examples to make the altimeter to give a proper altitude reading
123. 10:35But actually, the pilot can calibrate the altimeter to measure height instead of altitude
124. 10:41In this case as we know, the vertical distance between the mean sea level and the airport level is the elevation
125. 10:50and assuming standard pressure conditions, we would have something like this
126. 10:56So how can we make our altimeter indicate the height above this airport?
127. 11:03Well, in this particular case, the elevation is 2000ft
128. 11:08And as we can see, the isobar that identifies the 2000ft level is 27,92
129. 11:16So if we adjust the altimeter with 27,92 as barometric reference, it will indicate the height above the airport
130. 11:27As we are not longer using the sea level pressure as a reference, this pressure has other name
131. 11:33the pressure at the airport elevation is known as QFE
132. 11:38Which isn´t used widely in aviation
133. 11:42But it is important to understand the concept
134. 11:45As it happens with the QNH, if the conditions are not standard, the QFE value can change
135. 11:51An therefore, if we want to measure the height properly, we need to get updated QFE information
136. 11:59You may be wondering by now, how do you actually change the barometric reference in the altimeter?
137. 12:06Well, that is performed through the Kollsman´s window
138. 12:10it is a little window in the altimeter which allow us to change the barometric reference
139. 12:17it´s calibrated in InHg or hPa
140. 12:20We can change the value by turning the knob labeled "baro adjust"
141. 12:25So in summary
142. 12:28If we adjust sea level pressure (QNH) as barometric reference
143. 12:33The altimeter will read altitudes
144. 12:37While if we adjust the airport level pressure (QFE) as barometric reference
145. 12:43The altimeter will read height above the airport level
146. 12:47Then it is logical that if we set QFE while on the ground, the altimeter will read 0ft
147. 12:54Because we would be right on the reference level
148. 12:58Other thing we have to know, is how to read the altimeter needles
149. 13:03So let´s see the different needles we can find
150. 13:07Here we can see 3 main needles
151. 13:11First we have the triangular needle, which represents tens of thounsads of feet
152. 13:17So if the needle points the number 1, it represents 10.000ft
153. 13:22if it points number 2, it represents 20.000ft
154. 13:24if it points number 3, it represents 30.000ft, and so on
155. 13:29Then we can find the short needle, which will represent thusands of feet
156. 13:35so if it points number 1, it represents 1.000ft
157. 13:39if it points number 2, it represents 2.000ft
158. 13:41if it points number 3, it represents 3.000ft and so on
159. 13:45And finally we find the long needle, which will represent hundreds of feet
160. 13:50so if it points number 1, it represents 100ft
161. 13:53if it points number 2, it represents 200ft
162. 13:55if it points number 3, it represents 300ft and so on
163. 13:58Let´s see some exapmples of how to read the altimeter
164. 14:02Here all the needles are pointing 0
165. 14:06So we are at 0ft from our barometric reference
166. 14:11Now, let´s suppose that we increase our altitude and now the altimeter reads this
167. 14:17Well, in this case the triangular needle indicates that we are between 0ft and 10.000ft
168. 14:24Then, the short needle indicates that we are between 1.000ft and 2.000ft
169. 14:30And the long needle indicates 400ft
170. 14:35and we know that measurement is in feet because the altimeter indicates so in the background
171. 14:41So in this situation the altimeter reads 1400ft
172. 14:46Now....let´see what does the altimeter indicates here
173. 14:52In this case the triangular needle indicates that we are between 0ft and 10.000ft
174. 14:58The short needle indicates that we are between 7.000ft and 8.000ft
175. 15:02And the long needle indicates 500ft
176. 15:04so the altitude reading is 7.500ft
177. 15:09Finally let´s see this example
178. 15:14Here, the triangular needle indicates that we are between 10.000ft and 20.000ft
179. 15:23The short needle indicates 2.000ft
180. 15:27and the long needle indicates 000ft
181. 15:29So that indicates that we are at 12.000ft
182. 15:34As you can see now, reading an altimetter is pretty similar to reading a watch, the only thing is that it has 3 needles instead of 2.
183. 15:43Now, this 3-needle configuration give as a result a little problem
184. 15:49If we observe this two altimeters we can see that they are very similar, because the long and short needles are in the same positions
185. 15:57However, the altimeter on the left is indicating 2.000ft
186. 16:01While the one on the right is indicating 12.000ft
187. 16:03So the onwly way to notice the difference, is by looking at the triangular needle
188. 16:11Which sometimes can be a little difficult to see, specially under high workload
189. 16:14So it´s likely that an unaware pilot may interpret the altitude wrong
190. 16:22This is why some altimeters incorporate a 10.000ft flag with black and white strips
191. 16:27Which is visible while flying below 10.000ft but it desappears above 10.000ft
192. 16:35This way, the pilot can easily determine if he is flying below or above 10.000ft
193. 16:42Other thing to take into account, is that every altimeter has a maximum calibration altitude
194. 16:48Since some barometric altimeters are more sensitive and precise than others
195. 16:55So every altimeter is certified to measure the altitude correctly until a certain altitude
196. 17:01In this example we can see in the instrument background, that it´s limit is 20.000ft
197. 17:09So above 20.000ft the reading of the altimeter may have some errors
198. 17:15And this happens because as we increase altitude, it is harder for the altimeter to distinguish pressure changes with altitude
199. 17:24So normally the precision decreases
200. 17:26This is why some barometric altimeter designs are more precise than others
201. 17:32For example, there´s the simple altimeter, which is mainly used in small aircrafts
202. 17:37Calibrated normally up to 20.000ft
203. 17:40The sensitive altimeter incorporates a second aneroid capsule and a temperature compensation system
204. 17:50and the servo-assisted altimeter incorporates an electric motor which facilitates the pressure measurement
205. 17:59So it is installed in the most advanced aircrafts
206. 18:03Now, there are also some other caratule designs
207. 18:07Some of them replace two of their needles for a numeric indication of altitude, as we can see in this example
208. 18:14In this altimeter we can see that it only has one needle
209. 18:19and in replacement of the other two, it incorporates a numeric indication of altitude
210. 18:23which result in a more intuitive and fast interpretation
211. 18:29on the other hand, some altimeters are incorporated into the primary flight display
212. 18:34as in the case of the Airbus A320
213. 18:38Now we have to understand that the altimeters aren´t perfect
214. 18:42The installation and wear of gears leads to errors
215. 18:48and the altimeter is also subjected to position errors caused by maneuvers or changes in configuration
216. 19:00This is why there are altitude calibration tables
217. 19:04The aircraft manufacturer publishes an altitude calibration table in the manual
218. 19:08to determine the necessary altitude corrections to be applied in order to calculate the calibrated altitude
219. 19:13This tables take into account the altitude, speed and configuration of the aircraft, to determine the correction values
220. 19:20However, these are normally small corrections that do not exceed 50ft
221. 19:25That´s why in real operations, these tables are rarely used
222. 19:31Now, as we can see here, this table applies only to the normal static port
223. 19:38So, what happens if the static port gets blocked?
224. 19:43Well...in this case, the altimeter reading will be frozen
225. 19:49So independently of the changes in altitude, the needles of the instrument won´t move
226. 19:59Let´s see what happens inside the instrument
227. 20:03When we are flying and the static port gets blocked, the static pressure will be trapped inside the case
228. 20:11and then the aneroid capsule is going to stay in a fixed position
229. 20:18So, how do we solve this?
230. 20:22In most aircrafts, there is an alternate static source
231. 20:26So when a static port blockage is suspected, the pilot should activate the alternate source
232. 20:31and then consult the altitude correction tables for the alternate static source
233. 20:36Because as we already know, the pressure sensed by the alternate source is slightly different than the real one
234. 20:43Here we can see an example of an altitude calibration table for the alternate static source
235. 20:49as we can see, it´s pretty much the same as a regular altitude calibration table
236. 20:55Aircraft configuration and altitude are taken into account in order to give the correction values
237. 21:02In the next video, we will see altimetry and altimeter setting procedures
238. 21:07as well as the concept of "flight level"
239. 21:10I hope this information was useful for you
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