Definition
A combined control on a Distance Measuring Equipment (DME) unit that powers the receiver on or off and adjusts the audio volume of the station's Morse code identifier. Rotating the knob from the off position powers up the unit, and continued rotation increases the volume of the identification tone used to confirm the correct DME station has been tuned.
Plain English
One knob that does two jobs on a DME: it turns the unit on and off, and it sets how loud the station's identifying beeps sound in your headset.
Context Anchor
Seen on or near a DME control/display unit when setting up navigation equipment before or during instrument flight.
Why Pilots Care
Allows the pilot to power up the DME for slant-range distance and groundspeed data while setting identification tone volume to confirm the correct station is received.
Intuition Check
Do not read “volume” as changing the distance information. Here, volume means the loudness of the DME identification audio, while ON/OFF controls electrical power to the unit.
Example Sentence 1
After tuning the DME frequency, the pilot rotated the ON/OFF/Volume Switch clockwise to power the unit and bring up the Morse code ident.
Example Sentence 2
During the approach briefing the pilot confirmed the ON/OFF/Volume Switch was set so the DME audio remained audible without overpowering other radio traffic.