The
aircraft is travelling at 300 knots, i.e. 5 NM per minute.
One minute before the overhead the aircraft is 5 NM GROUND Range from the
beacon.
The aircraft is flying at FL300, roughly 5 NM.
The right-angled triangle that is formed by height (5 NM) and GROUND Range (5
NM) has a hypotenuse which forms the SLANT Range, which is the DME Range.
DME Range^2 = Ground Range^2 + Height^2
DME Range^2 = 5^2 + 5^2
DME Range^2 = 25 + 25
DME Range^2 = 50
DME Range = Square Root 50
DME Range = 7.1 NM
As the aircraft approaches the beacon, the accuracy of the Groundspeed
displayed reduces and under-reads.
Therefore, the DME Range will be 7.1 NM and the GS will be slightly less than
300 knots.