How do you calculate the maximum distance to communicate with tower control? I'm stuck on the following riddle found on the geocaching web site:

A helicopter pilot is flying at an altitude of 1981 meters. He is heading inbound an airfield 335 meters high. There are no particular obstacles (such as mountains, ...) between the helicopter and the airfield. What maximal distance (in nautical miles) can the pilot expect in order to be able to communicate with the airfield control tower?

Unfortunately I don't know anything about air control rules and multiple Google searches came to no avail.

Please could you give me a clue (not the answer as it would spoil the fun) how to answer this riddle?

Edit

For those who are interested here is the link to the geo-cache
The formula for VHF range taught in flight schools1 is 

$$ d = 1.25 * \sqrt{h} $$

where d is the range in nautical miles and h is the height in feet of the receiver above the transmitter.

So in your case it would be (spoiler: mouse-over to see hidden text):


   $$ 1.25 * \sqrt{(1981 - 335) * 3} \approx 87.84  $$


However, keep in mind that in some countries the range of Tower VHF radio tranmitters is limited so they only achieve a certain range. Uncontrolled airfields in Germany usually have a range of 30nm, which is achieved by limiting their power.

1: "Motorflug kompakt" - Winfried Kassera / 2012 - p. 296