# Maximum Theoretical Range RNAV

This is one of the easier topics on both the Radio Navigation and Communications syllabus. Most questions require using the formula:

$M = 1.23 (\sqrt{h_1} + \sqrt{h_2})$

where $M$ is the MTRMTR —Maximum Theoretical Range in nautical miles, $h_1$ is height of the aircraft in feet, and $h_2$ is the height of the beacon in feet.

• Getting the input data incorrect. The heights are in feet, so watch out for data given in metres.
• Getting the formula wrong. For example, adding the heights together then doing the square root.
• Rearranging the formula incorrectly. The reverse formula is: $h_1 = \Big(\frac{M}{1.23} - \sqrt{h_2}\Big)^2$.
• Getting the output units incorrect. The MTRMTR —Maximum Theoretical Range is in nautical miles, so watch for the examiner giving answers in kilometres.

The range above is exceedable in the case of super refraction, where atmospheric conditions benf the signal back towards the earth.