# How would you use the mental clock code to solve this dead reckoning problem?

Can someone explain the question below in details?

You plan to fly a cross country flight at 2 000 feet at a true airspeed of 120 knots. Your track is 270° (True) and the wind is from 225° (True) at 24 knots. Using the Mental Dead Reckoning Clock Code, what true heading would you steer, and what would be your magnetic heading if the local magnetic variation is 4° West?

• "Mental Dead Reckoning Clock" Not even a whizz wheel? – Dave Gremlin Apr 28 '19 at 17:02
• I'm afraid not. It's a question from the PPL theory questions book. I know how to determine the 'max drift angle' and but don't know how to include this material to find the magnetic heading. – Canberk Apr 28 '19 at 17:10

## 1 Answer

### Step 1: Calculate maximum drift.

Max drift $$= \frac{wind speed}{TAS (NM/min)} = \frac{24}{120/60} = \frac{24}{2} = 12$$

That means if you have a direct crosswind, you'll need to correct 12$$^{\circ}$$.

### Step 2: Determine how much of the wind is crosswind

The mental clock code is a way of estimating crosswind component from total wind. There are 60 minutes on a clock face. If the wind direction is 60$$^{\circ}$$ or more, it's considered all crosswind, just as 60 minutes is considered the whole clock face.

By extension, a wind angle of 30$$^{\circ}$$ means 30 minutes, which is half a clock, so half the wind would be considered crosswind. 15 degrees would be a quarter of a clock, therefore a quarter of the wind, and so on.

Your maximum drift is 12 degrees with a full crosswind, but you don't have a full crosswind. Instead you have a 45$$^{\circ}$$ angle, so 45 minutes of a clock, which is $$\frac{3}{4}$$.

$$12 * \frac{3}{4} = 9$$ so you'll need to apply 9 degrees of crosswind correction toward the wind. $$270 - 9 = 261^{\circ}$$

### Step 3: Correct for variation

"East is least, West is best". If variation is west, add to true to get magnetic.

$$261 + 4 = 265^{\circ}$$