Updated: Dec 30, 2020
What is the difference between Slant Range and Straight Line Distances? When is Slant Range Distance most accurate?
Here's what you need to know.
SLANT RANGE DISTANCE
Slant Range Distance is most commonly used with conventional DME Systems (Distance Measuring Equipment). It measures the distance between an aircraft and a ground based navigation system (such as a VOR). Because it takes into account the altitude of the aircraft and it's distance to the station it is called Slant Range Distance and may not always be accurate. See depiction below:
SLANT RANGE ERRORS
Traditional DME (that measures Slant Range Distance) is most accurate when the aircraft is further from the ground based navigation system and at a lower altitude. This is because the Slant is not as great. For example, if an aircraft is 5,000' directly above the ground based navigation system, it will indicate a distance of 1 Nautical Mile (instead of 0 Nautical Miles) due to the aircraft's altitude. See depiction below:
STRAIGHT LINE DISTANCE
Most modern aircraft are equipped with GPS Units and GPS gives distance information to pilot's in Straight Line Distance. This means, it does not take into account the altitude of the aircraft (does not measure the slant) and is therefore more accurate. It compares the aircraft's Latitude and Longitude with the Latitude and Longitude of the ground based navigation system. See depiction below:
Remember that Slant Range Distance is most commonly associated with traditional DME Systems and is most accurate when the aircraft is at lower altitudes and further from the ground based navigation system.
Author - Nate Hodell
CFI/CFII/MEI/ATP - Creator of wifiCFI - Owner of Axiom Aviation Flight School.
This information is included in the Navigation Aids: DME Lessons on wifiCFI. Sign up today to watch videos, listen to podcasts, take lesson quizzes, join live webinars, print lesson quicktakes, and more by clicking this link >