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Today we will talk about the importance in aviation de la International Standard Atmosphere – ISA or International Standard Atmosphere.
Post written by: William Casalins
Aeronautic engineer
FIA Foundation
A boy of about 20 years, short hair, impeccably dressed in a chair in an aviation school classroom, he asks a veteran professor who swayed around in the middle of the class explaining his aerodynamics subject.
- Teacher, please, Tell me the truth, why are you trying so hard to explain to us about ISA?, it's really so important? It seems more like a filler topic., but you insist very much that it be resolved with such exactness.
- Mijo, It is obvious that you think that there are much more important subjects for an aviator, like engines, flight controls, meteorology or navigation, maybe that's how it is, but soon you will remember your physics teacher when he told you: “pears with pears and apples with apples”.
- I don't understand teacher.
- Suppose there is an airplane factory in Quito and one in Guayaquil, You are going to buy a plane for the company and you fly the one from Quito first, take out 4 tons in a good rate of climb. Then you are going to fly the one in Guayaquil with very similar price and dimensions. comes out plenty with 5 tons, which one would you buy?
- The one from Guayaquil, undoubtedly
- Ok, You go to your base in Quito and you tell your boss very proud that you bought the best plane and he congratulates you.
- You arrive at noon to make your first flight from your base and you discover to your horror that your magnificent plane doesn't even get the 4 tons of the other and that the climb is rather precarious, do you think they deceived you?
- Or something bad happens to the plane
No, nothing happens to the plane, You yourself saw how it behaved the day it was delivered to you at the factory, what could have happened??
- Well, we all know that the higher the plane, the less it performs.
- And then why did you fail in the comparison and buy the plane with the least features?; you compared pears with pears and apples with apples?
- Yes, That depends on the elevation and temperature of the field..
- And other things, but how could you know in advance that that was going to happen and that you are judging pears by pears? That's when you realize the importance of standardizing things and in particular the performance of the aircraft., You can know it in advance just by looking at the technical sheet and without even having to fly it., you can interpret the performance curves anywhere in the world and be able to compare with other aircraft.
There are many factors that influence the performance of an aircraft., not just the height, also humidity and temperature and in countries that have seasons the uncertainty can be worse than in tropical countries, At one time at sea level you can be less 5 and in another one more 35 degrees Celsius temperature and the performance is extremely different, Densities affect a lot and can also hinder a good takeoff from a critical airport..
Then the engineers give you the tables and you, if you took advantage of the class, You will have no doubt in taking the exact load on a given track regardless of the weather conditions or the geography of the place..
- I mean, Teacher, can that prevent me from having an accident??
- Yes and a premature dismissal too for choosing the wrong plane.
Student and teacher finish the comment with smiles and convinced that it was the best question of the day.
Professor Omar Gonzales, realizes that they are in a EUREKA moment of effervescence and heat, and the condemned man does not plan to waste it; He quickly approaches the board and says: the ISA is not today's class, but right now they are going to realize the importance of.
He shows a table of the optimal height of an airplane with different deviations from ISA and makes a couple of doodles on the board..
- do you see the difference?
- Everyone says yes, it's obvious.
- Well we already know that it is obvious, Now let's see how we find the deviation to be able to explain the table.
We are in Bogotá, which is more or less 8000 lifting feet
According to I.S.A.. we should be at -1 degree centigrade, but on the tower thermometer it says 9 degrees, then, what is the deviation?
- 10 degrees Celsius everyone said in unison.
- Correct, So what table are we going to use??
- ISA more 10, do you see that it does work?
But at another time of day the Bogota thermometer says -6 degrees Celsius, what is the deviation at that moment?
- Less 5 Celsius degrees
Everyone in chorus said.
- Exact, and in flight it is done the same way. This is how the aviators of yesteryear could cross the pond without running out of gas in the attempt.
This article is dedicated to the dedicated aeronautical professors who, day after day, deliver their knowledge and experiences to the new generation of aviators who no longer swallow whole, The competition is great since there are countless videos and simulators, The teacher is forced to study more and show his expertise in the face of the new challenges that these boys impose on the class..
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