Radio Nav Question

[Phenom100]

A DME station is located 1000 feet above MSL. An aircraft flying at FL 370 in ISA conditions which is 15 NM away from the DME station, will have a DME reading of:

How do you work this out, as i get the answer as 13.7 nm but Bristol give 16 NM as the correct answer. Cheers Guys

[PingDit]

Ok, I'll give it a go.....
Draw yourself a right-angled triangle, where;
a=36000 (we've subtracted the 1000'asl from FL370)
b=15nm (15x6078[ft in a nm])=91170
where c=hypotenuse or slant range we're looking for.
now, square-up each side:
36000x36000=1296000000
91170x91170=8311968900
Add the squares...1296000000+8311968900=9607968900
sq root of last answer = 98020
now divide this by 6078 (to return to nm)
= 16.127nm
i.e. pythagorus' theorem
:)

[edymonster]

DME distance is the slant range. So draw a triangle. one side will be 15nm as the aircraft is 15nm away and the other side will be 36000ft which is approx 6nm (because the dme station is not at MSL). So use pythag for this 15 squared plus 6 squared equals 261. Then square root of 261 is approx 16nm.

[Phenom100]

Thanks guys, i didnt expect to get a reply straight away, as it was so late. It all makes sense now.

Thanks once again.

Danny C