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(h1+h2)M = 1.23 (\sqrt{h_1} + \sqrt{h_2})

where MM is the MTRMTR —Maximum Theoretical Range in nautical miles, h1h_1 is height of the aircraft in feet, and h2h_2 is the height of the beacon in feet.

Mistakes that be made include:

  • 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: h1=(M1.23h2)2h_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.

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What is the maximum range that an aircraft at FL195 can contact a beacon at 583 ft?
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