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When flying at 6000 feet above ground level, the DME indicates 5 NM. What is the horizontal distance from the aircraft to overhead the DME?
  • A
    4.9 NM
  • B
    4.6 NM
  • C
    5.2 NM
  • D
    4.3 NM

DME measures the straight-line distance from the aircraft to the ground station.
This is called "slant range" and is slightly more than the actual horizontal distance because of the difference in elevation between the aircraft and the station.
The most extreme case of "slant range error" occurs when the aircraft passes directly over the station; instead of reading zero, the DME shows the altitude of the airplane above the station (in nautical miles).
Slant range error also affects groundspeed and time-to-station displays when you're close to the station.
Displayed DME groundspeed drops below actual groundspeed as you approach the station and then climbs back to normal after you pass it.
Displayed DME time-to-station may not count all the way down to zero as you fly over the station.

The DME range (slant range) can be calculated using the following formula:

True Range = √ ( Slant range 2 – Aircraft height 2 )

NOTE: Range and Height must be in same units e.g. Nautical Miles!

In our case:

1 NM = 6 080 ft
6 000 ft correspond at 0.99 NM
True Range = √ ( Slant range 2 – Aircraft height 2 )
True Range = √ (52 – 0.99 2 )
True Range = √ (25– 0.98 2 )
True Range = √ 24.02
True Range = 4.9 NM

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