Interpolations

[SATXaviator]

I am working problems for the FIA...and getting killed on questions that require interplotions..My basic math skills must be rusty...ie. Presure altitude of 6000ft given. Chart shows Presure alt. of 5000ft and 7500ft. The difference is 2/5 or 40% ....Question How do you get this number???? Help Please....my brain is turning to mush...

 

[E_Dawg]

Yeah those killed me too... take a look at the Pilot's handbook of aeronautical knowledge (think that's the one). It's got a great description on how to interpolate. Also if you have the Gliem book it shows you step by step for each problem.

In reality I always just use the next highest number.

 

[Grabo172]

The way I do it is break down the missing areas into segments... ie you are looking for 6000ft and you are given 5000ft and 7500ft, so break it down into 500's. Try to find a common factor for both the given numbers and the one you are looking for.

lets say the 5000ft value is 2500
and the 7500ft value is 3500

there are 5, 500ft segments so each 500ft adds 200 { (3500-2500)/5 }

SO,
to get to 6000ft, take the 5000ft value and add 400 (200 * 2, since there are 2, 500 ft segments)

And get the answer 2900

Does that help any or just make it more confusing?

 

[SATXaviator]

my head just exploded.....

I have the gleim....but it just is not sticking...I finally did get one right. I keep getting close.....but on the FIA the answers are somtimes very close to each other...and those tricky wrong answers..that you think are right..


It is just fuzzy getting the Percent difference between values...

the common factor thing is confusing me... what is the formula for percent difference. x= percent difference, given are two values.

given 5000
given 7500
x=%difference,

dont forget to show your work.

 

[Grabo172]

sorry, my brain works in strange and weird ways!

 

[mtsu_av8er]

Here's my method...excuse the mess....

Let me know if you can follow the math...

 

[Rodelu]

If you like a somewhat different reasoning:
a) Consider going from 5000 to 7500, (2500) as 100%
b) The question now is what % of that, is going from 5000 to 6000 (1000)
c) Write down:

2500=100
1000="X"

To figure out X: 1000x100 and divide by 2500 = 40 (%)
You multiply across and divide by the number "left out"

Hope it helps and doesn't confuse you more

 

[jetman]

i would draw a line 7.5'' long
each inch been the equivalent of 1000' and each half inch=500'
7.5-5.0=2.5' = 2500'
2.5' = 5 half inches =100%
1/2 ' = 500' = 20%
1' = 1000' = 40%
No idea if this may help

 

[MidlifeFlyer]

[ QUOTE ]
I am working problems for the FIA...and getting killed on questions that require interplotions..My basic math skills must be rusty...ie. Pressure altitude of 6000ft given. Chart shows Pressure alt. of 5000ft and 7500ft. The difference is 2/5 or 40% ....Question How do you get this number???? Help Please....my brain is turning to mush...

[/ QUOTE ]Probably adding to the confusion, but here's an alternate method you mind find easier than the percentage method you're being taught. (I never could do it) This will be long, but doing it is faster than explaining it. The catch is that it's very easy =if= you understand why it works

Let's fill out your scenario with numbers. Say it's a takeoff distance problem

P-Alt = 5000, Takeoff distance = 1500'
P-Alt = 7500, Takeoff distance = 1800'
What is the takeoff distance for P-Alt = 6000'

What you're really trying to figure out is how the takeoff distance changes for each foot of altitude within the range.

The P-Altitude range you are working with is from 5000 to 7500, or a 2500' range.

Over that 2500' range, the takeoff distance increases from 1500 to 1800, a change of 300'

So, over the 2500' range, the takeoff distance varies 300'.

You want the takeoff distance per foot of altitude. When you see "per" think division. "Takeoff distance per foot of altitude" translates to "Takeoff Distance/Altitude"

Plug in the numbers, 300/2500 and you get a change of 0.12' of takeoff distance per ft of altitude.

The mathematical representation of this is
D= Takeoff distance
A= altitude

D1-A2
¯¯¯¯¯ gives you the change in distance/altitude
A1-A2

You know the number for 5000' It's 1500
You want the number for 6000'. That's 1000' higher than 5000
We've calculated the change/altitude to be 0.12.
1000 X 0.12 = 120'
The takeoff distance for 6000' = 1500 + 120 = 1620.


The system works for =any= interpolation.

 

[jdflight]

Good lord you guys are specific. 6000 is approximately halfway between 5 and 7.5. There you go. Now knock off a little because it wasn't quite half.

 

[EatSleepFly]

[ QUOTE ]
Good lord you guys are specific. 6000 is approximately halfway between 5 and 7.5. There you go. Now knock off a little because it wasn't quite half.

[/ QUOTE ]

Thats always been my technique!

 

[sorrygottarunway]

heck, when you get in the airplane the calculations mean nothing anymore anyway!

 

[MidlifeFlyer]

[ QUOTE ]
Good lord you guys are specific. 6000 is approximately halfway between 5 and 7.5. There you go. Now knock off a little because it wasn't quite half.

[/ QUOTE ]Depends if you are talking knowledge tests or real world.

The knowledge test usually requires an artificial level of accuracy.

The realities of the real world in a 20 year old Cessna single usually requires you to go to the next worse performance rather than even bother estimating. Even your method is too detailed for that!