Definition
On a DME (Distance Measuring Equipment) control unit, the mode switch is the selector that determines which function the unit performs and which information is displayed. Typical positions include OFF, distance (NM), groundspeed (GS), time-to-station (TTS), and a hold or remote-channel selection that lets the DME stay tuned to one station while the navigation radio is retuned to another.
Plain English
It is the control on the DME unit that you turn or press to choose what the DME shows you — distance to the station, your groundspeed toward it, or how long until you reach it. It also turns the unit on and off.
Context Anchor
Seen on or near the DME control head when setting up distance information for instrument navigation.
Derivation
‘Mode’ comes from the Latin modus, meaning ‘manner’ or ‘way.’ A mode switch literally selects the way the equipment operates — which job it is doing at the moment.
Why Pilots Care
Gives immediate access to the most useful piece of DME data for the current phase of flight without needing separate instruments.
Intuition Check
Do not read “mode” as a general flying technique here. In this context, it means the selected operating state of the DME equipment.
Example Sentence 1
After tuning the VORTAC, the pilot moved the mode switch from OFF to NM to display distance from the station.
Example Sentence 2
During the approach the instructor had the student switch to groundspeed mode to check for wind drift.