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In earlier versions of ICAO Doc 4444 (Amendment 9 and earlier), two aircraft were deemed to be laterally separated if they were departing a VOR on radials which diverged by at least 15 degrees and if at least one of the aircraft was at least 15 NM from the VOR.

However, as of Amendment 10, this distance requirement has increased to 17 NM (or more) if the distance measurement is based on DME:

When distance from VOR is based on co-located DME, distance of one aircraft should be at least 17 NM from VOR below FL190 and 18 NM from VOR at or above FL190.

What made ICAO change this separation?

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    $\begingroup$ Welcome to Aviation.SE! Can you quote the relevant section from ICAO Doc 4444? $\endgroup$
    – Bianfable
    Commented Jun 8, 2022 at 6:09
  • $\begingroup$ ICAO DOC 4444 AMDT 10 LATERAL SEPARATION AND MINIMA $\endgroup$ Commented Jun 8, 2022 at 6:53
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    $\begingroup$ The request is to quote it in your question rather than expect people to look it up. $\endgroup$
    – GdD
    Commented Jun 8, 2022 at 11:12
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    $\begingroup$ Voting to close as "unclear" until the question is clarified enough that I can understand what's being asked without needing to google-search then read a lengthy document. $\endgroup$
    – Ralph J
    Commented Jun 8, 2022 at 14:18
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    $\begingroup$ In ICAO Doc 4444, These is no mention of co-located DME. It is mentioned as an additional note in MATS Part I (India). However the reason may be the answer given by randomhead $\endgroup$ Commented Jun 9, 2022 at 2:55

1 Answer 1

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The change appears to be clarifying that if the 15 NM distance determination is based on ground distance, for example the aircraft has passed a fix which is known to be farther than 15 NM from the VOR, that is acceptable. The increased minimum distance only applies when the distance is measured using DME.

This is because DME measures slant-range distance between the antenna in the aircraft and the transponder on the ground. The calculated distance is the hypotenuse of a right triangle whose other sides are the actual over-the-ground distance and the height of the aircraft above the DME transponder. This hypotenuse will by necessity be greater than the over-the-ground distance. For example, when the aircraft is 15 NM from the DME transponder and at FL190, the DME measurement will be:

$$ \begin{align} \mathrm{DME}^2 &= x^2 + h^2 \\ &= \left(15 \mathrm{~NM} \cdot \frac{6076 \mathrm{~ft}}{1 \mathrm{~NM}} \right)^2 + \left(19000 \mathrm{~ft} \right)^2 \\ \mathrm{DME} &= \sqrt{ 91140^2 \mathrm{~ft}^2 + 19000^2 \mathrm{~ft}^2 } \\ &= 93100 \mathrm{~ft} \\ &= 15.3 \mathrm{~NM} \end{align} $$

Whereas if the aircraft is at FL450 above the exact same spot, the DME measurement will be 16.7 NM; at FL600 the measurement will be 18.0 NM. You can see that if the intention is to ensure that an aircraft is at least 15 NM from the VOR over the ground, and if DME is used to measure that distance, you need the DME measurement to read greater than 15 NM—and the amount by which it must be greater increases as the aircraft's altitude increases.

You will also note that even at very high altitudes the required DME distance is lower than what is prescribed in Doc 4444. I would guess that ICAO wanted to add some buffer to the allowable minimum distance in the interest of safety.

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