FAA Mechanic book is sloppy... 1

[RoarkS]

Folks, I usually live over in Aerospace, but I figured I should post this over here.
I'm trying to teach mechanics with this hot garbage. They just changed the regulations forcing us to teach with this:

https://www.faa.gov/sites/faa.gov/f...oks_manuals/aviation/amt_general_handbook.pdf

Go to Page 12-20 starting with 4. "temperature" thru 12-21.

Think of it as a find all the errors game.

I think I have most of them under control, but the one I really am stuck on is what are they doing with the 3 ratios on the top of 12-21... Supposed to be multiplication signs right?

--- also
The 25% increase for each step gives me some heartburn too. appx 125% maybe?

 

[itsmoked]

Nope. They're demonstrating that if you know any one set of values for a conductor you can use them in ratios to find the values of a different conductor made of the same material.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[RoarkS]

Okay so...

r/R=l/L=a/A

r=0.017 ohm
l=1 meter
a=1mm^2


R=?
L=50 meter
A=.25mm^2

l/L=0.02
a/A=4

They have equal signs between them... that's an untrue statement.

That would give me R could be 0.85 OR 0.00425

I must be ignorant of what's going on here.

I totally get how to do it with the equation they gave at the bottom of 12-20.

Using R=(p*L)/A, doing it twice, once to find p, then plug it in to get the final:

R=(((a*r)/l)*L)/A that for sure = 3.4 ohms

I figured out I can re-arrange that to be

a L r = A l R

Play with that I can get their third line where they get the answer from in the book.

r*(a/A)(L/l)=R agreed that works 3.4 ohms

I understand that... I don't understand why it's written r/R=l/L=a/A.

 

[PHovnanian]

It does seem to be a bit confusing.

R1/R2 = l1/l2 = A1/A2

seems wrong (top of page 12-21). The resistance of two similar pieces of wire vary proportionally with their lengths (R1/R2 = l1/l2). But the resistance of two similar pieces of wire vary inversely with their areas (R1/R2 = A2/A1).

Anyway, a better way to write this would probably be (R1*A1)/l1 = (R2*A2)/l2

But then I'm too old to learn "new math" at this point.

 

[RoarkS]

(R1*A1)/l1 = (R2*A2)/l2

That's pretty clean. I like it.

 

[itsmoked]

I agree that's a much clearer equation. I also agree that the proportion is upside down on the Area as PHovnanian shows.

Also much better than the " = " the relationships should've been shown with " ∝ " which means "proportional".

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Sparweb]

RoarkS,
You know how most AC's, TSO's and other policy documents from the FAA have a "submit corrections" form on the last page? This would be a good time to use that. Postage paid, the old-fashioned way, or fax machine if you're more modern.

I can see a lot of learners getting mixed up with that, like you did. I'm going to annotate my copy, and point it out to the others in the office who might refer to it.