Updated: Dec 8, 2020
Navigation Aids: DME Lesson by wifiCFI
DME (PHAK C16) (AIM 1-1-7)
DME is a means of measuring distance between an aircraft and a navigation system.
DME stands for Distance Measuring Equipment.
Often times, DME is co-located with VORs and other navigation aids.
A VOR co-located with DME (Distance Measuring Equipment).
TAC stands for TACAN and is for military use only.
To civilian pilots, VORTAC and VOR/DME mean the same thing.
On VFR Aeronautical Charts they are distinguished as shown in the FAA Chart User Guide.
Distance measuring equipment (DME) consists of an ultra high frequency (UHF) navigational aid with VOR/DMEs and VORTACs.
It measures, in NM, the slant range distance of an aircraft from a VOR/DME or VORTAC (both hereafter referred to as a VORTAC).
Although DME equipment is very popular, not all aircraft are DME equipped.
Many modern aircraft are not equipped with DME and instead use GPS distances.
How it Works:
In order to use DME, an aircraft must be equipped with DME equipment.
To know if your airplane is equipped with DME, check your POH (Pilot’s Operating Handbook).
The distance relayed to the pilot in Nautical Miles (NM) and is considered “Slant Range Distance.”
It is called Slant Range Distance because it measures the slanted distance from the aircraft to the NavAid.
Because DME measures in Slant Range, it can have some errors.
DME is most accurate when the aircraft is further away from the NavAid and at a lower altitude.
However, DME is less accurate when the aircraft is close and high.
In this example, we are 5,000’ above the NavAid.
Hence, DME will tell us that we are 1nm from the NavAid when we are actually 1nm above it.
However, as stated earlier, many modern aircraft are not equipped with traditional DME.
Instead, they are equipped with GPS Distance.
GPS Distance uses satellites to relay “Straight Line Distances” to the pilot.
Therefore, eliminating the errors associated with DME and Slant Range.
FAA Sources Used for this Lesson
Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 16
Aeronautical Information Manual (AIM) Chapter 1
FAA Chart User Guide