For some strange reason I cannot seem to find any information about what button to push on the instrument panel to active the DME. There is plenty of information about how DME works with interrogation and slant distance, but I cannot seem to locate any information about what button to actually push to activate DME when I am sitting in the cockpit (in a Cessna 172, for example) or where to look on the instrument panel to read the measured distance the DME is telling me. So if the DME is measuring 8 nautical miles, where on the instrument panel will that "8" be displayed?
Typically, there isn't a "button" as such, you tune the DME to the frequency of the ground station (or slave it to the NAV radio).
The knob on the right is used to tune a frequency (shown before MHZ) when in "frq" mode, which is usually a VOR/DME. The numbers before NM is your distance to/from the station in Nautical Miles.
For the classic DME instrument, see Jamiec's answer.
If your aircraft is equipped with a G1000 glass cockpit, the DME window needs to be enabled via the PFD menu using the buttons at the bottom of the PFD:
Displaying the DME Information Window:
- Press the PFD Softkey.
- Press the DME Softkey to display the DME Information Window.
- To remove the DME Information Window, press the DME Softkey again.
The Window will then be shown to the left of the HSI at the bottom of the PFD:
DME INFORMATION WINDOW
The DME Information Window is displayed above the BRG1 Information Window on the 360° HSI and in a box above and along side the Arc HSI. It shows the DME label, tuning mode (NAV1, NAV2, or HOLD), frequency and distance. When a signal is invalid, the distance is replaced by -.-- NM. Refer to the Audio Panel and CNS Section for information on tuning the DME.
Both Jamiec’s and Bianfable’s answers are right for the avionics equipment represented in their posts. Your actual answer will depend on what equipment you have in your aircraft. In my own recent experience, I have not seen an operable DME radio in a Cessna 172. I’ve only seen one operable ADF in a Cessna 172. In most cases, they will look like smaller versions of your NavCom radio. Mainly, I see at least one of the NavCom radios retrofitted to a GPS unit like the Garmin G430 or G530. This is a preferable expense than maintaining technology that is becoming increasingly obsolete. And, per AIM 1-2-3, RNAV/GPS can be substituted for DME.
If you are flying a glass panel aircraft like one equipped with a G1000, G500, or Dynon Skyview, the GPS distance to your selected point will be displayed on your PFD and is an option on your MFD. This can be found in the top info bar of your PFD. As long as the position of the NavAid is updated in your GPS database, you can select it as a fix and use this distance for navigational purposes.