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A DME station is located 1000 feet above MSL. An aircraft 15 NM away from the DME station, in ISA conditions at FL370, will indicate a DME range of approximately..
  • A
    15 NM.
  • B
    16 NM.
  • C
    17 NM.
  • D
    14 NM.

Refer to figure.

Distance Measuring Equipment (DME) is a navigation beacon that can indicate the aircraft’s position relative to the beacon. Aircraft send out a signal which is sent back by the DME ground equipment. Based on the delay of receiving the signal back, the aircraft can calculate the relative distance to the DME ground equipment.

Pay attention in this question to the location of the DME station. Flightlevels (FL) are always referenced to the standard pressure (1013 HPa) which is located at mean sea level (MSL).
The DME station is located 1000 feet above MSL. The airplane is at 37.000 feet referenced to mean sea level. Since the question asks for a DME range make sure to calculate with the height referenced to the height of the DME station.

In this case that will be: 37.000 -1000 = 36.000 feet.

Convert the height to nm by using the following table:

1 nm

6080 ft.

?

36.000 ft.

? = (36.000 ft x 1 nm)/(6080 ft) = 5,92 nm

(Continues on next page)
Knowing that:
A = 5,92 nm
B = 15 nm
C = ?

Using the ABC-formulae the following can be calculated:

A² + B² = C²
(5,92)² + (15)² = C²
C² = 260 nm
C = √260 = 16 nm

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