Aviation In Canada

Saturday, January 15, 2005

Distance with DME vs (RNAV) GPS

When flying IFR, and you're asked to report a distance, make sure you comply with the request and use the proper phraseology. Miles are miles, right? Not quite.

ATC uses reports from aircraft in non-radar environments to determine spacing and applies restrictions based on standards to prove separation. In the old days, DME was the only real method of measuring distances. It was recognized, though, that DME distances are slant range. This means the distance from the aircraft to the facility, rather than the distance between the aircraft's latitude and longitude and the coordinates of the DME facility. Mind you, this difference is often small, but it is significant when you're higher up, closer to the facility, or both. For example, if you pass overhead a DME facility at FL240, you should read something on the order of 4.0 NM as you fly over (approximately 6,000 feet per NM, with some tolerance for errors).

RNAV in general, with the more recent addition of GPS to the term, has changed this. Now, instead of relying solely on DME (or at all, for that matter), many aircraft are capable of reporting distances in miles instead of DME. Problem is, certain circumstances require additional separation for DME to account for slant range and known DME errors that don't occur with GPS. Hence, if a pilot is asked for his DME from a facility, he should respond with a DME reading, rather than an along track distance derived from RNAV or GPS, stating his reading as "20 DME", for example. If the controller doesn't specify what he wants, the pilot should report his distance qualified with the word describing where it came from. If he reads his DME, the pilot should say, "20 DME." If he gets it from RNAV (including GPS), he should say, "20 miles". Kind of subtle, but it may very well make a difference.