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An aircraft at FL300, in ISA conditions and with a ground speed of 300 kt, is about pass overhead a DME station at MSL. The DME receiver is capable of determining ground speed. One minute before the overhead, DME speed and distance indications are respectively..
  • A
    less than 300 kt and 5 NM.
  • B
    less than 300 kt and 7 NM.
  • C
    300 kt and 5 NM.
  • D
    300 kt and 7 NM.
Refer to figure.
Important to know with this question is that the groundspeed is only accurate when the aircraft is flying directly towards or away from the DME station. Meaning that the indication given by the DME will usually be less than what the aircraft is actually flying.

The DME on board the aircraft will indicate the (purple) slant range.

The aircraft is 1 minute from the DME, meaning that the ground distance towards the station will be:

300 nm 60 min
? 1 min



 ? = (1min x 300 nm)/(60 min) = 5 nm.

Knowing that the ground distance to the station is 5 nm, the slant range (DME indication) can be calculated by using the following:
FL300 = 30.000 ft = 4.9 nm = 5 nm.

The slant range can be used by using the following the pythagoras theorem:
Slant range = √((Ground distance)²+(Aircraft height)2)
Slant range = √((5 nm)²+(5 nm)²) = 7 nm.

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