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Distance measuring equipment

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Aircraft Systems, Flight Instructor

If an aircraft flying at an altitude of 30, 000 feet and the DME indicator display 10 miles to the ground station, what is the aircraft’s actual distance from the station and what is the slant-range error?

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1 Answers

  1. Best Answer


    John D Collins on Jun 03, 2020

    Converting 30000 feet into NM, divide by 6076.equals 4.94 NM above sea level. Assuming the DME ground station is at sea level, then the aircraft position can be described as a right triangle, with the hypotenuse (C) as 10 NM and the vertical leg as 4.94 NM (A). The actual lateral distance to a point directly overhead of the DME station is leg (B) and can be found by geometry where C squared = A squared plus B squared, or solving for B, is the square root of C squared minus A squared. I will let you do the calculation. Of course this is only an approximation because it assumes a flat earth and does not take into account the actual curvature of the earth, If the DME station is above or below sea level, you would need to adjust the vertical leg of the triangle to account for that.

    I would simply watch the DME and look for it to read 5.0 NM or the lowest value and mark the time between when it reached the lowest value to where it began to increase as the overhead position. Or simpler yet, use a GPS.

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