Need Help with Reciprocal Headings

jwp_145

jwp_145

GhostRider in the Sky
Hey folks, I used to always teach this little trick as a flight instructor, but since it's been a while now, I've forgotten it... say I'm wanting to figure out what the reciprocal is for a 135 Radial... I used to teach a Add 2 Subtract 2 trick... does anyone remember that one?
 
Add 200, subtract 20, more or less.

135 = 335 − 20 = 315
 
Maybe because I can do math really quickly in my head, but adding or subtracting 180 seems a lot easier.
 
Heath said:
For many brains, mine included, 200-20 is much faster.
Click to expand...

Yes I teach and use it.

For instance: if you want the reciprocal of 230....
Subtract 2 from the first digit=0
add 2 to the second digit= 5
answer 050

Same applies but reversed if total number is greater than 180.
For instance reciprocal of 050:
Take the first digit (0) and add 2= 2
take second digit (5) and subtract 2= 3
Answer:230

So it's either +2 -2 or -2+2
 
Focus on the middle digit. Add or subtract 8 to the middle digit (depending on if your current heading it greater than or less than 180) 3rd digit remains the same. The first digit your student will have to think about, and that will help promote situational awareness.

Example: 135
Middle Digit: Add 8 to the 3 (since 135 is less than 180) = 11, so middle digit is 1.
Last Digit: Stays the same making our last digits of our reciprocal "_15"
First Digit: Needs to be on the opposite side of the compass....ie, 3. (You're flying southeast, the reciprocal needs to be northwest. Hopefully this helps your student remember what general direction they are flying, and what the opposite direction is.)

So the reciprocal is 315


This method is super easy and very fast once you master it and it promotes situational awareness as your student will have to reference general directions and compass headings to confirm reciprocal direction

Eh, works for me. YMMV....
​
 
Holy crap... math in public?! No no no. Set it on the CDI... no math involved. They had this genius idea... they put a pointer both up AND down on them! Sometimes even on the sides!
 
jwp_145 said:
Hey folks, I used to always teach this little trick as a flight instructor, but since it's been a while now, I've forgotten it... say I'm wanting to figure out what the reciprocal is for a 135 Radial... I used to teach a Add 2 Subtract 2 trick... does anyone remember that one?
Click to expand...
If the first two digits of the heading you want to reciprocate are less than or equal to 18 then it is +2 - 2: say, heading of 165... +2 to first digit to 365 then -2 to second digit for 345.

If the first two digits of the heading you want to reciprocate are greater than 180 then it is -2 +2, for example a heading of 335: -2 for 135 then +2 to second digit for 155.

It would be easier if we had a base 12 numbering system. If I lived in the UK I would definitely be a member of The Dozenal Society
 
I'm with Alec. +180 if its <179, -180 if its >181 I don't know if most people do this, but I usually picture a longhand problem in my head and solve it really fast. So I'll see in my head:

120
+180
-----
300

Sounds crazy, but it only takes a second.
 
ATC RET 2003 said:
Isn't it on the instrument?
Click to expand...

Yes it is on the instrument...
But I was flying along V4 and I think it is like the 135 radial you are on... I was trying to do the +2-2 trick in my head to come up with 315, but since I was task saturated my brain just couldn't reember exactly how it was done.
And yeah I can just add 180 to the number in my head, but who wants to do that!

THanks for the help guys.
 
A good double check is that the reciprocal will always add up to the same number... a FE taught me that about 10 years ago.

360/180 - 3+6=9, 1+8=9
330/150 - 3+3=6, 1+5=6
030/210 - 3+0=3, 2+1=3
 
jwp_145 said:
I was trying to do the +2-2 trick in my head to come up with 315, but since I was task saturated my brain just couldn't reember exactly how it was done.
Click to expand...

Which is why the math is dumb. In the airplane... it's on the OBS/HSI. On the ground, who cares... you can even grab a calculator if you need to. I would never teach someone to do that in their head while flying. You have enough going on, arbitrarily adding one more thing doesn't make anything easier.
 
I just look at the DG and see what's directly opposite of the heading, radial, etc. Why make it any more compex than it needs to be?

Or is that considered "cheating"?
 
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