**“Twin Cessna 543MT, fly present heading, join the Graham 230 radial direct Graham.”**

In order to fly the 230 radial inbound, you will need to tune your CDI needle to the opposite of 230°.¹ How do you compute the opposite heading? Read on.

**The Trick Version 1:**
Add 200, then subtract 20:

230 + 200 =

*430*

*430* + 20 =

**450**
You might have noticed that a course of 450º does not make a whole lot of sense (unless you’re an engineer). Well, lets try again shall we?

**The Trick Version 2:**

Subtract 200, then add 20:

230 – 200 = *30*

*30* + 20 = **50**

Notice that we did just the opposite this time. We added 20 instead of subtracting, and we subtracted 200 instead of adding. The result makes a lot more sense too. The reciprocal course of 230° is 50°.

**Why it Works**

Because there are 360° in your heading indicator, the reciprocal heading will always be 180° away. If you add or subtract 180, you will always wind up with the opposite heading.

By subtracting 200, then adding 20, we are actually subtracting 180 in a more human-friendly way. Notice that 200 – 20 = 180. We break it up to make the math a little easier on the head.

The two versions of “the trick,” are there to provide a means to subtract 180 (for headings greater than 180º, and a method to add 180 (for headings less than 180°).

¹ For those of you having trouble understanding why you must fly the opposite of 230, recall that radials extend outward *from* a VOR. In order to track inbound, one must fly the radial’s reciprocal heading *to* the station.

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Nice refresher for me, Patrick.

I’ve always used the +2,-2 trick or vice versa.

Example: Reciprocal of 360 : subtract 2 from 3 which is 1, then add 2 to 6 which is 8 and and always keep the last digit which is 0, therefore = 180 degrees.

Example 2: Reciprocal of 270 : subtract 2 from 2 which is 0, then add 2 to 7 which is 9 and keep the last digit of 0, therefore = 090 degrees.

Example 3: Reciprocal of 010: add 2 to 0 which is 2, subtract 2 from 1 (kinda weird, I know), but is 9, keep last digit which is 0 = 190 degrees

That’s a really great way to think about it. By stripping off those zeroes it really makes the mental math a lot easier. Thanks for bringing this up.

example 3.add 2 to 0 = 2 , 2 from 1 = 9 (I agree) , keep the last 0. why isn’t answer 290? even though 190 is the reciprocal.