DME

[MSP Aviation]

When a DME reads 1 nautical mile, is it reading 6076 feet along the hypotenuse (direct from the DME to the aircraft), or are you 6076 feet from the DME horizontally along the ground?

If the DME measures direct, isn't the "groundspeed" that some DMEs give inaccurate? If I recall a bit of calculus, the rate of change of the hypotenuse isn't the same as that of the triangle's base.

[BEagle]

SoCal App, yes, that 1 nm per 1000ft is a very good US rule of thumb :ok: which few here have ever been taught. I was told it by a USAF exchange officer and taught it ever since!

MSP - also remember that a DME frequency-paired with an ILS will be adjusted to show range to the threshold, not to the beacon. If not, then the fact should be promulgated.

Except in the Brize Norton approach plates..... The Brize ILS is on 111.9 and the DME comes from a TACAN which co-incidentally is on the frequency assoicated with 111.9. Range will be shown to the TACAN, not to the threshold, so beware!

[Piltdown Man]

The range measured in the DME system is called "Slant" range. But does what it shows really matter? For example, the "four mile" check on an ILS is a gross error check. But when really crucial distances are put on charts there is a fudge factor built in as well as an understanding about what the DME will be showing at that particular point. So you are absolutely right, the numbers may be rubbish but it doesn't really matter.

PM

[MSP Aviation]

It's not a big issue. I think the only situation in which it might come into play is getting a groundspeed reading near the DME. I was just curious. Thanks :ok: