“I’ve heard we’re supposed to monitor 121.5 (Guard) while enroute. I don’t do it, and I don’t know any pilots who do. Should I do this, and if not how should I use my second radio during cruise?” — Mahmoud A.
“If you have two radios, put them both to use as long as that doesn’t distract you from your primary task of flying the airplane.
There are several frequencies you might monitor in flight. Guard, a.k.a.121.5, is certainly one of them. Not only does the AIM recommend it, it’s actually required by FDC NOTAM. The NOTAM states that, ‘all aircraft operating in the United States national airspace, if capable, shall maintain a listening watch on VHF guard 121.5 or UHF 243.0.’ The value of 121.5 is that someone is always listening—every ATC facility monitors it. You might be able to relay for another pilot in trouble, and when you need help yourself one day, the frequency will be ready to go.
Another option is monitoring the Center or Approach control covering your current area. The best way to do this is by using flight following. Even on a local practice flight, consider obtaining it for the extra set of eyes it provides. But if you’re not using flight following, you can still monitor to hear other traffic on the frequency. In this ADS-B world where ATC can see your N-number even without flight following, a controller might reach out to you.
This happened to a friend of mine in his Bonanza. Center called him directly about a potential traffic conflict, even though he might not have been listening. He documented the encounter in the video linked in the comments below.
Finally, a free radio can be used to monitor nearby CTAF frequencies. You might hear traffic departing or arriving, or maneuvering in your practice area. Just don’t clutter the frequency with unnecessary calls. Saying you’re ‘crossing the field southbound at 5500’ isn’t useful, and it will be heard at every airport on that frequency for hundreds of miles.”
Do you routinely monitor 121.5 during cruise flight?